What happened
During a flight training session involving touch-and-go landings, an Aerocruiser advanced ultralight experienced a mechanical failure shortly after departing from a private runway at the École de pilotage de Lavaltrie in Quebec. While flying at an altitude of approximately 300 feet, one of the blades of the two-blade wooden propeller separated from the hub.
In response to the failure, the pilot instructor shut down the engine and performed an emergency landing in an unprepared field. During the landing sequence, the aircraft struck a tree and overturned. The two occupants, an instructor and a student, sustained serious injuries during the impact on the ground.
The investigation
The investigation focused on the condition of the aircraft's Rotax 912 engine and its GSC International Inc. wooden propeller. The propeller had been in service for just over 300 hours. Investigators discovered that the operator had previously machined the propeller hub down by 0.010 inch to increase torque at the blade roots, a measure taken to address issues with pitch adjustment.
Laboratory analysis of the recovered blade revealed that the failure was a progressive one. Two specific anomalies were identified on the broken blade: a crack in the wood at the root and the presence of black adhesive tape around the urethane capsule at the blade root, which had been used to assist with pitch adjustment. Additionally, the non-fractured blade exhibited a crack at the exact same location at its root, suggesting both blades were subject to the same stress or damage mechanism.
Findings
- The propeller was manufactured to the required standards and possessed sufficient static load strength for its intended use.
- The engine did not exceed the manufacturer's specified overspeed limits during the incident.
- The blade separation was caused by a progressive failure originating from undetermined pre-existing damage at the blade roots.
- This pre-existing damage may have been caused by the blades striking a soft object, such as snow or water, or by improper torquing of the blade roots during installation.