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Kraftstoffvondesibel
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. 3 users liked this post. |
lancs
2025-06-19T17:47:00 permalink Post: 11906225 |
I appreciate the link to Musician's posts. I had read them before and have now reread them, and yes, they add valuable info although for me they did not discount the potential for a problem.
As I understand it, the fuel temperature is measured in the tanks, not just before the centrifugal pump and it's not inconceivable that the nacelles are warmer. I also understand that the Fuel Synoptic Display only turns to Amber if the temperature is too high. As it's not in red it might thought to be one of those "it will be OK" alerts, especially if it's not unusual in Indian summers.. 1.3 Fuel Inlet Temperature (C): At engine fuel pump inlet: GEnx‐1B Engine Series Temperatures \xb0C Minimum ‐ 53.8 Maximum 65.5 6 users liked this post. |
BrogulT
2025-06-19T17:48:00 permalink Post: 11906226 |
This explanation comes with a money-back guarantee and if I'm wrong I'll send out refunds. First, vapor lock is simply where a pump or other device becomes inoperative because it is designed to pump liquids but is presented with a gas (vapor) at it's inlet and thus cannot develop pressure and pump the fuel. Think of a very old car with a mechanical fuel pump on the engine block that draws fuel through a long tube from the fuel tank. If you shut the car off on a hot day, the residual heat may boil off the fuel in the lines and carburetor so that when you try to restart, there's no fuel anywhere and your pump has lost it's prime. It is key to note that even with a very crude system like this and volatile gasoline as a fuel, vapor lock usually only affects starting and not running engines. There are exceptions, of course. The three key factors are the absolute pressure at a particular point in the fuel system, the vapor pressure of the fuel at whatever temperature it is at and system design. System design has all but eliminated vapor lock as a serious issue in the gasoline automotive world. At near sea level, the outside pressure is about 1 bar (15psi) and at 50C typical jet fuel will have a vapor pressure of perhaps 0.02 bar. So the only way to cause it to vaporize jet fuel, even at 50C+, would be to subject it to a very, very strong suction. AFAIK there are no vulnerable points where you'd have suction during normal operation because the fuel pumps are presumably (I don't actually know) immersed in fuel and the entire system has greater than 1 bar pressure all the way to the high pressure pumps. Even without the electric pumps, the inlet to the mechanical pump is below tank level. So absent some major fuel line restriction, there aren't any points where you'd have strong suction aka very low absolute pressure. The discussions about fuel temperature also seem a big irrelevant to me--even at 60 or 70C the vapor pressure is still very low and I doubt you'd see significant vapors at all under 100C with any reasonable fuel system design and properly blended fuel . I'm assuming the fuel temperature limits are for other reasons, perhaps flash point or ignitabilty (TWA 800) or viscosity and lubricity concerns with the high pressure pump. Again, IDK, but vapor lock with Jet A seems very far fetched to me. I would note that improperly blended fuel could have a much higher vapor pressure and still work OK in most cases as long as positive pressure was maintained. So if the electrics and the pumps went offline and the fuel vapor pressure was way too high, I suppose there could be vapors formed in the suction line going to the mechanical pumps. But I don't have nearly enough knowledge to proclaim that as a possibility. I presume they've taken fuel samples at the source and tested them. Here's a paper on Jet A vapor pressure: https://www.researchgate.net/publica...Kerosene_Jet_A Last edited by BrogulT; 19th Jun 2025 at 19:34 . 6 users liked this post. |
CloudChasing
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. 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. |
ams6110
2025-06-19T18:48:00 permalink Post: 11906263 |
4 users liked this post. |
Tu.114
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. 1 user liked this post. |
EDML
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. 4 users liked this post. |
rigoschris
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. |
sSquares
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.
