FlightFollowerBoeing 737 Accident Involving Faulty Radio Altimeter Indications (2009)
Turkish Airlines Flight TK1951 was descending toward Amsterdam Schiphol on February 25, 2009 when a failure in the captain's left radio-altimeter system became part of a chain that led to a stall and crash short of the runway. The Boeing 737-800 was carrying 135 people; nine were killed.
During the approach to Runway 18R, the left radio altimeter suddenly changed from a normal height indication to minus 8 feet while the aircraft was still about 1,950 feet above the ground. The right radio altimeter continued to provide the correct height. The faulty signal was nevertheless treated by aircraft systems as valid data.
The erroneous altitude input caused the autothrottle to enter its retard-flare mode, which is normally used close to touchdown. Engine thrust therefore moved to idle while the aircraft was still well above the runway. The aircraft's speed began to fall while the crew continued the approach.
The investigation found that the crew did not recognize the significance of the automation's behavior quickly enough to prevent the loss of airspeed. The aircraft descended below a safe energy state, the stick shaker activated and the airplane stalled before the crew could recover.
The Dutch Safety Board's investigation did not reduce the accident to a single defective sensor. It examined the interaction between the radio-altimeter failure, the autothrottle logic, cockpit indications, crew workload, system design and the absence of procedures that would have helped the pilots diagnose the specific radio-altimeter failure.
The accident also exposed a history of recurring radio-altimeter anomalies. Investigation material showed that similar erroneous indications had occurred previously, including during earlier approaches, without producing a crash. The significance of TK1951 was that the erroneous signal was accepted by the automation at a critical stage of flight and the crew did not recover the resulting loss of airspeed in time.
“faulty radio altimeter”
Accident facts — How a Faulty radio Altimeter Doomed a Boeing 737
- Date
- 25 February 2009
- Aircraft
- Boeing 737-800
- Registration
- TC-JGE
- Flight
- Turkish Airlines Flight TK1951
- Route
- Istanbul Atatürk to Amsterdam Schiphol
- Location
- About 1.5 km north of Schiphol Runway 18R, Netherlands
- Occupants
- 135
- Fatalities
- 9
- Survivors
- 126
- Investigation authority
- Dutch Safety Board, with participation from NTSB/FAA and other agencies
- Investigation status
- Final investigation completed
Chronology of the event — How a Faulty radio Altimeter Doomed a Boeing 737
Sources
Source video
Dutch Safety Board investigation of Turkish Airlines Flight TK1951; FAA and NTSB supporting material.