What happened
On 17 June 2006, a Raj Hamsa X’Air microlight, registration G-CDHO, was conducting a training flight near the north bank of the River Thames, close to Tilbury Docks. During the flight, the aircraft's engine experienced a sudden failure when the magneto rotor separated from the engine. This separation caused significant damage to the propeller blades and resulted in the immediate loss of engine power.
The instructor, who was supervising the student pilot, managed to maneuver the aircraft into a gliding attitude. The crew successfully performed an uneventful forced landing in a large field adjacent to the river. Aside from damage to the engine and propeller, the aircraft remained undamaged, and both occupants escaped without injury.
The investigation
Following the incident, the AAIB investigated the engine and airframe. Examination of the engine revealed that the magneto rotor had been operating out of alignment, evidenced by heavy rubbing marks on the magneto coil formers.
Investigators also examined the gearbox casing and found damage to a stiffening rib. The condition of this rib suggested that the engine had likely tipped forward onto a work surface at some point during a period when it was being handled without the airframe attached. Metallurgical analysis of the crankshaft stub shaft showed a pre-existing crack that had spread through 65% of the shaft's cross-section via a torsional fatigue mechanism before the final failure.
Findings
- The engine failure was caused by the separation of the magneto rotor due to the failure of the crankshaft stub shaft.
- The stub shaft failed because of a crack that progressed through a torsional fatigue mechanism.
- Torsional loads on the shaft were significantly increased because the magneto rotor was operating out of alignment, causing it to contact the coil formers.
- The misalignment was likely caused by an impact on the magneto rotor, potentially occurring when the engine was being handled 'off-wing' following a previous workshop visit.
- The engine had previously undergone a crankshaft replacement in July 2005, and the failure occurred after only approximately two hours of operation following the most recent installation.