Schiphol airliner crash blamed on altimeter failure, pilot error

Thursday, March 5, 2009

Investigators in The Netherlands have announced that last month’s crash of a Boeing 737-800 was caused by a combination of an instrument failure and pilot error. Turkish Airlines Flight 1951 crashed upon approach to Amsterdam‘s Schiphol Airport, killing nine and injuring eighty of the 135 passengers and crew on board.

Pieter van Vollenhoven, head of the Dutch Safety Board, said yesterday that as the aircraft was at 1,950 feet the faulty altimeter indicated that the airliner’s height was at minus eight feet. Because the autopilot and autothrottle were running from this flawed data the plane automatically reduced engine power as it would in the final seconds before landing.

The plane’s altitude and airspeed continued to fall for a minute and a half without the pilots noticing, when at 450 feet and an airspeed 46 mph lower than it should be the aircraft’s stick shaker activated to warn the flight crew of an imminent aerodynamic stall. The pilots applied full engine thrust, but were too late to prevent the accident, and the aircraft crashed into a field and broke into three pieces.

“When the crew of the Turkish Airlines noticed what was going on, it was already too late to intervene effectively,” commented the Board’s press release. The Board also sent a warning to Boeing concerning possible problems with the 737-800’s altimeters. The United States airframe manufacturer has issued an alert to operators of all variants of the 737 worldwide – totalling almost 6,000 aircraft – reminding flight crews “to carefully monitor primary flight instruments during critical phases of flight.”

Boeing have also stated that the preliminary report “should not be construed as a final conclusion or outcome of the investigation,” and that the Board is still “in the early stages of this complex investigation.” Further, they told ABC News that “erroneous radio altimeter readings are detectable and recoverable. Should a radio altimeter failure occur, anomalous readings would be detected by monitoring systems or be apparent through flight deck effects.”

It is apparent from the flight data recorder – which carries details of the plane’s last eight flights – that the altimeter had also failed on at least two previous occasions, but the aircraft was able to land safely. Van Vollenhoven commented that misty conditions at the time of the crash would have prevented the pilots seeing their height earlier, and also said that the plane’s automatic flight systems should not be used when crucial instruments are defective.