2025-06-19T14:36:00
permalink Post: 11906073
I have read most of the thread (old and new). As a lawyer working in forensic investigations, I am constantly involved in problem-solving. My field of work also includes complex investigations related to insolvencies, which almost always require an analysis of the causes behind a specific, established outcome. In doing so, I naturally also have to deal with probabilities. However, it often turns out that the most likely or plausible explanation does not reflect what actually happened.
Many of the considerations I\x92ve read fail because the simultaneous failure of both engines is extremely unlikely, leading to a constant search for higher-order causes. It was suggested that an incorrect altitude setting led to an early thrust reduction. However, this would not explain the deployment of the RAT (Ram Air Turbine), especially since the thrust could have been readjusted. FADEC and TCAM are highly redundant systems, and TCAM failure is unlikely due to WOW (Weight on Wheels) logic, making a simultaneous engine failure after VR equally improbable.
With that said, and with regard to my question concerning the AD that relates to the fuel control switches (FCS), my thought\x97and it was nothing more than that\x97was that their activation becomes more probable if it can occur accidentally. That\x92s how I came across SAIB: NM-18-33.
Another user then brought up an iPhone. That notion would, of course, be dramatic\x97but how unlikely is it really that after approximately 10,000 actuations between December 2013 and June 2025, the two FCS no longer lock perfectly? Considering all of this, I find it quite conceivable that the A/T slightly reduced thrust in the first seconds after VR (e.g., if an incorrect target altitude had been entered) and that an object lying between the thrust levers and the FCS could have pushed the FCS into the \x93Off\x94 position. Due to the buttons on top of the switches, which provide some resistance, it\x92s even possible that the object both pulled and pushed them.
But all of this is speculation. The investigation report will bring clarity.
Even if my theory is not confirmed, I still believe that the positioning and mechanism of the FCS are suboptimal. Switches of such critical importance should be better protected, and movements in the area in front of the switches (like reducing thrust) should not follow the same direction as shutting off the fuel supply. A different switching direction alone would provide more safety\x97especially considering that the FCS are protected laterally by metal plates.
Many of the considerations I\x92ve read fail because the simultaneous failure of both engines is extremely unlikely, leading to a constant search for higher-order causes. It was suggested that an incorrect altitude setting led to an early thrust reduction. However, this would not explain the deployment of the RAT (Ram Air Turbine), especially since the thrust could have been readjusted. FADEC and TCAM are highly redundant systems, and TCAM failure is unlikely due to WOW (Weight on Wheels) logic, making a simultaneous engine failure after VR equally improbable.
With that said, and with regard to my question concerning the AD that relates to the fuel control switches (FCS), my thought\x97and it was nothing more than that\x97was that their activation becomes more probable if it can occur accidentally. That\x92s how I came across SAIB: NM-18-33.
Another user then brought up an iPhone. That notion would, of course, be dramatic\x97but how unlikely is it really that after approximately 10,000 actuations between December 2013 and June 2025, the two FCS no longer lock perfectly? Considering all of this, I find it quite conceivable that the A/T slightly reduced thrust in the first seconds after VR (e.g., if an incorrect target altitude had been entered) and that an object lying between the thrust levers and the FCS could have pushed the FCS into the \x93Off\x94 position. Due to the buttons on top of the switches, which provide some resistance, it\x92s even possible that the object both pulled and pushed them.
But all of this is speculation. The investigation report will bring clarity.
Even if my theory is not confirmed, I still believe that the positioning and mechanism of the FCS are suboptimal. Switches of such critical importance should be better protected, and movements in the area in front of the switches (like reducing thrust) should not follow the same direction as shutting off the fuel supply. A different switching direction alone would provide more safety\x97especially considering that the FCS are protected laterally by metal plates.
6 users liked this post.
2025-06-19T14:51:00
permalink Post: 11906087
OK, I promised some
informed speculation
when I got back, so here goes:
Disclaimer: never worked the 787, so my detailed knowledge is a bit lacking.
First off, this is perplexing - especially if the RAT was deployed. There is no 'simple' explanation that I can come up with.
GEnx-1B engines have been exceptionally reliable, and the GE carbon composite fan blades are very robust and resistant to bird strike damage (about 15 years after the GE90 entry into service, I remember a GE boast that no GE90 (carbon composite) fan blades had needed to be scrapped due to damage (birdstrike, FOD, etc. - now that was roughly another 15 years ago, so is probably no longer true, but it shows just how robust the carbon composite blades are - far better than the more conventional titanium fan blades).
