Uncontained fire originating in a fuel leak damaged the flight-control surfaces of an Antonov An-12 transport, leading it to dive into the ground and disintegrate, Russian investigators believe.
The aircraft, operated by Avis-Amur, had been conducting a charter flight from Komsomolsk-on-Amur to Keperveem, with a refuelling stop at Magadan, on 9 August 2011.
Forty minutes after departing Magadan - while southwest of Omsukchan - the crew requested to descend to 6,600m (21,650ft) and return to the airport, advising that there was a fuel leak.
The An-12 (RA-11125) turned left and started back to Magadan but the situation deteriorated and, just seven minutes after the decision to return, air traffic control lost contact with the flight.
All of the nine crew members and two passengers on board died.
Although the condition of the wreckage meant the precise source of the fire could not be determined, Russia's Interstate Aviation Committee states that hot engine parts probably ignited fuel leaking from the lines in the vicinity of the outboard left-hand powerplant.
The fire spread to the wing and the An-12 began rolling to the left. MAK says the crew was unable to counter the roll - despite "strenuous efforts" and turning the yoke fully to the right - probably because the left-hand aileron mechanisms had been damaged by the fire.
Without lateral control the aircraft rolled to more than 66° left and lost height rapidly. It turned several times around its longitudinal axis, at a rate of 60°/s, descending at 100m/s. The An-12 had been overflying high terrain and was only 860m above the ground at the time.
Overcast cloud, says MAK, and the rapid onset of the emergency prevented the crew from finding a suitable landing site.