Seconds to disaster: How the Air India flight crashed and what are the fuel switches under scrutiny?
Here is the sequence of events - by the seconds - on Jun 12, as detailed by Indian investigators in their preliminary report released on Saturday (Jul 12). All times in GMT.
TIMELINE
5.47am GMT (11.17am IST): Air India Dreamliner VT-ANB landed in Ahmedabad from New Delhi as AI423.
7.48.38am: The aircraft was observed departing from Bay 34 at the airport.
7.55.15am: The aircraft requested taxi clearance, which was granted by air traffic control. A minute later the aircraft taxied from the bay to Runway 23 via Taxiway R4, backtracked and lined up for take-off.
8.02.03am: The aircraft was transferred from ground to tower control.
8.07.33am: Take-off clearance issued.
8.07.37am: The aircraft started rolling.
8.08.39am: Aircraft lifted off. "The aircraft air/ground sensors transitioned to air mode, consistent with liftoff," the report said.
8.08.42am: Aircraft reached max airspeed of 180 knots. "Immediately thereafter, the Engine 1 and Engine 2 fuel cutoff switches transitioned from RUN to CUTOFF position one after another with a time gap of 01 sec," the report said.
"The Engine N1 and N2 began to decrease from their take-off values as the fuel supply to the engines was cut off.
"In the cockpit voice recording, one of the pilots is heard asking the other why did he cutoff. The other pilot responded that he did not do so.
"The CCTV footage obtained from the airport showed Ram Air Turbine (RAT) getting deployed during the initial climb immediately after lift-off
"The aircraft started to lose altitude before crossing the airport perimeter wall."
8.08.47am: Both engines' values "passed below minimum idle speed", and the RAT hydraulic pump began supplying hydraulic power.
8.08.52am: Engine 1 fuel cutoff switch transitioned from CUTOFF to RUN.
8.08.56: Engine 2 fuel cutoff switch also transitions from CUTOFF to RUN.
"When fuel control switches are moved from CUTOFF to RUN while the aircraft is inflight, each engines full authority dual engine control (FADEC) automatically manages a relight and thrust recovery sequence of ignition and fuel introduction," the report stated.
"Engine 1's core deceleration stopped, reversed and started to progress to recovery. Engine 2 was able to relight but could not arrest core speed deceleration and reintroduced fuel repeatedly to increase core speed acceleration and recovery."
8.09.05am: One of the pilots transmitted 'MAYDAY MAYDAY MAYDAY".
8.09.11am: Data recording stopped.
8.14.44am: Crash fire tender left the airport premises for rescue and firefighting.
WHAT ARE FUEL SWITCHES?
They are switches that regulate fuel flow into a plane's engines. They are used by pilots to start or shut down engines on the ground or to manually shut down or restart engines if an engine failure occurs during a flight.
Aviation experts say a pilot would not be able to accidentally move the fuel switches that feed the engines. But if moved, the effect would be immediate, cutting off engine power.
There are independent power systems and wiring for the fuel cutoff switches and the fuel valves controlled by those switches, according to US aviation safety expert John Cox.
WHERE ARE THE FUEL SWITCHES LOCATED?
The two fuel control switches on a 787, in Air India's case equipped with two GE engines, are located below the thrust levers.
The switches are spring-loaded to remain in position. To change one from run to cutoff, a pilot has to first pull the switch up and then move it from run to cutoff or vice versa.
There are two modes: "CUTOFF" and "RUN".
At the crash site, both fuel switches were found in the run position, and there had been indications of both engines relighting before the low-altitude crash.
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