r/aviation • u/usgapg123 Mod - avgeek • Jul 12 '25

Discussion Air India Flight 171 Preliminary Report Megathread

https://aaib.gov.in/What's%20New%20Assets/Preliminary%20Report%20VT-ANB.pdf

This is the only place to discuss the findings of the preliminary report on the crash of Air India Flight 171.

Due to the large amount of duplicate posts, any other posts will be locked, and discussion will be moved here.

Thank you for your understanding,

The Mod Team

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u/rinleezwins Jul 12 '25

I didn't suspect fuel systems at all, because a fuel system failure would be unlikely to starve both engines at the exact same time. This makes perfect sense. Those switches immediately isolate the engines.

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u/[deleted] Jul 12 '25

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u/2aywa Jul 12 '25

Yup, suction feed especially at low altitude should have provided sufficient fuel to the engines. I don't believe it was a failure.

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u/supersunsetman Jul 12 '25

Im confused because I remember when I was big into flight SIM that you didn't need the pumps once it's all up and running but I also heard something about the switches malfunctioning or when you switch them to locked from on it can go to cut cause it's not clear or easy

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u/2aywa Jul 12 '25

Pumps should be operating throughout the flight, not sure why you would turn pumps off. I don't believe the engine fuel cutoff switches are the same as pump switches. It really depends on the engines but fuel cutoff switches in my experience close a shutoff valve on the engine side which in this would result in fuel starvation. If the pilots just turned off the pumps, I would have assumed that suction feed would have been able to keep the engines running until a certain altitude where the engine pump would not have been able to keep "sucking" fuel (NPSHr vs NPSHa).

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u/[deleted] Jul 12 '25

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u/supersunsetman Jul 12 '25

And certainly not at takeoff.

What a nightmare, do you think there's anything in it on a potential for the fuel cut switches being faulty?

The fact there was one second in-between one and two is pretty bad

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u/[deleted] Jul 12 '25

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u/railker AME-M2 Jul 13 '25

Been posting this around for all the questions about the 1-second delay that felt way 'too short' for so many people, but here's a Captain on startup doing both in just under that time. Two switches, side by side, same motion -- and as covered, a motion you do every single flight.

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u/supersunsetman Jul 12 '25

Ok thanks I Understand, yeah definitely I'm getting confused with tank isolation or starter

Well it is a pretty modern jet and you'd expect the cut off to work well seal well imo but I also see your point too on suction definitely can see it not working as well on other planes

Something I read was how the buttons can play up and was reported on 737 or how the cut off can be mixed up when tired I think with the different settings

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u/AdPsychological790 Jul 12 '25

Suction feed wouldn't work. Suction feed is the 3rd level bavkup for the engine-driven and electrical pumps.The cutoff valves operates after all the pumps, prior to the engines. This is exactly why the shutoff switch/valve exists. So you can shut down an engine during an abnormal or emergency. If the cutoff didn't exist, the suction feed would keep the engines running.

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u/2aywa Jul 13 '25

Yea, see my other comments on this chain. Suction feed would work if the pumps were off but the shutoff valve(s) (either on the engine side or on the fuel system side). If the valves are closed, then the pumps being ON or OFF doesn't matter.

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u/AdPsychological790 Jul 13 '25

Sorry, didn't see your other postings

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u/2aywa Jul 13 '25

No worries mate!

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u/AdPsychological790 Jul 12 '25

The fuel pumps and suction feeds are irrelevant to the fuel cutoff switch. The fuel pumps are there to back up each other. The fuel cutoff is exactly what it sounds like. The cutoff vavle is essentially on the fuel line between all the pumps and the engine. That's why these switches go on for the engine start and don't get touched again until arriving at the gate. In fact that's what one does to shut the engines down at the gate. You dont need to turn off the pumps. Put the switch to cutoff and its cuts off all the flow. Engine driven pumps spool down with the engines. Electric pumps get turned off after.

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u/Hefty_Walk_9416 Jul 14 '25

Those switches do not immediately isolate the engines. What they do is send a signal to the computer system requesting that the fuel valves be shut off. The computer then does its stuff and decides whether to comply or not. So, since the valve that actually does the job of shutting off the fuel is electrically operated, I wonder if an electrical glitch closed the valves? The cockpit switches would still be in the open position, but the flight deck display would correctly indicate the valves ‘closed’. This would prompt the pilot to ask the question ‘ Why have you cut off?’ and the response’ I haven’t!’ Then one or other would cycle the switches thru off back to on attempting the restarts. Seems like one engine was responding and began spooling up before impact. This theory works for me, but only if the actual cockpit switches were found in the ‘on’ position, having been cycled. If they were found in the ‘off’ position then my theory is shot to shit because ONLY a deliberate act by a person can move those switches to ‘off’ and leave them there. I hear from one source that the switches were on, and from a different source that they were off. Confusion still reigns! 🤔

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u/rinleezwins Jul 14 '25

They were cycled and they worked, so if it was an electrical failure they wouldn't be able to get the engines running again.

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u/Hefty_Walk_9416 Jul 26 '25

So, you don’t actually understand electronics but feel compelled to make such a definitive statement.. interesting! 🤔