A false reading from a faulty altimeter caused an autopilot to sharply slow a Turkish Airlines jet short of the runway last month, sending it plunging into a muddy field and killing nine people, Dutch investigators said Wednesday.
The flight carrying 135 passengers and crew crashed a kilometer (less than a mile) from Amsterdam's Schiphol Airport as it was landing on Feb. 25. The pilots were among the dead.
One of the plane's altimeters, a device that measures altitude, had registered that the plane was flying below sea level and caused the autopilot to rapidly reduce power before the crash, officials said.
The Boeing 737-800's flight recorders showed false readings from the same altimeter on two flights before the crash, chief investigator Pieter van Vollenhoven said. He did not say if pilots had noticed the previous incorrect readings.
The Dutch Safety Authority said it had issued a warning to Boeing as a result of its investigation, asking 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," Van Vollenhoven said.
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 the Turkey's Pilot's Association, Ahmet Izgi, told Turkey's NTV news channel that the preliminary Dutch findings were "not satisfactory" and said it would be odd for the pilots to not react to a false altimeter reading in time to save the plane.
Landing on autopilot
Van Vollenhoven said landing the plane on autopilot was not unusual and the pilots could not see the runway as the plane began its descent because of clouds and a light rain.
At 1,950 feet the airplane's left altimeter suddenly and mistakenly registered an altitude of 8 feet below sea level and passed the reading on to the automatic control 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.
But the autopilot reduced gas to the engines and the plane lost speed, decelerating until, at a height of 450 feet it was about to stall. Warning systems alerted the pilots.
"It appears that then the pilots immediately gave gas, full gas, however it was too late to recover," Van Vollenhoven told reporters. He said it would be for courts to apportion blame.
The plane fell into a freshly plowed field, striking the ground tail first and breaking into three pieces.
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.
Those killed in the crash included five Turks and four Americans.
Turkish Airlines said the dead included the three pilots. It described the captain, Hasan Tahsin Arisan, as an experienced pilot and air force veteran.
Company will pay compensation
The company has said it will pay compensation to victims and survivors.
The American dead included three Boeing employees on a business trip unrelated to the flight.
As of Wednesday, 28 survivors are still hospitalized.
The investigation is expected to last until the end of the year. Van Vollenhoven said that from now on "the technical investigation will be directed at the functioning of the automatic pilot, the automatic gas control system, and their coupling to the radio altimeter."
The Turkish 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.
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.