Fuel supply to both engines of Air India’s Flight AI171 got cut off seconds after takeoff, leaving the two pilots confused, a preliminary crash probe report said on Saturday.
In the black box audio, one pilot is heard asking his colleague why he cut the fuel off, and the other pilot replies that he has not.
A few seconds later, fuel supply resumed to both engines and the first engine started recovering and gained power. But the second engine could not recover fully, and the aircraft kept losing speed and altitude.
The London-bound Boeing 787-8 Dreamliner went down about half a minute after takeoff from Ahmedabad airport on June 12. It crashed into a medical college hostel near the airport, killing all but one of the 242 people on board and 30-odd on the ground.
The Aircraft Accident Investigation Bureau (AAIB) released its preliminary report in the early hours of Saturday, providing insights into the possible causes of
the crash.
But it gave no final conclusion, blaming neither the pilots nor Boeing, and said it had no recommendations at this stage for B787-8 and GEnx-1B engine operators and manufacturers.
According to the report, both engines shut down within seconds of the takeoff, with the fuel control switches in the “cut-off” position. The aircraft began losing altitude even before crossing the airport perimeter wall.
Fuel control switches manage the flow of fuel to the plane’s engines. When they are in the “run” position, fuel flows into the engines; when they are in the “cut-off” position, fuel flow stops, shutting the engines down.
Most of the data came from the two Enhanced Airborne Flight Recorders (EAFRs) the aircraft was equipped with, one in the tail section and the other in the forward section. These devices record flight data and cockpit voice information.
“As per the data, the aircraft achieved the maximum recorded airspeed of 180 Knots IAS (indicated air speed) at about 08:08:42 UTC (1:38:42pm) and immediately thereafter, the Engine 1 and Engine 2 fuel cut-off switches transitioned from run to cut-off position one after another with a time gap of 01 sec,” the AAIB report says.
“The Engine N1 and N2 began to decrease from their takeoff values as the fuel supply to the engines was cut off.”
The preliminary report does not clarify how the switches may have changed to the cut-off position during the flight. Generally, fuel control switches are put in the cut-off position during landing.
As the cockpit voice recorder suggests, the pilots were themselves confused about it. It remains unclear which pilot asked the other why he cut off the fuel — to be told that he had not.
Captain Sumeet Sabharwal and First Officer Clive Kunder were experienced and medically fit. Sabharwal had 8,200 hours of flying experience while co-pilot Kunder had 1,100 hours of flying under his belt.
No significant bird activity was observed close to the flight path.
CCTV footage from the airport showed a Ram Air Turbine (RAT) getting deployed during the initial climb, immediately after liftoff.
A RAT is a small, propeller-driven turbine that deploys from the aircraft body in an emergency to generate either electrical or hydraulic power. When the aircraft loses all engine power, the RAT uses the speed of the plane to spin the turbine and generate power.
According to flight data, as both engines lost power and dropped below the minimum speed, the backup RAT system got activated and began supplying hydraulic power. A few seconds later, the pilots initiated a restart process and turned on the fuel switch for the two engines, one after the other.
While the first engine began recovering, the second could not recover fully.
Both engines had been installed recently, one on May 1 and the other on March 26.
“…The Engine 1 fuel cutoff switch transitioned from cutoff to run at about 08:08:52 UTC. The APU (a small backup engine) Inlet Door began opening at about 08:08:54 UTC (1:38:54pm), consistent with the APU Auto Start logic,” the report says.
“Thereafter at 08:08:56 UTC (1:38:56pm) the 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 in flight, each engine’s full authority dual engine control automatically manages a relight and thrust recovery sequence of ignition and fuel introduction.
“The EGT (exhaust gas temperature) was observed to be rising for both engines indicating relight. 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 re-introduced fuel repeatedly to increase core speed acceleration and recovery. The EAFR recording stopped at 08:09:11 UTC (1:39:11 pm).”
A few seconds before the EAFR recording stopped, the pilots communicated “Mayday, Mayday, Mayday” to the air traffic controller.
The air traffic control officer enquired about the call sign but did not get any response, and later observed the aircraft crashing outside the airport boundary.
Initial reports by Flightradar, an online flight tracker platform, show that the aircraft reached an altitude of 625 feet but suddenly began dropping with a vertical speed of 475 feet per minute.
The AAIB said the data downloaded from the forward EAFR was being analysed in detail.
It said retrieving the black box data was delayed because this had to be done with the help of a kit sourced from the National Transportation Safety Board of the US.
So far, only an initial analysis of the audio and flight data has been done.
The report says both engines were retrieved from the wreckage and have been quarantined at a hangar in the airport.
Statements from witnesses and the surviving passenger have been obtained and a complete analysis of the postmortem reports of the crew and passengers is being done to corroborate the findings with the engineering assessments.
Reacting to the report, minister of state for civil aviation, Murlidhar Mohol, told reporters that this was a preliminary report and the investigations would continue.
“This is not the final report. Until the final report comes out, we should not arrive at any conclusion,” Mohol said, stressing: “AAIB is an autonomous authority and works without interference of the civil aviation ministry.”