Posts about: "Fuel Cutoff Switches (detent)" [Posts: 55 Pages: 3]

With the greatest respect, I don't think any deliberate act has been proven. Allocating blame on assumptions should be avoided. People should work on facts, and then unconfirmed assumptions.

The only known fact is that engines for some reason were shut down. The other known fact is that there is a recorded indication that the start lever command was recorded as both cutoff in short succession then followed by idle detent causing a restart attempt.

I think the key unknowns here are important.
1. Was the start lever position recorded, or only the electrical signal? My money is on an electrical signal not position, therefore the manual positioning of the start levers is not conclusively proven.

2. The bulletin for locking mechanism for the start levers has been mentioned in the preliminary report. This is a huge unknown. Did that mean that in a failure that the levers could both move into cutoff at the same time? This occurred exactly as the aircraft was rotating and increasing pitch.

3. The information from the CVR if taken at face value must not be discounted. No pilot actually confirmed conclusively seeing the other move the switch. This could have been an assumption noticing an engine failure, seeing the start levers positions in cutoff or indeed seeing the start levers moved deliberately. This is a huge critical unknown.

4. One pilot asked 'why did you cutoff' and the other responded 'I didn't'. This is not a confirmation of anything for sure. In fact it is equally a suggestion no pilot moved the switches, equally as it is saying they did. This is not conclusive. This raises an important question, was this genuine, did either pilot have any reason in their life to attempt to do this and then cover their actions? There is zero suggestion here either way, but hopefully investigators will already be looking at their backgrounds and state of affairs.

5. There is no indication who or what commanded the engines back to idle detent again. There is proof of both engines attempting to restart. I would take this as confirmation of teamwork existing on the flight deck, and this would suggest a lack of deliberate interference by either pilot. Should that have been deliberately done, so much more could have happened. It just hasn't been alluded to in the preliminary report.

My own impression is that it did not appear to be a situation of a deliberate act to crash. I say this as they appear to have both tried to save the aircraft and immediately restart the engines and recover taking some critical timely actions demonstrating competent reaction.

The timing of the commanding fuel to cutoff is critical imo. The aircraft was pitching. If an accidental command to cutoff occurred then I feel this however unlikely would have been made instead of another action. The only likely thing happening at that exact time or due 2-3 seconds after lift off was gear up. The CVR and recorded timing of engine failure in relation to that challenge and response in SOP will be key imo. Did the challenge for gear up occur before the idle detent electrical signals? Also, the focus should be on that bulletin for the locking mechanism for the start levers.

I just cannot imagine an experienced crew doing this by accident and the information just doesn't suggest a deliberate act.

My post is just to highlight huge lists of unknowns. The media appears to have made their mind up that the flight crew did this. I just highlight that is definitely not confirmed yet. We have a whole list of unknowns and conflicting communication that raises more questions than they solve.

I would urge zero conclusions be made, and RIP the crew and passengers. There are zero conclusions to what happened yet in my opinion, and I think the suggestion to relatives of the flight crew and families tragically killed that this is a known pilot deliberate action simply cannot be made at this point. We must not blame the flight crew prematurely or pretend we know what happened. I think the media should take note and back off from all the assumptions.

A bit of psycology. If one were intent on crashing the plane and did switch to cut off, they would most likely fight like hell to keep them off.

Kudos to the crew for trying to re-light the engines so quickly.

My 10 cents is on the switches and there is not a aireworthiness directive out there for nothing.

The data frame is recording the state of the fuel control switch system, being OFF or RUN. It does not monitor the physical position of the switch itself. It is normally reasonable to infer that the state has followed the switch but at the risk of sounding like a broken record, electrical systems on aircraft are prone to issues with water ingress, and we have learnt that computer systems are also prone to bit flipping from cosmic radiation. I doubt that this is anything like Kev's big day out at Learmonth, with QF072, but I also remain concerned over the potential for deluge of pooled water into the E&E bay, which has happened before. The evidence to put that to bed will be a spectral analysis of the time from liftoff onwards, looking for the signature of a switch being repositioned to OFF and then back to RUN. The Fuel Control Switches have an audible snap in most cases when repositioned. I would also spend a lot of time looking at the engine start case to detect the switch being positively locked into the RUN position.

When reading any of the data when it comes out, pay some attention to the sampling rate of the data being provided, it is quite possible to make erroneous assumptions where that is not taken into consideration. The data buses used to get data from the aircraft system to the recorder, and the recorders themselves use sequential sentences, and varyious rates.

IMHO.

Yes can we just accept this as a fact because the bizarre scenarios some folks are coming up with are just damn ludicrous and frankly destroy the purpose of pprune.

If you\x92ve ever moved those switches you know how they work and you know it\x92s impossible for anything to knock them or move them out of the detent.

Over the years there have been many \x93slips\x94 that have led to accidents, I\x92m yet to come across one which involved both FCS inadvertently being cutoff. The design works, believe it or not.

While this is thread is still deeply into hamster wheel status, there are some valuable inputs being made. I'll try to continue with that...

