Boeing Warns of Possible 737 Altimeter Fault
Frances Fiorino fiorino@aviationweek.com 
Boeing is advising operators of all 737-series and BBJ aircraft to carefully monitor primary flight instruments and not engage autopilot/throttle systems during approach and landing in event of a radio altimeter malfunction.
The manufacturer issued the March 4 Multi-Operator Message (MOM 09-0063-01B) in response to preliminary findings of Dutch investigators in their probe of the Feb. 25 Turkish Airlines 737-800 crash at Amsterdam Schiphol Airport. The MOM includes all 737-series, from -100 to -900.
The initial accident findings, issued the same day, indicate the Dutch are focusing on the link between radio altimeter deviation and the autothrottle system as the possible cause of the landing accident.
Based on flight data recorder readouts, investigators determined that all was normal onboard Flight 1951--until the aircraft reached 1,950 feet during its final approach to Runway 18R with 135 people onboard.
At that point, the radio altimeter readout on the captain's (left) side suddenly switched to -8 feet. The altimeter, interpreting the 737 to be just a few feet above touchdown, commanded the autothrottles to power down and configure for landing.
The "landing gear must go down" warning signal alerted the crew to the problem, but data indicate the signal was not regarded as a problem, according to the prelminary report.
The 737-800 responded to the command, decelerating to minimum flying speed, with a stall warning sounding at 150 meters (490 ft.). The flight crew applied full power, but the aircraft was too low to allow recovery. The aircraft hit the ground traveling at 175 kilometers (94 knots) airspeed about 1.5 km. north of Schiphol. Normal landing speed is 260 km. or 140 kt.
Enormous braking forces--in part caused by the nose wheel becoming embedded in the ground--caused the 737 to break in three parts, with its tail, landing gear and engines detaching. Nine of the 135 onboard were killed, including the flight crew.
The deviation occurred only on the captain's (left) radio altimeter, according to the Dutch initial report, which also states that the malfunction had occurred twice previously, both times during landing. However, it was unclear from the report if the events occurred on the same aircraft.
An AviationWeek check of the FAA database indicates the agency issued no airworthiness directives (AD) on 737-series radio altimeter malfunctions. FAA spokesman Les Dorr confirmed there were no ADs to date, but noted the FAA is monitoring the progress of the Flight 1951 probe and will take whatever action becomes necessary.
Cockpit automation, designed to increase safety by altering pilot workload, may also hold hidden hazards. In late 2008, Airbus issued both an operations telex and engineering bulletins to A330 and A340 operators. They suggested remedial actions to take in the event of flight control system failures as had occurred on two Qantas A330 aircraft in 2008.
Both in-flight incidents, one on Oct. 4, and another on Dec. 27 last year, involved faults in the air data inertial reference system units (ADIRU) which led to erroneous values being fed to primary computers--and in turn, to uncommanded attitudes. In one case, the aircraft plunged 650 ft. in 20 seconds. The Australian Transport Safety Bureau continues to investigate both events.
The Flight 1951 probe may lead to additional corrective actions. The Dutch have asked Boeing to investigate whether the radio altimeter/autothrottle-disconnect procedure outlined in its March 4 MOM might be applicable to in-flight situations.
Photo: AP/Wide World
