What happened
On 4 June 2015, a Kolb Twinstar Mk III Xtra, registration G-CGZA, was conducting a private flight over Swinefleet Common in Yorkshire. During the flight, the aircraft's propeller unexpectedly detached from the engine. The loss of the propeller caused subsequent damage to the aircraft's tailboom. Despite the mechanical failure, the pilot managed to execute an emergency landing without further incident. There were no injuries to the pilot during the event.
The investigation
Investigators examined the remains of the engine and the propeller components. Although the propeller itself was not recovered, the pilot provided photographs of the fracture faces of the propeller retaining bolt shanks. These bolts had remained attached to the engine crankshaft. Analysis of these fracture surfaces revealed clear evidence of high cycle fatigue as the mechanism of failure.
Findings
- The propeller separated from the engine during flight without prior warning.
- The separation caused structural damage to the tailboom.
- The primary cause of the component failure was identified as high cycle fatigue within the propeller retaining bolt shanks.