What happened
On 29 July 2001, a Streak Shadow SA-SLA, registration G-MGPH, was conducting a private flight from Norton to North Weald. While cruising at approximately 1,000 feet near Cockfield, Suffolk, the pilot heard a loud bang, followed immediately by an engine overspeed. The pilot promptly shut down the engine and performed a forced landing in a nearby wheat field.
During the landing sequence, the aircraft's nosewheel struck a tractor rut, causing it to detach, and the aircraft slid on its nose before coming to a halt. There were no injuries to the pilot or the passenger. Upon inspection, the crew discovered the propeller had separated from the engine, and the left flap had sustained damage. One propeller blade was also missing.
The investigation
The AAIB examined the engine, gearbox, and failed components. The engine and gearbox showed no signs of shock loading or prior damage. Investigators found the broken shanks of the six propeller attachment bolts still located in the gearbox flange.
Metallurgical analysis of the bolts revealed that several had failed due to fatigue caused by bending. This bending indicated that significant repetitive side loads were being applied to the bolts. Such loads should not occur if the propeller is correctly seated and clamped against the gearbox flange. The investigation also noted that the specific mounting method used on this factory-built aircraft—utilising threaded holes and stiff nuts—could lead to a false torque reading, where the nut appears tight even if the propeller is not properly compressed against the flange.
Findings
- The propeller separated from the gearbox flange following the progressive fatigue failure of the mounting bolts.
- The primary cause was likely insufficient clamping force between the propeller hub and the gearbox flange, caused by incorrect torque tightening or loosening during service.
- The specific assembly method used on G-MGPH allowed for a situation where the mounting bolts could appear correctly torqued without the propeller being fully seated.
- The remaining bolts failed due to overload once the structural integrity of the assembly was compromised.