Comment by bunderbunder
19 hours ago
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.
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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.
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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.
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> 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?
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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.
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> 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.
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