Originally explained on the blancolirio channel on YouTube -
The timing and manner of the break make a lot more intuitive sense when you consider that the engine is essentially a massive gyroscope. As the plane starts to rotate, the spinning engine resists changes to the direction of its spin axis, putting load on the cowling. When the cowling and mount fail, that angular momentum helps fling the engine toward the fuselage.
From a failure analysis perspective that is much less relevant though. The first failure was the rear engine mount if it had been a secondary failure it would have been deformed first and then broken, and it clearly is not. It just tore in half on one of the four connections and then the rest deformed slightly due to overstress.
It was however relevant to the survivability of the accident: if the left engine wouldn't have detached, or would have detached in a more "manageable" way, the other engine (probably the tail engine from how it looks) wouldn't have been affected too, and the pilots would have had a better chance to take off. Plus the whole "when an engine detaches, it shouldn't start a fire in the wing it was attached to" part of course...
Yes, agreed, the secondary safety wasn't there either. There was a Boeing accident near Amsterdam with the plane crashing into an inhabited area, it had dropped two engines but kept flying, at least for a while...
A 747 with 2 engines out that's already got some altitude and speed is much more "survivable" (still extraordinarily difficult) than a trijet trying to take off on 1 engine, which is impossible.
Planes can safely land with 0 engines, though this is obviously "not ideal." See: Gimli Glider.
Yup. That's exactly what experts said of American Airlines flight 191 which was basically the same engine mount, same failure. Engine flipping over the wing.
The failure of the pylon appears to be different. On AA 191, the pylon rear bulkhead cracked and came apart. In the case of UPS flight 2976, the pylon rear bulkhead looks to be in one piece, but the mounting lugs at the top of the rear bulkhead cracked.
American 191's engine mount failed because of improper maintenance. It remains to be seen whether this failure had the same cause or if it was something else, such as metal fatigue.
A failure due to metal fatigue would still be a failure to properly maintain the aircraft, right? I know by "improper maintenance," you're referring to actual improper things being done during maintenance, and not simply a lack of maintenance. But I'm reading things like "the next check would've occurred at X miles," and, well... it seems like the schedule for that might need to be adjusted, since this happened.
Yes, when I said "improper" I meant the American 191 maintenance crew took shortcuts. The manual basically said "When removing the engine, first remove the engine from the pylon, then remove the pylon from the wing. When reattaching, do those things in reverse order." But the crew (more likely their management) wanted to save time so they just removed the pylon while the engine was still attached to it. They used a forklift to reattach the engine/pylon assembly and its lack of precision damaged the wing.[0]
Fatigue cracking would be a maintenance issue too but that's more like passive negligence while the 191 situation was actively disregarding the manual to cut corners. The crew chief of the 191 maintenance incident died by suicide before he could testify.
> The crew chief of the 191 maintenance incident died by suicide before he could testify.
To be clear, a crew chief (Earl Russell Marshall) did. But he wasn't directly involved in maintenance of the specific DC-10 that crashed. Or at least, I haven't found a source saying he was, and some sources say he wasn't. https://www.upi.com/Archives/1981/03/26/The-wife-of-an-airli...
If the (FAA-approved) maintenance schedule says "the next check should occur at X miles" and X miles hasn't happened yet, then it's not going to be classified as improper maintenance -- it's going to be classified as an incomplete/faulty manual.
Now, of course, if that maintenance schedule was not FAA-approved or if the check was not performed at X miles, that's going to be classified as improper maintenance.
According to various comments the plane was nowhere near the cycling for a special detailed inspection of the aft pylon mount lugs: SDI is at 29200 cycles and the plane had 21043.
There was a lubrication task in October, but according to tech comments that would just in greasing the zero fittings, no taking apart anything.
Those pictures of that torn up part are pretty hefty, that's a clean break, no stretching as far as I can see it just tore the material in half, you can see the grain. There does not seem to be any torsion either so most likely that was the first part to go, if the problem had been in the engine then I would expect this part to be mangled, not pulled apart. What stress damage there is occurred shortly after that first break. A valid question would be whether or not that crack was there before take-off or not.
I'm very curious what the metallurgic analysis of the mirror part on the other wing will come up with, especially whether there are any signs of stress fractures in there. If there are that will have substantial consequences for the rest of the still flying MD11's, about 50 or so are still in service.
The preliminary report mentions fatigue cracks on both sides of the aft lug, and one side of the forward lug, with the other showing no trace of fatigue, only overstress.
From this it seems like the aft lug was way fucked, and the forward lug was hanging on for dear life, until it could not.
Yes, that's exactly how I read it. The aft lug was the first to go, the forward lug shows signs of stress so it held on longer.
I don't think they're going to be skimpy on the metallurgy report so looking forward to the analysis of the mirror parts on the other wing. Those will tell without a doubt whether it was maintenance related or age related fatigue. Right now I would bet on the latter but the former could also still be a factor, for instance, that bearing might not have had enough lubricant.
It depends. This aircraft was made near the beginning of the MD-11 production and if the original analysis for the fatigue life of this location was wrong, then you would expect to see that appear in older aircraft first. If that ends up being the case then it's not an inspection or maintenance issue, it's an engineering failure. Given aerospace accident history I would say that is less likely than some maintenance issue but we won't know for sure for a bit.
Even if it was an inspection or maintenance issue (which given the kind of failure and available data looks increasingly doubtful, though it can not yet be ruled out) this part failed in a catastrophic way when it should have had ample engineering reserve over and beyond the load to which it was subjected. It just snapped clear in half those breaks are indicative of a material that has become brittle rather than that the part deformed first and then broke due to excess stress.
In other words, a slow motion video of a camera aimed at that part during the accident would have shown one of the four connections giving way due to fatigue cracks and then the other three got overstressed and let go as well, in the process damaging the housing of the spherical bearing.
The part at the bottom of page 9 is the key bit. Now I very much want to see the state of the mirror part on the other wing, that will show beyond a doubt whether it was maintenance or an over-estimation of the design life of that part.
It would also be interesting to have a couple of these pulled from the fleet and tested to destruction to determine how much reserve they still have compared to the originally engineered reserve.
According to the preliminary report, 3 of the 4 showed fatigue cracks, and the 4th overstressed. So yes, agree a random sample of these parts should be pulled from the fleet and tested - but something pretty crazy was happening here re: fatigue.
That it was so far from the maintenance schedule to be inspected AND that the fatigue cracks seem to have formed in areas that would be hard to visually inspect anyway points to either a engineering problem (especially bad, since the DC10 problem of a similar nature happened in roughly the same parts, albeit due to different abuse - you’d think the engineers would overdo it there, if nothing else), or some specific type of repeated abuse that particular pylon received, which is pointing more to a design problem.
Re-reading the 1979 report might be helpful here. This isn't my field, but it seems that the engine is attached "hard" to the pylon, then the pylon is attached via a bearing mount system to the wing frame. The bearings wear out, and hence have to be replaced (not sure how often, but they were doing it on the entire fleet prior to the 1979 crash). The 1979 investigators thought that the fatigue cracks were caused by removal of the entire pylon/engine assembly as one unit (because that put excess stress on the aft bearing, they suspected due to support being provided from below by a fork lift). After the 1979 accident engines had to be removed first, then pylon, supposedly removing that cause for mount cracking. Perhaps there was another cause.
Before the 1979 accident, engines also had to be removed first.
Airlines have to follow the approved maintenance manual procedures; that manual called for engine removal and installation from a pylon that was on the wing. American was improvising a maintenance procedure without the legal authority to do so, resulting in 191.
There is nothing here to say it being a maintenance issue is doubtful. It could quite literally be a similar issue to Flight 191, we don't know yet.
It did have ample engineering reserve beyond the load it was subject to... before fatigue damage initiated a crack which then grew until there was no reserve left. The question is why the fatigue crack initiated prematurely? Maintenance damage? Analysis mistake? We don't know yet.
If you read the original 1979 report in full, I think you'll begin to realize that this "improper maintenance" thing was a cover-up. Actually quite similar to the 737MAX -- find someone or something to blame other than the design of the aircraft, then move on.
The picture of that part that is torn into two pieces certainly seems to suggest so, that's a clean break, not an overstressed part deforming and then breaking.
Ironically, AA flight 191 could have been salvageable, because the engine detaching didn't start a fire. However, it led to loss of hydraulic pressure on that wing, which led to the flaps/slats retracting on just the left wing, which led to the plane becoming uncontrollable. After that accident, the DC-10 was retrofitted with hydraulic fuses to prevent something like this happening again. Unfortunately, that didn't help the UPS crew, because in their case, the detachment caused more damage to the wing...
Flipping backwards is what caused the engine to fly to the right and land to the right of the takeoff runway. The stills in the NTSB preliminary report clearly show the engine flying over the aircraft, to the right, and then heading straight down.
Because making every jet engine in two different models would make them a lot more expensive. It would also cause maintenance issues because of non-interchangeable parts.
Not an aviation expert at all, so take this with a grain of salt, but I think "the spinning engine resists changes to the direction of its spin axis" offers two important insights:
* why it failed at rotation (the first/only sudden change of direction under full throttle) rather than as soon as it was mounted onto the plane, while taxiing, as soon as they throttled up, mid-flight, or on landing. This is important because at rotation is the worst possible time for this failure: no ability to abort take-off, no ability to land safety under no or severely limited power, little time to react at all, full fuel. Knowing these failures are likely to manifest then stresses the importance of avoiding them.
* why it failed in such a way that it damaged the rest of the plane.
Not so much what was wrong with the mounting in the first place, if that's what you're asking. Presumably it was designed to withstand the forces of this moment and clearly has done so many times before.
Well, some force flung it inboard and above the fuselage (gods, that CCTV stills sequence.) Knowing that the engine rotates CCW, there are not many candidates.
> There are lots of candidates for a failing engine yeeting itself in any direction.
For the precise trajectory, certainly; for the general direction, not so much. Could you describe a combination of forces that would have thrown that engine to the left of the direction of travel? (We're talking about this accident, not any engine anywhere.)
My broad comment is that gyroscopic precession having any critical role in this is incredibly far fetched. That said, I've never flown or worked on a turbofan so ¯\_(ツ)_/¯.
Gyroscopic forces might have changed the direction of travel a few degrees, but the motive force comes from the engine's thrust, the power of its spinning blades pushing air. An engine cut loose at full power moves forward like a missile.
Yes, obviously; MD-11s aren't flinging engines off the wing every single takeoff. A 34 year old airframe may or may not actually match design strength, though.
Yes, but the point is that this moment of the takeoff is when a failure that's been waiting to happen is most likely both because of the thrust and the gyroscopic resistance.
Did I understand the report correctly that the part was scheduled to be replaced in the future after a certain number of hours, it just hadn't hit the threshold yet ?
If you're referring to this quote (excerpted from the AVHerald article linked elsewhere in the thread), I don't think so:
> At the time of the accident, N259UP had accumulated a total time of about 92,992 hours and 21,043 cycles [..] A special detailed inspection (SDI) of the left pylon aft mount lugs would have been due at 29,200 cycles and of the left wing clevis support would have been due at 28,000 cycles
This isn't talking about replacement, only inspection; and it wasn't going to happen in the near future: 7k cycles at four flights/day means inspection is due in 5 years.
It wasn't doing four flights per day. As a long-distance cargo aircraft it was doing two flights per day, and I doubt it was flying every single day of the week.
So we are talking about at least 10 years before that inspection was due.
I'd be very surprised to read that the aft lug that cracked (and the bearing it contained) were made of aluminum. They were almost certainly steel or Inconel.