Not saying it happened here! |
MatthiasC172
2025-06-19T22:06:00 permalink Post: 11906425 |
TCMA restart
2 users liked this post. |
EDML
2025-06-19T22:23:00 permalink Post: 11906444 |
ONCE AGAIN: I don't think that is what happened - it's just an example of a chain of events triggered by worn out and/or older equipment. Both failures would never happen on a new aircraft. 2 users liked this post. |
AirScotia
2025-06-19T22:34:00 permalink Post: 11906450 |
*On the ground* you get into a latched state, once TCMA deploys: after activation the relays stay latched to prevent a re-runaway. A full power reset of the affected EEC channel(s) and relay logic - normally done only at the gate - is required before fuel can flow again. So you can\x92t easily relight.
Technically, then, if TCMA deployed erroneously during takeoff, there would be no way for the pilots to restart the engines? 2 users liked this post. |
Kraftstoffvondesibel
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? |
user989
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
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.
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.
29 users liked this post. |
Lead Balloon
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. Last edited by Lead Balloon; 20th Jun 2025 at 00:59 . 3 users liked this post. |
framer
2025-06-20T01:15:00 permalink Post: 11906521 |
User989 thanks for a nice summary
I am at risk of turning into one of those folks who gets their mind locked on one possibility and keeps banging on about it but here goes;
2. Fuel contamination
No other aircraft affected, no measures taken at airport. Simultaneous flameout due to contaminated fuel very unlikely. 1/ No other aircraft being affected 2/ No measures taken at the airport 3/ No AD’s from the regulators 4/ No grounding of 787’s 5/ Flight profile 6/ Rat deployment etc etc I agree with your statement that dual flameout due fuel contamination is very unlikely, but we ARE dealing with something that is very unlikely. I favour the theory because an error in treating the fuel is so predictably human and simple, and a dual engine failure being related to fuel is also a simple and obvious idea, and it satisfies all we know both about the aircraft’s behaviour, and the authorities behaviour post accident. I posted a report earlier of a 787-8 powered by the same engine type have both engines roll back sub-idle within a minute of each other while airborne due to this, so we know it can happen in theory. Now……I want to be clear that I’m not saying I think I know what happened, I’m an average Joe with my hands full just flying the line, but I am a bit surprised that the idea of ‘fuel contamination specific to that airframe’ doesn’t get discussed more on this thread. Thanks again for the clear summary of discussion thus far. 6 users liked this post. |
Pinkman
2025-06-20T01:43:00 permalink Post: 11906532 |
User989 thanks for a nice summary
I am at risk of turning into one of those folks who gets their mind locked on one possibility and keeps banging on about it but here goes; If the authorities determined that the accident aircraft had been treated by maintenance for microbial growth in the fuel tanks within the last week or so, and they suspected that that procedure was carried out in a way that could result in fuel contamination, then that would explain 1/ No other aircraft being affected 2/ No measures taken at the airport 3/ No AD\x92s from the regulators 4/ No grounding of 787\x92s 5/ Flight profile 6/ Rat deployment etc etc I agree with your statement that dual flameout due fuel contamination is very unlikely, but we ARE dealing with something that is very unlikely. I favour the theory because an error in treating the fuel is so predictably human and simple, and a dual engine failure being related to fuel is also a simple and obvious idea, and it satisfies all we know both about the aircraft\x92s behaviour, and the authorities behaviour post accident. I posted a report earlier of a 787-8 powered by the same engine type have both engines roll back sub-idle within a minute of each other while airborne due to this, so we know it can happen in theory. Now\x85\x85I want to be clear that I\x92m not saying I think I know what happened, I\x92m an average Joe with my hands full just flying the line, but I am a bit surprised that the idea of \x91fuel contamination specific to that airframe\x92 doesn\x92t get discussed more on this thread. Thanks again for the clear summary of discussion thus far. Last edited by Pinkman; 20th Jun 2025 at 02:32 . 7 users liked this post. |
cloudhawke
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).
"
|
Lead Balloon
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. 2 users liked this post. |
Someone Somewhere
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. 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. |
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