Russian investigators have detailed a serious loss-of-control incident last month involving a Utair Boeing 737-500 on approach to Moscow Vnukovo.
Federal air transport regulator has revealed that the aircraft was subjected to excessive pitch – up to 45° – and bank of 95° before the crew regained control.
The aircraft (VQ-BJP) had been following a precision approach to runway 06 after a service from Krasnodar on 13 October.
Rosaviatsia states that conditions were overcast, with a cloud base of just 60m, and that the aircraft was in cloud and "out of sight" of ground.
The aircraft was following an arrival pattern which involved passing the IBTER waypoint, south-west of the airport, at a height of 600m before turning right to align with the runway and descending to 400m by the final approach point.
Rosaviatsia says this turn and descent were conducted with the autopilot and autothottle engaged, the landing-gear deployed, and the flaps set at 15°. The engines were operating at 35-40% of their N1 level.
After the turn the aircraft levelled at 400m but its pitch started increasing to 10° and airspeed declined to 133-135kt. The autothrottle raised the thrust level to 75% of N1.
As the 737 approached the entry point to the glidepath, at around 130kt, the crew extended the flap setting to 30° and the autopilot disengaged.
Rosaviatsia says the engine thrust setting had increased further by this point, to 95% of N1, and the power setting – combined with an increasing pitch of 19° – resulted in the jet entering a "smooth climb", while the airspeed declined to 128kt.
The pilot's control column, it states, was pushed nose-down and the thrust levers were pulled back to a lower power setting, but without a disconnection of the autothrottle.
Pitch continued to increase to a maximum of 45° and airspeed bled away to less than 100kt, triggering a stick-shaker alert. The inquiry says the control column registered "alternating deflection" and the aircraft experienced banks of up to 95°.
After the jet reached the maximum pitch the control column was pushed forward and remained this way until the aircraft emerged from the upset.
It had climbed to a height of 750m during the event and, once the crew regained control, the aircraft executed a go-around at 350m. The second approach was uneventful.
Rosaviatsia says that, despite the excessive operating parameters, the aircraft did not breach loading and speed limits for its configuration. None of the 111 passengers and five crew members was injured and the jet was undamaged.
Investigators are still analysing the circumstances of the incident which took place in daylight at around 09:17.