I know Veritasium gets posted here a lot, but a few days ago he posted a deep-dive into the the engineering of jet engine turbine blades. Turns out they're made from a single crystal of a superalloy that entangles itself at a molecular level such that it actually gains strength as it's heated, only losing strength above 1200 degrees C / 2200 degrees F. Below that temperature, as long as the strain on the part is below the plastic deformation threshold, it's not really losing any strength at all over time.
No; roughly, yes. Based on the crystal structure of the metal, fatigue works differently.
> The fatigue limit or endurance limit is the stress level below which an infinite number of loading cycles can be applied to a material without causing fatigue failure.[1] Some metals such as ferrous alloys and titanium alloys have a distinct limit,[2] whereas others such as aluminium and copper do not and will eventually fail even from small stress amplitudes.
Grounding all MD-11s and DC-10s is a major move. I guess it makes sense as a big factor was the fatigue cracks on the pylon (lugs), despite the pylon not being behind on inspections. I am wondering what the inspections of pylons in other planes will yield, likely that will determine whether the grounding will continue.
But beyond figuring out why the engine mount failed, I am very interested in what caused the actual crash. "Just" losing thrust in a single engine is usually not enough to cause a crash, the remaining engine(s) have enough margin to get the plane airborne. Of course this was a major structural failure and might have caused additional damage.
EDIT: It seems there was damage to the engine in the tail, even though this was not specified in the preliminary report, likely because it has not been sufficiently confirmed yet.
And if the failure of a wing engine can cause the rear engine to fail, that would raise concerns about all "two in front one in back" trijets. Similar to how putting the Space Shuttle orbiter's heat shield directly in the line of fire for debris that comes off he rocket during launch turned out to be a bit of a problem.
At this point there aren’t any trijet designs like that being built, and it’s unlikely we’ll ever see a new trijet design. It served a role in the transition from four engines to two, but now with ETOPS-370 there’s no commercially viable route that can’t be served with an appropriate twinjet.
There are several passenger trijets still existing - they are just not commercial airliners. Dassault for one is quite fond of the design; the Falcon 900, 7X and 8X are trijets, and I'm pretty sure the latter two are still in production. I wouldn't be at all surprised to see another trijet design from them probably around 2030.
And the failure of an engine mounted on the left wing can cause debris to cross through the fuselage structure and cause a failure of the engine mounted on the right wing, or to fly thousands of feet in any particular direction, as happened to American Airlines in both a ground run incident, and in their Flight 883 accident.
The industry also responded to those crashes. For example, the El Al 1862 incident prompted a redesign of the engine strut that was subsequently mandated as a retrofit for all 747s.
And here's a more detailed description of that ground run incident. It also found that the failure was related to a design flaw, and mandated that aircraft be grounded for inspection and rework. https://skybrary.aero/accidents-and-incidents/b762-los-angel...
I'm not a regulator or aerospace engineer or anything like that so I can't really say which actions are or are not appropriate. But I do want to observe that these are all unique failures with unique risk profiles that can't all be painted with a single broad brush. All I was trying to do in the previous post was speculate on why a MD-11 failure could result in a grounding of the DC-10 and KC-10A as well. The first thing that came to mind is that I think those are the only remaining trijets of that general shape that are still around. Though I suppose another possibility is that they all share an identical pylon design or something like that.
> Though I suppose another possibility is that they all share an identical pylon design or something like that.
They're very closely related planes (MD-11 is an upgraded DC-10; KC-10A is a military version of the DC-10), so that wouldn't be surprising. Likely the KC-10A has the same pylon, and the MD-11 has one that's similar enough that it's worth being cautious.
Yeah, the trijet design seems failed in general. Unless you can design it to tolerate any wing+tail dual engine failure -- in which case, why have the tail engine at all?
It wasn’t failed. It was designed for a very specific reason and served that purpose well.
Once the reason went away, better designs took over.
They were designed to allow smaller jets to fly over the ocean further than a two engine jet was allowed (at the time). Airlines didn’t want to waste all the fuel and expense of a huge 4 engine jet, but 2 wouldn’t do. Thus: the trijet.
The rules eventually changed and two engine jets were determined to be safe enough for the routes the trijets were flying.
Using two engines that were rated safe enough used less fuel, so that’s what airlines preferred.
It was never designed to be used anywhere else as a general design. Two engines did that better.
You've framed this as disagreeing with me, but I don't think you are. I agree the design made sense in the 1960s, when we didn't know any better and requirements were different.
No, you really can't. Even if it were the same size a dramatically more powerful engine would need a larger "tail" to maintain control in case of an engine out scenario. But a 50% more powerful engine is also likely to be much bigger meaning that major components like the landing gear (and everything around them). A 50% more powerful engine is also likely to be much heavier necessitating its support structures (a.k.a. the wing or tail) be redesigned.
The 737 MAX suffered a number of bad design decisions to accommodate its newer, more powerful engines. Its engines topped out at about 8% more powerful than the 737 NG engines.
Look at thrust on the 737 Max vs thrust on the original 737.
There's a lot of other changes, of course, but more powerful wing engines let you build a bigger plane in the same kind of shape. Changes in flight rules are also significant; if twin jets can't serve all your routes, you most likely want trijets to cover the routes that can't be served by twins and don't demand a quad ... with current flight rules and current engines, twin engine covers pretty much everything.
Essentially every new design is a twinjet, so it's clearly possible to make appropriate decisions in that design space. And both Boeing and Airbus have given up on quadjets.
The MD-11 isn't a new design. It's a stretched version of a first generation widebody whose design dates back to the mid-1960s. Before the MD-11 was developed, McDonnell-Douglas toyed with the idea of a dual engine variant before settling on a three engine version of the DC-10. Trijets in general came about because the engines of the day were too unreliable and too small to work in twin engine configuration at that scale.
The plane which ended up being the final nail in the MD-11's coffin, the 777, didn't start development until the 90s. Of its three initial engine choices, two were derivatives of engines that were around when the trijets came to be. The initial version of that Rolls Royce engine was so late (and so unreliable) that it essentially killed the Lockheed trijet. The third option, the GE90, was the largest turbofan engine at its introduction until it was succeeded in 2020 by the GE9X.
Scaling these earlier engines up to fit an MD-11 sized twin was never an option.
When I replied to this thread[0], with this comment[1], both the comment I was replying to and my comment were talking about trijets in the abstract, not MD-11s in particular, and the current year, or perhaps as early as the 1990s, but definitely not as early as the 1960s.
Several comments, including yours, seem to have misconstrued that to mean I think the MD-11 in particular could be retrofitted into a twinjet. That's, uh, wildly mistaken, and not something I've ever claimed. I just think trijets in general are a design dead-end. Again, that doesn't mean it didn't make sense in 1960s when the DC-10 was being designed.
>both Boeing and Airbus have given up on quadjets.
It is possible “to make appropriate decisions” up to a certain size. They didn’t stop making new quadjets because the design doesn’t work as well as a twin engine, but because airlines don’t need/want aircraft that large. You wouldn’t build a successor to the A380 as a twin engine.
At some point it comes down to probabilities. With so many flights going on, one in a million incidents become a certainty. For example UA232 [1] suffered failure in all 3 redundant hydraulic systems due to an uncontained engine failure. Any of the 3 systems would have been enough to retain control of the aircraft. Of course this lead to some investigations on why all 3 systems could be impacted at the same time and what can be done to limit failures.
Besides the technical aspects that flight is an impressive example of resilience and skill. Bringing that plane down to the ground in nearly one piece was essentially impossible and a one in a million chance in itself.
Airlines operate to a much stricter standard than one in a million. If one in a million flights ended in a fatal crash, the US alone would see about 3 airline passenger deaths per day on average. The actual average over the past 10 years is under 0.02 deaths per day.
It's true that you can never get to zero. There's always a chance of some catastrophic failure. The lesson of modern airline safety is that you can get extremely close to zero by carefully analyzing and learning from the failures, which is exactly why these thorough investigations are done. The lesson from UA232 was to make sure one failure can't take out all of the hydraulic systems.
In this specific instance, "the engine fell off and took out another engine, leaving the aircraft with insufficient power to climb" is definitely not in the realm of "probabilities will get you eventually." It's very much in the realm of a mechanical failure that should not happen, combined with a bad design flaw that turns that failure from a mere emergency into pretty much guaranteed death.
Cargo is held to a lower standard than passenger service, but I suspect this will still spell the end of the DC-10 and MD-11, at least in the US. Engines will fail, and for an aircraft of this size, that needs to be survivable in all phases of flight just for the safety of people on the ground.
> The lesson of modern airline safety is that you can get extremely close to zero by carefully analyzing and learning from the failures, which is exactly why these thorough investigations are done.
I have heard it said that "every air safety rule is written in blood."
If the engine had just failed, they would very likely have been fine. Experienced crew, would likely have handled it. But the engine came off the wing, and then another engine was damaged. At that point there was no recovery possible.
This is understating it. Any minimally competent crew should be able to handle a single engine failure on takeoff (in a normal scenario, not this one). It’s absolutely within the performance envelope of the aircraft and is something that crew train for. If pilots were not routinely able to handle this kind of failure, we’d see a lot more crashes.
The pilots did (apparently) exceptionally well keeping the plane level even with unbalanced weight and nearly no thrust; perhaps they had been over water they’d even have been able to ditch successfully.
This is still understating it. Any barely competent crew should be able to handle a single engine failure on takeoff (in a normal scenario, not this one).
> Airlines operate to a much stricter standard than one in a million. If one in a million flights ended in a fatal crash, the US alone would see about 3 airline passenger deaths per day on average.
I think you conflated flights (several 10Ks per day) with passengers (several million per day).
One in a million flights is one accident every few decades.
> at least in the US. Engines will fail
As per the report, this appears to be a structural failure, not an engine failure.
If randomly distributed, one in a million flights crashing and killing all passengers means that one in a million passengers dies.
The US sees about 25,000 airline flights per day, or around 9 million per year. So with one in a million flights crashing, we'd expect roughly 9 crashes per year.
Even if they end the grounding of the MD-11/DC-10 I'd be shocked if any airlines still using them will continue to use them.
Seems like the risk/reward just isn't really there for the few of them still in service, and if anything happened it would be a PR nightmare on top of a tragedy.
UPS and FedEx each have around 25 MD-11s, Western Global has 2 I think, the Orbis Flying Eye Hospital is an MD-10, some cargo airline in Botswana has one, and 10 Tanker has some DC-10 firefighting tankers.
Given that the report only mentioned a single other seemingly related accident in 1979 I am not sure that objectively this is a reason to discontinue flying these planes. The fact that these planes have been in service since the early 70s is a testament to their safety and reliability in itself. Of course public perception, especially with the videos of huge fireballs from hitting one of the worst possible locations, might put enough pressure on airlines to retire the planes anyway.
I agree on the end of an era. Hearing something else besides just Airbus- or Boeing-something always gives me a bit of joy. Even though MDs and DCs are of course Boeings in a sense now as well.
I managed to find some statistics on hull losses per million departures [1, p. 13]. Seems like indeed MD-11s have a highish rate of incidents by that metric compared to other types, even if they are not catastrophically less safe than other planes. That metric stacks the statistics a bit against cargo planes, which most (all?) MD-11s are now. These planes tend to fly longer haul instead of short hop, so you get more flight time/miles but less departures. There are also likely some other confounding factors like mostly night operations (visibility and crew fatigue) and the tendency to write off older planes instead of returning them to service after an incident. Plus these aircraft have been in operation long enough that improvements in procedures and training would impact them less than more modern types, as in they already had more accidents before these improvements.
The DC-10 had a number of other problems, but the MD-11 has always had a reputation of being an unforgiving aircraft especially when compared to the DC-10. It's less about training and more that the MD-11 was simply too many design compromises piled on to an old design.
