What happened
On 8 November 1998, an Airbus A310, registration 5Y-BFT, was performing a scheduled departure from London Heathrow Airport toward Nairobi. During the initial climb, shortly after reaching approximately 600 feet, the aircraft experienced a sudden drop in the nose and a loss of altitude. The flight crew managed to stabilize the aircraft and continue the departure.
During the climb, the crew attempted to retract the landing gear. This action triggered a Master Warning and an ECAM message indicating the landing gear was not down. Simultaneously, the aircraft's airspeed began to decay toward V2, and engine thrust reduced. The commander responded by pushing the thrust levers forward, re-selecting navigation modes, and lowering the aircraft's nose to regain airspeed. The aircraft subsequently recovered its climb.
The investigation
Investigators examined radar data, which confirmed the aircraft lost roughly 300 feet in altitude and 30 knots in airspeed during the event. Analysis of the Flight Control Computers (FCC) revealed that a loss of the localiser signal due to noise caused the flight director to revert from its takeoff mode to basic heading and vertical speed modes. This reset occurred so rapidly that it may have gone unnoticed by the crew.
Because the aircraft was in a specific phase of takeoff, the autothrottle system was programmed to reduce thrust to maintain V2 speed once the gear was retracted. The investigation found that the crew's attention was diverted by the landing gear warning, leading them to miss the reduction in thrust.
Regarding the flight recorders, the investigation found that the Quick Access Recorder (QAR) cartridge had been incorrectly assembled, which had significantly degraded the recording performance and resulted in the loss of relevant data from the time of the incident.
Findings
- The primary cause of the flight director's abnormal behavior was a mode reversion caused by signal noise affecting the localiser signal, which triggered an automatic reset of the Flight Control Computers.
- The reduction in thrust was a result of the autothrottle re-engaging in speed mode following the gear retraction, while the crew was preoccupied with a landing gear warning.
- The landing gear warning itself was likely triggered by the combination of the gear being in the process of retraction, the aircraft being below 750 feet, and the thrust not being at the required takeoff rating.
- The flight crew's decision to ignore the flight director's pitch-down command prevented a more severe loss of altitude.