What happened
On 24 August 2002, a Miles M-65 Gemini 1A, registration G-AKKH, was performing a private local flight over Old Warden Aerodrome in Bedfordshire. While the pilot was descending at a low throttle setting, a loud thumping sound was heard. The pilot then witnessed the left propeller detach from the engine and strike the aircraft's nose before flying away. Despite the loss of the propeller, the pilot was able to return the aircraft to the airfield and land without further incident. There were no fatalities and no injuries to the pilot or the passenger on board.
The investigation
Investigators recovered the propeller and hub sleeve from a nearby field. While the hub remained attached to the crankshaft, the propeller retaining bolts were largely missing, though two bolt heads were found still attached to the aircraft's cowling. Metallurgical analysis of these two recovered bolt heads revealed that both had suffered fatigue due to reverse bending, with the damage originating in the radius between the bolt shank and the head. The patterns of damage were consistent with the bolts being insufficiently tightened.
Further examination of the propeller boss and hub showed evidence of 'fidgeting'—a process where the propeller moves against the hub. This caused localized tearing of the friction disc and polishing/hammering marks on the bolt holes. The investigation also highlighted significant ambiguity in historical maintenance documentation. While some manuals suggested the use of Belleville washers to stabilize clamping force, other parts lists and the actual configuration of the aircraft did not include them. Without these washers, the clamping force is highly susceptible to changes in atmospheric conditions, as the moisture content of the wooden propeller affects its thickness.
Findings
- The left propeller separated from the Miles M-65 Gemini 1A because the propeller attachment bolts had failed due to fatigue.
- The fatigue was caused by cyclic loading resulting from insufficient pre-load on the bolts.
- The lack of Belleville washers in the assembly meant the clamping force was vulnerable to fluctuations in humidity and temperature.
- Maintenance was performed in a warm, dry hangar, which likely resulted in the bolts being tightened while the wood was in a state that would later lead to a reduction in pre-load as the wood dried or environmental conditions changed.
- The current Light Aircraft Maintenance Schedule (LAMS) allows for much longer inspection intervals than the original 1946 manufacturer's manual, which required checks every 10 hours.