The MD-11 had a pretty short service life as a passenger aircraft because it simply wasn't very fuel efficient compared to the competition, safety wasn't really the motivating factor. However fuel consumption was behind some of the poor design choices McDonnell/Boeing made. In broad strokes: McDonnell/Boeing shrunk the control surfaces to improve fuel consumption "necessitating" poorly designed software to mask the dodgy handling and higher landing speeds. This exacerbated a DC-10 design "quirk" where hard landings got out of hand very quickly and main landing gear failure would tend to flip the plane.
Yeah you can train around this but when something else goes tits up you've got a lot less leeway to actually recover safely.
I think that the Mad Dogs only exist as freighters (~or their derivative KC-10 tankers~-Edited to correct that they retired last year) these days. I think the last pax service for any of them was over a decade ago.
And air freight just gets a lot less public attention, I think they are going to keep flying them if they don't get grounded.
Yes, but there are many MD-11's still flying as freighters. There are four fire-fighting DC10's out of ~8 still flying, but there are 25 Mad Dogs (MD-11) at UPS, 38 with FedEx, and Western Global has 4, so there are plenty of MD-11F's around.
(Blancolirio points out that the DC-10 tanker is what they modernized to relatively recently -- before that they were flying even more dangerous WW2 airframes for firefighting.)
There were only about 18 aircraft of all types still flying at the time and none are dedicated passenger aircraft. Indefinite grounding is the most sensible move.
Now, Boeing, et. al. are trying to evaluate the deficiencies in existing D checks and put together an inspection regime (i.e., NDT) that would proactively identify fatigue that would economically permit continued serviceable operation. If this feat turns out to be impossible for technical or cost reasons, then and only then will the grounding will become permanent.
> EDIT: It seems there was damage to the engine in the tail, even though this was not specified in the preliminary report, likely because it has not been sufficiently confirmed yet.
Yes, the initial videos were showing the tail engine flaming out. And in the 1979 crash, the engine also severed hydraulic lines that hold the slats extended. So they folded in due to the aerodynamic pressure, essentially stalling the wing.
Based on the original descriptions of the crash, I assumed the engine fell off.
From the photos, it’s clear it went up over the wing and impacted the fuselage with a (at least) minor explosion, which would have thrown foreign objects into the third engine in the tail for sure.
Losing 2/3 of the engines isn’t survivable on takeoff for this class of plane, at the weights they were at.
It's an engine - the thing pushing the entire plane forwards. Provided it is running (and at takeoff that's definitely the case), an engine being liberated from its plane suddenly has a lot less mass holding it back, so the logical thing to do is to shoot forwards. And because the wing is attached to the upper side of the engine, anything short of an immediate failure of all mounting points is probably also going to give it an upwards trajectory.
Add in air resistance, and you get the "swing across the wing and back" seen in the photos.
Sure, but if the engine grenades it can take it’s mounts with it and not shoot off like a bottle rocket in front of and over the plane, dropping down and under the plane instead (or even just sit there). Same with a compressor stall, or whatever.
It’s clear from the photos this wasn’t the engine failing at all, and in fact the engine kept producing a ton of thrust (probably until it ran out of fuel as it pulled it’s fuel line apart while departing the wing), and instead the thing that is supposed to be so incredibly strong that it restrains all this chaos failed.
Which is a pattern in this family of aircraft, but definitely not a common or normal thing in general eh?
Most aircraft, the engine stays with the airframe even if it turns into a giant burning pile of shrapnel and dead hopes and dreams.
Engine pylons are actually usually designed to fail in a particular way to ensure the separation happens as safely as possible; obviously that didn't happen here, which will probably be something the NTSB will have to investigate why.
The up and over is usually actually the safer direction I think? But in this case it also moved laterally, which is possibly what fouled the tail engine and made it unrecoverable. Will be interesting to see the final report.
Fully functioning engines departing from aircraft isn't common but it's not unheard of either. Off the top of my head it's happened a few times on the 747 and 737.
All cargo companies run a wide fleet of many different plane types, particularly to avoid this very problem of being grounded by the FAA. But yes, these were still widely used in cargo transports. Although newer 2 engine planes can haul the same kg and use a lot less fuel.
It wasn't just one engine off, aside from possibly damaging tail engine you also have damage to the wings and control surfaces that might've just not got enough lift because of that.
"Your IP address 104.28.103.15 has been used for unauthorized accesses and is therefore blocked!
Your IP address belongs to Cloudflare and is being used by many users, some of which are hackers and hide behind the cloud/proxy to avoid being tracked down. Hence the automatic defense closed access from that IP address.
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That's a pretty nice message. Most sites that filter VPNs and proxies just kill the connection, give a generic error, or subject you to endless captchas.
So could everyone that blocks network traffic for various reasons, but usually they don't because they're not doing it in the primary application layer, but using a WAF or reverse proxy or something else in front of their application... and also most DGAF to cater specifically to the users they block.
Again, you're usually lucky to even get a return packet.
> On the aft lug, on both the inboard and outboard fracture surfaces, a fatigue crack was observed where the aft lug bore met the aft lug forward face. For the forward lug's inboard fracture surface, fatigue cracks were observed along the lug bore. For the forward lug's outboard fracture surface, the fracture consisted entirely of overstress with no indications of fatigue cracking
If I'm parsing this correctly, they're saying that fatigue cracks should have been visible in the aft pylon mount, and that the forward mount was similarly fatigued but showed no damage on the outside?
> If I'm parsing this correctly, they're saying that fatigue cracks should have been visible in the aft pylon mount, and that the forward mount was similarly fatigued but showed no damage on the outside?
If you can get to the report, Figure 7 shows the left pylon, with the forward and aft lug enlarged in the inset. Both lugs cracked on two sides. They're saying both cracks on the aft lug as well as the inboard crack on the forward lug were observed to be fatigue cracks, but the forward lug outboard fracture was observed to be entirely a stress crack.
Outboard and inboard are just away from and towards the center of the plane. On the left pylon, that's left and right, respectively. So, it looks like the left side crack in the forward lug developed from overstress, but the other three cracks were from fatigue. My expectation is that fatigue should be apparent upon the right kind of inspection, if timely, even if the metal has yet to fracture.
Very fast. Quite sad to see it happen. Also quite puzzling is how the Air India disaster still does not have a root cause analysis done (though supposedly it will be released end of this year)
> Also quite puzzling is how the Air India disaster still does not have a root cause analysis done
Nothing puzzling. Straight-up cover-up.
Now, the interesting part would be to know what is being covered-up. Pilot error? Pilot suicide? Or a critical system malfunction Boeing cannot afford?
The Indian authorities has blamed the pilots in every single crash. AND there is not enough evidence to guarantee that was the case. It is one of many possibilities.
40 years between severe accidents is fine in terms of expected failures. It's also not a good comparison because in the 70s maintenance crew were using a forklift to remive engines, improperly stressing the engine pylon. This was done as a shortcut
How do you figure? They're very similar planes. The left engine and its pylon detached in both cases during takeoff rotation. Both incident reports stated that proper maintenance would have prevented the detachment.
The way the situation played out is different but the failure mode seems to be very similar if not the same.
The NTSB report itself even references AA-191 as the only "similar event".
AA-191 was caused by improper maintenance (dreamed up by people who were made to cut corners and was never compliant with manufacturer spec) damaging the pylons holding the engine.
If someone did the same thing again, that would be rather unfortunate. Just more deaths for profit, even though we know it was dangerous.
The parts that seem to have fatigued and failed were only like 80% of the way through their inspection period. They were to be inspected after 28k cycles. They were at 21k cycles.
It sure looks the same from "Engine pulled itself off and flew away" angle, but if there is any similarity under the surface that's very bad. Flying was much much less safe in the 70s.
The report doesn't say that because it's just reporting the facts not drawing conclusions. In my opinion, if a catastrophic failure happens that is a maintenance failure. Either the inspections failed to notice the fatigue or the inspection guidelines were too optimistic.
Maintenance was informed by the earlier incident. It's why we haven't seen even more DC-10/MD-11 failures sooner. Designs too have kinda been informed by this -- it's not like Boeing or Airbus make trijets anymore.
> The referenced AA Flight 191 is shockingly similar. It makes me wonder if aviation really is back sliding into a dangerous place.
I think it's cut throat capitalism at its best. Surely it was much too safe before, let's see how far back we can scale maintenance on the operations front but also how far back can you scale cost during development and production and then see where it takes us. If that changes the risk for population from 0.005 to 0.010, the shareholders won't care and it's great for profits.
I think we can see both but especially the latter with Boeing.
The entire MD-11 project was a budget-limited rush-job to try to capture some market share before the A340 and 777 came into service.
It produced an aircraft that failed to meet its performance targets, was a brute to fly and was obsolete the moment its rivals flew.
Douglas* by the early 1990s was a basket-case of warmed-over 1960s designs without the managerial courage to launch the clean-sheet project they needed to survive.
Rather the opposite: if the cause is similar to AA 191, why weren't the actions taken after AA 191 to prevent a repeat effective? If we can get a repeat of that incident, what's preventing the industry from repeating the mistakes from all those other incidents from the past decades? Why aren't they learning from their past mistakes - often paid for in blood?
I understood the post I responded to to be referring to the cause as the engine detaching from the same type of plane, not the root cause for why the engine detached. Per the “investigation section” in the wikipedia article, I would be surprised if it was the same root cause:
There's no such thing as "This is fixed forever". If lax maintenance oversight has led to companies re-introducing known dangerous maintenance procedures or departing from known good ones, then we will be back in the 70s in terms of airplane safety and people will have to die again to relearn those lessons.
Someone's always trying to claw you in the less safe direction. It's a constant battle to not regress.
But IDK, hopefully this plane just got some sort of "unlucky" about fatigue somehow, and it doesn't have far reaching consequences.
I'm surprised at how many years the plane went without having that part inspected. It looks like the failure was due to fatigue cracks, but the last time the part was inspected was in 2001?
I believe the part was at least visually inspected in 2021:
> A review of the inspection tasks for the left pylon aft mount found both a general visual inspection (GVI) and a detailed visual inspection of the left pylon aft mount, required by UPS's maintenance program at a 72-month interval, was last accomplished on October 28, 2021.
These all seem like OCR errors...? Why would there be OCR in this workflow? Did they print this out and then generate a PDF from a scan instead of the original source? To maintain an air gap maybe?
El Al 1862 was another flight [1] that had an engine fall off, taking another engine out with it. The pilots managed to fly around for a few minutes and attempt a landing, but there was too much structural damage.
It doesn't seem aircraft are designed to survive these types of catastrophic failures.
They seem to have lost the tail engine too. Yes, it is a significant problem that engine failures aren't independent, so trijets are kind of a bad design.
Not only did it happen at the worst possible moment, it took out a second engine on it's way out and over the plane. Two engines should've been enough to get off the ground and potentially land the plane, but one engine on a trijet isn't enough.
Yeah, pilots I know saw puffs of flame coming out of the engine, and said that that's a tell-tale sign of a compressor stall. Which could have been caused by debris from the separating left engine striking the turbine.
The video of the aircraft crossing the road wings level (well after #1 separated) and maintaining relatively controlled flight until too much energy bled off suggests to me the aircraft was likely to be controllable to a landing if sufficient thrust was available.
Yeah, if they had had more altitude, I would guess that this would have looked even more like the AA 191 crash from 1979, with the left wing stalling and causing a roll and pitch down.
That in turn reminds me of the DHL flight out of Baghdad in 2003 that was hit by a missile [0]. Absolutely amazing that they managed to keep it together and land with damage like that.
An important factor in AA 191 is that the engine leaving did significant damage to the hydraulic lines in that wing - including those for the leading-edge slats. At the time the plane was not equipped with any mechanism to keep the slats extended, so after hydraulic pressure was lost airflow over the wings caused them to retract, which significantly lowered that wing's stall speed.
