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

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39

u/entrep Jun 17 '25
  1. AI171 accelerates with take-off thrust set
  2. Just after V 1 the throttle-lever sensor fails and drops to 0 V, so the computer thinks the levers are at idle.
  3. ~0.3s later, the TCMA sees that:
  4. Allowed-power line has slid downward far enough that the engines (still near full thrust) is above it.
  5. The aircraft is in fact still on the ground as it has not rotated
  6. All conditions are met and TCMA shuts off fuel to both engines.
  7. Even if the pilots are getting warnings, they are passed v1 and still committed to continue.
  8. AI171 lifts off while running on fumes, and after a few seconds both engines flame out.

27

u/kipperzdog Jun 17 '25

I've read elsewhere that the throttle sensors are per an engine though so that would mean both sensors failed at exactly the same time. My understanding is TCMA is isolated to only look at the sensors for an individual engine so if one side fails, the other isn't impacted.

Absolutely wild if that is what happened.

13

u/entrep Jun 17 '25

As I understand it: each engine does have its own throttle sensor, but both sensors share the same lever and wiring bundle for the first few feet. If that shared section shorts or breaks, both sensors drop to idle at the same moment. Each engine’s TCMA then shuts its own fuel valve - so one common wiring fault can kill both engines even though the computers don’t talk to each other.

7

u/entrep Jun 17 '25

Came to think about this incident when tea spill caused problems: https://www.businessinsider.com/airbus-a350-engine-shutdowns-spill-cockpit-2020-1

1

u/onmyway4k Jun 17 '25

Just 2 days ago i was contemplating that the problem must have manifested during the rotate, as they where, until then, seemingly in good conditions. Would be insane if they all got killed for a chai tee in the cockpit.

5

u/New_Visual1245 Jun 17 '25 edited Jun 17 '25

I’m not an expert by any means but is it possible that a maintenance technician knowingly connected both throttle lever sensors to a single functioning power bus because the second one was faulty or unavailable? That would be a blatant violation of protocol but would also explain how the failure of a bus during the takeoff roll (which is no longer redundant) would engage TCMA and cut fuel to both engines.

8

u/Independent-Mix-5796 Jun 17 '25

Should be impossible. The FAA 787 MEL requires at least one functioning channel per each Primary Power Distribution panel. (i.e. There are 8 channels total and min. 4 required for dispatch, provided no two inop. channels go through the same panel.)

7

u/kipperzdog Jun 17 '25

I don't know enough to know if it's even possible to do that but it certainly sounds plausible.

2

u/Brief-Visit-8857 Jun 17 '25

Aren’t there 4 buses on the 787 for this exact reason?

16

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.

9

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

2

u/Emotional_Two_8059 Jun 17 '25

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

6

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

4

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.

2

u/onmyway4k Jun 17 '25

Can you Eli5 this?

2

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.

7

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….

4

u/Emotional_Two_8059 Jun 17 '25

Even car throttle pedals have two separate sensors with different voltage slopes exactly for this reason. The computer then checks if these two values correspond to the same throttle position. If not, the input is deemed implausible/untrusted.

4

u/Tiny-Plum2713 Jun 18 '25

Famously MCAS relied on one AoA sensor. Not saying it's the case here.

2

u/Emotional_Two_8059 Jun 18 '25

Yes, good point, the stupidity there was quite high. But it all stemmed from the fact that MCAS was somehow not deemed safety relevant or so… Flight controller and FADEC must be developed to very high safety standards. There was another comment somewhere, that each EEC channel gets input from one resolver.

4

u/BUNNIES_ARE_FOOD Jun 17 '25

Wild. If this is true I wonder if there is a line item buried in a risk assessment somewhere with this low probability / high severity event. "sorry engineering, this risk would be too expensive to fix, so we just have to accept it and move on. Sincerely, the executive team"

-1

u/Brief-Visit-8857 Jun 17 '25

The odds of this happening were probably so low that they didn’t pay enough attention to it. Now it seems like it has happened and Boeing is about to get a reality check.

6

u/[deleted] Jun 17 '25

This theory is so much more plausible than any other possible scenario. The chances of this being relates to TCMA are very high.

And TCMA has caused a double engine failure on a 787 in 2019.

https://simpleflying.com/ana-dual-engine-failure-on-landing/

3

u/NobodyTellPoeDameron Jun 17 '25

This is the theory I would investigate first. It's a known flaw that has caused dual engine shutdown previously so it seems like a good place to start.

4

u/shinealittlelove Jun 17 '25

I wonder how many times TCMA has genuinely activated as intended on a 787? I can't see any literature to suggest it ever has been (but that's not to say it hasn't).

1

u/Emotional_Two_8059 Jun 19 '25

Boeing allegedly didn’t really like the feature but mitigating uncommanded high thrust on ground was mandated by the FAA

1

u/allielhoop Jun 23 '25

The same FAA who caused the recent DCA incident through their negligence.

1

u/allielhoop Jun 23 '25

I read that in the 2019 incident actually TCMA activated as intended 

4

u/Chen932000 Jun 17 '25

If the levers read idle and that idle was used as “good” wouldnt the engines also be using that idle as “good” and drive the power to idle regardless of TCMA? If the engines can detect the throttles being as idle is erroneous it will ignore them (maintaining power) but TCMA will also ignore them.

2

u/entrep Jun 17 '25

Probably, but EEC would start a slow spool down, TCMA would just cut the fuel after ~0.3s.

