[bunderbunder]

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.

[ralph84]

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.

[filleduchaos]

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.

[Gare]

The Falcons have all three engines tail mounted, so not the same "type" of trijet as MD-11.

[secult]

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.

[buildsjets]

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...

[bunderbunder]

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.

[jefftk]

> 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.

[TylerE]

No military operates the KC-10 anymore. There are a grand total of two remaining in use as aerial firefighters.

[jefftk]

> 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.

[loeg]

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?

[MBCook]

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.

[inferiorhuman]

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.

[MBCook]

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!

[loeg]

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.

[harpiaharpyja]

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.

[potato3732842]

> in which case, why have the tail engine at all?

"you know what this motorized piece of anything needs, less power"

-nobody, ever

[loeg]

You know you can just make the wing engines 50% more powerful, right?

[inferiorhuman]

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.

[rangestransform]

It wasn’t about the engines being powerful but about the fan being bigger and therefore more efficient

[inferiorhuman]

The effects of a bigger fan are the same: the engine is bigger and more powerful necessitating changes to the airframe.

[psunavy03]

> just make the wing engines 50% more powerful

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

[toast0]

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.

[loeg]

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.

[inferiorhuman]

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.

[542354234235]

>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.

[db48x]

It would be way cheaper to replace the airplane with a modern twin-engine plane than to retrofit new engines onto an old plane.

[lazide]

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

[rft]

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

[wat10000]

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.

[chasil]

> 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...

[SoftTalker]

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.

[foldr]

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.

[bombcar]

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.

[ImPostingOnHN]

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).

[16bytes]

> 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.

[wat10000]

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.