After AA 191 the DC-10 was equipped with a locking system: loss of pressure now results in the slats getting stuck in their current position. The MD-11 will undoubtedly also have this system, so a direct repeat of AA 191 is unlikely.
From the wing down I assumed it may have depended if the engine coming out unintentionally means redundant hydraulic lines and mounts are also getting disconnected causing a complete loss of control not that it would have helped much at that point beyond minimizing ground damages.
TIL about this eerily similar DC-10 crash in 2011:
Shortly after liftoff, 20 feet (6.1 m) above and 7,000 feet (2,100 m) down the runway, the No. 2 engine separated from the wing and struck the No. 1 engine's inlet cowling, causing it to produce drag and reduced thrust. Even with full right aileron and rudder, the plane started to descend and drift to the left. The captain lowered the nose and leveled the wings, which was followed by the plane making multiple contacts with the runway. After touchdown, the plane drifted left and departed the runway, crossing a taxiway before coming to rest in a saltwater marsh. A fire erupted which consumed the top of the cabin and the cockpit. All three crew members survived.
...is this a bot comment? The accident you linked is very clearly of a Boeing 707, which has zero relation to a DC-10 and is most decidedly not a trijet.
As I told my friends, this preliminary report annoys me. It annoys me for the same reason it seemingly annoys the NTSB: American 191 is nearly identical on the surface, right down to the engine detachment and resultant loss of the aircraft, in almost the exact same spot on the airframe, ~45 years later.
Needless to say they’re going to be scrutinizing everything to determine what the cause is and the sequence of events that created the accident, but I also suspect everyone involved is just as annoyed at this as I am, given that this exact situation should have been fixed already.
Actually, the two accidents are only superficially similar, and there is no basis for saying that "this exact situation" should have already been fixed.
And no, this is not just pedantry. A bulkhead fracturing from impact damage and its mounting point failing from fatigue cracking are nowhere near the same thing, even if they might lead to the same outcome.
I’m not sure where you’re getting these conclusions from, because AA191 was an issue where the engine separated from the wing and took hydraulics with it, creating a sequence of failures that resulted in the loss of the airframe. The UPS flight also had a separation of the engine from the wing and created some sequence of events that resulted in the loss of the airframe, but they’ve made it very clear in the report that the how and the why are still very much open questions.
My point - and from what I see in the report, a grievance shared by the NTSB, given their citation of AA191 - is that both aircraft come from the same lineage (DC-10/MD-11), the same manufacturer (McDonnell-Douglas, now Boeing), had the same failure (engine separating from wing assembly during takeoff roll), and with all involved parties throwing up their hands and swearing they followed all the rules and maintenance schedules.
And that’s too many coincidences to just dismiss outright from the get-go.
> I’m not sure where you’re getting these conclusions from
I'm not sure how to say this in a way that does not come across as rude, and I apologise in advance if it does, but I got those conclusions (in actuality, statements of fact) from actually reading the reports and understanding the terms and diagrams in them, where it is pretty clearly detailed that the sequence of events does not start with engine separation? You can quite literally read for yourself in both reports (as opposed to relying on a shallow pop culture understanding of the parts involved) the differences in the initial damage to the pylon.
Like, it is understandable that not everyone is an aviation nerd, but it is quite a bit silly to go "the engine separated so they're the same thing which should have been fixed by now". Or to put it another way - do you drive a car? Do you understand the car you drive, as well as cars in general - do you understand that e.g. there are many different things that can cause a flat tyre and so it makes little sense to point to two road accidents as being the same because they both "start" with a flat tyre, even if they involve the same vehicle + tyre manufacturer?
> My point - and from what I see in the report, a grievance shared by the NTSB, given their citation of AA191
I feel quite comfortable in saying that you are projecting your grievance on the investigating body, actually, especially considering that you've manufactured a narrative out of whole cloth where "all involved parties [threw] up their hands and [swore] they followed all the rules and maintenance schedules". In reality, the investigation detailed within about a month that American Airlines (as well as Continental Airlines and United) was most decidedly not following the maintenance guidance from McDonnell Douglas, which had directly led to the fracture of the aft pylon bulkhead (again, a different part from the one that failed in the UPS crash). But of course, that does not fit into the easy-to-swallow "McDonnell Douglas bad" story that people prefer over actually reading and understanding these reports for themselves.
Gyroscopic precession took the left engine to the right. In AA 191 the right engine departing to the right did not affect the center engine. Sadly the engine failure procedure at the time mandated slowing down to V2 which was below the stall speed with slats retracted. There's now revised procedure and hydraulic fuses.
I expect all remaining aircraft will be getting new rear pylon lugs with shortened inspection intervals - provided the replacement cost is below the value of continued usage.
Looks like it's been moved to https://www.ntsb.gov/investigations/Documents/DCA26MA024%20P...
Originally explained on the blancolirio channel on YouTube -
The timing and manner of the break make a lot more intuitive sense when you consider that the engine is essentially a massive gyroscope. As the plane starts to rotate, the spinning engine resists changes to the direction of its spin axis, putting load on the cowling. When the cowling and mount fail, that angular momentum helps fling the engine toward the fuselage.
I think far simpler explanation is "the back part failed first and engine is making thrust so it just flipped over on now-hinge mounting
That's why it flipped upwards, but not why it flipped towards the body of the plane / to the right.
Yes, and that lateral movement is very important since the debris seems to have caused at least one other engine to the right to fail as well.
From a failure analysis perspective that is much less relevant though. The first failure was the rear engine mount if it had been a secondary failure it would have been deformed first and then broken, and it clearly is not. It just tore in half on one of the four connections and then the rest deformed slightly due to overstress.
It was however relevant to the survivability of the accident: if the left engine wouldn't have detached, or would have detached in a more "manageable" way, the other engine (probably the tail engine from how it looks) wouldn't have been affected too, and the pilots would have had a better chance to take off. Plus the whole "when an engine detaches, it shouldn't start a fire in the wing it was attached to" part of course...
Yes, agreed, the secondary safety wasn't there either. There was a Boeing accident near Amsterdam with the plane crashing into an inhabited area, it had dropped two engines but kept flying, at least for a while...
https://en.wikipedia.org/wiki/El_Al_Flight_1862
Pictures of that accident are not for the faint of heart so think twice before clicking.
A 747 with 2 engines out that's already got some altitude and speed is much more "survivable" (still extraordinarily difficult) than a trijet trying to take off on 1 engine, which is impossible.
Planes can safely land with 0 engines, though this is obviously "not ideal." See: Gimli Glider.
(Mostly putting that out there for others.)
Yup. That's exactly what experts said of American Airlines flight 191 which was basically the same engine mount, same failure. Engine flipping over the wing.
The failure of the pylon appears to be different. On AA 191, the pylon rear bulkhead cracked and came apart. In the case of UPS flight 2976, the pylon rear bulkhead looks to be in one piece, but the mounting lugs at the top of the rear bulkhead cracked.
Admiral Cloudberg has a great article on AA 191 that covers exactly what happened: https://admiralcloudberg.medium.com/rain-of-fire-falling-the...
American 191's engine mount failed because of improper maintenance. It remains to be seen whether this failure had the same cause or if it was something else, such as metal fatigue.
A failure due to metal fatigue would still be a failure to properly maintain the aircraft, right? I know by "improper maintenance," you're referring to actual improper things being done during maintenance, and not simply a lack of maintenance. But I'm reading things like "the next check would've occurred at X miles," and, well... it seems like the schedule for that might need to be adjusted, since this happened.
Yes, when I said "improper" I meant the American 191 maintenance crew took shortcuts. The manual basically said "When removing the engine, first remove the engine from the pylon, then remove the pylon from the wing. When reattaching, do those things in reverse order." But the crew (more likely their management) wanted to save time so they just removed the pylon while the engine was still attached to it. They used a forklift to reattach the engine/pylon assembly and its lack of precision damaged the wing.[0]
Fatigue cracking would be a maintenance issue too but that's more like passive negligence while the 191 situation was actively disregarding the manual to cut corners. The crew chief of the 191 maintenance incident died by suicide before he could testify.
[0] https://en.wikipedia.org/wiki/American_Airlines_Flight_191#E...
> The crew chief of the 191 maintenance incident died by suicide before he could testify.
To be clear, a crew chief (Earl Russell Marshall) did. But he wasn't directly involved in maintenance of the specific DC-10 that crashed. Or at least, I haven't found a source saying he was, and some sources say he wasn't. https://www.upi.com/Archives/1981/03/26/The-wife-of-an-airli...
If the (FAA-approved) maintenance schedule says "the next check should occur at X miles" and X miles hasn't happened yet, then it's not going to be classified as improper maintenance -- it's going to be classified as an incomplete/faulty manual.
Now, of course, if that maintenance schedule was not FAA-approved or if the check was not performed at X miles, that's going to be classified as improper maintenance.
A more likely metric for this particular inspection would be hours or cycles, in other words starts and landings, not 'miles'.
According to various comments the plane was nowhere near the cycling for a special detailed inspection of the aft pylon mount lugs: SDI is at 29200 cycles and the plane had 21043.
There was a lubrication task in October, but according to tech comments that would just in greasing the zero fittings, no taking apart anything.
Those pictures of that torn up part are pretty hefty, that's a clean break, no stretching as far as I can see it just tore the material in half, you can see the grain. There does not seem to be any torsion either so most likely that was the first part to go, if the problem had been in the engine then I would expect this part to be mangled, not pulled apart. What stress damage there is occurred shortly after that first break. A valid question would be whether or not that crack was there before take-off or not.
I'm very curious what the metallurgic analysis of the mirror part on the other wing will come up with, especially whether there are any signs of stress fractures in there. If there are that will have substantial consequences for the rest of the still flying MD11's, about 50 or so are still in service.
The preliminary report mentions fatigue cracks on both sides of the aft lug, and one side of the forward lug, with the other showing no trace of fatigue, only overstress.
From this it seems like the aft lug was way fucked, and the forward lug was hanging on for dear life, until it could not.
Yes, that's exactly how I read it. The aft lug was the first to go, the forward lug shows signs of stress so it held on longer.
I don't think they're going to be skimpy on the metallurgy report so looking forward to the analysis of the mirror parts on the other wing. Those will tell without a doubt whether it was maintenance related or age related fatigue. Right now I would bet on the latter but the former could also still be a factor, for instance, that bearing might not have had enough lubricant.
Zerk fittings, not zero (stupid autocorrect), the grease fittings.
Grease nipples. They had a perfectly fine name already, which autocarrot was perfectly happy to spell.
It depends. This aircraft was made near the beginning of the MD-11 production and if the original analysis for the fatigue life of this location was wrong, then you would expect to see that appear in older aircraft first. If that ends up being the case then it's not an inspection or maintenance issue, it's an engineering failure. Given aerospace accident history I would say that is less likely than some maintenance issue but we won't know for sure for a bit.
Even if it was an inspection or maintenance issue (which given the kind of failure and available data looks increasingly doubtful, though it can not yet be ruled out) this part failed in a catastrophic way when it should have had ample engineering reserve over and beyond the load to which it was subjected. It just snapped clear in half those breaks are indicative of a material that has become brittle rather than that the part deformed first and then broke due to excess stress.
In other words, a slow motion video of a camera aimed at that part during the accident would have shown one of the four connections giving way due to fatigue cracks and then the other three got overstressed and let go as well, in the process damaging the housing of the spherical bearing.
The part at the bottom of page 9 is the key bit. Now I very much want to see the state of the mirror part on the other wing, that will show beyond a doubt whether it was maintenance or an over-estimation of the design life of that part.
