What happened
On September 18, 2006, an AS 350 BA Ecureuil, registration F-GUPF, was performing a non-scheduled public transport flight between Paris/Issy-les-Moulineaux and Fontenay/Trésigny. While flying over the A4 motorway, the pilot noticed red hydraulic alarms and a corresponding audible warning, indicating a drop in pressure in the flight control hydraulic assistance system.
Following established emergency procedures, the pilot reduced speed and diverted to Lognes aerodrome. After requesting and receiving permission to approach the grass runway, the pilot initially prepared for a running landing. However, during the final approach, the pilot decided to deviate from the runway to land in a grassy area between the paved runway and the north taxiway to avoid blocking the active runway. During this lateral translation, the pilot lost control of the aircraft. The main rotor blades, the nose, and the skids subsequently struck the paved runway surface, resulting in the destruction of the aircraft.
One passenger was ejected from the aircraft during the impact because they were not restrained; all other occupants sustained minor injuries.
The investigation
The investigation focused on the mechanical failure and the pilot's decision-making process. Investigators examined the wreckage and found that the drive belt for the hydraulic pump had broken. While the cause of the belt failure could not be determined, the belt was a flat-type belt rather than the newer, more reliable Poly V-type kit.
Technical examinations of the engine, flight controls, and hydraulic pump showed no other malfunctions. The investigation also reviewed the pilot's training and the manufacturer's updated safety documentation. It was noted that while the aircraft's flight manual contained updated warnings regarding the dangers of low-speed maneuvers without hydraulic assistance, the operator's manual of operations had not been updated to reflect these critical changes, leaving the pilot unaware of the specific risks associated with the maneuver performed.
Findings
- The primary cause of the hydraulic failure was the rupture of the hydraulic pump drive belt.
- The pilot's decision to perform low-speed maneuvers without hydraulic assistance was the direct cause of the loss of control.
- Several aggravating factors contributed to the accident, including the high aircraft mass, the transition toward a hover, and a tailwind component.
- The pilot's confidence in managing the failure was bolstered by a successful approach phase, but the suddenness of the control instability during the low-speed translation led to overcontrolling the aircraft.
- The pilot was unaware of the manufacturer's specific warnings regarding the risks of low-speed flight in the absence of hydraulic assistance due to a lack of updated information in the company's operating manual.