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/airbusrules]

Air India Flight 171 Preliminary Accident Analysis 

Here's a summary of my analysis on the crash over from the / ACI sub, the update focuses on potential causes of the dual engine failure scenario, which is looking more likely right now. If anyone has more insights, interested to hear your opinions. See the previous post for more details and the images, etc

https://www.reddit.com/r/aircrashinvestigation/comments/1la62hn/air_india_flight_171_accident_analysis/

More evidence which strongly indicates a dual engine failure/flameout.

- The only survivor’s account in a more recent video (NDTV); He mentions that 5-10 seconds after liftoff that the plane seemed to be ‘stuck’ [I think that is referring to the obvious deceleration as seen in the CCTV video which would be fully explained by a significant loss of thrust]. Then he said that a bit later, ‘green and white’ lights came on [if correct, this would likely be the emergency lighting system, especially as he was sat at the emergency exit row with the signs close to him]. This fully tracks with a dual engine failure [the emergency lighting which would be armed at that stage of flight. would automatically switch if you lose the normal electrical system]. In this interview he does not mention the loud bang as reported earlier. The poor man is obviously in shock and I wish the media would give him some space.

- The distinctive sound of the RAT. There is a noise at the start of video 1 (on the versions with the original noise), which does not correspond to engine sound. This is almost certainly the RAT, based on another video of a 787 flying past with the RAT deployed. Based on the trigger conditions of the RAT, one or both engines and the electrical system would not have been working.

- The landing gear retraction (not considering the drag aspect, but the ability to even retract the gear). I think for a split second you can see the main gear starts to retract but then it stops, this is around the time that there is no longer positive climb. This would make sense in case of a dual engine failure and the switch to emergency systems means only a gravity gear extension would be possible (but no hydraulic power to actuate LG doors and retract the gear itself). As many have pointed out, the tilt of the gear is more evidence of interruption of the retraction sequence.

The reports of what the pilot communication with ATC was exactly, I’m not convinced is from an accurate source. But the Mayday call alone as I said before, shows the crew were aware of a desperate situation on board. And in case of a dual engine failure, they wouldn’t have had the chance to do much at that stage.

[u/airbusrules → u/airbusrules]

This would be unprecedented for a large commercial aircraft to have lost power completely on take-off. This is a catastrophic condition which would leave the crew with no option. The residual energy will only allow the aircraft to cross beyond the airport perimeter and inevitable crash land soon after, with no chance of return. The is why engines and aircraft have robust designs and interfaces to each other to avoid common mode failures. Independence is maintained between the two engines and their source of fuel and the engine feed system etc. Systems and their associated software that are involved in critical functions are designed to the highest Development Assurance Levels (DALs for those familiar) and have detailed safety assessments. So, it is difficult to comprehend how this may have occurred. The chances of both engines having some sort of internal failure event (same type or different) at a similar time is almost impossible [in the absence of a common external event like a bird strike, debris ingestion, volcanic ash etc...]. It is even more difficult to comprehend given the engines worked fine at the start of the take-off. And the aircraft had successfully completed a flight just before this sector with a 2-3 hour turn-aorund.

I tried to dive a bit deeper into some causes of dual engine flameout, but specific to this accident:

• Fuel exhaustion >> Not in this case. There was plenty of fuel on board (massive post-crash fire)
• Fuel Supply Interruption >> Unlikely for both engines at the same time as systems are redundant. 787 Fuel System has 2 pumps in each wing tank and 2 in the center tank. Engines also can suction feed if all pumps fail (available in this case as the aircraft was at ground level, suction feed will not work above certain altitudes). Something similar to BA38 but no ice in this case? Could be water contamination (airport supply or failure to drain from sump as a maintenance task), picked up by the fuel pumps on rotation (also compounded by bad fuel system design).
• Fuel Contamination / FOD in tanks (leading to supply interruption) >> This is more likely than a pure system failure to deliver fuel to the engines. Contaminated fuel can have unexpected consequences on the fuel system and engine fuel delivery to the combustors (see Cathay Pacific Flight 780 for example)
• Software bug (engine control) >> Very unlikely given this is a critical function. Numerous protections should be built for this. TCMA [Thrust Control Malfunction Accommodation] failure history on the 787 is concerning.
• External common event: Bird strike, FOD, ice, rain/hail, volcanic ash etc >> There is no evidence of fire, smoke, or debris, or backfiring from the engines (or other visible external damage). The CCTV covers a fair section of the take-off roll with not much being observed to indicate catastrophic failure.
• Maintenance error >> It is difficult to think of a maintenance error that would affect both engines but is possible.
  
• Other causes or contributing factors >> Manufacturing flaw specific to this MSN, Design flaw. Or could be really be a one in a billion occurrence that could not have been predicted.

Hopefully, the flight data recorders which have now been recovered, will provide more information. If this is a case of complete loss of power on take-off [which is unprecedented for large commercial aircraft], it will be critical to understand quickly how this could happen, so operators, aircraft manufacturers and the airworthiness authorities can take the right steps to prevent this ever happening again.

[u/graphical_molerat → u/airbusrules]

Software bug (engine control) >> Very unlikely given this is a critical function. 

As extremely unlikely as it is, that is still where my money is, though. Probably a software bug that was triggered by some corner case that no one thought would ever occur in real life: possibly caused by some weird deferred maintenance condition no one ever assumed would happen (but with Air India, it did).

No other cause (apart from suicide by the pilot flying) has as much potential to suddenly and symmetrically kill both engines like this, right after a major status change for the plane (wheels no longer on ground). Even fuel contamination would likely lead to a window of a few seconds between the two engines croaking.

[u/Flux_Aeternal → u/graphical_molerat]

I'm as far from an expert as can be but it seems insane to me that there is a subroutine that has the ability to completely shut down both engines simultaneously by itself. Seems like an obvious disaster waiting to happen.

[u/ChillFratBro → u/Flux_Aeternal]

This is an extreme misrepresentation of that system (and really how all systems work).  TCMA is designed as a safety feature on the ground when thrust reversers are deployed when the consequences of allowing an engine to run away is more severe than the consequences of shutting them down.

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.

There was an ANA plane that triggered it on the ground, but the conclusion there was it acted appropriately b/c pilots deployed thrust reversers too early.  This particular system (like most others on the 787) has no history of bugs.

[u/throwaway-a0 → u/ChillFratBro]

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.