What happened
On June 14, 2014, a Robin DR400-120, registration F-GCRU, was performing a flight between Lognes and Nangis for personal purposes. During the attempt to depart from Nangis for the return leg, the engine failed to start, and the pilot observed the electrical instruments and dashboard lights extinguishing. The propeller appeared to be stuck during several start attempts. Suspecting a battery failure, the pilot requested a ground power unit from the aero club. After a club pilot arrived and manually moved the propeller, the engine started normally, and the crew departed for L quite.
Approximately five minutes south of Lognes, while cruising at 1,150 feet, the engine RPM suddenly dropped by 250. Although the pilot maintained level flight at full power, the engine eventually began to vibrate before failing completely and the propeller seizing. The pilot performed a forced landing in a nearby rapeseed field using full flaps. As the aircraft decelerated through the tall crops, the momentum caused the plane to nose over and invert onto its back. The pilot and two passengers were uninjured, though they had to break a rear window to evacuate the aircraft.
The investigation
The investigation focused on the mechanical state of the Lycoming O-235 engine and the decision-making process leading to the flight. Investigators examined the engine components following its disassembly. The engine had undergone its last overhaul in May 2011 and had accumulated 1,506 hours since that time.
Technical analysis of the engine revealed that the number 3 connecting rod had detached from the crankshaft, subsequently piercing the right half of the crankcase. The rod cap was broken, and the associated half-bearings were destroyed. Furthermore, the number 3 main bearing half-bearings were damaged, with clogged oil passages and destroyed locating pins. While the lubrication system and oil quality showed no anomalies, the exact sequence of these internal failures could not be determined.
Findings
- The engine failure was caused by the detachment of the number 3 connecting rod from the crankshaft and subsequent damage to the central main bearing.
- The pilot elected to proceed with the flight despite the unresolved engine starting difficulties at Nangis.
- The late hour and the limited time remaining before sunset likely influenced the decision to depart without a mechanic present to verify the aircraft's airworthiness.
- The height of the rapeseed crops in the landing field contributed to the aircraft flipping onto its back during the landing roll.