What happened
On Saturday, April 20, 2002, a Robin DR 400-140 B aircraft, registered F-GTPK, was conducting a cross-country flight from Toulouse Lasbordes toward Biarritz. The aircraft was carrying a pilot and three passengers. Approximately 50 minutes into the flight, while cruising at 2,500 feet with the engine running at 2,500 RPM, the pilot experienced a loud explosion followed by moderate smoke entering the cabin. The engine lost all power, though the propeller continued to rotate due to the relative wind. The pilot successfully performed a forced landing in an uncultivated field near Vidou. There were no fatalities or injuries during the incident.
The investigation
The investigation focused on the mechanical failure of the Lycoming O-320-D2A engine, which had a total of 1,774 flight hours. Investigators examined the engine components and reviewed recent maintenance history. It was noted that during a 50-hour inspection on March 21, 2002, metallic particles were found in the oil filter and strainer. Following Lycoming Service Instructions, a follow-up inspection was conducted on April 18, 2002, after 10 additional flight hours. During that inspection, the four cylinders were removed to check the piston pin plugs, as particles had reappeared. Some plugs showed abnormal wear and were replaced. The aircraft had flown only 4 hours and 15 minutes since that maintenance work.
Findings
Technical analysis of the engine revealed that the failure was caused by the breakage of the connecting rod big end for rod number 4, followed by the breakage of the big end for rod number 3. The investigation determined that the initial failure of the lower big end of the rod number 4 cap originated from a fatigue crack at the junction of the rod body and the machining fillet. The area where the crack started exhibited a rough machining finish characterized by micro-tearing of the metal.