What happened
On 25 February 2006, a DHC-1 Chipmunk 22A, registration G-AORW, was returning to Prestwick from a flight to Islay. While the aircraft was approximately 0.5 miles offshore, the pilot noted a slight vibration through the airframe, though all engine temperatures and pressures appeared normal. Shortly after, the engine stopped without warning.
Because the aircraft was at too low an altitude to reach the runway and a built-up area lay directly ahead, the pilot performed a forced landing on an unoccupied section of Prestably Beach. There were no fatalities and no injuries to the pilot or the passenger during the incident.
The investigation
Following the landing, the aircraft was recovered to a hangar. An examination of the engine revealed that the crankshaft had failed near the No 2 piston location. Investigators found that the crankshaft installed in the engine was an older type that had been superseded by Mod 2602.
Historical data from the engine manufacturer, Bristol Siddeley Engines Ltd, indicated that older crankshafts were susceptible to cracking when subjected to periods of abnormal operation, such as aerobatics. While the specific operational history of this component could not be fully verified, the aircraft had previously been used for air racing and aerobatic manoeuvres. Laboratory analysis confirmed that the failure was caused by a high cycle fatigue process, though the exact initiation point was obscured by smearing on the fracture surface.
Findings
- The engine suffered a crankshaft failure near the No 2 piston.
- The crankshaft was an older model that lacked the material and surface hardening introduced by later modifications.
- The failure was caused by crack progression through high cycle fatigue.
- The aircraft's previous use in aerobatics and air racing may have contributed to the increased bending loads that led to the failure.