It would also be interesting to have a couple of these pulled from the fleet and tested to destruction to determine how much reserve they still have compared to the originally engineered reserve.
According to the preliminary report, 3 of the 4 showed fatigue cracks, and the 4th overstressed. So yes, agree a random sample of these parts should be pulled from the fleet and tested - but something pretty crazy was happening here re: fatigue.
That it was so far from the maintenance schedule to be inspected AND that the fatigue cracks seem to have formed in areas that would be hard to visually inspect anyway points to either a engineering problem (especially bad, since the DC10 problem of a similar nature happened in roughly the same parts, albeit due to different abuse - you’d think the engineers would overdo it there, if nothing else), or some specific type of repeated abuse that particular pylon received, which is pointing more to a design problem.
Re-reading the 1979 report might be helpful here. This isn't my field, but it seems that the engine is attached "hard" to the pylon, then the pylon is attached via a bearing mount system to the wing frame. The bearings wear out, and hence have to be replaced (not sure how often, but they were doing it on the entire fleet prior to the 1979 crash). The 1979 investigators thought that the fatigue cracks were caused by removal of the entire pylon/engine assembly as one unit (because that put excess stress on the aft bearing, they suspected due to support being provided from below by a fork lift). After the 1979 accident engines had to be removed first, then pylon, supposedly removing that cause for mount cracking. Perhaps there was another cause.
Before the 1979 accident, engines also had to be removed first.
Airlines have to follow the approved maintenance manual procedures; that manual called for engine removal and installation from a pylon that was on the wing. American was improvising a maintenance procedure without the legal authority to do so, resulting in 191.
> you’d think the engineers would overdo it there, if nothing else
No kidding, especially given the lack of redundancy in the design.
There is nothing here to say it being a maintenance issue is doubtful. It could quite literally be a similar issue to Flight 191, we don't know yet.
It did have ample engineering reserve beyond the load it was subject to... before fatigue damage initiated a crack which then grew until there was no reserve left. The question is why the fatigue crack initiated prematurely? Maintenance damage? Analysis mistake? We don't know yet.
If you read the original 1979 report in full, I think you'll begin to realize that this "improper maintenance" thing was a cover-up. Actually quite similar to the 737MAX -- find someone or something to blame other than the design of the aircraft, then move on.
The picture of that part that is torn into two pieces certainly seems to suggest so, that's a clean break, not an overstressed part deforming and then breaking.
Ironically, AA flight 191 could have been salvageable, because the engine detaching didn't start a fire. However, it led to loss of hydraulic pressure on that wing, which led to the flaps/slats retracting on just the left wing, which led to the plane becoming uncontrollable. After that accident, the DC-10 was retrofitted with hydraulic fuses to prevent something like this happening again. Unfortunately, that didn't help the UPS crew, because in their case, the detachment caused more damage to the wing...
Flipping backwards is what caused the engine to fly to the right and land to the right of the takeoff runway. The stills in the NTSB preliminary report clearly show the engine flying over the aircraft, to the right, and then heading straight down.
Maybe stupid question: Why not have the #1 engine spin in the opposite direction so that it doesn't go towards the fuselage?
Because making every jet engine in two different models would make them a lot more expensive. It would also cause maintenance issues because of non-interchangeable parts.
There might be some truth in that. But the report doesn't confirm that theory.
What theory? That the mount failed? Or the rotation of the engine in the photos going up and over the fuselage?
It seems like both are true, but doesn't necessarily prove WHY the mount failed.
Not an aviation expert at all, so take this with a grain of salt, but I think "the spinning engine resists changes to the direction of its spin axis" offers two important insights:
* why it failed at rotation (the first/only sudden change of direction under full throttle) rather than as soon as it was mounted onto the plane, while taxiing, as soon as they throttled up, mid-flight, or on landing. This is important because at rotation is the worst possible time for this failure: no ability to abort take-off, no ability to land safety under no or severely limited power, little time to react at all, full fuel. Knowing these failures are likely to manifest then stresses the importance of avoiding them.
* why it failed in such a way that it damaged the rest of the plane.
Not so much what was wrong with the mounting in the first place, if that's what you're asking. Presumably it was designed to withstand the forces of this moment and clearly has done so many times before.
> Presumably it was designed to withstand the forces of this moment and clearly has done so many times before.
The report seems to suggest metal fatigue in the motor mount may be a possible culprit.
Not the motor mount but the pylon mount. The pylon was found attatched to the engine with both engine mounts attached.
But yes, the report mentions stress factures where the aft pylon mount failed.
That the engine was flung into the fuselage due to gyroscopic forces.
Well, some force flung it inboard and above the fuselage (gods, that CCTV stills sequence.) Knowing that the engine rotates CCW, there are not many candidates.
> Knowing that the engine rotates CCW, there are not many candidates
There are lots of candidates for a failing engine yeeting itself in any direction.
> There are lots of candidates for a failing engine yeeting itself in any direction.
For the precise trajectory, certainly; for the general direction, not so much. Could you describe a combination of forces that would have thrown that engine to the left of the direction of travel? (We're talking about this accident, not any engine anywhere.)
> Could you describe a combination of forces that would have thrown that engine to the left of the direction of travel?
Foreign object gets yeeted to the right. Internal component gets yeeted to the right. Engine exploded on its right side.
I think each of those is more likely than gyroscopics since the engine went to the left. Not left and up.
> [...] the engine went to the left. Not left and up.
Whatever you're describing, it's not this accident. Over and out.
You're correct–I didn't look at the photos.
My broad comment is that gyroscopic precession having any critical role in this is incredibly far fetched. That said, I've never flown or worked on a turbofan so ¯\_(ツ)_/¯.
Gyroscopic forces might have changed the direction of travel a few degrees, but the motive force comes from the engine's thrust, the power of its spinning blades pushing air. An engine cut loose at full power moves forward like a missile.
This is a preliminary report. Its purpose is to present initial evidence/information.
The final reports are always much more comprehensive.
I'm presenting it "useful not true" - not an RCA.
I assume such forces are calculated and added in when deciding hot thick to make those mounting brackets.
Yes, obviously; MD-11s aren't flinging engines off the wing every single takeoff. A 34 year old airframe may or may not actually match design strength, though.
Yes, but the point is that this moment of the takeoff is when a failure that's been waiting to happen is most likely both because of the thrust and the gyroscopic resistance.
Yep. Now do 3 decades of metal fatigue.
Did I understand the report correctly that the part was scheduled to be replaced in the future after a certain number of hours, it just hadn't hit the threshold yet ?
If you're referring to this quote (excerpted from the AVHerald article linked elsewhere in the thread), I don't think so:
> At the time of the accident, N259UP had accumulated a total time of about 92,992 hours and 21,043 cycles [..] A special detailed inspection (SDI) of the left pylon aft mount lugs would have been due at 29,200 cycles and of the left wing clevis support would have been due at 28,000 cycles
This isn't talking about replacement, only inspection; and it wasn't going to happen in the near future: 7k cycles at four flights/day means inspection is due in 5 years.
"7k cycles at four flights/day"
It wasn't doing four flights per day. As a long-distance cargo aircraft it was doing two flights per day, and I doubt it was flying every single day of the week.
So we are talking about at least 10 years before that inspection was due.
Aluminum has limited loading cycles
I'd be very surprised to read that the aft lug that cracked (and the bearing it contained) were made of aluminum. They were almost certainly steel or Inconel.
Wouldn't that be true of all cast metal objects?
Or are some metals impervious?
I know Veritasium gets posted here a lot, but a few days ago he posted a deep-dive into the the engineering of jet engine turbine blades. Turns out they're made from a single crystal of a superalloy that entangles itself at a molecular level such that it actually gains strength as it's heated, only losing strength above 1200 degrees C / 2200 degrees F. Below that temperature, as long as the strain on the part is below the plastic deformation threshold, it's not really losing any strength at all over time.
https://www.youtube.com/watch?v=QtxVdC7pBQM
No; roughly, yes. Based on the crystal structure of the metal, fatigue works differently.
> The fatigue limit or endurance limit is the stress level below which an infinite number of loading cycles can be applied to a material without causing fatigue failure.[1] Some metals such as ferrous alloys and titanium alloys have a distinct limit,[2] whereas others such as aluminium and copper do not and will eventually fail even from small stress amplitudes.
https://en.wikipedia.org/wiki/Fatigue_limit
Grounding all MD-11s and DC-10s is a major move. I guess it makes sense as a big factor was the fatigue cracks on the pylon (lugs), despite the pylon not being behind on inspections. I am wondering what the inspections of pylons in other planes will yield, likely that will determine whether the grounding will continue.
But beyond figuring out why the engine mount failed, I am very interested in what caused the actual crash. "Just" losing thrust in a single engine is usually not enough to cause a crash, the remaining engine(s) have enough margin to get the plane airborne. Of course this was a major structural failure and might have caused additional damage.
EDIT: It seems there was damage to the engine in the tail, even though this was not specified in the preliminary report, likely because it has not been sufficiently confirmed yet.
And if the failure of a wing engine can cause the rear engine to fail, that would raise concerns about all "two in front one in back" trijets. Similar to how putting the Space Shuttle orbiter's heat shield directly in the line of fire for debris that comes off he rocket during launch turned out to be a bit of a problem.
At this point there aren’t any trijet designs like that being built, and it’s unlikely we’ll ever see a new trijet design. It served a role in the transition from four engines to two, but now with ETOPS-370 there’s no commercially viable route that can’t be served with an appropriate twinjet.
There are several passenger trijets still existing - they are just not commercial airliners. Dassault for one is quite fond of the design; the Falcon 900, 7X and 8X are trijets, and I'm pretty sure the latter two are still in production. I wouldn't be at all surprised to see another trijet design from them probably around 2030.
The Falcons have all three engines tail mounted, so not the same "type" of trijet as MD-11.
Don't forget about Tupolev Tu-154. It didn't stop flying as a commercial airplane because of safety, rather because of noise emission limits.
And the failure of an inboard wing mounted engine can cause the failure of an outboard wing mounted engine on the same side, as in the case of El Al 1862. https://www.faa.gov/lessons_learned/transport_airplane/accid...
And the failure of an engine mounted on the left wing can cause debris to cross through the fuselage structure and cause a failure of the engine mounted on the right wing, or to fly thousands of feet in any particular direction, as happened to American Airlines in both a ground run incident, and in their Flight 883 accident.
https://www.dauntless-soft.com/PRODUCTS/Freebies/AAEngine/
https://aerossurance.com/safety-management/uncontained-cf6-a...
The industry also responded to those crashes. For example, the El Al 1862 incident prompted a redesign of the engine strut that was subsequently mandated as a retrofit for all 747s.
And here's a more detailed description of that ground run incident. It also found that the failure was related to a design flaw, and mandated that aircraft be grounded for inspection and rework. https://skybrary.aero/accidents-and-incidents/b762-los-angel...
I'm not a regulator or aerospace engineer or anything like that so I can't really say which actions are or are not appropriate. But I do want to observe that these are all unique failures with unique risk profiles that can't all be painted with a single broad brush. All I was trying to do in the previous post was speculate on why a MD-11 failure could result in a grounding of the DC-10 and KC-10A as well. The first thing that came to mind is that I think those are the only remaining trijets of that general shape that are still around. Though I suppose another possibility is that they all share an identical pylon design or something like that.
> Though I suppose another possibility is that they all share an identical pylon design or something like that.
They're very closely related planes (MD-11 is an upgraded DC-10; KC-10A is a military version of the DC-10), so that wouldn't be surprising. Likely the KC-10A has the same pylon, and the MD-11 has one that's similar enough that it's worth being cautious.
No military operates the KC-10 anymore. There are a grand total of two remaining in use as aerial firefighters.
