What happened
On 28 August 1999, a T21 Slingsby Turbo, registration BGA 326, was conducting a private flight over Louth, Lincolnshire. The aircraft, which had been airborne for approximately two hours, was in the process of descending toward a farm strip when the pilot reduced engine power. During this maneuver, a loud noise occurred, immediately followed by a complete loss of engine power.
Upon recognizing the engine failure, the pilot noted that the aircraft's fabric skin was flapping loose on the rear fuselage. Given the proximity to the destination farm strip, the pilot elected to perform an emergency landing. The aircraft landed without further incident, and there were no injuries to the two crew members on board.
The investigation
The investigation focused on the mechanical failure of the engine's propulsion components. This specific aircraft had been modified with a two-stroke piston engine mounted above the wing, a configuration that permitted sustained flight but lacked self-launching capabilities. The propulsion system utilized a propeller bolted to a layshaft, with a reduction gearwheel driven by a toothed rubber belt.
Post-landing inspections determined that the propeller and the reduction gearwheel had become detached from the layshaft. This separation was caused by the failure of the single bolt used to secure the assembly.
Findings
- The primary cause of the engine power loss was the failure of the single securing bolt on the propeller drive.
- Evidence suggested the bolt had not been properly seated, appearing to have "bottomed out" within its threaded hole during installation.
- This improper installation likely subjected the bolt to abnormal loads and excessive wear during flight operations, leading to its eventual failure.