Russian investigators have revealed that substantial nose-down trim on the crashed Flydubai Boeing 737-800 was applied through the manual trim switches on the pilot’s control column.
The inquiry into the 19 March accident at Rostov-on-Don had already disclosed that the aircraft had been subjected to a nose-down control yoke input at a height of 900m (2,950ft) while executing a go-around.
It had also stated that the stabiliser had undergone a simultaneous 5° shift towards a nose-down attitude.
But an updated analysis from the Interstate Aviation Committee says the stabiliser movement coincided with an input from the stabiliser trim switches on the control column – used to shift the stabiliser and trim the aircraft manually when the autopilot is not active.
This switch input lasted for 12s, according to flight-data recorder information, while the cockpit-voice recorder also picked up the noise of the manual trim-wheels – located on the centre pedestal – rotating.
The substantial nose-down attitude commanded by the yoke movement and the stabiliser trim caused the aircraft to pitch sharply into a steep dive from which it failed to recover.
Flight FZ981’s crew had conducted a successful missed approach to Rostov some 2h earlier, aborting after a windshear warning. The aircraft entered a long holding pattern before attempting its second approach, during which it carried out its fatal go-around.
The Interstate Aviation Committee says both approaches had been conducted with autopilot and autothrottle disengaged.
It states that a sudden 20kt increase in airspeed, to 176kt, during the second approach could have pointed to another windshear encounter, and might have prompted the go-around. As the pilots aborted the second approach, they set maximum take-off thrust – about 101-102% of N1 – and climbed at a rate of about 3,900ft/min.
The crew set 15° of flap and retracted the landing-gear. As the 737 climbed through 600m (1,970ft), with nose-up pitch of 18°, the pilot pushed forward on the control column, which increased the aircraft’s airspeed to more than 200kt – and caused an automatic partial retraction of the flaps to 10°.
Subsequent nose-up control column input led the aircraft to continue climbing at 3,150ft/min until it reached 900m, when the nose-down command and nose-down trim input pushed the jet into its final dive.
Investigators state that the aircraft took off from Dubai with a weight of 68t and a 17.3% centre of gravity, and weather forecasts – both for Rostov and the alternates of Krasnodar and Volgograd – were “not inhibitory”, adding: “The crew took a justified decision to depart.”
No details have been released on the crew’s rest and duty records. But the Interstate Aviation Committee says that continuing investigative work will include assessment of the pilot’s psycho-emotional and physiological state.