What happened
On 9 February 2004, a Falcon 900EX, registration VP-BMS, was operating a private flight from Kilimanjaro to London (Luton). The flight crew had previously noted an intermittent fault with a hydraulic pump and, believing the Minimum Equipment List (MEL) permitted departure, decided to defer repairs. During the flight, the crew observed a reduction in the No 1 hydraulic system fluid quantity, which eventually reached zero, though system pressure remained normal.
During the approach to Luton, the loss of the No 1 hydraulic system caused the leading-edge slats to partially retract. The crew attempted to extend the landing gear using both the normal and emergency systems, but the gear position indicators remained red, showing no green 'down and locked' lights. The crew subsequently diverted to London (Stansted) Airport.
While landing on Runway 05 at Stansted, the aircraft was configured for a full flap landing. During the landing roll, the right main landing gear partially retracted. This caused the aircraft to veer to the right, eventually leaving the paved surface and coming to rest in the grass, approximately 139 metres from the runway centreline. There were no fatalities or injuries among the two crew members and two passengers.
The investigation
The AAIB investigation examined the aircraft's hydraulic systems, the flight recorders, and the procedures used by the crew. Investigators found that a leak in the No 1 hydraulic pump had caused the loss of fluid. While the crew attempted an emergency gear extension by pulling the manual release handles, the investigation determined that the aerodynamic forces generated by the crew's maneuvers were insufficient to engage the mechanical downlocks.
Analysis of the flight data recorder (FDR) showed that the rudder inputs used to create a sideslip were not substantial enough to lock the main gear. Furthermore, the investigation highlighted that the crew relied on a training checklist rather than the approved Flight Manual (AFM). The training checklist was found to be less detailed regarding the time required for gear locking and contained instructions that were not entirely consistent with the aircraft's actual physical requirements.
Findings
- The crew misinterpreted the Minimum Equipment List, incorrectly believing they could depart with an unserviceable hydraulic pump.
- The loss of the No 1 hydraulic system was caused by a leak in the No 1 hydraulic pump.
- The landing gear failed to lock down because the aerodynamic loads generated during the sideslip and acceleration were insufficient to engage the mechanical downlocks.
- The crew's reliance on a training checklist, which was marked 'FOR TRAINING PURPOSES ONLY', contributed to the failure to follow the correct procedures for emergency gear extension.
- Simulator training had provided the crew with an unrealistic expectation of how little rudder input was needed to lock the gear.
Safety action
- A recommendation was made to Dassault Aviation to review the Master Minimum Equipment List to ensure the distinction between pump failures and caution light failures is unambiguous.
- A recommendation was made to review the hydraulic system indications following a system failure.
- Recommendations were made to FlightSafety International regarding the accuracy of training documentation and the fidelity of flight simulators in representing the physical forces required for emergency gear extension.