Not saying it wasn't somehow birdstrike related, just that is very unlikely (then again, all the other explanations I can come up with are also very unlikely
).
Using improper temp when calculating TO performance - after some near misses, Boeing added logic that cross-compares multiple total temp probes - aircraft TAT (I think the 787 uses a single, dual element probe for aircraft TAT, but stand to be corrected) and the temp measured by the engine inlet probes - and puts up a message if they disagree by more than a few degree tolerance - so very, very unlikely.
N1 power setting is somewhat less prone to measurement and power setting errors than EPR (N1 is a much simpler measurement than Rolls EPR) - although even with EPR, problems on both engines at the same time is almost unheard of.
The Auto Thrust (autothrottle) function 'falls asleep' at 60 knots - and doesn't unlock until one of several things happens - 250 knots, a set altitude AGL is exceeded (I'm thinking 3,000 ft. but the memory is fuzzy), thrust levers are moved more than a couple of degrees, or the mode select is changed (memory says that last one is inhibited below 400 ft. AGL). So an Auto Thrust malfunction is also extremely unlikely. Further, a premature thrust lever retard would not explain a RAT deployment.
TO does seem to be very late in the takeoff role - even with a big derate, you still must accelerate fast enough to reach V1 with enough runway to stop - so there is still considerable margin if both engines are operating normally. That makes me wonder if they had the correct TO power setting - but I'm at a loss to explain how they could have fouled that up with all the protections that the 787 puts on that.
If one engine did fail after V1, it's conceivable that they shut down the wrong engine - but since this happened literally seconds after takeoff, it begs the question why they would be in a big hurry to shut down the engine. Short of an engine fire, there is nothing about an engine failure that requires quick action to shut it down - no evidence of an engine fire, and even with an engine fire, you normally have minutes to take action - not seconds.
The one thing I keep thinking about is someone placing both fuel switches to cutoff immediately after TO. Yes, it's happened before (twice - 767s in the early 1980s), but the root causes of that mistake are understood and have been corrected. Hard to explain how it could happen ( unless, God forbid, it was intentional ).
Disclaimer: never worked the 787, so my detailed knowledge is a bit lacking.
First off, this is perplexing - especially if the RAT was deployed. There is no 'simple' explanation that I can come up with.
GEnx-1B engines have been exceptionally reliable, and the GE carbon composite fan blades are very robust and resistant to bird strike damage (about 15 years after the GE90 entry into service, I remember a GE boast that no GE90 (carbon composite) fan blades had needed to be scrapped due to damage (birdstrike, FOD, etc. - now that was roughly another 15 years ago, so is probably no longer true, but it shows just how robust the carbon composite blades are - far better than the more conventional titanium fan blades).
Not saying it wasn't somehow birdstrike related, just that is very unlikely (then again, all the other explanations I can come up with are also very unlikely
).
Using improper temp when calculating TO performance - after some near misses, Boeing added logic that cross-compares multiple total temp probes - aircraft TAT (I think the 787 uses a single, dual element probe for aircraft TAT, but stand to be corrected) and the temp measured by the engine inlet probes - and puts up a message if they disagree by more than a few degree tolerance - so very, very unlikely.
N1 power setting is somewhat less prone to measurement and power setting errors than EPR (N1 is a much simpler measurement than Rolls EPR) - although even with EPR, problems on both engines at the same time is almost unheard of.
The Auto Thrust (autothrottle) function 'falls asleep' at 60 knots - and doesn't unlock until one of several things happens - 250 knots, a set altitude AGL is exceeded (I'm thinking 3,000 ft. but the memory is fuzzy), thrust levers are moved more than a couple of degrees, or the mode select is changed (memory says that last one is inhibited below 400 ft. AGL). So an Auto Thrust malfunction is also extremely unlikely. Further, a premature thrust lever retard would not explain a RAT deployment.
TO does seem to be very late in the takeoff role - even with a big derate, you still must accelerate fast enough to reach V1 with enough runway to stop - so there is still considerable margin if both engines are operating normally. That makes me wonder if they had the correct TO power setting - but I'm at a loss to explain how they could have fouled that up with all the protections that the 787 puts on that.
If one engine did fail after V1, it's conceivable that they shut down the wrong engine - but since this happened literally seconds after takeoff, it begs the question why they would be in a big hurry to shut down the engine. Short of an engine fire, there is nothing about an engine failure that requires quick action to shut it down - no evidence of an engine fire, and even with an engine fire, you normally have minutes to take action - not seconds.
