Investigators have disclosed that a Turkish Airlines Boeing 777-300ER lost nearly 8,000ft in altitude after its crew – surprised by a severe turbulence encounter over Iceland – attempted to recover with aggressive control inputs and poor co-ordination.
Both pilots engaged their control columns, opposing one another with rapid and sharp movements, while the crew applied large thrust changes which were countered by the autothrottle.
Icelandic investigation authority RNSA says the twinjet, which had been cruising at 35,000ft, descended at rates up to 17,100ft/min and the stick-shaker triggered several times.
During the upset the aircraft entered an overspeed state, reaching 368kt – some 18kt above maximum operating velocity – and Mach 0.92.
The crew eventually arrested the descent just below 27,300ft, less than 1min 50s since the initial indication of turbulence and 1min after the aircraft departed from its cruise altitude.
RNSA says the vertical wind speed was “only a fraction” of the aircraft’s sink rate, indicating that “something else” besides the turbulence was needed to explain the “intensity” of the descent.
Although satellite images and other data showed high-altitude mountain waves were present over Iceland several hours before the incident, there were no meteorological SIGMET warnings in effect.
Two captains and a first officer undergoing line training were on board the flight from Toronto, on 13 February 2023. The first officer – with just 38h on type – was flying but the commanding captain was away from the cockpit resting when the turbulence encounter began.
While cockpit-voice recorder information was unavailable, analysis of the flight-data recorder indicated a downdraught and an increased tailwind, with the engine power rising to offset a dip in airspeed.
Turbulence led to oscillations in acceleration and angle-of-attack, as well as vertical fluctuations from the updraught and downdraught. The crew reduced the thrust to idle as the airspeed increased from 281kt towards 300kt, and its Mach number from M0.83 to the near maximum of M0.89.
“Periodic [thrust-lever] reductions to idle were commanded, likely by the flight crew, over the next minute, and each time was countered by a throttle increase command by the autothrottle,” says the inquiry.
The crew also deployed the speedbrakes in a bid to slow the aircraft.
Although the 777 decelerated and briefly regained its target airspeed, RNSA says the speedbrakes were left extended – having been “possibly forgotten”. Combined with another reduction of thrust, almost to idle, this led the aircraft to begin descending, even as the autothrottle tried to advance thrust again.
Cockpit warnings were issued and “brief, sharp increases” in angle-of-attack also triggered intermittent stick-shaker activity.
The first officer responded by pushing the control column forward, causing the autopilot to disengage and the jet to start pitching nose-down.
As the aircraft descended at 1,360ft/min – with the stick-shaker activating again – the captain intervened with an opposing nose-up input. But he did not clarify that he was taking control, and the first officer was unaware that both pilots were trying to fly the jet.
The airspeed dropped to 256kt, and M0.76, and maximum thrust was commanded with the aircraft nearing 10° nose-down as the first officer continued to push on the control column.
Such were the opposing forces that the control columns broke out momentarily. “Both pilots were simultaneously acting as pilot flying, while neither pilot was monitoring the flight, suggesting a breakdown of crew resource management,” says the inquiry.
The captain briefly reduced the pitch to 2.5° nose-down but another stick-shaker activation prompted the first officer to lower the nose again. The jet reached a nose-down attitude of 18.3° and the descent rate, at this point, exceeded 9,000ft/min.
For a moment the first officer pulled on his control column, causing a sharp increase in pitch rate, and fourth stick-shaker activation, before he resumed pushing on the column. The opposing inputs from the two pilots – each applying around 440N (100lb) – again led to a breakout.
The aircraft exceeded its maximum operating speed just below 30,700ft and descent rate reached nearly 16,000ft/min. As it rose to 17,100ft/min, the captain pulled harder on the control column and the pitch started to increase.
Breakout results in the pilots’ column inputs being averaged, and they essentially cancelled one another out as the jet neared 27,500ft, and the pitch reached zero at 27,295ft. The airspeed reduced and overspeed warnings ceased.
Opposing control column forces stopped and the 777 began climbing again. The autopilot was re-engaged and the speedbrakes were retracted.
Reykjavik air traffic control had called the crew several times during the descent, says the inquiry, but only after the aircraft had recovered did one of the pilots respond – noticeably “out of breath” – with a remark that the flight had experienced severe turbulence.
The 777 climbed to 31,500ft where it met further prolonged turbulence and the crew descended to 28,000ft.
Once the conditions improved the commanding captain returned to the cockpit, communicating with air traffic control and taking over as the flying pilot. By this point air traffic control had received a SIGMET for the area in which the 777 was flying and the crew accepted an offer to be vectored out of it.
Five cabin crew members and two passengers received minor injuries. The inquiry points out that, owing to light turbulence, the crew had turned on the seatbelt signs 10min before the incident.
While the possibility of diverting to Glasgow or Copenhagen was discussed, the crew opted to continue to Istanbul, where medical assistance was requested. This resulted in the cockpit-voice recorder, which only had 2h duration, being overwritten.
