What happened
On February 25, 2009, a Turkish Airlines Boeing 737-80 and registration TC-JGE crashed approximately 1.5 km north of runway 18R at Amsterdam Airport Schiphol. The flight, traveling from Istanbul, was performing an instrument landing system (ILS) approach when the aircraft impacted farmland. The accident resulted in 9 fatalities (four crew and five passengers) and 86 injuries among the 128 passengers and 7 crew members on board. The aircraft was destroyed upon impact.
During the approach, the aircraft was vectored to intercept the localizer at roughly 5.5 NM from the threshold. As the aircraft descended, the crew transitioned to a vertical speed mode to intercept the glide path. During this phase, the aircraft's speed began to decay. Despite the crew's attempts to manage the descent, the aircraft's pitch increased significantly, leading to a high angle of attack and subsequent impact.
The investigation
The Dutch Safety Board (OVV) examined the wreckage, flight data recorders, and cockpit voice recorder. The investigation focused on the technical performance of the aircraft's automated systems, specifically the autothrottle and radio altimeters. Investigators analyzed the flight path, the aircraft's response to control inputs, and the environmental conditions at the time of the accident, which included mist and low clouds.
Technical scrutiny revealed that while the right radio altimeter functioned correctly, the left radio altimeter provided erroneous data, reporting a height of -8 feet while the aircraft was still at high altitude. This faulty data triggered several aural configuration warnings and influenced the behavior of the automated flight systems.
Findings
- The left radio altimeter malfunctioned, transmitting an incorrect altitude of -8 feet to the cockpit and automated systems.
- This erroneous altitude data caused the autothrottle system to enter retard mode prematurely, moving the thrust levers to idle during the approach.
- Because the thrust levers were at idle, the aircraft's airspeed continued to drop significantly below the target speed.
- In an attempt to maintain the glide path despite the loss of speed, the automatic flight system commanded an increasing nose-up pitch and applied nose-up stabilizer trim.
- The aircraft experienced a high angle of attack, triggering stick shaker warnings at approximately 460 feet before the crew attempted a manual recovery.
- Data from previous flights indicated that this specific aircraft had experienced similar left radio altimeter malfunctions on at least two other occasions.
Safety action
Following the discovery of the altimeter malfunction, the Dutch Safety Board issued a warning to Boeing regarding the 737 Dispatch Deviations Guide. The Board suggested that procedures regarding the use of automatic pilots and autothrottle systems should be reviewed for all phases of flight when radio altimeters are malfunctioning. In response, Boeing issued a Multi Operator Message (MOM) on the same day as the report regarding radio altimeter malfunctions.