A common malfunction with Boeing radio altimeters, compounded by several errors by pilots, led to last year's fatal crash by a Turkish Airlines 737 plane as it dropped short of the runway at Amsterdam's airport, according to investigators' final report.
AMSTERDAM, The Netherlands — A common malfunction with Boeing radio altimeters, compounded by several errors by pilots, led to last year’s fatal crash by a Turkish Airlines plane as it dropped short of the runway at Amsterdam’s airport, according to investigators’ final report released Thursday.
Flight TK1951 — a Boeing 737 carrying 135 passengers and crew from Istanbul — crashed a mile from Schiphol Airport on Feb. 25, 2009. The three pilots were among nine people killed. Three of the four Americans who died were Boeing employees on a business trip unrelated to the flight.
The Dutch Safety Board investigation affirmed its initial findings that a faulty altimeter, one of two on the airplane that determine its height above the ground, played a key role in the crash.
The left altimeter, which feeds data to the critical flight controls, erroneously registered the plane’s altitude as being below sea level when it began its final descent from 2,000 feet. That caused the plane’s autopilot to reduce the throttle to an idle too soon.
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The plane was on an unusually steep approach and should have been slowing anyway, which may have played a role in the pilots’ initial failure to notice they had lost too much speed, the report said.
“If the altimeter in the Boeing hadn’t been malfunctioning, the accident wouldn’t have taken place,” Safety Board Chief Pieter van Vollenhoven said.
He criticized Boeing, saying the manufacturer was aware that altimeter malfunctions are common, but until the crash it considered them a technical problem and not a safety threat.
The investigators’ report concludes that “the prime responsibility” for the radio altimeter system’s problem “lay not with Turkish Airlines but with Boeing.”
Boeing spokeswoman Sandy Angers said Thursday that the altimeter problem had been known, but any possible effects were considered “detectable and recoverable,” and therefore not a safety issue.
She said Boeing has improved airplane systems and specified flight-crew training, among other moves, to prevent a defective altimeter from causing future accidents.
Unlike that 2002-built jet, Angers said, the latest 737s have a feature that compares readings in the left and right altimeters. If there’s a discrepancy, it disconnects the autothrottle.
Angers was unable to say how many 737s lack this feature today, but she said it can be retrofitted on many 737 jets now in service.
In the Dutch crash, the pilots failed to react until the plane’s yoke gave a “stick shake,” a warning it was about to stall.
At that moment, the airplane’s condition was precarious, but salvageable if the crew had responded appropriately, according to the investigators.
The first officer did respond correctly by gunning the engines. But as the captain took control, because of the faulty altimeter reading, the still-engaged autothrottle reduced the engines back to idle.
It wasn’t until nine seconds after the “stick shake” warning that one of the pilots pushed the throttle levers fully forward again, but at that point it was too late.
The jet crashed into a freshly plowed field, striking the ground tail first and breaking into three pieces.
John Nance, a veteran pilot and aviation-safety expert, said the crash was caused by the convergence of the faulty altimeter with a series of human errors, including “not paying attention, lack of situational awareness,” disregard of standard operating rules while approaching the runway and undue deference by the two co-pilots to the captain in command.
Seattle Times aerospace reporter Dominic Gates contributed to this report.