What happened
On 16 April 2012, a Hawker Cygnet replica, registration G-EBJI, was conducting a flight test programme at Old Warden Aerodrome in Bedfordshire. The aircraft, which was operating under a Light Aircraft Association (LAA) Permit Flight Release Certificate, was approximately two and a half hours into its testing phase.
While the pilot was flying on the left-hand downwind leg of Runway 2-1 at an altitude of 800 feet, a momentary vibration was felt, followed by the propeller detaching from the engine and falling away from the aircraft. The pilot immediately closed the throttle, switched the ignition to the OFF position, and executed a successful forced landing on the runway. There were no injuries to the pilot, though the propeller was not recovered following an extensive search of the area.
The investigation
The investigation focused on the mechanical failure of the engine's crankshaft. The aircraft was powered by a JAP J.99 piston engine, which had been recently rebuilt with several new components and a custom-built wooden propeller.
Metallurgical examination of the fractured crankshaft revealed that the failure originated from a machined keyway feature at the rear end of the crankshaft. The presence of beachmarks and heavy discoloration on the fracture surface indicated that the fatigue crack had been developing for a significant period. While the engine's history included two propeller-related incidents in the mid-1960s, it could not be determined if these or another unrecorded event caused the initial crack.
Furthermore, the investigation uncovered a 1939 Aeronca Service Memorandum (M-36) which had previously addressed similar fatigue cracking in related engine series and recommended inspections at 25-hour intervals. However, this memorandum was not listed in the LAA SPARS document used by inspectors, meaning the specific area of the crankshaft was not flagged for inspection during the engine's rebuild.
Findings
- The propeller detached because the engine crankshaft suffered a fatigue fracture.
- The crack initiated at the rear end of the machined keyway on the crankshaft.
- The fracture surface showed evidence that the crack had been present for a long duration.
- The LAA SPARS reference document did not include the 1939 Aeronca Service Memorandum M-36, which would have directed inspectors to check this specific area for cracks.