THE HAGUE, Netherlands – A faulty flight instrument aboard a Turkish jetliner lowered the plane's airspeed, setting off warning signals in the cockpit and prompting the pilots to try and accelerate before it crashed in the Netherlands, officials said Wednesday.
The Turkish Airlines plane carrying 135 passengers and crew crashed less than a mile from the runway at Amsterdam's Schiphol Airport shortly before it was due to land on Feb. 25. Nine people were killed.
The pilots, who died in the crash, were landing on automatic pilot when the altimeter, a device that measures altitude, registered that the plane was flying lower than it actually was and instructed the plane to decelerate, officials said.
The Boeing 737-800 had twice before experienced problems with its altimeter, said Dutch Safety Authority chief investigator, Pieter van Vollenhoven, at a news conference at The Hague.
He said the Safety Authority has warned Boeing of the problem and asked the company to alert customers that when altimeters are not functioning properly "the automatic pilot and the gas system coupled to them may not be used for approach and landing."
Boeing said it was reminding all operators of its 737s to carefully monitor primary flight instruments during critical phases, adding that it was carefully monitoring the fleet.
The Deputy Chairman of Turkey's Pilot's Association Ahmet Izgi said during an interview with Turkey's NTV said the preliminary findings were "not satisfactory" and that it was odd the pilots didn't react to the altimeter.
"It is also not logical that it was not changed after malfunction," he told Turkey's NTV news channel.
The pilot's association had earlier suggested the crash was due to "wake turbulence" from a large plane, a Boeing 757, that had landed at Schiphol Airport two minutes earlier. Wake turbulence forms behind an aircraft as it passes through the air.
Using data recovered from the jet's black boxes, Van Vollenhoven described how investigators believe the crash happened. He said it would be for courts to apportion blame.
At 1950 feet, "the airplane's left radio altimeter suddenly registered a change in altitude" to negative 8 feet (about 2 meters). "It didn't only register it, but passed it on to the automatic steering system," Van Vollenhoven said.
According to conversation recorded between the plane's captain, first officer and an extra first officer on the flight, the pilots noticed the faulty altimeter but didn't consider it a problem and didn't react, Van Vollenhoven said.
Gas to the engines was reduced and the plane lost speed, decelerating until, at a height of 450 feet, the plane was about to stall. Warning systems alerted the pilots.
"From the "black box" (data recorders) it appears that then the pilots immediately gave gas, full gas, however it was too late to recover," Van Vollenhoven said.
He said that the pilots had been unable to see the runway at the time the plane began its descent due to weather conditions — cloudy with a light rain. Van Vollenhoven said it was not unusual to land a plane on autopilot.
Eyewitnesses said it seemed to fall from the sky. Passengers who survived had noticed the pilot gunning the engines at the last minute. Some didn't realize the landing had gone wrong until other passengers began opening emergency doors.
Four Americans were killed in the crash, including three Boeing employees. As of Wednesday, 28 survivors were still hospitalized.
The investigation is expected to last until the end of the year.
Boeing's 737 is the world's best-selling commercial jet, with more than 6,000 orders since the model was launched in 1965.
The 737-800, a recent version of the plane, has a "very good safety record," said Bill Voss, president of the independent Flight Safety Foundation in Alexandria, Va.