The one thing I keep thinking about is someone placing both fuel switches to cutoff immediately after TO. Yes, it's happened before (twice - 767s in the early 1980s), but the root causes of that mistake are understood and have been corrected. Hard to explain how it could happen ( unless, God forbid, it was intentional ).
3 users liked this post.
2025-06-19T14:52:00
permalink Post: 11906088
I\x92ve had a look at the Honeywell spec sheet for TL type switches. They are a common type with many available configurations and are essentially a normal looking snap action switch. The locking feature is an add-on which can be specified. I have to say that from the diagrams, the locking mechanism doesn\x92t look that robust and I\x92d guess that it is subject to wear which was probably the basis of the SAIB. Given that this is a mechanical locking device seeing frequent use possibly with less than full mechanical sympathy it is possible to see how wear would occur making the lock less effective. This does beg the question of whether a check on the mechanism has made it into maintenance routines. ( Note - the locking mechanism appears to be the same for all TL switch types) . Those familiar with the switches - what do you think?
For the avoidance of doubt, the above is a technical observation on the switch type NOT a causation theory for this accident.
For the avoidance of doubt, the above is a technical observation on the switch type NOT a causation theory for this accident.
6 users liked this post.
2025-06-19T15:04:00
permalink Post: 11906096
this isn\x92t the type of switch fitted to the 787 as a fuel control switch, totally irrelevant but has generated yet more nonsense. The switches are spring loaded (or so it feels) in addition to having a massive block to prevent inadvertent operation in either direction. Anyone suggesting they could be accidentally \x93knocked off\x94 is so clueless about their operation it\x92s actually painful to rebut
4 users liked this post.
2025-06-19T16:05:00
permalink Post: 11906149
Cutoff Switches
this isn’t the type of switch fitted to the 787 as a fuel control switch, totally irrelevant but has generated yet more nonsense. The switches are spring loaded (or so it feels) in addition to having a massive block to prevent inadvertent operation in either direction. Anyone suggesting they could be accidentally “knocked off” is so clueless about their operation it’s actually painful to rebut
Not looking over my shoulder here but I should add that I know and respect Honeywell switches. Those I have worked with are well designed and well made plus Honeywell incorporate improvements based on field experience. As posted by another contributor upthread I am puzzled by the odd ‘down’ = engine off configuration though.
Last edited by Europa01; 19th Jun 2025 at 16:36 .
4 users liked this post.
2025-06-19T16:12:00
permalink Post: 11906161
this isn\x92t the type of switch fitted to the 787 as a fuel control switch, totally irrelevant but has generated yet more nonsense. The switches are spring loaded (or so it feels) in addition to having a massive block to prevent inadvertent operation in either direction. Anyone suggesting they could be accidentally \x93knocked off\x94 is so clueless about their operation it\x92s actually painful to rebut
That's a TL series switch with 4 poles (the "4" in "4TL"), a "type D" lock (meaning locked out of centre position per the Honeywell data sheet - the "D" in "3D." This is a photo of one:
You can find the manufacturer's datasheet here: https://octopart.com/datasheet/4tl83...ywell-25749542
Problems with critical switches aren't new on 787-8s. For instance, in addition to the SAIB above, there's this AD: https://www.federalregister.gov/docu...pany-airplanes
Looking at the photo above, it isn't just wear that's potentially an issue, but foreign object impingement. There don't appear to be gaitors fitted to these switches in the 788, so the locking mechanisms are potentially susceptible to a build-up of material if not kept clean. There are a range of other failure modes possible, whilst the SAIB specifically describes found-in-the-field problems with these switches.
Yes, they're chunky, and very positive when new. That doesn't mean they shouldn't be discussed.
7 users liked this post.
2025-06-19T16:28:00
permalink Post: 11906174
(re:skwdenier and the photo of the switch. )
And those wires, which all i can find points to having (non-english speaker) screw terminals. within the LRU shared by both switches. Those things go bad over time even when installed in non-movable equipment.
For those that argue that these are some kind of very special switches. They aren't. Sure they are really nice switches, but that series is available through normal distributors like mauser or elfa-distrelec in europe for, and the price isn't sensationally high compared to other switches.
I am curious as to how the LED light power is threaded through to the switch top. Why do they have a LED, in the first place? Seems a bit gimmicky.
