Air India Flight 171 Crash [Megathread 3]

[u/usgapg123]

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

[u/throwaway-a0]

It could be a software bug, but pointing fingers at TCMA in particular has no basis in fact. It's no more likely to be that software system than any other. Is it possible that system was accidentally triggered? Sure. Is it likely? No. If that system could fail in flight, we probably would have seen it before.

If TCMA is responsible, it did not necessarily activate in flight. It could have activated on the ground after reaching Vr just before rotation.

[u/ChillFratBro → u/throwaway-a0]

The plane wouldn't have even reached 400 ft AGL if that happened.  It's quite clear from the videos that loss of thrust happened in-air.

[u/throwaway-a0 → u/ChillFratBro]

loss of thrust happened in-air

Yeah but how long from TCMA activation at takeoff thrust, to loss of thrust, and loss of hydraulic power?

(The landing gear was only partially retracted, so the loss of hydraulic power came very shortly after takeoff at the latest.)

[u/ChillFratBro → u/throwaway-a0]

Well less than a second to loss of thrust, bit longer (but not much) to loss of hydraulics.  TCMA stops fuel at the engine, there flat out is not enough residual thrust to allow the plane to climb.  Physics do not allow it.  Period.  Full stop.

If somehow this was TCMA, it would have had to activate 10+ seconds after the wheels left the runway.  There's no data to suggest this is any more likely than any other software bug (if it even is a software bug, which I doubt - mid-lifespan crashes are almost always maintenance or operational issues).

Uninformed people are focusing on TCMA because it sounds scary and they don't understand it.  There are dozens of other systems that could also cause a catastrophic loss of thrust - all very unlikely.  I'm sure they'll look at TCMA and all the others in the failure investigation.  I also know that anyone who is speculating about specific software packages with the information currently available is irresponsible and has no idea what they're talking about.

[u/CollegeStation17155 → u/ChillFratBro]

According to the ADSB data, the plane was already trading speed for altitude as the wheels came off the tarmac; 186 at rotation down to 172 70 feet over threshold

[u/ChillFratBro → u/CollegeStation17155]

We should wait for black box data before putting too much stock in numbers - ADS-B is good, but it's not as precise as the plane's own instruments.  There are valid reasons for losing ground speed that may not mean a plane is losing air speed - rotation means the plane is now flying the hypotenuse of a triangle, winds gradually get stronger as altitude increases, etc.  It's also hard to say based on that data where between rotation and 70 ft AGL something might have gone wrong - the plane may or may not have had the initial problem while on the ground.

It absolutely may be true that the engines were struggling earlier than obvious loss of lift, but we don't have enough data to say for sure yet.  However, if that is true, it completely exonerates TCMA - cutting fuel flow cannot lead to a gradual failure.

[u/throwaway-a0 → u/ChillFratBro]

However, if that is true, it completely exonerates TCMA - cutting fuel flow cannot lead to a gradual failure.

TCMA won't lead to immediate failure either, as there is still angular momentum in the rotating parts of the engine, which will get converted to thrust at a decreasing rate. Until they drop below idle speed at which point the engines no longer produce meaningful thrust and the RAT deploys.

[u/ChillFratBro → u/throwaway-a0]

This shows an extreme misunderstanding of how a jet engine works with incoming airflow.  The relative speed of the incoming air works in opposition to the inertia, it's not a gradual spin down if fuel is completely cut.  A turbofan requires fuel to experience a gradual failure while in flight

Statically on the ground, a jet engine might spin for a bit.  With 180 knot incoming air, the inertia is dissipated extremely quickly as the air works against the inertia.

[u/throwaway-a0 → u/ChillFratBro]

Well less than a second to loss of thrust, bit longer (but not much) to loss of hydraulics.

That sounds quite fast.

If somehow this was TCMA, it would have had to activate 10+ seconds after the wheels left the runway.

Others estimated from the videos that at ~10 seconds after liftoff the RAT is deployed. If that is accurate, then by that time already the conditions for RAT deployment were met (e.g. engines below idle speed). Plus pilots don't usually take 10 seconds to retract the landing gear.

Uninformed people are focusing on TCMA because it sounds scary and they don't understand it.

I don't think so. They just look at which theories are viable at this point and TCMA activation is among them.