Accident Overview
While in cruise flight, the engine experienced a sudden partial loss of power accompanied by "clanging" noises. The engine continued to operate at a maximum of 900–1,100 revolutions per minute (rpm), but insufficient power was available to maintain altitude. The pilot contacted the tower of a nearby airport but determined that the runway could not be reached. Due to power lines and a blind curve on a paved road, the pilot opted to land adjacent to the road. The airplane impacted a ditch and came to rest inverted.
Engine History and Condition
The engine had been overhauled 2.0 flight hours before the accident. A field overhaul had been performed due to a bent pushrod, stuck valve, and broken lifter for the number 3 cylinder. During that overhaul, all four rocker arms had been magnetic particle inspected.
Metallurgical Examination
Examination of the engine revealed that the number 3 cylinder exhaust rocker arm had separated into two pieces. A metallurgical examination by a Safety Board metallurgist determined that the number 3 exhaust rocker arm had fractured around the central pivot hole due to fatigue. The fatigue originated at the outer surface of the oil hole. The corners of the hole were sharp and had not been deburred, which increases the likelihood of fatigue cracking. The outer surface of the fatigued area measured 0.31 inches long on the left side of the oil hole and 0.21 inches long on the right side.
Inspection Findings
The fractured exhaust rocker arm had been magnetic particle inspected for cracks during the field overhaul. The Safety Board metallurgist noted staining on the cracks from the solution used during the magnetic particle inspection. Based on the staining, the cracks would have been detectable in size (approximately 0.2 inches) at the time of the inspection.