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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.

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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.

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?

> 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?

> just make the wing engines 50% more powerful

You realize this is not quite how aerospace engineering works, right?

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.

Now it is, yes. At the time, it would have required 4 total engines, which is a different matter altogether.

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.

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.

Airlines currently don't want them (which is not even 100% accurate since airlines pulled A380s out of storage, and continue to push back plans to retire them). You started this by saying "You know you can just make the wing engines 50% more powerful, right?". You weren't talking about commercial decisions, you were talking about engineering decisions and capabilities. So, no you can't just make twin engines bigger in all situations. If airlines want large capacity aircraft again, they will be quad jets, not super powerful twin 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.

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.

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.

  It was never designed to be used anywhere else as a
  general design. Two engines did that better.

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.

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.

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.

> 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.

> 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).

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.

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!

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.

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.

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.

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.

Airlines haven't been using them, or at least not 1st world airlines. Just freight and wilderness fire fighters.

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.

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.

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.

> 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.

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.

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.

> 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

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