As survivors described their ordeal, a clearer picture was emerging tonight over what happened to the Turkish Airlines Boeing at Schiphol airport, but not why.

The twin-engined jet, one of the work-horses of world aviation, appears to have stalled before falling the last few dozen feet and hitting the ground close to landing on the south-facing runway 18R at Schiphol airport.

Descriptions by passengers of a sudden drop "as if the plane fell into turbulence" would suggest that the aircraft did not fly into the ground under control. That could have been a possibility if landing instruments failed or a pilot misjudged the approach.

It appears that the aircraft unexpectedly lost flying speed as it was descending at about 160 mph down the glide slope between 2,000 and 1,000 feet about two miles from the runway threshold.

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An account from a witness suggested that the aircraft may have suffered a power loss because he said the engines were making no noise.

The crew would have reacted immediately, seeking to recover by reapplying power - if it was available - and lowering the nose. There was no time for alerting the controllers with whom they were in radio contact.

An aircraft stalls when the wings lose lift due to slow airspeed or a sudden manoeuvre. At such a low altitude, recovering from a stall is hard, especially in a jet, which responds slowly to commands.

Most of the passengers survived because the aircraft, although tail-down, was in still in a relatively level attitude when it struck the ploughed field. The pilots would have been trying to regain control until the last second.

There was little wind. Visibility was mediocre in mist, but the pilots would easily have seen the runway approach lights once they came below the 800 foot cloud ceiling.

The possible causes are numerous. They include a loss of engine power because of such things as fuel starvation, ice, a bird strike or a control problem. A British Airways Boeing crash-landed - without stalling - short of the Heathrow runway last year after ice accumulated on the flight from Beijing deprived the engines of fuel.

This month's crash of a Continental commuter airliner near Buffalo, New York, was apparently caused when a combination of slow speed and ice on the tail caused it to stall and fall near vertically, killing all aboard. Engine icing was deemed by some pilots to be a possible factor in the Amsterdam crash, though the air temperature was five degrees Celsius.

If the engines were running normally, such a stall could be induced simply by flying too slowly, though airliners have multiple systems to warn pilots if they are approaching the danger zone. A failure in the flight controls is another possibility.

There was also speculation that the aircraft could have been hit by wake turbulence, from a preceding heavy airliner. Violent mini-tornadoes can loiter for minutes in the wake just under the path of slow-flying heavy jets. Airliners are separated by controllers to avoid encountering such dangerous turbulence. They have been known to be sent out of control after following too closely behind and slightly below a preceding large aircraft.

Charles Bremner, Paris Correspondent for The Times, has been a qualified pilot for 25 years

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