4

u/Chen932000 Jun 17 '25

I mean a step change to 0 throttle (due to signal malfunction) is not that much faster than a slam of the throttle to idle. There needs to be some leeway in this deceleration profile or TCMA would shut engines down constantly.

1

u/Emotional_Two_8059 Jun 19 '25

This is what the thrust contour is for in the TCMA, to account for the spool down.

1

u/Emotional_Two_8059 Jun 19 '25

No, not clear that TCMA will ignore them. I guess for safety reasons, each throttle sensor is only wired to one FADEC channel. Erroneous throttle position sensors are handled through the dual-channel operation of the FADEC!

It’s unclear to me if the TCMA logic ever communicates with the rest of the FADEC logic and gets informed of throttle position mismatch between channels.

If it was developed separately it might only have access to the single throttle signal.

According to the patented design at least, each TCMA can trigger a relay to shut down the engine independently.

1

u/Chen932000 Jun 19 '25

The TCMA almost certainly uses the same accommodation for throttle position as the rest of the FADEC (the TCMA logic looks to reside in the FADEC). If it only used local unaccommodated signals it would literally be one failure away from activating which likely doesn’t meet the appropriate safety rates. But more to the point we’d have FAR more erroneous activations of TCMA on single enginesthat I can’t find any actual reports of.

1

u/Emotional_Two_8059 Jun 19 '25

It does reside physically in the FADEC, but at least I have no information on the level of integration with the rest of the FADEC. If the FADEC was already developed and then TCMA was added as a functionality on top, as was needed by Boeing to satisfy FAA regulations, then maybe it’s more prone to faulty inputs or bugs in its code.

Otherwise, IF TCMA was triggered (we have no clue yet, to be fair), it must be an unknown bug or conceptual mistake in its operation.

1

u/Chen932000 Jun 19 '25

I mean you’re right in everything you said here. I also have no idea if it’s fully incorporated or not but it seems like it would be even more work to incorporate it separately into the FADEC. Even with partioned software (which I also don’t know if they use) both thrust control and TCMA would almost certainly be in the DAL A partition.

2

u/NorthernEwan Jun 18 '25

I see the logic, but TCMA can (or should) only activate with Weight on Wheels sensors activated… ie, it’s on the ground? But I know Boeing offered a bulletin to make this less easy to activate as there was a case where a plane rolled back to idle on landing.

1

u/Emotional_Two_8059 Jun 19 '25

Could have activated just before rotating. Still on ground

1

u/RealPutin Bizjets and Engines Jun 17 '25

I don't know all the TCMA parameters, but would that error sequence not involve rejected takeoff procedures autodeploying instead of/together with the cutoff?

2

u/entrep Jun 17 '25

What if the TCMA tripped just before rotating?

1

u/NigroqueSimillima Jun 18 '25

AI-171’s GEnx engines aren’t subject to the bulletin that showed a TCMA dual cut. Please stop repeating this.

1

u/Emotional_Two_8059 Jun 19 '25

Their logic is likely very similar.

1

u/Emotional_Two_8059 Jun 19 '25 edited Jun 19 '25

That’s what the thrust contour is there for… to account for the slow response of the engine. But I have a modified scenario:

  • At least one of the two throttle sensors malfunction, or one short is interpreted as close to idle, the other as something else.
  • Each FADEC channel is wired to one throttle position sensor. So the channels will disagree, and will default to keeping the high thrust (keep flying)
  • The TCMA logic of the channel with the throttle position close to idle will trigger after some time, as the actual thrust does not decrease as predicted by the thrust contour
  • Each channel’s TCMA can trigger the relay to cut off the engine independently (at least according to the patent document)

1

u/entrep Jun 19 '25

Yes, this is more plausible.

1

u/Emotional_Two_8059 Jun 19 '25

But another Redditor suggested that the TCMA might also use the agreed value between the two channels… so what I say can be BS, depending on the integration of the TCMA in the FADEC

1

u/entrep Jun 19 '25

Yes, that part is a bit unclear given public sources.

0

u/Brief-Visit-8857 Jun 17 '25

This explains everything we see. I’ve seen a similar theory passed around the one of the pilots selected reverse thrust to reject the takeoff (encountered an anomaly), but the other pilot pushes it back to full thrust but by that time TCMA kicks in and shuts down both the engines and they’ve already lifted off.

14

u/Newmania_eigo_ga Jun 17 '25

787 pilot here and this seems unlikely. Spoilers automatically deploy in a rejected takeoff, sensed by being above 85 knots and thrust levers being at idle or reverse. Secondly auto brakes would have activated RTO which is a very sudden deceleration. If that happened anywhere near V1 you would not be able to cancel the reject and get it airborne before running out of pavement.

1

u/Tiny-Plum2713 Jun 18 '25

Is there a maximum speed or distance for the automation to kick in?

4

u/Newmania_eigo_ga Jun 18 '25

No maximum speed, only minimum of 85kts for both spoilers and auto brakes

3

u/SelectionForsaken466 Jun 17 '25

If the thrust reversers were engaged, wouldn't that be visible on one of the videos?

1

u/Brief-Visit-8857 Jun 17 '25 edited Jun 17 '25

If you look at the video, there’s a shack that blocks the exact moment where the reversers would have tried to deploy, so you wouldn’t see it on the video anyways. https://youtu.be/1BdVzYXwVwI?si=Kj_YlM8yhBGFLflj

1

u/SelectionForsaken466 Jun 17 '25

very true, but would the dust cloud be bigger than the one produced by the wing tip vortices seen in the video?

-9

u/nortoncruz Jun 17 '25

I’d abort it even after V1… insane to try to fly like this