What happened
On June 11, 2011, a Robin DR 400-125, registered F-BVCY, was performing a training flight departing from the Moulins Montbeugny aerodrome. While climbing to an altitude of 2,900 feet, approximately 2 nautical miles from the airfield, the instructor noted abnormal noises and vibrations accompanied by a loss of engine power. The instructor took control of the aircraft, performed a 180-degree turn, and executed an emergency landing on the same runway from which the flight had departed.
The investigation
The investigation focused on the Lycoming (O-2ASS J2A) engine, which had accumulated 809 flight hours since its last overhaul in 2009. During that overhaul, new cylinder blocks had been installed to comply with EASA SIB 2009-24, which addressed previous issues with cylinder base cracks. Initial inspections confirmed that the fuel supply and magnetos were functioning correctly.
Further examination of the engine revealed a significant crack in the left crankcase half near the base of cylinder number 2. This crack had caused an oil leak. While the cylinder bases themselves were intact, investigators found evidence of fretting on the internal mating surfaces of the crankcase at the crankshaft bearing locations.
Findings
- The primary cause of the engine power loss and vibrations was the deterioration of the crankshaft crankcase near the base of the second cylinder.
- The crack in the crankcase half led to an oil leak and subsequent mechanical instability.
- The presence of fretting on the internal surfaces suggests insufficient tightening of the through-bolts, potentially occurring during the previous maintenance session when the cylinders were replaced.
- The instructor's prompt response prevented more severe consequences during the flight.