ahead of any initial report from the Aircraft Accident Investigation Bureau of India, I am attempting an educated look into the crystal ball.
DISCLAIMER: This is a private analysis without any guarantee for the accuracy of the reported facts and conclusions.
a) observation (high confidence) from available video footage:
- takeoff run after backtrack from VAAH (Ahmedabad) RWY 23. Visual impression that A/C got airborne rather late. Plume of dust indicating significant engine thrust at initial climb over the runway end.
- regular initial climb attitude.
- about 12 seconds after rotation, change in pitch attitude, climb terminated within few seconds, no apparent stall, controlled glide into terrain.
- RAT was deployed on final glide, no audible engine thrust noise, APU inlet open at crash site pictures.
- wing trailing flaps deployed at crash site pictures.
- a mayday call indicating thrust problems has been reported by authorities, though literal transcript has not yet been published.
- gear extended until impact, forwarded tilted gear trucks may indicate initiation of gear-up sequence
- sole survivor reported green/white (emergency) cabin lights and flickering.
b) apparent final state of A/C at crash:
- RAT (auto-)deploy, (auto-)initiation of APU startup, flickering lights / switch to emergency cabin lights: loss of electrical power fed from engine driven generators
- Control surfaces operational (RAT can assure minimum hydraulics + electric control) to maintain wings level and to control glide speed
- Insufficient/no thrust on both engines to maintain or gain altitude
c) discussion of possible faults and consequences:
hypothesis 1 “a/c misconfiguration for take-off“: erroneous settings of takeoff weight, ambient temperature/pressure, engine de-rating factor, flaps setting, possible miscalculation of V1, VR. Highly unlikely due to B788 automated crosschecking of weight on wheels, flaps settings. Would have triggered warning messages prior to T/O. After all, the A/C had a rather nominal takeoff run and initial climb with no observable “out of envelope” behavior. Hypothesis 1 is inconsistent with observed electrical system + dramatic thrust loss.
hypothesis 2 “unlawful interference, intentional action“: contradicted by a reported technical mayday call. Manually commanded dual-engine shutdown is a complex process and is unlikely performed after rotation in an efficient manner to affect both engines in an apparent simultaneous manner. Intentionally pulling back both thrust levers to flight idle would not be enough to trigger the electrical backup system, i.e. RAT deployment.
hypothesis 3: “a/c misconfiguration on initial climb“: initial climb would have normally seen “gear-up” action and in theory could have been confounded with “flaps up” action by pilot error, resulting in a loss of lift. This unlikely misconfiguration however could have been healed without catastrophic consequences by applying max thrust and manually keeping the A/C within the flight envelope. Hypothesis 3 is also inconsistent with observed electrical system + dramatic thrust loss.
now, leaving us with only scenarios which are able to trigger a rapid, simultaneous dual engine loss of thrust:
hypothesis 4: “fuel contamination / temperature“: A contamination of fuel (water, any sort of dirt) residual to the main tank (feeding the engines) from present or past refueling cannot be rules out, but it is highly unlikely that: i) this would lead to a perfectly synchronous and extremely quick and “efficient” flame-out of both engines without apparent visible clues (smoke, thrust bursts and altitude variations), ii) it has not been experienced (at least in parts noticed by crew or engine telemetry) by any other aircraft refueled from the same hypothetical contaminated airport(s). Although ambient temps in Ahmedabad were high (METAR: VAAH 120830Z 24003KT 6000 NSC 37/17 Q1000 NOSIG), I would also rather discount the possible issue of fuel vapor lock (elevated fuel temperature can result in the development of fuel vapor which cannot be processed by liquid fuel pumps) due to the large amount of fuel very likely drafted from the underground hydrant fuel system available at AMD or residual fuel from the prior flight and short rotation from DEL.
hypothesis 5: “systems commanded dual engine cutoff“: Both engines have their own EEC/FADEC (electronic engine control / full authority digital engine control) units with 2-fold redundancy and electricity generated by engine-mounted redundant generators. As long as the engine is turning, EECs are powered and are able to control fuel flow as commanded by cockpit-side thrust levers and fuel master switch sensors (with redundant sensors and wiring). On the fuel delivery side, each engine has it’s own mechanically coupled fuel pump which is able to directly draft fuel from the main tank – without support of main tank electrical boost pumps. Therefore, even with a total loss of power on all of the A/C electrical busses, each engine – once started – will autonomously continue to operate according to thrust-lever input, until fuel starvation. Now, B787 series got a unique further software package integrated into EEC called TCMA (Thrust Control Malfunction Accomodation System, https://patents.google.com/patent/US6704630B2/en). It actually does what the title is promising: Use a software logic to “correct” any physical misbehavior of an engine with regard to its commanded thrust level, more precisely: It is designed to radically shut down (closing of fuel master valve) a “runaway” engine if all of the following conditions are met: 1) the aircraft is on the ground, 2) thrust lever is at idle position, 3) engine is above idle rpm. TCMA has two independent channels (for each engine, each of the channels has authority to close the fuel valve) which each evaluate the 1-3 conditions. TCMA seems to be the only “authority” – apart from pilot action in the cockpit – to close the fuel valves and to simultaneously and rapidly shut down both engines. From the TCMA conditions, in the present scenario – takeoff thrust – only (3) was true at all times, (1) should have transitioned from true to false after leaving the ground while (2) is supposed to remain false at all times during takeoff.
Now, the real speculation starts: If the aircraft was faced with some additional, electrical issue (moisture, electric arc, power surge … these kind of things) at the very end of the takeoff run (past V1, such that the crew did not consider the option to abort – but possibly was distracted for an instant, explaining the slightly longer than expected ground run) or after rotation (in anticipation of, or at gear retraction) and that electrical interference and/or a software restart condition compromised the readout of the WOW (weight on wheels) and thrust lever sensors such that WOW remained at it’s ground state and thrust lever (both!) readouts were shortly flickering to an idle or null readout (and as such were accepted as valid input !) this could have closed both engine’s fuel valves shortly into the climb, leaving just some residual fuel for the initial positive rate.
I can’t resist my gut feeling, but the “capabilities” of TCMA remind me of MCAS (Maneuvering Characteristics Augmentation System) of the B737 MAX series (two fatal crashes: Lion-610, 29.10.18, Ethiopian-302, 10.03.19), where a “full authority” automated software system (which was introduced to control a secondary problem) detected an incorrect state from erroneous sensor input and executed an unexpected fatal action – unrelated to the problem it was designed for.