> Incorrect. The KC-10 was based on the Boeing 707. No connection to the DC-10 at all other than having 10 in the name.
I think you're confused? It's nothing like the 707, and is a trijet. https://en.wikipedia.org/wiki/McDonnell_Douglas_KC-10_Extend...
EDIT re your edit: it's still a military version, even if militaries have retired it.
Yeah, the trijet design seems failed in general. Unless you can design it to tolerate any wing+tail dual engine failure -- in which case, why have the tail engine at all?
It wasn’t failed. It was designed for a very specific reason and served that purpose well.
Once the reason went away, better designs took over.
They were designed to allow smaller jets to fly over the ocean further than a two engine jet was allowed (at the time). Airlines didn’t want to waste all the fuel and expense of a huge 4 engine jet, but 2 wouldn’t do. Thus: the trijet.
The rules eventually changed and two engine jets were determined to be safe enough for the routes the trijets were flying.
Using two engines that were rated safe enough used less fuel, so that’s what airlines preferred.
It was never designed to be used anywhere else as a general design. Two engines did that better.
In the case of the quad jets, Boeing tried the 747-SP and had minimal marketing success.
In the case of the trijets the MD-11 lived on as a freighter because it had a much higher capacity than anything else smaller than a 747.
Not quite. Dassault still makes a three engined bizjet and in theory the Chinese fly a three engined stealth jet.I didn’t know there was a three engine business jet, my knowledge is mostly passenger airliners and even then just from an amateur perspective.
Other than being able to identify a couple of famous ones I don’t know a ton about military airplanes either.
Thanks!
You've framed this as disagreeing with me, but I don't think you are. I agree the design made sense in the 1960s, when we didn't know any better and requirements were different.
A design that was once useful but no longer has a use is not the same thing as a failed design. Which is what the disagreement seems to be about.
> in which case, why have the tail engine at all?
"you know what this motorized piece of anything needs, less power"
-nobody, ever
You know you can just make the wing engines 50% more powerful, right?
No, you really can't. Even if it were the same size a dramatically more powerful engine would need a larger "tail" to maintain control in case of an engine out scenario. But a 50% more powerful engine is also likely to be much bigger meaning that major components like the landing gear (and everything around them). A 50% more powerful engine is also likely to be much heavier necessitating its support structures (a.k.a. the wing or tail) be redesigned.
The 737 MAX suffered a number of bad design decisions to accommodate its newer, more powerful engines. Its engines topped out at about 8% more powerful than the 737 NG engines.
It wasn’t about the engines being powerful but about the fan being bigger and therefore more efficient
The effects of a bigger fan are the same: the engine is bigger and more powerful necessitating changes to the airframe.
> just make the wing engines 50% more powerful
You realize this is not quite how aerospace engineering works, right?
Look at thrust on the 737 Max vs thrust on the original 737.
There's a lot of other changes, of course, but more powerful wing engines let you build a bigger plane in the same kind of shape. Changes in flight rules are also significant; if twin jets can't serve all your routes, you most likely want trijets to cover the routes that can't be served by twins and don't demand a quad ... with current flight rules and current engines, twin engine covers pretty much everything.
Essentially every new design is a twinjet, so it's clearly possible to make appropriate decisions in that design space. And both Boeing and Airbus have given up on quadjets.
The MD-11 isn't a new design. It's a stretched version of a first generation widebody whose design dates back to the mid-1960s. Before the MD-11 was developed, McDonnell-Douglas toyed with the idea of a dual engine variant before settling on a three engine version of the DC-10. Trijets in general came about because the engines of the day were too unreliable and too small to work in twin engine configuration at that scale.
The plane which ended up being the final nail in the MD-11's coffin, the 777, didn't start development until the 90s. Of its three initial engine choices, two were derivatives of engines that were around when the trijets came to be. The initial version of that Rolls Royce engine was so late (and so unreliable) that it essentially killed the Lockheed trijet. The third option, the GE90, was the largest turbofan engine at its introduction until it was succeeded in 2020 by the GE9X.
Scaling these earlier engines up to fit an MD-11 sized twin was never an option.
When I replied to this thread[0], with this comment[1], both the comment I was replying to and my comment were talking about trijets in the abstract, not MD-11s in particular, and the current year, or perhaps as early as the 1990s, but definitely not as early as the 1960s.
Several comments, including yours, seem to have misconstrued that to mean I think the MD-11 in particular could be retrofitted into a twinjet. That's, uh, wildly mistaken, and not something I've ever claimed. I just think trijets in general are a design dead-end. Again, that doesn't mean it didn't make sense in 1960s when the DC-10 was being designed.
[0]: https://news.ycombinator.com/item?id=45996656
[1]: https://news.ycombinator.com/item?id=45996787
No, big twins weren't possible and didn't make sense in the 60s. The engines were simply far too unreliable.
>both Boeing and Airbus have given up on quadjets.
It is possible “to make appropriate decisions” up to a certain size. They didn’t stop making new quadjets because the design doesn’t work as well as a twin engine, but because airlines don’t need/want aircraft that large. You wouldn’t build a successor to the A380 as a twin engine.
Yes, and? If no one wants them, it's a commercial dead end.
It would be way cheaper to replace the airplane with a modern twin-engine plane than to retrofit new engines onto an old plane.
No one is talking about retrofitting new engines on MD-11s.
Now it is, yes. At the time, it would have required 4 total engines, which is a different matter altogether.
At some point it comes down to probabilities. With so many flights going on, one in a million incidents become a certainty. For example UA232 [1] suffered failure in all 3 redundant hydraulic systems due to an uncontained engine failure. Any of the 3 systems would have been enough to retain control of the aircraft. Of course this lead to some investigations on why all 3 systems could be impacted at the same time and what can be done to limit failures.
Besides the technical aspects that flight is an impressive example of resilience and skill. Bringing that plane down to the ground in nearly one piece was essentially impossible and a one in a million chance in itself.
[1] https://en.wikipedia.org/wiki/United_Airlines_Flight_232
Airlines operate to a much stricter standard than one in a million. If one in a million flights ended in a fatal crash, the US alone would see about 3 airline passenger deaths per day on average. The actual average over the past 10 years is under 0.02 deaths per day.
It's true that you can never get to zero. There's always a chance of some catastrophic failure. The lesson of modern airline safety is that you can get extremely close to zero by carefully analyzing and learning from the failures, which is exactly why these thorough investigations are done. The lesson from UA232 was to make sure one failure can't take out all of the hydraulic systems.
In this specific instance, "the engine fell off and took out another engine, leaving the aircraft with insufficient power to climb" is definitely not in the realm of "probabilities will get you eventually." It's very much in the realm of a mechanical failure that should not happen, combined with a bad design flaw that turns that failure from a mere emergency into pretty much guaranteed death.
Cargo is held to a lower standard than passenger service, but I suspect this will still spell the end of the DC-10 and MD-11, at least in the US. Engines will fail, and for an aircraft of this size, that needs to be survivable in all phases of flight just for the safety of people on the ground.
> The lesson of modern airline safety is that you can get extremely close to zero by carefully analyzing and learning from the failures, which is exactly why these thorough investigations are done.
I have heard it said that "every air safety rule is written in blood."
https://www.cnn.com/2024/01/02/travel/tokyo-plane-crash-safe...
If the engine had just failed, they would very likely have been fine. Experienced crew, would likely have handled it. But the engine came off the wing, and then another engine was damaged. At that point there was no recovery possible.
This is understating it. Any minimally competent crew should be able to handle a single engine failure on takeoff (in a normal scenario, not this one). It’s absolutely within the performance envelope of the aircraft and is something that crew train for. If pilots were not routinely able to handle this kind of failure, we’d see a lot more crashes.
The pilots did (apparently) exceptionally well keeping the plane level even with unbalanced weight and nearly no thrust; perhaps they had been over water they’d even have been able to ditch successfully.
This is still understating it. Any barely competent crew should be able to handle a single engine failure on takeoff (in a normal scenario, not this one).
> Airlines operate to a much stricter standard than one in a million. If one in a million flights ended in a fatal crash, the US alone would see about 3 airline passenger deaths per day on average.
I think you conflated flights (several 10Ks per day) with passengers (several million per day).
One in a million flights is one accident every few decades.
> at least in the US. Engines will fail
As per the report, this appears to be a structural failure, not an engine failure.
If randomly distributed, one in a million flights crashing and killing all passengers means that one in a million passengers dies.
The US sees about 25,000 airline flights per day, or around 9 million per year. So with one in a million flights crashing, we'd expect roughly 9 crashes per year.
Even if they end the grounding of the MD-11/DC-10 I'd be shocked if any airlines still using them will continue to use them.
Seems like the risk/reward just isn't really there for the few of them still in service, and if anything happened it would be a PR nightmare on top of a tragedy.
Definitely an end of an era!
UPS and FedEx each have around 25 MD-11s, Western Global has 2 I think, the Orbis Flying Eye Hospital is an MD-10, some cargo airline in Botswana has one, and 10 Tanker has some DC-10 firefighting tankers.
That’s the entire worldwide fleet.
Given that the report only mentioned a single other seemingly related accident in 1979 I am not sure that objectively this is a reason to discontinue flying these planes. The fact that these planes have been in service since the early 70s is a testament to their safety and reliability in itself. Of course public perception, especially with the videos of huge fireballs from hitting one of the worst possible locations, might put enough pressure on airlines to retire the planes anyway.
I agree on the end of an era. Hearing something else besides just Airbus- or Boeing-something always gives me a bit of joy. Even though MDs and DCs are of course Boeings in a sense now as well.
One other accident that was similiar, but these planes have had a ton of crashes for other reason.
I managed to find some statistics on hull losses per million departures [1, p. 13]. Seems like indeed MD-11s have a highish rate of incidents by that metric compared to other types, even if they are not catastrophically less safe than other planes. That metric stacks the statistics a bit against cargo planes, which most (all?) MD-11s are now. These planes tend to fly longer haul instead of short hop, so you get more flight time/miles but less departures. There are also likely some other confounding factors like mostly night operations (visibility and crew fatigue) and the tendency to write off older planes instead of returning them to service after an incident. Plus these aircraft have been in operation long enough that improvements in procedures and training would impact them less than more modern types, as in they already had more accidents before these improvements.
[1] https://www.boeing.com/content/dam/boeing/boeingdotcom/compa...
The DC-10 had a number of other problems, but the MD-11 has always had a reputation of being an unforgiving aircraft especially when compared to the DC-10. It's less about training and more that the MD-11 was simply too many design compromises piled on to an old design.
The MD-11 had a pretty short service life as a passenger aircraft because it simply wasn't very fuel efficient compared to the competition, safety wasn't really the motivating factor. However fuel consumption was behind some of the poor design choices McDonnell/Boeing made. In broad strokes: McDonnell/Boeing shrunk the control surfaces to improve fuel consumption "necessitating" poorly designed software to mask the dodgy handling and higher landing speeds. This exacerbated a DC-10 design "quirk" where hard landings got out of hand very quickly and main landing gear failure would tend to flip the plane.
Yeah you can train around this but when something else goes tits up you've got a lot less leeway to actually recover safely.
I think that the Mad Dogs only exist as freighters (~or their derivative KC-10 tankers~-Edited to correct that they retired last year) these days. I think the last pax service for any of them was over a decade ago.
And air freight just gets a lot less public attention, I think they are going to keep flying them if they don't get grounded.
The airforce retired the KC-10 in 2021.
Most of the DC-10s in service in the US are used for fire fighting.
And with Omega Air, for contracted air refuelling
https://www.omegaairrefueling.com/
Yes, but there are many MD-11's still flying as freighters. There are four fire-fighting DC10's out of ~8 still flying, but there are 25 Mad Dogs (MD-11) at UPS, 38 with FedEx, and Western Global has 4, so there are plenty of MD-11F's around.