Thrust lever position post-accident - it takes very little force to move the thrust levers, a little more than one pound-force at the knob. I'd be very surprised if the post impact position was the same as pre-impact (and lever angle is recorded on the FDR, so the investigators know where they were). In short - the post-accident lever position is not meaningful.

Fuel condition switch position detents - if the Indian investigators had any reason to believe that a failed or inoperative detent was a contributing fact, the authorities would have ordered a fleet-wide inspection (especially since it literally takes only seconds to do the inspection - the paperwork would take many times longer than the actual inspection).

Engine restart (i.e. "Quick Windmill Relight"): Even if the igniters were firing, at high power they won't actually spark (the electrical resistance or the air at several hundred psi prevents a spark) - so they won't spark until you get down somewhere near idle if you're close to sea level. Once the engine has dropped below the min idle, it takes a long time for it accelerate back to even an idle condition. At takeoff power, the compressor components get very hot - do a power cut the air coming in the compressor gets heated by the residual heat in the compressor. This in turn limits how fast you can add fuel in the burner without excessively back pressuring the compressor and causing a compressor stall. So it actually takes longer for the engine to accel to idle that it would during a normal (cold engine) start. The 5 seconds to 95% accel requirement referenced earlier is from a stable 'high' idle (we typically call it 'approach idle' since it's automatically selected when landing flaps are selected). Approach idle is ~10% N2 higher than the in-flight minimum idle, so that takes several more seconds. Bottom line, after initiating the Quick Windmill Relight, you're not going to have usable thrust for at least 30 seconds - probably closer to 60 seconds.

For all the complaining about this preliminary report, it actually goes into more detail than is typical.

BTW, my money is still on the 'muscle memory/action slip' or whatever you want to call it. I can easily imagine a scenario along the line of 'why did you turn off the fuel' - 'I didn't - oh wait - oh ...

Regarding the thrust lever position.

The EAFR data revealed that the thrust levers remained
forward (takeoff thrust) until the impact.

Would that position be consistent with a restart attempt?

Well said TDR, the report expressly states that the T/R levers are bent but in the stowed position, so it is quite probable that the thrust lever angle is not as they were in flight pre impact.

The last line, is not a zero possibility, but at present the whole wiring system for the fuel control switches would need to be evaluated for any potential common fault that may be intermittent before I would lean towards the cognitive slip type error. The poor old FO would have both hands on the yoke from V1, irrespective of the company's SOP and so would have been rather unpleasantly surprised by the loss of performance. I think the 10 seconds in this case is remarkably fast observation-orientation-decision-action sequence, and that one engine had already started to recover towards operation is a credit to the design of the engine.

Great comment by GroundedSpanner in post 561 Preliminary Air India crash report published
The switches act on multiple circuits. A single electrical / electromechanical fault shutting down both engines is Extremely Improbable.
A few more observations on switch comments in this thread:
Liquid spillage - Switches are sealed so highly unlikely and not both at once.
Switches between detents \x96 The surface the locking \x93pips\x94 sit on is slightly curved. You can get the switch to sit on it but only with difficulty. I can see no way how both could be in this position accidentally.
Naming \x96 These are normally called a \x93locking toggle switch\x94.
It is a difficult design problem. You have to have a means to shut off the fuel but a single fault should not shut the off OR stop you shutting it off.
(I design fuel control systems for CS 25 aircraft).

Incorrect Locking
The spring function is still active, but due to the displaced locking ring, the tab does not engage with the detent.
The two conditions for the switch remain unaltered - Cutoff / Run, but the Run position is not gated, held in position.

Edit; "Can these switches be carelessly set in the on position but not fully locked? I am thinking whether it is possible for the switches to be electrically on, but not mechanically locked."
Yes

The bottom section of either switch can be clearly seen in this post accident photograph. There is nothing visible where your theory would require a well visible detent.

The switches in the photo are of a Boeing 737. The crashed airplane was a B787. It has a different type of switch. May look from the outside the same as a B737 switch. Partnumber is different.
Also Air India did not find any faulty switches.

https://www.reuters.com/business/aer...es-2025-07-16/

The discussion on faulty switches is a red herring. One of the pilots moved the switches. Either deliberately or by mistake.

Thank you for "telling it like it is"! Just to clarify one point for doubters--the "click" or "clunk" that the FCS makes--is it due to the internal toggle mechanism or due to the locking detent clicking back in after you let go of it? If you smoothly pull the lever up, switch from RUN to CUTOFF (or vice versa) while holding the spring part of the lever up and then very slowly and gently release it so that it settles in quietly, will it still make the noise? I'm assuming it will because large, heavy duty 4-pole toggle switches typically have a pretty good snap to them, both in feel and sound--but I haven't heard one of these specific switches.

Transcript of the relevant part:

"The unit is designed with a spring-loaded action when it's pulled up. Its natural state wants to stay down at all times - that is one locking mechanism. The secondary locking mechanism is the gate - when the switch is pulled up and transitioned into its next position, it gets locked by the gate, or the detent, or the bridge (whatever you want to call it)."

Obviously the narrator doesn't mean that there are two independent locking mechanisms - it's the combined action of the spring and the detent that supplies the desired locking characteristics.