Since they have such inherent damage potential in systems that are otherwise carefully isolated through redundancies and physical seperation, why aren't they? Why aren't the 2 cut off switches placed in 2 different locations in the cockpit. Seems like such a cheap insurance.
Sorry if I am adding noise, I know these are just one of many possibilities, and I am not knowledgeable enough to weigh these different theories against each other, but this switch thing is just something I know personally have more failure potential than what I thought were acceptable for this application.
And those wires, which all i can find points to having (non-english speaker) screw terminals. within the LRU shared by both switches. Those things go bad over time even when installed in non-movable equipment.
For those that argue that these are some kind of very special switches. They aren't. Sure they are really nice switches, but that series is available through normal distributors like mauser or elfa-distrelec in europe for, and the price isn't sensationally high compared to other switches.
I am curious as to how the LED light power is threaded through to the switch top. Why do they have a LED, in the first place? Seems a bit gimmicky.
Since they have such inherent damage potential in systems that are otherwise carefully isolated through redundancies and physical seperation, why aren't they? Why aren't the 2 cut off switches placed in 2 different locations in the cockpit. Seems like such a cheap insurance.
Sorry if I am adding noise, I know these are just one of many possibilities, and I am not knowledgeable enough to weigh these different theories against each other, but this switch thing is just something I know personally have more failure potential than what I thought were acceptable for this application.
3 users liked this post.
2025-06-19T18:05:00
permalink Post: 11906239
Fuel valves and TCMA software updates?
Commanded engine cutoff - the aisle stand fuel switch sends electrical signals to the spar valve and the "High Pressure Shutoff Valve" (HPSOV) in the Fuel Metering Unit, commanding them to open/close using aircraft power. The HPSOV is solenoid controlled, and near instantaneous. The solenoid is of a 'locking' type that needs to be powered both ways (for obvious reasons, you wouldn't want a loss of electrical power to shut down the engine). The fire handle does the same thing, via different electrical paths (i.e. separate wiring).
As I've noted previously, a complete loss of aircraft electrical power would not cause the engines to flameout (or even lose meaningful thrust) during takeoff. In the takeoff altitude envelope, 'suction feed' (I think Airbus calls it 'gravity feed') is more than sufficient to supply the engine driven fuel pumps. It's only when you get up to ~20k ft. that suction feed can become an issue - and this event happened near sea level.
Not matter what's happening on the aircraft side - pushing the thrust levers to the forward stop will give you (at least) rated takeoff power since the only thing required from the aircraft is fuel and thrust lever position (and the thrust lever position resolver is powered by the FADEC).
The TCMA logic is designed and scrubbed so as to be quite robust - flight test data of the engine response to throttle slams is reviewed to insure there is adequate margin between the TCMA limits and the actual engine responses to prevent improper TCMA activation. Again, never say never, but a whole lot would have had to go wrong in the TCMA logic for it to have activated on this flight.
Now, if I assume the speculation that the RAT deployed is correct, I keep coming up with two potential scenarios that could explain what's known regarding this accident:
1) TCMA activation shutdown the engines
or
2) The fuel cutoff switches were activated.
I literally can come up with no other plausible scenarios.
In all due respect to all the pilots on this forum, I really hope it wasn't TCMA. It wouldn't be the first time a mandated 'safety system' has caused an accident (it wouldn't just be Boeing and GE - TCMA was forced by the FAA and EASA to prevent a scenario that had never caused a fatal accident) - and there would be a lot embarrassing questions for all involved. But I personally know many of the people who created, validated, and certified the GEnx-1B TCMA logic - and can't imagine what they would be going through if they missed something (coincidentally, one of them was at my birthday party last weekend and inevitably we ended up talking about what we used to do at Boeing (he's also retired)). Worse, similar TCMA logic is on the GEnx-2B (747-8) - which I was personally responsible for certifying - as well as the GE90-115B and the 737 MAX Leap engine - the consequences of that logic causing this accident would be massive.
As I've noted previously, a complete loss of aircraft electrical power would not cause the engines to flameout (or even lose meaningful thrust) during takeoff. In the takeoff altitude envelope, 'suction feed' (I think Airbus calls it 'gravity feed') is more than sufficient to supply the engine driven fuel pumps. It's only when you get up to ~20k ft. that suction feed can become an issue - and this event happened near sea level.
Not matter what's happening on the aircraft side - pushing the thrust levers to the forward stop will give you (at least) rated takeoff power since the only thing required from the aircraft is fuel and thrust lever position (and the thrust lever position resolver is powered by the FADEC).
