Initial flight-data recorder analysis indicates that the pilots of the Tatarstan Boeing 737-500 which crashed at Kazan pushed the aircraft into a steep dive after the jet pitched up during a go-around.
Russia’s Interstate Aviation Committee states that the crew engaged go-around thrust after assessing the aircraft’s position relative to the runway. The autopilot was disconnected and the aircraft flown manually.
As the engines increased power the crew retracted the 737’s flaps from the 30° position to 15°. But the thrust from the underwing engines pitched the aircraft nose-up, and it climbed at a pitch of 25°.
This led the airspeed to bleed away from 150kt to 125kt, says the inquiry.
The pilots, after retracting the landing-gear, countered the climb and the loss of airspeed by pushing the control column forward – pushing the aircraft into a dive at a height of just 700m (2,300ft).
Investigators point out that the aircraft did not exceed its angle-of-attack limit, indicating that there was no stall.
During the “intense” dive the aircraft reached 75° pitch down, says the inquiry, and it slammed into the ground at over 240kt just 45s after initiating the missed approach.
Both CFM International CFM56 engines were functioning until the moment of impact, and there is no immediate indication of system failure.
While the flight-data recorder has generated detailed information the cockpit-voice recording mechanism was found to be missing after its container was opened. Investigators are conducting a search for the device.
The inquiry has not established whether somatogravic illusion might have played a role in the night-time accident.
Somatogravic illusion occurs when the brain, in the absence of visual references, misinterprets the sensations caused by rapid acceleration, during a climb, as excessive pitch. This can cause pilots to react with sharp nose-down input, enough to push the aircraft into a dive at low altitude.
The investigators are examining the airline’s pilot training procedures, assisted by Russian pilots experienced in operating the 737.
Tatarstan Airlines has grounded its other Boeing 737 as a precaution in the wake of the accident.
The carrier says it is "temporarily suspending" services with the twinjet following the loss of the 737-500, and all 50 occupants, on 17 November.
Tatarstan Airlines has a 737-400 in a fleet which also comprises four Airbus A319s and two Tupolev Tu-154s, as well as 15 Cessna 208 Grand Caravans.