Investigators revealed that the Turkish airline Boeing 777-300ER lost nearly 8,000 feet at altitude after its crew was surprised by a serious turbulent encounter through Iceland.
Both pilots engaged in the control pillars, opposed each other with rapid and sharp movements, and the crew applied the large thrust changes rebutted by Autothrottle.
The Icelandic Research Bureau's RNSA says the Twinjet, which was cruised at 35,000 feet, had descended at a speed of up to 17,100 feet/min, causing the stick shaker several times.
During the upset, the aircraft entered an overspeed state, reaching 368kt (approximately 18kt above the maximum operating speed) and MACH 0.92.
The crew ultimately arrested a descent of less than 27,300 feet. This was less than 50 seconds, less than 1 minute, since the first sign of turbulence and a minute after the aircraft departed from cruise altitude.
According to the RNSA, vertical wind speeds are “just a small portion” of the aircraft's sink rate, indicating that “something else” is needed other than turbulence to explain the “strength” of the descent.
Satellite images and other data showed that high-altitude mountain waves were present in Iceland a few hours before the incident, but weather-based Sigmet warnings were not effective.
Two captains and one officer who are undergoing line training were on a flight from Toronto on February 13, 2023. The first mate, who was only 38 hours in the type, was away from resting the cockpit when the turbulent encounter began.
Cockpit voice recorder information was not available, but flight data recorder analysis showed that dodd knockout and tailwind increased, and engine power increased, offsetting the dip with airspeed.
Turbulence led to vibrations of acceleration and attack, as well as vertical fluctuations from prosperity and dragging. The crew reduced the thrust to idle as airspeed increased from 281kt to 300kt and its Mach number increased from M0.83 to near M0.89.
“Perhaps the flight crew ordered regular (thrust lever) reductions over the next minute, each time they were countered by the auto throttle increase command,” the inquiry said.
The crew also deployed SpeedBrakes to slow down the aircraft.
The 777 slowed the target's airspeed and temporarily regained its responsiveness, but the RNSA says the speed break has been kept extended. Even if Autothrottle tried to advance the thrust again, this combined with another near-idling thrust reduction, which led to the aircraft beginning to descend.
A cockpit warning was issued, and “short and sudden increases in attack angles” also triggered intermittent stick shaker activity.
The first mate responded by pushing the control post forward, the autopilot was released and the jet began to begin nose down.
The aircraft descended at 1,360 feet/min, which re-energized the stick shaker, and the captain intervened in the nose-up input on the opposite side. However, he did not make it clear that he was in control, and the assistant officer was unaware that both pilots were trying to fly the jet.
Atmospheric speed dropped to 256kt and M0.76, with the superior officer continuing to push the control pole, leading to maximum thrust as the aircraft approached 10° nosedown.
This was the opposite force, with a temporary break-out of the contrast pillar. “Both pilots acted as pilot flights at the same time, but neither pilot was monitoring the flight, suggesting a breakdown of crew resource management,” the study states.
The captain temporarily reduced the pitch to a 2.5° nose down, but the activation of another stick shaker made the assistant officer drop his nose again. The jet reached the bottom of the nose of 18.3°, at which point the descent rate exceeded 9,000 feet/min.
For a while, the assistant officer pulled his control column, causing a sharp increase in pitch rate and activation of the fourth stick shaker, reopening the column. Opposite inputs from two pilots applying about 440n (100 pounds) each led to a breakout once again.
The aircraft exceeded its maximum operating speed below 30,700 feet, with descent speeds reaching approximately 16,000 feet per minute. Upon rising to 17,100 feet/min, the captain pulled harder on the control column and the pitch began to increase.
The breakout resulted in averaging the pilot's column input, bringing the jet closer to 27,500 feet and zero at 27,295 feet, which essentially cancelled each other. Air speeds have slowed down and excessive speed warnings have stopped.
The forces of the opposing control pillars stopped and the 777 began to climb again. Autopilot was once again involved and speed brakes were withdrawn.
The investigation says Reykjavik's air traffic control had called the crew several times during the descent, but only after the aircraft recovered it was one of the pilots responded – significantly “breathing out” – with statements that the flight had experienced severe turbulence.
The 777 climbed 31,500 feet and met further prolonged turbulence, causing the crew to descend to 28,000 feet.
Once conditions were improved, the commander returned to the cockpit, communicated with Air Traffic Control and took over as a flying pilot. By this point, Air Traffic Control had received the Sigmets of the area where the 777 was flying, and the crew had accepted the offer to be protected from it.
Five cabin crew members and two passengers suffered minor injuries. Investigations point out that because of the turbulence, crews had turned on seat belt signs 10 minutes before the incident.
The possibility of detours to Glasgow or Copenhagen was debated, but the crew chose to follow Istanbul, where medical assistance was requested. This overwritten the cockpit voice recorder as it only lasted for 2 hours.