Here are 4 of them. All grounded now.
https://www.10tanker.com/gallery
Mad dogs are MD-80s.
The KC-10 went out of service last year. None are operating.
Yeah, but DC-10 based tankers for wildfire fighting were still flying until the recent grounding: https://en.wikipedia.org/wiki/DC-10_Air_Tanker
(Blancolirio points out that the DC-10 tanker is what they modernized to relatively recently -- before that they were flying even more dangerous WW2 airframes for firefighting.)
Damnit, I knew that! Just forgot it in the moment.
There were only about 18 aircraft of all types still flying at the time and none are dedicated passenger aircraft. Indefinite grounding is the most sensible move.
Now, Boeing, et. al. are trying to evaluate the deficiencies in existing D checks and put together an inspection regime (i.e., NDT) that would proactively identify fatigue that would economically permit continued serviceable operation. If this feat turns out to be impossible for technical or cost reasons, then and only then will the grounding will become permanent.
Airlines haven't been using them, or at least not 1st world airlines. Just freight and wilderness fire fighters.
Most of them are used as cargo planes. Which have dramatically lower usage rates than passenger planes (and they are retired passenger planes)
Sucks for the pilots flying them for sure tho.
> EDIT: It seems there was damage to the engine in the tail, even though this was not specified in the preliminary report, likely because it has not been sufficiently confirmed yet.
Yes, the initial videos were showing the tail engine flaming out. And in the 1979 crash, the engine also severed hydraulic lines that hold the slats extended. So they folded in due to the aerodynamic pressure, essentially stalling the wing.
Based on the original descriptions of the crash, I assumed the engine fell off.
From the photos, it’s clear it went up over the wing and impacted the fuselage with a (at least) minor explosion, which would have thrown foreign objects into the third engine in the tail for sure.
Losing 2/3 of the engines isn’t survivable on takeoff for this class of plane, at the weights they were at.
> I assumed the engine fell off
It's an engine - the thing pushing the entire plane forwards. Provided it is running (and at takeoff that's definitely the case), an engine being liberated from its plane suddenly has a lot less mass holding it back, so the logical thing to do is to shoot forwards. And because the wing is attached to the upper side of the engine, anything short of an immediate failure of all mounting points is probably also going to give it an upwards trajectory.
Add in air resistance, and you get the "swing across the wing and back" seen in the photos.
Sure, but if the engine grenades it can take it’s mounts with it and not shoot off like a bottle rocket in front of and over the plane, dropping down and under the plane instead (or even just sit there). Same with a compressor stall, or whatever.
It’s clear from the photos this wasn’t the engine failing at all, and in fact the engine kept producing a ton of thrust (probably until it ran out of fuel as it pulled it’s fuel line apart while departing the wing), and instead the thing that is supposed to be so incredibly strong that it restrains all this chaos failed.
Which is a pattern in this family of aircraft, but definitely not a common or normal thing in general eh?
Most aircraft, the engine stays with the airframe even if it turns into a giant burning pile of shrapnel and dead hopes and dreams.
Engine pylons are actually usually designed to fail in a particular way to ensure the separation happens as safely as possible; obviously that didn't happen here, which will probably be something the NTSB will have to investigate why.
The up and over is usually actually the safer direction I think? But in this case it also moved laterally, which is possibly what fouled the tail engine and made it unrecoverable. Will be interesting to see the final report.
Fully functioning engines departing from aircraft isn't common but it's not unheard of either. Off the top of my head it's happened a few times on the 747 and 737.
> Grounding all MD-11s and DC-10s is a major move
Not really. There are zero left in passenger service, they pretty much only serve cargo now.
Losing cargo jet capacity right before the holidays may cause some issues.
All cargo companies run a wide fleet of many different plane types, particularly to avoid this very problem of being grounded by the FAA. But yes, these were still widely used in cargo transports. Although newer 2 engine planes can haul the same kg and use a lot less fuel.
It wasn't just one engine off, aside from possibly damaging tail engine you also have damage to the wings and control surfaces that might've just not got enough lift because of that.
Captain Steeeve thinks it was actually the starboard engine that also failed due to evidence of compressor stalls in some of the footage
https://youtu.be/CmXLQHhUtv4?t=499
Adding summary analysis from AVHerald [0]
[0] https://avherald.com/h?article=52f5748f&opt=0
"Your IP address 104.28.103.15 has been used for unauthorized accesses and is therefore blocked! Your IP address belongs to Cloudflare and is being used by many users, some of which are hackers and hide behind the cloud/proxy to avoid being tracked down. Hence the automatic defense closed access from that IP address.
"Make sure to not use a proxy/cloud service for visiting AVH (e.g. Apple Users turn off your private relay) but your native IP address, then access should be possible without a problem again."
No thank you, AV Herald.
That's a pretty nice message. Most sites that filter VPNs and proxies just kill the connection, give a generic error, or subject you to endless captchas.
They could've blocked just the comments, allowing at least read-only access to the site, instead of blocking it off entirely
So could everyone that blocks network traffic for various reasons, but usually they don't because they're not doing it in the primary application layer, but using a WAF or reverse proxy or something else in front of their application... and also most DGAF to cater specifically to the users they block.
Again, you're usually lucky to even get a return packet.
I block all traffic from Cloudflare outright on my servers.
Every so often they sneak in new blocks of IP addresses though so you're playing whack-a-mole with a particularly scummy opponent.
They're pretty upfront about their ranges:
https://www.cloudflare.com/en-au/ips/
Or if you prefer:
https://www.cloudflare.com/ips-v4/#
> On the aft lug, on both the inboard and outboard fracture surfaces, a fatigue crack was observed where the aft lug bore met the aft lug forward face. For the forward lug's inboard fracture surface, fatigue cracks were observed along the lug bore. For the forward lug's outboard fracture surface, the fracture consisted entirely of overstress with no indications of fatigue cracking
If I'm parsing this correctly, they're saying that fatigue cracks should have been visible in the aft pylon mount, and that the forward mount was similarly fatigued but showed no damage on the outside?
> If I'm parsing this correctly, they're saying that fatigue cracks should have been visible in the aft pylon mount, and that the forward mount was similarly fatigued but showed no damage on the outside?
If you can get to the report, Figure 7 shows the left pylon, with the forward and aft lug enlarged in the inset. Both lugs cracked on two sides. They're saying both cracks on the aft lug as well as the inboard crack on the forward lug were observed to be fatigue cracks, but the forward lug outboard fracture was observed to be entirely a stress crack.
Outboard and inboard are just away from and towards the center of the plane. On the left pylon, that's left and right, respectively. So, it looks like the left side crack in the forward lug developed from overstress, but the other three cracks were from fatigue. My expectation is that fatigue should be apparent upon the right kind of inspection, if timely, even if the metal has yet to fracture.
It sounds like the aft lug failed first, and then the not quite as compromised forward lug failed in overload.
Appreciate the transparency in these reports. The technical breakdowns always highlight how complex aviation safety is.
Link doesn't seem to be available now:
> Page not found
> The page you're looking for doesn't exist.
I found a link to the PDF that seems to work https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/G...
Also in case that link stops working I got it from this page https://www.ntsb.gov/investigations/Pages/DCA26MA024.aspx
EDIT: nevermind immediately after posting this comment it is now giving a 403 error
Your first link is working fine
Yeah working again for me too, they're probably having some sort of server problems
If anyone saved a copy locally, it'd be great if you could share it somewhere. (I, for once, did not, and the tab is gone now :-/ ).
Revised URL -https://www.ntsb.gov/investigations/Documents/DCA26MA024%20P...
Very fast. Quite sad to see it happen. Also quite puzzling is how the Air India disaster still does not have a root cause analysis done (though supposedly it will be released end of this year)
> Also quite puzzling is how the Air India disaster still does not have a root cause analysis done
Nothing puzzling. Straight-up cover-up.
Now, the interesting part would be to know what is being covered-up. Pilot error? Pilot suicide? Or a critical system malfunction Boeing cannot afford?
> quite puzzling is how the Air India disaster still does not have a root cause analysis done
Not that puzzling: the most likely explanation is pilot suicide and the Indian government does not want to acknowledge that.
The Indian authorities has blamed the pilots in every single crash. AND there is not enough evidence to guarantee that was the case. It is one of many possibilities.
A commenter in HN thread covering the initial crash mentioned that the left engine detaching might have been the cause https://news.ycombinator.com/item?id=45821537
The referenced AA Flight 191 is shockingly similar. It makes me wonder if aviation really is back sliding into a dangerous place.
40 years between severe accidents is fine in terms of expected failures. It's also not a good comparison because in the 70s maintenance crew were using a forklift to remive engines, improperly stressing the engine pylon. This was done as a shortcut
I don't know if it's "sliding back" as much as it is that this plane is also fundamentally from the 1970s.
The MD-11 was developed after that crash. Shouldn't its design and maintenance procedures have been informed by the incident?
The MD-11 is nothing but a re-engined and a re-named DC-10. They share the same type certificate.
https://www.easa.europa.eu/en/document-library/type-certific...
Aside from the engine detaching, it doesn't appear that this incident is in any way similar to the previous incident.
How do you figure? They're very similar planes. The left engine and its pylon detached in both cases during takeoff rotation. Both incident reports stated that proper maintenance would have prevented the detachment.
The way the situation played out is different but the failure mode seems to be very similar if not the same.
The NTSB report itself even references AA-191 as the only "similar event".
AA-191 was caused by improper maintenance (dreamed up by people who were made to cut corners and was never compliant with manufacturer spec) damaging the pylons holding the engine.
If someone did the same thing again, that would be rather unfortunate. Just more deaths for profit, even though we know it was dangerous.
The parts that seem to have fatigued and failed were only like 80% of the way through their inspection period. They were to be inspected after 28k cycles. They were at 21k cycles.
It sure looks the same from "Engine pulled itself off and flew away" angle, but if there is any similarity under the surface that's very bad. Flying was much much less safe in the 70s.
The root cause does not appear (at this stage) to be the same: incorrect maintenance in AA191 as opposed to fatigue cracking here.
Where does this report say proper maintenance would have prevented the incident?
The report doesn't say that because it's just reporting the facts not drawing conclusions. In my opinion, if a catastrophic failure happens that is a maintenance failure. Either the inspections failed to notice the fatigue or the inspection guidelines were too optimistic.
Maintenance was informed by the earlier incident. It's why we haven't seen even more DC-10/MD-11 failures sooner. Designs too have kinda been informed by this -- it's not like Boeing or Airbus make trijets anymore.
> The referenced AA Flight 191 is shockingly similar. It makes me wonder if aviation really is back sliding into a dangerous place.
I think it's cut throat capitalism at its best. Surely it was much too safe before, let's see how far back we can scale maintenance on the operations front but also how far back can you scale cost during development and production and then see where it takes us. If that changes the risk for population from 0.005 to 0.010, the shareholders won't care and it's great for profits.
I think we can see both but especially the latter with Boeing.
The entire MD-11 project was a budget-limited rush-job to try to capture some market share before the A340 and 777 came into service.
It produced an aircraft that failed to meet its performance targets, was a brute to fly and was obsolete the moment its rivals flew.
Douglas* by the early 1990s was a basket-case of warmed-over 1960s designs without the managerial courage to launch the clean-sheet project they needed to survive.
* as a division of MDC
And then they merged with Boeing...
Are you referring to AA 191 in 1979? That seems like low enough frequency event to not be worried about it.
The murder suicides in the last few decades seem more concerning.
