What happened
On 14 October 2015, an Aeronca C3, registration G-AEFT, was conducting a private flight from Bodmin to Roche. While flying at approximately 2,500 ft and performing a gentle left orbit, the aircraft's propeller suddenly detached and fell to the left. The pilot immediately initiated emergency procedures and transmitted a distress call to Newquay Airport.
Recognising a nearby landing strip at the Roserrow golf club, the pilot navigated toward the area. After performing a low pass to check for obstacles, the pilot executed a successful forced landing at the strip. The aircraft sustained no damage during the landing, and the pilot escaped with no injuries. A nearby Cessna 152 pilot assisted by relaying the news of the safe landing to Newquay.
The investigation
The investigation focused on the cause of the propeller separation. The propeller was recovered from a building site near Polzeath, where it had landed. An examination of the assembly revealed that the tapered end of the crankshaft had fractured. This failure was noted to be highly similar to a previous incident involving a J.A.Prestwich J99-engined aircraft in 2012.
Historical records indicated that the JAP J99 engine, a licensed version of the Aeronca E-113c, had been subject to a 1939 service memorandum regarding potential cracking at the keyway slot. However, this had never been made a mandatory requirement. The investigation found that the fracture on G-AEFT was a high-cycle fatigue failure, with the origin of the crack appearing to coincide with corrosion pitting on the crankshaft.
Findings
- The propeller detached due to a fatigue crack developing in the tapered end of the crankshaft.
- Corrosion pitting at the origin of the crack likely contributed to the structural failure.
- Previous visual inspections performed by the owner were insufficient to detect the developing crack, suggesting that a dye-penetrant test would have been more effective.
- The existing safety information regarding the engine was not mandatory, leaving the risk of crankshaft failure unaddressed by formal airworthiness directives.