What happened
On May 16, 2016, a Cirrus SR2/2, registration N19BV, was conducting a private VFR flight from Belle-Ile-en-Mer to Lille Lesquin. The aircraft was occupied by the pilot and one passenger. During the flight, the pilot descended to 1,400 ft to fly beneath a cloud layer. Shortly after, the pilot noticed a battery warning light and decided to divert to Amiens.
As the aircraft approached Amiens, the engine power decreased and eventually failed. During the descent, the aircraft's emergency parachute was deployed. The aircraft struck the ground at 13:03 in Ailly sur Somme. The impact resulted in two serious injuries and the destruction of the aircraft. The wreckage was found in a field with a high vertical impact component, indicating the aircraft struck the ground with a nose-down attitude.
The investigation
The BEA examined the wreckage, the engine, and the aircraft's Avidyne EX5000 avionics suite. Data from the MultiFunction Display (MFD) revealed that a battery voltage drop occurred at 12:33, which the pilot did not immediately detect. The investigation also focused on the engine's Continental IO-550-N components, specifically the alternator assembly.
Investigators found that the alternator driven gear had been thrown against the engine housing, puncturing an oil duct and spraying oil across the fuselage and canopy. Analysis of the alternator shaft revealed a fatigue fracture. Furthermore, the investigation scrutinized maintenance records, noting that the alternator had been serviced in 2011, but the required 500-hour/two-year inspections had not been performed since.
Findings
- The primary cause of the engine failure was a fatigue fracture of the alternator shaft.
- The fracture was caused by abnormal loads at the mechanical coupling, stemming from the incorrect installation of a thrust washer; the washer's copper-colored face was installed toward the nut rather than the alternator.
- This improper assembly led to excessive wear and a loss of tightness in the assembly.
- The failure of the alternator caused a sudden reduction in electrical voltage, eventually leading to the engine power loss.
- The emergency parachute was deployed at an estimated height of less than 300 ft, which was insufficient to allow the aircraft to recover a level attitude before impact.