What happened
On 19 November 2001, a Westland Scout AH1, registration G-BXSL, was performing a private flight near Cambridge. After departing Cambridge Airport following a refuelling stop, the aircraft was cruising at 1,500 feet when the pilot heard a noise resembling a compressor stall, accompanied by a drop in rotor RPM.
The pilot responded by lowering the collective lever and reducing the throttle to ground idle. A 180-degree turn into the wind was performed to execute a run-on autorotational landing into a field approximately 8 miles south-west of Cambridge. As the helicopter decelerated, the landing gear skids sank into the soft ground, causing the aircraft to tip forward. This motion caused the rotor blades to strike the ground and the tail boom, resulting in the aircraft resting on its right side. There were no injuries to the two crew members on board, but the helicopter was damaged beyond economic repair.
The investigation
Investigators examined the wreckage and the engine components. While the compressor and turbine appeared undamaged, an examination of the fuel system revealed significant issues. A fuel sample taken from the low-pressure fuel filter contained a large amount of cloudy, yellow-stained water.
Further testing of the centre tank sump drain by the AAIB confirmed the presence of water and fungal fragments. While the fuel from the airport and farm bowsers was found to be within acceptable limits, the water in the aircraft's centre sump had likely been present for a considerable period. The investigation also looked into the maintenance protocols for the aircraft, which was being operated under a Permit to Fly.
Findings
- The engine failure was likely caused by water contamination of the fuel.
- The water in the fuel system contained fungal fragments, indicating it had been present for some time.
- Due to a maintenance oversight, the specific requirement to drain and check the centre tank sump for water was not included in the pilot's flight reference cards or the maintenance programme.
- The physical difficulty of accessing the centre sump drain and the lack of a specialized sampling tool discouraged effective sampling.
Safety action
Following the investigation, the maintenance organisation reviewed its procedures to include fuel contamination checks in the pilot's flight reference cards. Technical log sheets were also updated to ensure the results of fuel sampling from both the boost pumps and the centre tank sump are formally recorded.