Investigators of the Turkish Boeing 737-800 approach crash at Amsterdam have found that a sudden reduction in engine thrust coincided with a step-change in the reading from one of the aircraft's two radio altimeters.
The Dutch Safety Board, in preliminary findings released today, adds that a similar problem had occurred twice during landing in the course of eight previous flights contained on the 737's flight recorders.
It states that the "irregularity" on the fatal 25 February approach to Amsterdam Schiphol occurred at 1,950ft, as the aircraft descended to runway 18R after an otherwise-normal flight from Istanbul.
The left-hand radio altimeter suddenly changed its reading from 1,950ft to minus 8ft, and transmitted the information to the autopilot which was engaged at the time.
"This change had a particular impact upon the automatic throttle system which provides more, or less, engine power," says the inquiry board.
Cockpit-voice recorder information shows that a landing-gear warning alerted the crew to the malfunctioning altimeter. Initial indications, says the inquiry board, suggest the warning was not regarded as a problem.
But it states that the aircraft responded to the step-change as if the jet was just a few metres above the runway, and engine power reduced.
"It seems that the automatic system - with its engines at reduced power - assumed it was in the final stages of the flight," says the board. "As a result, the aircraft lost speed."
The crew comprised a captain, a first officer for whom the flight was a training exercise, and another first officer behind.
"Initially the crew did not react to the issues at hand," says the board. But as the aircraft's speed fell, and it approached possible stall, the stick-shaker activated at 150m (490ft) and the throttle immediately increased to full power - too late, however, to arrest the descent and recover the aircraft.
The jet, travelling with a forward speed of just 95kt, struck the ground with its empennage first and rapidly decelerated, breaking into several sections.
Investigations are to concentrate on the functioning of the radio altimeters and their connection to the autothrottle. The Dutch Safety Board says the recorded data shows no evidence of deviating readings from the 737's right-hand altimeter.
It says "extra attention" should be paid to the role of the radio altimeter when using the autopilot and autothrottle, and the board is requesting greater emphasis in the 737 operating manual that the autopilot and autothrottle should not be used for landing if the connected altimeter is faulty.
Turkish Airlines did not make any mention of the altimeters in an earlier statement about recent maintenance performed on the aircraft.
Five passengers and four crew on board the aircraft were killed in the crash, and 28 of the 80 passengers injured remain in hospital.