Turkish Airlines has expressed reservations about conclusions of the Dutch inquiry into the crash of a Boeing 737-800 on approach to Amsterdam, notably over the crew's failed attempt to recover from the fatal stall.
The aircraft's autothrottle reduced the thrust to idle early in the approach, after receiving incorrect altitude information from a faulty left-hand radio altimeter, causing the jet to lose airspeed.
This went unnoticed by the crew until the aircraft threatened to stall, the stick-shaker activating with the 737 just 460ft above the ground.
The Dutch Safety Board inquiry found that the first officer, the flying pilot, responded immediately to the stick-shaker by pushing the control column and thrust levers forward, in line with stall-recovery procedures.
But cockpit-voice recorder analysis shows that the captain also called that he was taking control, just as the thrust levers had been pushed halfway forward. "Assumingly, the result of this was that the first officer's selection of thrust was interrupted," says the final report into the crash.
Crucially, as the first officer relinquished the thrust levers to hand over control, the still-engaged autothrottle immediately retarded them again to the idle position.
"Directly thereafter, the autothrottle was disengaged," says the inquiry report. "But for a period of seven seconds the thrust levers were not moved forwards from the idle position."
While the investigation could not determine whether the captain had placed his hands on the thrust levers, it says that nine seconds passed between the activation of the stick-shaker and the movement of the thrust levers to maximum.
By this point the aircraft had already stalled and the remaining height of 350ft was "insufficient for the recovery procedure", it adds.
Turkish Airlines says the autothrottle "kicked back unexpectedly" and that Boeing "had not mentioned", in its documentation, a need to disconnect the autothrottle during the procedure.
The carrier also describes the relationship between the left-hand radio altimeter and the autothrottle as "error-prone", adding that it was not explained in Boeing's documentation for flight crews until after the crash.
While acknowledging that disengagement of the autothrottle is not described in the recovery procedure, the Dutch Safety Board highlights the problems of incomplete knowledge of the aircraft's interdependent systems. It points to the crew's suffering from "automation surprise" with respect to the autothrottle's behaviour - during both the original loss of thrust on approach and the attempted stall recovery.
Turkish Airlines also claims that simulator tests show that a height of at least 500ft is required for the 737-800 to recover successfully from a stall, and that the ill-fated jet was already below this level when the pilots initiated the recovery procedure.
Source: Flight International