What happened
On October 3, 1987, Crossair flight LX 733, operated by the Saab SF 340 aircraft HB-AHG, was preparing for departure from Bern-Belp Airport to Paris CDG. The weather conditions were favorable with dry runways and light winds. As the aircraft accelerated down runway 32, a master warning illuminated indicating low oil pressure on the right engine just before reaching the decision speed (V1).
The pilot-in-command (PIC) reacted immediately by pulling back the thrust levers and applying maximum braking. However, during this process, he inadvertently lifted the lever latches before the left throttle reached the flight idle stop. This caused the left engine to remain at a higher thrust setting than the right engine, which was correctly set to ground idle. The resulting asymmetric thrust pushed the aircraft to the right.
The PIC attempted to correct the drift using rudder but applied it in the wrong direction (right instead of left) and failed to use nosewheel steering. Despite heavy braking, the aircraft could not stop within the remaining runway length. It crossed the right edge of the runway with its main landing gear, continued off the pavement with the nose gear, and came to a halt approximately 90 meters beyond the runway end in the grass. No injuries were reported among the crew or passengers.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the technical status of the aircraft, the operational procedures, and the cockpit voice recorder (CVR) data. The investigation revealed that the oil pressure warning was caused by a short circuit in the right nacelle due to chafing against the engine cowling, exacerbated by vibrations during takeoff.
Maintenance records showed that this specific fault had been reported four times since August 17, 1987, but remained unresolved. Crucially, the crew was not informed of these previous faults because Crossair did not yet utilize a "Hold Item List" (HIL) system to communicate deferred defects to flight crews. Additionally, a service bulletin designed to prevent such false warnings by installing a separate oil pressure sensor had not yet been implemented.
Data from the digital flight data recorder (DFDR) confirmed that the rejected takeoff was initiated two seconds after the warning appeared. The deceleration rate was significantly lower than expected for a maximum braking scenario, indicating insufficient brake pressure application initially. The CVR analysis showed that the PIC did not verbally announce "stop" or "rejected," and the co-pilot only said "go, go" when it became clear the aircraft would overrun.
Findings
The primary cause of the accident was the pilot's failure to execute the correct rejected takeoff procedure. Specifically:
- Incorrect Thrust Lever Management: The PIC lifted the throttle latches too early, locking the left engine in a position above flight idle while the right engine was at ground idle. This created significant asymmetric thrust.
- Lack of Nosewheel Steering: The pilot did not use nosewheel steering to maintain directional control, relying solely on rudder which was insufficient and incorrectly applied.
- Insufficient Mental Preparation: The takeoff briefing had not adequately reviewed the specific actions required for a rejected takeoff, particularly regarding the management of thrust levers in beta mode.
Contributing factors included:
- The unresolved oil pressure fault, which created a stressful environment for the crew.
- The absence of a formal system (HIL) to ensure the crew was aware of known aircraft defects.
- A takeoff briefing that failed to emphasize the critical steps for handling engine failures or warnings before V1.
Safety action
The investigation highlighted the need for improved communication of deferred maintenance items to flight crews. Crossair subsequently implemented the Hold Item List (HIL) system, which became mandatory for all Swiss aircraft on July 1, 1988. This ensured that any known defects affecting flight safety were explicitly communicated to the crew before departure.
Additionally, the report suggested that training programs should reinforce the mental preparation required for rejected takeoffs, ensuring that both pilots are clear on their roles and the specific mechanical actions needed to manage thrust levers correctly under pressure.