Rather the opposite: if the cause is similar to AA 191, why weren't the actions taken after AA 191 to prevent a repeat effective? If we can get a repeat of that incident, what's preventing the industry from repeating the mistakes from all those other incidents from the past decades? Why aren't they learning from their past mistakes - often paid for in blood?
I understood the post I responded to to be referring to the cause as the engine detaching from the same type of plane, not the root cause for why the engine detached. Per the “investigation section” in the wikipedia article, I would be surprised if it was the same root cause:
https://en.wikipedia.org/wiki/American_Airlines_Flight_191
I assume the erroneous maintenance procedures that led to the loss of AA191 were rectified a long time ago.
>rectified a long time ago.
There's no such thing as "This is fixed forever". If lax maintenance oversight has led to companies re-introducing known dangerous maintenance procedures or departing from known good ones, then we will be back in the 70s in terms of airplane safety and people will have to die again to relearn those lessons.
Someone's always trying to claw you in the less safe direction. It's a constant battle to not regress.
But IDK, hopefully this plane just got some sort of "unlucky" about fatigue somehow, and it doesn't have far reaching consequences.
Link to page that links to the report, as of now: https://www.ntsb.gov/investigations/Pages/DCA26MA024.aspx
I'm surprised at how many years the plane went without having that part inspected. It looks like the failure was due to fatigue cracks, but the last time the part was inspected was in 2001?
I believe the part was at least visually inspected in 2021:
> A review of the inspection tasks for the left pylon aft mount found both a general visual inspection (GVI) and a detailed visual inspection of the left pylon aft mount, required by UPS's maintenance program at a 72-month interval, was last accomplished on October 28, 2021.
I’m seeing 2021 on page 10 - an I missing something?
surprised to see typos in aviation terms and acronyms: ADS-8 (page 3) and 747-BF (page 5)
Swapping B and 8 in both cases, which is typically something that happens with OCR. Weird.
pretty weird...
These all seem like OCR errors...? Why would there be OCR in this workflow? Did they print this out and then generate a PDF from a scan instead of the original source? To maintain an air gap maybe?
it would seem so. so the question is why they would maintain an air gap for a safety report
It's a good policy. Document formats like to include lots of random junk, better to be safe.
Also it prevents redactions from being disclosed.
Reminds me of xerox scanner fun, maybe someone scanned it to pdf to publicise?
Nontheless the pdfs have been replaced and the newer ones don't seem contain these errors anymore.
With many eyes, all typos are embarrassing.
The new document is an image.
I was under the impression that a plane could deal with an engine failure at any point in flight - including during takeoff.
Dropping an engine entirely is a similar situation to a failure - with the benefit that you now have a substantially lighter if imbalanced aircraft.
Should this plane have been able to fly by design even with an engine fallen off?
El Al 1862 was another flight [1] that had an engine fall off, taking another engine out with it. The pilots managed to fly around for a few minutes and attempt a landing, but there was too much structural damage.
It doesn't seem aircraft are designed to survive these types of catastrophic failures.
[1] - https://en.wikipedia.org/wiki/El_Al_Flight_1862
They seem to have lost the tail engine too. Yes, it is a significant problem that engine failures aren't independent, so trijets are kind of a bad design.
Not only did it happen at the worst possible moment, it took out a second engine on it's way out and over the plane. Two engines should've been enough to get off the ground and potentially land the plane, but one engine on a trijet isn't enough.
Yes, but when the engine came off, it also disrupted the third engine in the tail. Can't take off in this model with 2 out of 3 engines gone.
Yeah, pilots I know saw puffs of flame coming out of the engine, and said that that's a tell-tale sign of a compressor stall. Which could have been caused by debris from the separating left engine striking the turbine.
Debris, or even just smoke from the wing fire.
It was specifically the distinct, rhythmic puffing. I'm not sure you could expect the same pattern from debris or a wing fire.
I think they were saying that smoke/particulates could be sufficient to upset the rear engine-- things short of what we ordinarily call "debris".
I'm just saying that smoke alone can cause a compressor stall -- it doesn't necessarily require larger debris.
Ah, gotcha. My bad.
Even if they had the thrust (doubtful) there wouldn't be enough lift with a gaping hole in the leading edge of one wing.
The video of the aircraft crossing the road wings level (well after #1 separated) and maintaining relatively controlled flight until too much energy bled off suggests to me the aircraft was likely to be controllable to a landing if sufficient thrust was available.
..for a moment. If there was sufficient hydraulics damage it might've stopped being controllable.
And even if they worked the fire might've damaged the plane enough.
For example https://www.faa.gov/lessons_learned/transport_airplane/accid...
when they lost tail engine, all of the hydraulics went down
To be clear, we don't yet know if the UPS flight lost hydraulics or not. There are several somewhat redundant hydraulic systems.
(Also, as a result of the Sioux City crash you linked, there were several ADs issued requiring changes to hydraulics in these airframes.)
I thought the leading edge of the wing was intact in this case? I may be misremembering.
Yeah, if they had had more altitude, I would guess that this would have looked even more like the AA 191 crash from 1979, with the left wing stalling and causing a roll and pitch down.
That in turn reminds me of the DHL flight out of Baghdad in 2003 that was hit by a missile [0]. Absolutely amazing that they managed to keep it together and land with damage like that.
[0] https://en.wikipedia.org/wiki/2003_Baghdad_DHL_attempted_sho...
An important factor in AA 191 is that the engine leaving did significant damage to the hydraulic lines in that wing - including those for the leading-edge slats. At the time the plane was not equipped with any mechanism to keep the slats extended, so after hydraulic pressure was lost airflow over the wings caused them to retract, which significantly lowered that wing's stall speed.
After AA 191 the DC-10 was equipped with a locking system: loss of pressure now results in the slats getting stuck in their current position. The MD-11 will undoubtedly also have this system, so a direct repeat of AA 191 is unlikely.
> significantly raised the stall speed
(yeah, it's one of those weird metrics where "bigger is worse", so you're absolved)
It didn’t fall off, it flew up and then landed back on the plane.
https://www.reddit.com/r/aviation/comments/1p276xx/ntsb_issu...
And also ripped open a giant hole in the fuel tank which allowed all of the fuel to be released and ignited.
It nuked the tail engine so actually TWO engines failed.
From the wing down I assumed it may have depended if the engine coming out unintentionally means redundant hydraulic lines and mounts are also getting disconnected causing a complete loss of control not that it would have helped much at that point beyond minimizing ground damages.
This engine didn’t just fail, it failed catastrophically and took out another engine with it.
It's just time to kill the MD-11 entirely. These 3-engine aircraft are too risky to continue flying.
TIL about this eerily similar DC-10 crash in 2011:
Shortly after liftoff, 20 feet (6.1 m) above and 7,000 feet (2,100 m) down the runway, the No. 2 engine separated from the wing and struck the No. 1 engine's inlet cowling, causing it to produce drag and reduced thrust. Even with full right aileron and rudder, the plane started to descend and drift to the left. The captain lowered the nose and leveled the wings, which was followed by the plane making multiple contacts with the runway. After touchdown, the plane drifted left and departed the runway, crossing a taxiway before coming to rest in a saltwater marsh. A fire erupted which consumed the top of the cabin and the cockpit. All three crew members survived.
https://en.wikipedia.org/wiki/Omega_Aerial_Refueling_Service...
Obviously the DC-10 is not the MD-11, but the MD-11 is a direct descendant, including the trijet configuration.
The article you linked is talking about a 707, not a trijet. In particular, engine 2 on the MD-11 is the tail engine, not a wing engine.
...is this a bot comment? The accident you linked is very clearly of a Boeing 707, which has zero relation to a DC-10 and is most decidedly not a trijet.
The surveillance video mentioned in page 2 -- from which the series of still images are shown -- is that available publicly?
I haven't seen that one, this video [1] includes a different angle taken from a vehicle on the airport.
[1] https://youtu.be/POKJUJk_2xs?t=342
As I told my friends, this preliminary report annoys me. It annoys me for the same reason it seemingly annoys the NTSB: American 191 is nearly identical on the surface, right down to the engine detachment and resultant loss of the aircraft, in almost the exact same spot on the airframe, ~45 years later.
Needless to say they’re going to be scrutinizing everything to determine what the cause is and the sequence of events that created the accident, but I also suspect everyone involved is just as annoyed at this as I am, given that this exact situation should have been fixed already.
* Annoyed = seething rage
Actually, the two accidents are only superficially similar, and there is no basis for saying that "this exact situation" should have already been fixed.
And no, this is not just pedantry. A bulkhead fracturing from impact damage and its mounting point failing from fatigue cracking are nowhere near the same thing, even if they might lead to the same outcome.
I’m not sure where you’re getting these conclusions from, because AA191 was an issue where the engine separated from the wing and took hydraulics with it, creating a sequence of failures that resulted in the loss of the airframe. The UPS flight also had a separation of the engine from the wing and created some sequence of events that resulted in the loss of the airframe, but they’ve made it very clear in the report that the how and the why are still very much open questions.
My point - and from what I see in the report, a grievance shared by the NTSB, given their citation of AA191 - is that both aircraft come from the same lineage (DC-10/MD-11), the same manufacturer (McDonnell-Douglas, now Boeing), had the same failure (engine separating from wing assembly during takeoff roll), and with all involved parties throwing up their hands and swearing they followed all the rules and maintenance schedules.
And that’s too many coincidences to just dismiss outright from the get-go.
> I’m not sure where you’re getting these conclusions from
I'm not sure how to say this in a way that does not come across as rude, and I apologise in advance if it does, but I got those conclusions (in actuality, statements of fact) from actually reading the reports and understanding the terms and diagrams in them, where it is pretty clearly detailed that the sequence of events does not start with engine separation? You can quite literally read for yourself in both reports (as opposed to relying on a shallow pop culture understanding of the parts involved) the differences in the initial damage to the pylon.
Like, it is understandable that not everyone is an aviation nerd, but it is quite a bit silly to go "the engine separated so they're the same thing which should have been fixed by now". Or to put it another way - do you drive a car? Do you understand the car you drive, as well as cars in general - do you understand that e.g. there are many different things that can cause a flat tyre and so it makes little sense to point to two road accidents as being the same because they both "start" with a flat tyre, even if they involve the same vehicle + tyre manufacturer?
> My point - and from what I see in the report, a grievance shared by the NTSB, given their citation of AA191
I feel quite comfortable in saying that you are projecting your grievance on the investigating body, actually, especially considering that you've manufactured a narrative out of whole cloth where "all involved parties [threw] up their hands and [swore] they followed all the rules and maintenance schedules". In reality, the investigation detailed within about a month that American Airlines (as well as Continental Airlines and United) was most decidedly not following the maintenance guidance from McDonnell Douglas, which had directly led to the fracture of the aft pylon bulkhead (again, a different part from the one that failed in the UPS crash). But of course, that does not fit into the easy-to-swallow "McDonnell Douglas bad" story that people prefer over actually reading and understanding these reports for themselves.
Not an aviation expert, nor I want to be one, but the images look pretty intense.
It is incredible to me how quickly some lives can go from "another day as usual" to "gone" in a matter of seconds.
It's so similar to AA191
That's terrible. If the NTSB had flagged this flaw before then someone failed with an inspection regime or maintenance.
The NTSB doesn't ever accept the "sometimes bad things happen, shrug" excuse and kudos to the professionals there.
McDonnell-Douglass right there that's where the problems start.
Gyroscopic precession took the left engine to the right. In AA 191 the right engine departing to the right did not affect the center engine. Sadly the engine failure procedure at the time mandated slowing down to V2 which was below the stall speed with slats retracted. There's now revised procedure and hydraulic fuses.
I expect all remaining aircraft will be getting new rear pylon lugs with shortened inspection intervals - provided the replacement cost is below the value of continued usage.