Incident Overview
A pilot observed an oil leak around the propeller hub during takeoff. The aircraft returned to the airport, where maintenance personnel found a cracked propeller governor oil tube fitting. The part was replaced, and the flight continued. During a later refueling stop, the fitting was checked and found not leaking.
Second Occurrence and Emergency Landing
While cruising over mountainous terrain, the pilot again noticed an oil leak. Air traffic control provided vectors to a nearby airport. Before reaching the airport, the engine stopped. The pilot attempted an emergency landing, during which the aircraft struck a tree.
Investigation Findings
Examination of the engine revealed that the oil line fitting had cracked again. Metallurgical examination determined that both fittings (the original and the replacement) failed due to low-stress, high-cycle fatigue induced by a high mean tensile stress. The oil line was found to be unrestrained, although the service manual specifies two clamps to secure it. The resulting vibration from the unrestrained line created the stress loads that caused the fittings to fail.
Probable Cause
The failure of the oil line fittings was due to low-stress, high-cycle fatigue induced by a high mean tensile stress, caused by vibration from the unrestrained oil line that should have been secured with two clamps as per the service manual.