Russian investigators have determined that the upper surfaces of the wing and horizontal stabiliser on a UTair ATR 72-200 were not inspected for ice before the aircraft stalled on take-off from Tyumen.
The aircraft had arrived at Tyumen just after 23:40 on 1 April 2012 and was parked overnight for more than 7h, during which it was exposed to wet snowfall, strong winds and freezing temperatures.
Although a technician had examined the aircraft from the ground, says the Interstate Aviation Committee, the critical surfaces could not have been seen without the aid of a ladder or elevated platform.
The ATR's crew arrived at 06:30 on 2 April but, although the captain performed a walk-around, he similarly did not pay sufficient attention to the presence of ice deposits on the wing and stabiliser - despite the de-icing carried out on aircraft parked on adjacent stands.
Ground-ice warnings had been transmitted to various organisations, including the airline and its maintenance department, UTair-Technik.
Analysis by the inquiry calculated that icing reduced the aircraft's lift coefficient by about 25% and "substantially" increased its drag. The aircraft departed with flaps at 15°, at 127kt, but the climb resulted in a "significantly elevated" angle of attack.
Flap retraction started as the aircraft reached 640ft and 139kt. But at 690ft, with the airspeed at 150kt, the aircraft began to bank right by 40° - with a stall alarm sounding - then rolled into a left bank which, the inquiry says, the crew was "unable to prevent" despite full aileron deflection.
The inquiry says that the angle of attack increased to more than 25° and that the ATR's elevator deflection was "mainly" pitch-up.
It descended at over 3,900ft/min (20m/s) in a 55° left bank and struck the ground about 1,500m from the end of the runway and 400m left of the centreline. Ten of the 39 passengers, but none of the four crew members, survived the impact.
The inquiry says that a recovery technique, using 15° flap, might have resulted in a 300-400ft loss of height, but adds that recovery without flap deployment would have been "impossible".
It adds that the accident illustrated a "lack of effective control" by UTair and UTair-Technik over de-icing procedures and training.
Over a 90min period either side of the accident nine other flights - four of which were operated by UTair - departed the airport, and all except the transit flights underwent de-icing.