Investigators probing the Yakovlev Yak-42 crash at Yaroslavl have revealed the presence of an apparent braking force on the aircraft during the take-off run, but have yet to understand its origin.
The aircraft had entered runway 23 at taxiway 5 - about 300m (984ft) from the beginning of the strip - for the flight to Minsk on 7 September.
Russia's Interstate Aviation Committee (MAK) said the Yak-42 accelerated to around 89kt, in line with the engine power setting, but that the pitch did not increase when the crew attempted to lift the nose-wheel at 100kt despite elevator deflection of 9-10°.
About 6s later the engine power was increased to a higher thrust setting. But MAK said that, despite this setting, the acceleration "slowed significantly". It added that this might be explained by "the appearance of an additional braking force", although further tests - possibly using a similar aircraft - are needed to determine the magnitude.
Parts of the braking system retrieved from the wreckage are undergoing special examination, said MAK.
The Yak-42's centre of gravity was within limits; investigators have already disclosed that the aircraft's weight was not excessive, that its flaps were set to their 20° take-off position, and that its stabiliser was set at the 8.7° pitch-up position.
MAK said the pilots had checked all the flight controls, including the elevator which deflected cleanly to a pitch-up position of 21°. The last check was carried out 1min 40s prior to take-off.
Although the elevator deflected to the pitch-up position during the take-off roll, said MAK, the aircraft failed to lift off. Its speed increased to 124kt and evidence at the crash scene indicates that the jet eventually became airborne 400m beyond the runway end, with the elevator deflected 13-14° and the stabiliser set at 9.5° pitch-up.
But the aircraft failed to gain sufficient height to clear the localiser antenna, colliding with the structure and suddenly pitching up to 20° for a few seconds. The aircraft did not achieve a height of more than 5-6m (16-20ft) before banking left and hitting trees and terrain.
Wreckage analysis shows the flaps and slats were in the take-off configuration, the spoilers were retracted and the stabiliser was positioned about 10° pitch-up. There was no disconnection of elevator control cabling.
"Technical investigators are considering all possibilities for the additional braking force during take-off and the reasons why the aircraft failed to lift off in time from the runway," MAK added.