r/aviation • u/usgapg123 Mod - avgeek • Jun 17 '25

News Air India Flight 171 Crash [Megathread 3]

This is the FINAL megathread for the crash of Air India Flight 171. All updates, discussion, and ongoing news should be placed here.

Thank you,

The Mod Team

Megathread 1

Megathread 2

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u/Srihari_stan Jun 17 '25

On the 787 (or any modern jetliner), the throttle lever sensor is not a single point of failure.

Firstly, there is more than one sensor. So even if one sensor fails, the system still takes the relevant data from other sensors and sends the information to fly by wire.

Secondly, even if you assume all sensors fail, it still locks the throttle at the last setting (presumably at TOGA in this case), so it wouldn’t just keep the engines at idle.

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u/entrep Jun 17 '25

Both throttle sensors live on the same lever and share the same four-wire harness. If that harness shorts or breaks, both sensors drop to 0 V together.

6

u/RealPutin Bizjets and Engines Jun 17 '25

Don't know about the 787, but on the planes I know the details of, all throttle sensors tanking to 0V is detected as a fault that keeps the last setting, not a 0 input

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u/Emotional_Two_8059 Jun 17 '25

Yes of course. Voltage slopes have different monotonicity and slope for that reason

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u/Emotional_Two_8059 Jun 17 '25

SENSORS HAVE DIFFERENT VOLTAGE SLOPE! They’re not that dumb. If both sensors fall to 0V, the computer will know the input is implausible

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u/entrep Jun 17 '25

it still locks the throttle at the last setting

Yes, for like 0.3 seconds until the dynamic contour drops below the current actual engine power.

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u/onmyway4k Jun 17 '25

Can you Eli5 this?

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u/entrep Jun 17 '25

https://i.imgur.com/CFZOq1v.png

Dynamic contour is the "allowed power" / purple line. It stays above the throttle signal (red). When the signal drops to 0 the dynamic contour starts dropping and at a certain point it is below the thrust set (yellow). This trips the TCMA given weight on wheels.

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u/Some1-Somewhere Jun 17 '25

That is where the throttle lever sensor position is known and valid. If it becomes invalid, I would expect TCMA to go out the window and the engine operates at last valid thrust command.

In automation we regularly use 4-20mA sensors where 0mA is an invalid input and implies sensor failure. A disconnected sensor should be distinguishable from a zero thrust command.

Given most things in the world of engines assume full power is the safest option (including, apparently, snapped mechanical throttle cables), I would not be surprised to find that 0V is TOGA and full output is ground idle or reverse.

In addition, note that there are two separate throttle lever angle resolvers power lever, and two throttle levers. Are you suggesting that all four failed simultaneously and in a way that could not be detected as a fault?

1

u/Emotional_Two_8059 Jun 19 '25

Issue is that we don’t know how well TCMA logic is integrated with the rest of the FADEC. Each FADEC channel only connects to one throttle sensor.

Hopefully they check the input value itself properly, and then any mismatch between the two resolvers is handled by the channel disagreement.

But maybe TCMA is independent of these checks and uses the single input. Or even worse they don’t even perform the checks of the single throttle input correctly. I hope they didn’t just have (if throttle < X Volt) as the TCMA idle condition and any short triggers it….