Interestingly, he goes on to say that it was the failure of the spring on a 737 switch that led to the SB being issued.

I didn't find that whole explanation particularly coherent or thorough and certainly not "authoritative". There are two 'locking' mechanisms that keep the switch in whatever particular position it is in. One is the external gate consisting of the spring loaded lever with the tab part that engages with the gate on the body of the switch. The other is the internal toggle mechanism that provides a snap action between the two positions. Neither mechanism is infinitely strong but either is sufficient to prevent the switch from moving due to vibration, turbulence, light touch, etc. What he says about the bulletin involving only the 737 and the problem being the spring, that's just sloppily inaccurate. The bulletin specifies replacement for one part number that is only found on the 737 but it also clearly calls for (voluntary) inspections of the other models including the 787. The actual defect that was found in the 737 switches is documented earlier in this thread and appears to involve rotation of the lever or tab so that it doesn't drop down and engage with the gate. IDK first hand, but there are photos. If this rotation was caused by a defect in the spring part, this hasn't been explained.

I posted a week or so ago, on the forums generally, suggesting that the moderation of the Air India threads had gotten excessive, and at that particular point, it might even be an opinion I stand by. But reading through the last 100+ posts, other than the above few comments, it's just garbage. I'm sorry, to those who think they're saying something meaningful, but it's just hot garbage.

For the legal types - stop quoting what is, and isn't, the burden of proof. This is a forum on the Internet. The topic being discussed is an international investigation into a plane crash. For both of those things, there is no possible relevance to the burden of proof in legal cases in <whatever your country is>.

For the psychologists (both actual ones and those pilots who are moonlighting as one) - stop quoting psychological theory. Many of us know it, others don't, but we all have such an incredible lack of information about the pilots concerned that it's pointless mentioning it, except in passing. No one has time for pages of guesswork or instructions to the investigators on what they should be looking at.

For the tech guys who are still talking about the possibility of malfunctioning switches, please, stop. The pilot saying "why did you do x" is not consistent with a switch failure, you don't say that just because of an unexpected run-down, you say something more generic, like "holy smokeballs, dual engine failure". The specificity of the comment implies clearly observed action, specific to the switches, which aligns with the data record.

For everyone - just because something isn't absolutely mathematically impossible does not mean you can or should bring it out and eat up pages doing so. Investigations do not have to cover every single possible "what if", because in an unlimited universe no investigation would ever conclude. What they have to do is come up with the most likely scientifically derived assessed outcome for an already fairly unlikely event. I can accept people saying "We don't know for sure that the switches were operated deliberately" even though that's probably more likely than not. I can't accept the kind of argument which isn't dissimilar to: "Maybe there was a flaw with the switch detent, and a book fell off the console, and the previous crew had spilled water on it, making it slippery, and before that when the console was installed it was 1.5 degrees more inclined than it should be, and that caused the rollback. I accept that we're already in "unlikely event" scenario, but once the impossible has been eliminated, refusing to accept that there's probably 2-3 likely scenarios left, and instead inserting incredibly unlikely theoretical stuff... well that's just bad faith.

Finally, it's personal taste, but I remain a bit disappointed with the deliberate blindness being applied to inconvenient truths by some on here. The two currently most likely rational explanations are simple - deliberate pilot action, accidental pilot action. Either way, it's pilot action . As frustrating as some of the wild theories are, the obfuscation being pushed out by the "circle the wagons" brigade is just as bad. Any pilot knows that their life - and those of their passengers - relies on clear sightedness and not avoiding uncomfortable data points. So let's trust our instruments. There should be no absurd rush to judgement but also, please, let's not hide behind the "they were professionals, treat them with respect" stuff, endlessly. Someone selected fuel on both engines to cut-off, mere moments after they left the ground. It won't be an engine restart procedure (too low, too early), it won't be some bizarre technical problem that hasn't happened in millions of flight hours on other 787s worldwide. The CVR says "why did you do X" and the data recorder says "X happened". The two line up. What we don't know for absolute certain is why, so the only sensible things now being examined are:

1) Deliberate pilot action (can only assume harmful intent at Vr + 3 sec)
2) Accidental pilot action (due to severe cognitive disconnect, or "wallet in freezer" stuff)

I'm one of many being frustrated by the lack of CVR recording being released, because that will almost certainly identify whether it was 1 or 2. But it would be abnormal for that to happen, and I respect that until they're sure, if the CVR points to something, they will want it confirmed before releasing it, because whether it's 1 or 2, it's bad news for the families of that particular pilot.

Oh, and to whoever said "Ask yourself if we are ready to hear it?".... come on. Hundreds of people died. Ignoring a piece of evidence related to that because it might be uncomfortable hearing it is unacceptable, and a bit childish. It might be uncomfortable, but professionals follow the evidence, wherever it may lead, however uncomfortable it might be.

Sorry this sounds so angry. The cause is that having stuck my neck out and said "Why are you guys moderating so much" to the mods, a free run of the thread has revealed such nonsense being posted that I'm embarrassed about having said anything. Lock away, I say!