What happened
On March 20, 2005, a Socata MS 893 E Rallye Commodor, registration F-BUNB, was conducting a pleasure flight departing from Grenoble Le Versoud. Approximately ten minutes into the flight, while cruising at 4,000 feet, the pilot noticed oil gradually covering the windshield. The pilot immediately initiated a turn to return to the departure airfield, beginning a slow descent at 120 knots and notifying air traffic control of the situation.
During the descent, the pilot observed a drop in oil pressure while oil temperature remained normal. Shortly thereafter, the propeller moved to a low pitch position, and the pitch control became inoperative. Black oil streaks appeared on the engine cowling. As the oil pressure continued to decline, the engine began making abnormal noises before suffering a total power loss. The pilot declared an engine failure and performed an emergency landing on a straight path between two fields, eventually coming to a stop in a field.
The investigation
Investigators examined the Lycoming O-360-A1A engine to determine the cause of the failure. The inspection revealed that the oil supply line to the propeller governor had developed a crack at the elbow, near the oil supply connection. This failure was linked to the condition of the retaining clamp used to secure the line to the oil case.
Further examination of the internal engine components showed that connecting rods number 2 and 3 had broken at the head. The anti-friction metal on the bearings for rods 2 and 3 had melted, and the bearing for rod 4 was also heavily damaged. Metallurgical analysis of the retaining clamp showed that both attachment legs had failed due to fatigue cracking. One leg failed under tensile stress, while the other failed under alternating bilateral bending stress.
Findings
- The primary cause of the oil leak was the failure of a damaged mounting clamp securing the propeller governor oil supply line.
- Fatigue cracking in the clamp's attachment legs was driven by engine vibrations, which may have been exacerbated by impact marks found on the propeller.
- The loss of the clamp allowed the oil line to move, leading to the fracture at the elbow and the subsequent loss of oil pressure.