What happened
On the afternoon of August 25, 2022, a SCHEMPP HIRTH - JANUS CM1 glider, registered F-CVAS, was performing a local flight at Albertville aerodrome. The pilot, accompanied by a passenger, initiated a self-launch from runway 23. During the initial climb, at an altitude of approximately 50 meters, the pilot began a right-hand turn. Shortly after, at a maximum height of about 60 meters, the engine stopped. The glider subsequently lost altitude, pitched down, and collided with the ground near the edge of the A43 motorway. The impact resulted in two fatalities and the destruction of the aircraft.
The investigation
Investigators examined the wreckage and the aircraft's propulsion system, a Rotax 535C engine. The investigation revealed that the engine's electrical generator rotor and the crankshaft were no longer mechanically coupled due to a ruptured keyway. This failure was traced to a loose nut on the conical end of the crankshaft that lacked any thread-locking compound, such as Loctite. The lack of thread lock allowed the nut to loosen progressively, subjecting the keyway to significant shear stress during engine starts until it eventually failed during flight, causing the ignition system to malfunction and the engine to shut down.
Further examination of the engine showed evidence of previous piston seizures and the use of pistons from a different engine model, the Rotax 503. Maintenance records indicated that a 2018 engine rebuild had been performed by a non-approved workshop and utilized these incompatible parts. Additionally, the investigation found that the required 300-hour inspection of pistons and segments had not been documented as completed, and there were inconsistencies in the recorded engine operating hours.
Findings
- The engine shutdown was caused by the rupture of the keyway connecting the generator rotor to the crankshaft.
- The primary cause of the keyway failure was the progressive loosening of the crankshaft nut due to the absence of thread-locking compound.
- The use of non-approved maintenance procedures and parts from different engine models may have contributed to engine instability and previous seizures.
- Inconsistencies in maintenance documentation and the use of an unapproved workshop hindered the proper tracking of the engine's airworthiness.