What happened
On 11 June 2010, an Aerola Alatus-M motor glider, registration G-CFDT, was conducting a private flight from Davidstow Airfield in Cornwall. Following a successful powered takeoff, the pilot began reducing engine power while climbing to 1,000 ft. During this phase, the aircraft experienced intense vibrations accompanied by a loud bang and a sudden increase in engine RPM.
Upon inspection, the pilot discovered that the pylon-mounted propeller had separated from the engine. The separation caused the propeller to strike the engine bay doors, resulting in damage to the right wing panel and the fuselage. Despite the incident, the pilot managed an uneventful glide approach and landed the aircraft safely. There were no injuries to the pilot.
The investigation
Investigators examined the remains of the propeller mounting structure and found the remains of a 10 mm mounting bolt. Analysis of the bolt shank revealed a fracture surface consistent with a bending overload, with no evidence of fatigue crack propagation. The propeller shaft itself showed signs of mechanical wear and scoring on its inner diameter.
Furthermore, the propeller lock was found to be deformed, and its rubber end cap was missing. Paint transfer between the propeller and the lock indicated that the lock was not in the fully deployed position when it was struck by the rotating propeller. The investigation also noted that the aircraft's previous owner had modified the engine bay doors to provide more clearance following a prior propeller strike incident.
Findings
- The primary cause of the propeller separation was the failure of the aluminium propeller shaft due to bending overload.
- It is probable that the shaft had failed during a previous propeller strike event, which likely increased the likelihood of the propeller moving out of its plane of rotation.
- The increased vibration during the power reduction phase caused the propeller lock to move into the arc of the propeller, leading to the impact that further destabilized the assembly.
- The physical separation of the propeller was facilitated by the failure of the retaining bolt and the subsequent impact with the engine bay doors.
Safety action
Following this accident and reports of similar shaft failures in other Alatus-M aircraft, the manufacturer has developed a new stainless steel shaft to be used in all new gliders. The manufacturer also recommends that existing owners replace the aluminium shafts with the stainless steel version and 'cut back' the engine bay doors to ensure sufficient clearance.