The TCMA logic is designed and scrubbed so as to be quite robust - flight test data of the engine response to throttle slams is reviewed to insure there is adequate margin between the TCMA limits and the actual engine responses to prevent improper TCMA activation. Again, never say never, but a whole lot would have had to go wrong in the TCMA logic for it to have activated on this flight.
Now, if I assume the speculation that the RAT deployed is correct, I keep coming up with two potential scenarios that could explain what's known regarding this accident:
1) TCMA activation shutdown the engines
or
2) The fuel cutoff switches were activated.
I literally can come up with no other plausible scenarios.
In all due respect to all the pilots on this forum, I really hope it wasn't TCMA. It wouldn't be the first time a mandated 'safety system' has caused an accident (it wouldn't just be Boeing and GE - TCMA was forced by the FAA and EASA to prevent a scenario that had never caused a fatal accident) - and there would be a lot embarrassing questions for all involved. But I personally know many of the people who created, validated, and certified the GEnx-1B TCMA logic - and can't imagine what they would be going through if they missed something (coincidentally, one of them was at my birthday party last weekend and inevitably we ended up talking about what we used to do at Boeing (he's also retired)). Worse, similar TCMA logic is on the GEnx-2B (747-8) - which I was personally responsible for certifying - as well as the GE90-115B and the 737 MAX Leap engine - the consequences of that logic causing this accident would be massive.
I seem to remember Fred Dibner talking about how railway cars brake by draining the piston not by pressurising it, so trains will stop when supply lines break.
The electrical system updates to 787s for ADs and SBs - do any of these include software updates? For example the integer overflow causing GCU failsafe rectified under AD 2018-20-15. If so, who is writing and implementing these software updates? The original engineers? Their apprentices who had years long handovers? Or have they been outsourced and offshored? When these updates occur, does the entire system get tested and ratified or just the bit the bug fix is meant to fix? Because I\x92ve seen new bugs introduced by bug fixes in areas seemingly nothing to do with the original problem.
2025-06-19T18:48:00
permalink Post: 11906263
4 users liked this post.
2025-06-19T18:49:00
permalink Post: 11906264
There were simultaneous engine failures, but those were due to massive birdstrikes (
US1549
) or due to epidemic engine failures on Il-62s of various versions (like
LOT 007
or
LOT 5055
).
Fuel related total engine failures like Aeroflot 366 or Air Transat 236 at least had the decency to have the engines starve one after another as the fuel in the individual tanks depleted.
But all those are probably highly irrelevant when considering the Air India accident. An engine disintegration or a heavy birdstrike would have been visible on the videos, a sizeable bird would have left some remains. And gradual fuel starvation would have shown some yaw.
As much as I despise the thought, the issue that got AI171 must have come from within the aircraft, although this most decidedly does not infer any wrongdoing by any crewmember.
Fuel related total engine failures like Aeroflot 366 or Air Transat 236 at least had the decency to have the engines starve one after another as the fuel in the individual tanks depleted.
But all those are probably highly irrelevant when considering the Air India accident. An engine disintegration or a heavy birdstrike would have been visible on the videos, a sizeable bird would have left some remains. And gradual fuel starvation would have shown some yaw.
As much as I despise the thought, the issue that got AI171 must have come from within the aircraft, although this most decidedly does not infer any wrongdoing by any crewmember.
1 user liked this post.
2025-06-19T18:55:00
permalink Post: 11906268
One thing that just came to my mind: We are scratching our heads why that happened after the type is in service for almost 15 years with millions of flight hours and surely hundred thousands of T/Os and landings.
What if it takes something to be worn/used after many years to get that kind of failure? The AI 787 was 11 years old. We have been discussing the fuel switches, but there are thousands of other parts that might contribute to such a failure in connection with some other problem.
What if it takes something to be worn/used after many years to get that kind of failure? The AI 787 was 11 years old. We have been discussing the fuel switches, but there are thousands of other parts that might contribute to such a failure in connection with some other problem.
4 users liked this post.
2025-06-19T19:01:00
permalink Post: 11906271
One thing that just came to my mind: We are scratching our heads why that happened after the type is in service for almost 15 years with millions of flight hours and surely hundred thousands of T/Os and landings.
What if it takes something to be worn/used after many years to get that kind of failure? The AI 787 was 11 years old. We have been discussing the fuel switches, but there are thousands of other parts that might contribute to such a failure in connection with some other problem.
