Comment by Jtsummers 14 hours ago
> And at least in theory the MD-11 should be flyable with just two engines
Flying with two engines and taking off without an engine in a loaded aircraft are two very different things. A lot more thrust is needed during takeoff than after.
> The crew wouldn't have had any way to know that one of their engines had not simply failed, but was straight-up gone with their wing on fire to boot.
I don't know about the MD-11 itself, but other aircraft from that time period have sensors to detect and report overheat and fire in various parts of the aircraft, including engines and wings.
Well, there's a very big difference between "Engine fire: some of the combustion chamber's heat and flame has breached containment" and, say, "Engine fire: the engine has exploded, catastrophically damaging your wing which is now visibly on fire". However, both things are reported in the cockpit as ENG FIRE.
There's also a very big difference between "Engine failure: something has damaged or jammed enough components that the turbines are no longer spinning fast enough to produce thrust or drive the generators" and "Engine failure: the engine is no longer attached to the aircraft, which is why it is no longer producing thrust". However, both things are reported in the cockpit as ENG FAIL.
(Un)fortunately, cockpit warnings prioritise the what (so to speak) and not the how or why. On one hand, this makes decision-making a lot simpler for the crew, but on the other...well, in rare cases the lack of insight can exacerbate a disaster. Depending on when exactly the engine gave out, this poor crew might have been doomed either way, but they might have been able to minimise collateral damage if they knew just how badly crippled the aircraft was. And there was a very similar accident to this one (actually involving the predecessor of the MD-11, the DC-10), American Airlines 191 - one of the engines detached from the aircraft, damaging the leading edge of its wing in the process, causing that wing to stall when the crew slowed down below the stall speed of the damaged wing in a bid to climb. If they could have somehow known about the damage, the accident might have been avoided entirely as the crew might have known to keep their speed up.
> There's also a very big difference between "Engine failure: something has damaged or jammed enough components that the turbines are no longer spinning fast enough to produce thrust or drive the generators" and "Engine failure: the engine is no longer attached to the aircraft, which is why it is no longer producing thrust". However, both things are reported in the cockpit as ENG FAIL.
What is the difference?
I’m sure they knew there was an issue, but I don’t think the sensors can differentiate between “your engine is on fire, but if you can shut it down quickly, everything will be cool.” and “half your entire wing is on fire and your engine is pouring flame out the front/top instead of the back”
This puts an impractical amount of faith in the sensor wiring when the whole pylon and cowling are shredded.
It is a very practical amount of fait.
There are two fire detection loops for each engine.[1] Even if both fails (because they get shredded as you say it) the system will report an engine fire if the two loops fail within 5s of each other. (Or FIRE DET (1,2,3,or APU) FAIL, if they got shredded with more than 5s in between without any fire indications in between.)
The detection logic is implemented directly below the cockpit. So that unlikely to have shredded at the same time. But even if the detection logic would have died that would also result in a fire alarm. (as we learned from the March 31, 2002 Charlotte incident.)[2]
In other words it is a very reliable system.
1: page 393 https://randomflightdatabase.fr/Documents/Manuel%20Aviation/...
2: https://www.fss.aero/accident-reports/dvdfiles/US/2002-03-31...
I don't know what the MD-11 would have had, again I didn't work on it. But the systems used for other aircraft would have reported an alarm based on what I saw in the video, at least they were designed to do that. The LRU receiving the sensor inputs wouldn't typically be in the wing and would be able to continue reporting the alarm condition even if the sensors fail. In fact, the lack of current from the sensor (for the systems I worked on) would have been enough to trigger the alarm if the sensor were completely eliminated.
No reading is not quite the same as "hot", but I'm sure it did contribute to discerning simple compressor stall to whatever this was.
Every multi-engine airliner is designed to be able to take off safely even if an engine fails at a critical moment. What might have happened in this case is that the mechanism of failure of one engine caused damaged or interfered with the operation of another engine (via smoke, debris, etc.), and taking off with two engines degraded is not part of the design criteria.
I specifically remember watching a flight test doing an aggressive takeoff and having the voiceover say that aircraft (two engine) need to have enough power to take off full with one engine. And so can take off very steeply empty with two engines. Would that not also be the case for these planes?
Yes, planes are designed to be able to take off with a lost engine. Usually this will extend the roll a bit because the speeds are different for engine out operations. This isn't the first MD-11 with an engine out take off, 5 years ago a FedEx MD-11 took off with a failure in the left engine[1]. Slightly different case, obviously, but it's certainly something that is accounted for when designing planes.
Yes, the takeoff roll will be longer, the climb will be much more shallow, but it is possible to take off with one out of two engines (and obviously also with two out of three). Of course, after successful takeoff, the plane should turn around and land as soon as possible.
In this case however, with the wing already on fire (the engine is below the wing, so flames coming out of it would be visible behind and under the wing, not in front), I'm afraid that even if they had managed to take off, the fuel tank would have exploded or burned through the wing before they would have had a chance to land. Actually, this looks similar to the 2000 Concorde crash...
That being said, depending on how you lose the engine it can really mess up the takeoff; AA191 was lost when an engine detached from the plane on takeoff and took out part of the wing and hydraulic system with it. https://en.wikipedia.org/wiki/American_Airlines_Flight_191
> when an engine detached from the plane on takeoff...
https://cdn.discordapp.com/attachments/757091156717862935/14...
Source: https://reddit.com/r/flying/comments/1ooms7t/ksdf_accident/n...
All planes are definitely capable of taking off safely even if they lose an engine at the worst time. Whatever happened here, I would be shocked if lack of thrust in the 2 remaining engines was a significant factor unless someone really screwed up the load calculations and they were overweight for conditions.
Single engine planes (GA, and some military planes) don’t handle this condition well at all.
In fact, for awhile (maybe still the case), the #1 killer of skydivers was single engine failure on takeoff from the jump plane (and similar aircraft failures), not accidents ‘while skydiving’.
Taking off with one engine inoperative (on a multi-engine aircraft, obviously - you aren't going to get anywhere with your only engine gone) is completely normal/within design parameters, albeit undesirable.
In fact, it being normal almost certainly contributed to the scale of this accident, since a single engine failure during the takeoff roll isn't considered enough of an emergency to reject the takeoff at high speed (past a certain speed, you only abort if the aircraft is literally unflyable - for everything else, you take the aircraft & emergency into the air and figure it out there). The crew wouldn't have had any way to know that one of their engines had not simply failed, but was straight-up gone with their wing on fire to boot.