What happened
On 10 May 2004, a Schweizer 269C helicopter, registration G-TASS, was conducting a mountain flying instructional flight near Bowscale Tarn, Cumbria. The flight, involving a pilot and an instructor, was part of a training session in the Lake District. While flying along a shoulder of high ground, the engine RPM dropped rapidly to zero, resulting in a sudden engine failure.
As the aircraft was at a low altitude, there was insufficient height to reach the valley floor. The instructor took control to establish an autorotation profile, attempting to reach a grassy slope. During the descent, the aircraft struck a bracken-covered protrusion, causing the tail rotor to hit the ground and the main rotor to strike the tail boom. The helicopter then hit the hillside with significant force and rolled onto its right side. Both crew members sustained serious injuries.
The investigation
The AAIB examined the wreckage and the engine components to identify the cause of the power loss. While the engine itself showed no mechanical defects and the fuel system appeared functional, investigators focused on the mixture control mechanism. They discovered that the mixture control cable had become disconnected from the lever on the fuel injector servo.
Microscopic analysis of the cable end revealed that it had been cut too short with wire cutters during the attachment process. This error significantly weakened the connection. Testing showed that while a standard attachment could withstand much higher loads, this specific connection failed under a force of only 5 lbs. The investigation also noted that the attachment method used was an older design that had since been superseded by a more secure version in newer aircraft.
Findings
- The engine failure occurred while the aircraft was unable to reach a suitable landing site.
- The mixture control cable had been improperly attached, having been cut too short, which made the connection susceptible to release under light loads.
- It is possible that engine or airframe vibrations caused the weakened cable to pull through the lever, leading to insufficient fuel flow.
- There was no evidence of fuel contamination or internal engine mechanical failure.