What if it takes something to be worn/used after many years to get that kind of failure? The AI 787 was 11 years old. We have been discussing the fuel switches, but there are thousands of other parts that might contribute to such a failure in connection with some other problem.
2025-06-19T19:05:00
permalink Post: 11906278
There were simultaneous engine failures, but those were due to massive birdstrikes (
US1549
) or due to epidemic engine failures on Il-62s of various versions (like
LOT 007
or
LOT 5055
).
Fuel related total engine failures like Aeroflot 366 or Air Transat 236 at least had the decency to have the engines starve one after another as the fuel in the individual tanks depleted.
But all those are probably highly irrelevant when considering the Air India accident. An engine disintegration or a heavy birdstrike would have been visible on the videos, a sizeable bird would have left some remains. And gradual fuel starvation would have shown some yaw.
As much as I despise the thought, the issue that got AI171 must have come from within the aircraft, although this most decidedly does not infer any wrongdoing by any crewmember.
Fuel related total engine failures like Aeroflot 366 or Air Transat 236 at least had the decency to have the engines starve one after another as the fuel in the individual tanks depleted.
But all those are probably highly irrelevant when considering the Air India accident. An engine disintegration or a heavy birdstrike would have been visible on the videos, a sizeable bird would have left some remains. And gradual fuel starvation would have shown some yaw.
As much as I despise the thought, the issue that got AI171 must have come from within the aircraft, although this most decidedly does not infer any wrongdoing by any crewmember.
- GoAir320 at Delhi
- Transasia AT72 at Taipei
- Alitalia A332 at Seoul
- SA Airlink JS41 at Durban
Not saying it happened here!
2025-06-19T23:16:00
permalink Post: 11906475
If so , the likelyhood of this having anything to do with the switches, their harness, or connectors drops way down. (although most theories are dealing with statistical "impossibilities", what better time than after decades for such to occur.)
The switches are double on's or 4 pole, that means they are (can be) connected to 2 different systems individually. Anyone know how that system looks? Why 2 signals?
2025-06-19T23:26:00
permalink Post: 11906480
Summary of main theories
DISCLAIMER: Poster (a) is one of the (apparently quite numerous) lawyers following this thread; (b) a long-time forum lurker and aviation enthusiast who loves studying FCOMs for fun (to each his own, I guess); (c) has followed and read this thread from the start.
What I cannot do is add new theories or uncover any new facts the actual experts have not already thought of. However, since summarizing and structuring information is one thing lawyers tend to regularly do (and sometimes even do well), here is my attempt at a useful contribution to this thread: an attempt to summarize the main theories discussed here since day one (which I think hasn't been done for quite some time) in the hope that a birds-eye view will be helpful to those who have not read everything since the beginning or might even trigger some new flash of inspiration for someone more knowledgable than me. I have focused on the cons since there does not seem to be enough evidence to come to any positive conclusion.
I shall try to be concise and to refrain from personal evaluations of my own. Of course, no disrespect whatsoever is intended towards all those who have contributed to this thread and to the individual theories, one or combinations of which may turn out to have led to this tragic outcome. That arguments can be made against every single theory that has been propagated seems to be the result of the highly improbable and unusual nature of this deplorable event and certainly not due to any lack of knowledge or reasoning skills in this forum.
DEAR MODS: If I have distorted anything or if, meaning well, should have achieved the opposite \x96 I guess you know where the delete button is\x85
Anyway, here goes:
A. Misconfiguration or wrong takeoff data
Widely refuted, since
Still brought up from time to time. However, widely disregarded due to
It should be pointed out that the question of "RAT in or out" was for a while the most contentious in this thread.
C. Low-altitude capture
Still argued, even if refuted by many since
Various possible reasons for this have been discussed:
I. Bird strike/FOD
Unclear which maintenance measures could possibly have been performed that would have resulted in simultaneous loss of both engines. No apparent relationships between malfunctions reported by previous passengers and essential systems.
IV. Large-scale electrical fault (e.g. due to water in E&E bay)
The engines will continue to run if electrical power is lost. FADECs are powered independently.
V. Shutdown of engines by TCMA
A parallel is drawn to the ANA incident. However, this would require not only a fault in the air/ground logic but also a sensed discrepancy between T/L position (not necessarily idle) and thrust output on both engines simultaneously.
