Pilots of the crashed Red Wings Tupolev Tu-204 twice selected maximum reverse thrust without the reverser system engaging, and unwittingly catapulted the aircraft forward under high power.
Russian investigators have detailed the crew's failed attempts to slow the twinjet before it overran Moscow Vnukovo's runway 19 at 116kt and struck a highway embankment.
The aircraft had been configured for landing - with flaps at 37° and slats at 23° - and overflew the threshold at a height of 15m, travelling at 140kt, says a preliminary analysis from the Interstate Aviation Committee (MAK).
It touched down about 900-1,000m after the threshold, at a speed of 124kt, some 5s after the throttle was reduced to idle thrust. The aircraft took 10s to descend the last 4m before runway contact, a touchdown which registered at 1.12g on the flight-data recorder.
The inquiry points out that the Tu-204 landed with winds gusting up to 22kt from the right, and the aircraft was exhibiting a slight left bank of up to 1.5°.
While the left-hand main landing-gear registered a compression signal, the right-hand gear did not. The investigators also note that the Tu-204's spoilers did not automatically deploy.
As the nose-gear was lowered the pilots moved the reverser control lever to the maximum setting "in one motion", says MAK.
But neither of the engines' reverser systems responded. By selecting maximum reverse thrust, without the reverser system activating, the pilots effectively commanded high forward thrust from the Aviadvigatel PS-90 powerplants.
The Tu-204 only slowed to 108-110kt, about 7-8s after landing, before it started to accelerate again, reaching 130kt.
This acceleration further reduced the weight on the landing gear and, as the aircraft travelled along the runway, it oscillated in the roll axis, from 4.5° left to 2.6° right. The result was that the left- and right-hand landing-gear alternately compressed, says MAK, but simultaneous compression of both main gear "did not occur", rendering attempts to brake "ineffective".
"Pressure in the brakes was applied only when the landing-gear compressed," it states.
Maximum reverse remained selected for 8s before the control lever was disengaged. But 5s later the flight engineer called "Reverse!" and the pilots re-engaged maximum reverse.
MAK says this had the same effect as before, powering the aircraft forward. At this point it was around 950-1,000m from the runway end and still travelling at 125-130kt. Reverse thrust was again disengaged, after 4s, and the crew tried resorting to an automatic braking system.
But 32s after landing the Tu-204 ran out of available runway. In a last-ditch effort the flight engineer used an emergency system to cut the engine power as the aircraft overran. Ironically, the effect of crossing rough ground and snow caused the aircraft's landing gear to compress, deploying the spoilers and releasing the thrust-reverser mechanism.
Five of the eight occupants, including the three cockpit crew, were killed in the 29 December accident. The crash prompted an airworthiness directive instructing crews to engage reverse thrust in stages, by initially selecting a low-thrust setting and checking that the reversers have activated before committing to maximum reverse thrust.