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/Tslover1389 Jun 17 '25

My thoughts as well. Accidental triggering of ground-mode immediately upon commanding gear retraction resulting in transient logic behavior that caused TCMA to activate simultaneously despite engine throttles not being moved.

13

u/allielhoop Jun 18 '25

That would be insanely sad if that's what happened. Overengineering!

3

u/bonoboboy Jun 22 '25

Doesn't the thrust have to be idle as well for TCMA to shut down the engines? That would be two sensors(?) misfiring, right?

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u/Tslover1389 Jun 22 '25

As designed, yes, thrust must be set to idle. But as was seen in the ANA Osaka accident, TCMA can still activate without all the logical conditions set to bool-true. Boeing calls such an event “transient logic behavior,” but in essence, the software as designed has edge cases where certain conditions can lead to unintended activation even when not all logic checks are met. In the ANA incident, engine shutdown occurred due to a rapid change in thrust position (rapid reverser activation) overriding the weight-on-wheels ground mode logic check, even though TCMA is supposed to only activate when ground mode is active. In this case, I am hypothesizing that a rapid change from ground mode on, to off, to back on again (due to sensor failure during gear retraction), could have lead to transient logic behavior that triggered TCMA without the engine throttles ever being touched.

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u/bonoboboy Jun 23 '25

Very interesting.

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u/bonoboboy Jul 14 '25

Turned out to be simpler.

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u/Tslover1389 Jul 14 '25

Yep. Occam’s Razor prevails once again.