VI. (Inadvertent) shutdown by flight crew
What I cannot do is add new theories or uncover any new facts the actual experts have not already thought of. However, since summarizing and structuring information is one thing lawyers tend to regularly do (and sometimes even do well), here is my attempt at a useful contribution to this thread: an attempt to summarize the main theories discussed here since day one (which I think hasn't been done for quite some time) in the hope that a birds-eye view will be helpful to those who have not read everything since the beginning or might even trigger some new flash of inspiration for someone more knowledgable than me. I have focused on the cons since there does not seem to be enough evidence to come to any positive conclusion.
I shall try to be concise and to refrain from personal evaluations of my own. Of course, no disrespect whatsoever is intended towards all those who have contributed to this thread and to the individual theories, one or combinations of which may turn out to have led to this tragic outcome. That arguments can be made against every single theory that has been propagated seems to be the result of the highly improbable and unusual nature of this deplorable event and certainly not due to any lack of knowledge or reasoning skills in this forum.
DEAR MODS: If I have distorted anything or if, meaning well, should have achieved the opposite \x96 I guess you know where the delete button is\x85
Anyway, here goes:
A. Misconfiguration or wrong takeoff data
Widely refuted, since
- rotation, takeoff and initial climb seem normal;
- likely extreme errors would have been required to have such tragic effect (the fuel tanks should have been only about half full, so not close to MTOW);
- there is strong evidence that at least some flaps were extended for takeoff (post-crash photo, perhaps also visible in video from behind)
Still brought up from time to time. However, widely disregarded due to
- the fact that with two working engines an inadvertent flap retraction should easily be recoverable, even with gear down;
- strong indications that hydraulic and electric power were lost (audible/visible indications of RAT extension, survivor statement, lack of engine noise, position of MLG bogies).
It should be pointed out that the question of "RAT in or out" was for a while the most contentious in this thread.
C. Low-altitude capture
Still argued, even if refuted by many since
- inconsistent with apparent loss of hydraulic/electric power;
- PF would have been flying manually (however, A/T reaction would have been unexpected for the PF);
- should have been recoverable (unless one assumes that the crew (a) remained unaware of the changed FMA annunciations although alerted by the unexpected FD commands; and (b) was so startled that an A/T thrust reduction was not noticed and corrected, even though the PF was apparently sufficiently alert not to follow the FD commands).
Various possible reasons for this have been discussed:
I. Bird strike/FOD
- Would have to have occurred simultaneously due to lack of rudder/aileron input indicating symmetric thrust.
- No remains/traces on runway, no visual indications (flocks of birds, flames, structural engine damage).
1. Loss of electric fuel pumps
Suction feed would have provided sufficient fuel pressure.
2. Fuel contamination
No other aircraft affected, no measures taken at airport. Simultaneous flameout due to contaminated fuel very unlikely.
3. Vapour lock
Unlikely to occur in this scenario. Even if (momentarily) no sufficient fuel pressure from the center tank, the engines would have been fed by the wing tanks.
III. Improper maintenance
Unclear which maintenance measures could possibly have been performed that would have resulted in simultaneous loss of both engines. No apparent relationships between malfunctions reported by previous passengers and essential systems.
IV. Large-scale electrical fault (e.g. due to water in E&E bay)
The engines will continue to run if electrical power is lost. FADECs are powered independently.
V. Shutdown of engines by TCMA
A parallel is drawn to the ANA incident. However, this would require not only a fault in the air/ground logic but also a sensed discrepancy between T/L position (not necessarily idle) and thrust output on both engines simultaneously.
VI. (Inadvertent) shutdown by flight crew
1. Spontaneous execution of memory items (fuel control switches OFF, then ON; deploy RAT) due to assumed engine malfunction
In contrast to mistakenly shutting down the wrong engine after having correctly diagnosed the problem as per SOP, this would require not only a simple error in execution but a counter-intuitive unilateral action immediately after takeoff against basic principles of SOP or CRM.
2. No indications whatsoever of an intentional shutdown for nefarious reasons
(Would also be inconsistent with the content of the alleged mayday call.)
VII.
Malfunction/mishandling of the fuel cutoff switches (most recent)
1.
Wear or improper operation of the switches, so that they do not lock but can shift back into the OFF position.
Argued to be impossible due to robust switch design, preventing switch release in any other than a locked position.
Actuation of the switches by an item placed before them which was pushed onto the switches by retarding thrust levers seems equally unlikely due to force required to pull the switches out of the locked position.
Actuation of the switches by an item placed before them which was pushed onto the switches by retarding thrust levers seems equally unlikely due to force required to pull the switches out of the locked position.
2.
Spilled drink leading to short in the wiring
Hardly conceivable that before takeoff open liquid containers would be placed anywhere where they could spill onto the pedestal.
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2025-06-20T00:49:00
permalink Post: 11906514
SLF but I think this makes sense. If pulling from takeoff thust back to idle with WoW would cause TCMA activation, we'd see engine shutdowns on every rejected takeoff.
I also wonder about this theory that one of the pilots called for reject and pulled the thrust levers back, and the other overruled him and continued the takeoff. Is this plausible? CRM aside, if max braking and spoilers are triggered in this scenario, it doesn't seem so to me.
I also wonder about this theory that one of the pilots called for reject and pulled the thrust levers back, and the other overruled him and continued the takeoff. Is this plausible? CRM aside, if max braking and spoilers are triggered in this scenario, it doesn't seem so to me.
Last edited by Lead Balloon; 20th Jun 2025 at 00:59 .
3 users liked this post.
2025-06-20T02:46:00
permalink Post: 11906545
tdracer addressed the shutoff valve operation earlier: "the aisle stand fuel switch sends electrical signals to the spar valve and the "High Pressure Shutoff Valve" (HPSOV) in the Fuel Metering Unit, commanding them to open/close using aircraft power. The HPSOV is solenoid controlled, and near instantaneous.
The solenoid is of a 'locking' type that needs to be powered both ways (for obvious reasons, you wouldn't want a loss of electrical power to shut down the engine).
"
2025-06-20T03:41:00
permalink Post: 11906563
I am not suggesting you are wrong and, as I've said before, the descriptions and schematic in the patent application are just 'big hands / small maps' concepts. However, if TCMA functionality "is simply a bit of software in the FADECs", merely sending a 1 or 0 or other signal into a point in the pre-existing FADEC that already had control over fuel cutoff (with the TCMA software merely monitoring data busses, rather than direct sensor outputs, to work out thrust lever position and whether or not the aircraft is 'on the ground' for TCMA purposes) I for one would really like to know that for sure and get my head around the implications.
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2025-06-20T04:18:00
permalink Post: 11906574
Just so I have this clear, are you saying that the implementation of the TCMA functionality involved
no
new components being added to the pre-existing FADEC? Are you saying, in effect, that the two switch relays described in the TCMA patent application, which relays and their configuration achieves the described two channel redundancy, were already there as components or are mere depictions of what the software does itself?
I am not suggesting you are wrong and, as I've said before, the descriptions and schematic in the patent application are just 'big hands / small maps' concepts. However, if TCMA functionality "is simply a bit of software in the FADECs", merely sending a 1 or 0 or other signal into a point in the pre-existing FADEC that already had control over fuel cutoff (with the TCMA software merely monitoring data busses, rather than direct sensor outputs, to work out thrust lever position and whether or not the aircraft is 'on the ground' for TCMA purposes) I for one would really like to know that for sure and get my head around the implications.
I am not suggesting you are wrong and, as I've said before, the descriptions and schematic in the patent application are just 'big hands / small maps' concepts. However, if TCMA functionality "is simply a bit of software in the FADECs", merely sending a 1 or 0 or other signal into a point in the pre-existing FADEC that already had control over fuel cutoff (with the TCMA software merely monitoring data busses, rather than direct sensor outputs, to work out thrust lever position and whether or not the aircraft is 'on the ground' for TCMA purposes) I for one would really like to know that for sure and get my head around the implications.
It uses existing thrust-lever-angle inputs, existing N1 inputs, and (presumably) existing WoW inputs, does software stuff inside the ECU, and if necessary uses the existing overspeed cutout outputs to stop the engine.
3 users liked this post.
2025-06-20T04:24:00
permalink Post: 11906577
What if it takes something to be worn/used after many years to get that kind of failure? The AI 787 was 11 years old. We have been discussing the fuel switches, but there are thousands of other parts that might contribute to such a failure in connection with some other problem.
The TWA 800 airframe was 25 years old at the time of the accident, where arced wiring was implicated in that crash. In the aftermath, industry-wide sampling of aircraft found cracked insulation on wiring, non-factory swarf added by follow-on maintenance, instances where wiring had been re-routed or manipulated in a manner that placed increased strain on looms, etc. Of course the problem with wiring-related problems is that they can produce faults that no engineer could have foreseen or have developed countermeasures for.
Not to be an alarmist, but much has been written about wiring issues on the aging fleet. I tend to believe that maintenance people in earlier generations were more conscientious about their work, where now more than ever corners are cut (beginning at the factory).
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