PIPER PA-32-300 — Brentwood, TN

What happened

The airplane was in cruise flight about 5,400 ft when the pilot declared an emergency due to a partial loss of engine power. The pilot and controller discussed options for the forced landing, but ultimately the pilot announced that the engine was “stopped,” and that the airplane was “heading down.”

A witness described the airplane maneuvering about 75 ft above the ground before it impacted wires, crossed the road on which he was driving, and came to rest in trees on the opposite side of the road. He reported that there was no engine noise and that the airplane made only a “whistling” sound. After the airplane came to rest suspended in trees, the witness watched as it caught fire, “shifted,” then fell from the trees before it exploded and became engulfed in flames.

Examination of the engine at the accident site revealed that the crankshaft could not be rotated by hand, and the oil suction screen was completely occluded with debris that comprised metallic particles and pieces of rubber material consistent in appearance with pieces of rubber magneto drive cushion (bumper). Further examination revealed that the crankshaft was fractured at the rear edge of the No. 6 connecting rod journal. The No. 6 connecting rod and rod cap were damaged, and the No. 6 rod bearing was extruded.

Review of maintenance records indicated that the engine underwent a field overhaul about 10 years before the accident, during which the crankshaft was replaced. At the time of the accident, the engine had accrued about 101 hours since overhaul.

Examination of the crankshaft fracture revealed a fatigue crack that initiated at the surface in the fillet radius between the No. 6 connecting rod journal and the forward cheek. Similar fatigue cracks were observed at the forward end of the connecting rod journal with features consistent with excessive loads induced during contact between the connecting rods and the engine case. The No. 6 rod bearing was likely destroyed due to interruption of the oil to the bearing from fatigue crack propagation.

The crankshaft was made obsolete 30 years before the engine’s most recent overhaul, and although it was still authorized for use in multiple variants within this series of engines, it was not authorized for use in the accident engine model. Additionally, the connecting rod bearings installed on the accident engine were not authorized for use in the accident engine model.

It is likely that the use of a crankshaft that was not approved for use in the accident engine make and model resulted in abnormal contact between the connecting rods and their journals, imparting stress to the crankshaft and resulting in its failure. It is possible that the renitriding of the crankshaft may have affected its ability to tolerate high loading and contributed to the failure. The observed nitride layer was thicker than the original specification, and the core hardness was lower than the specification. Combined, these factors increased the potential to generate cracks.

While the oil filter screen was occluded with material consistent with a magneto bumper, there was no evidence of general oil starvation elsewhere within the engine, which would be consistent with an occluded screen; therefore, it is likely that the magneto bumper dropped into the engine due to large vibrations following the crankshaft failure, and broken pieces then became trapped in the oil screen.

Further examination revealed that the connecting rod in the number 4 position was labeled 3, and the one in the number 3 position was labeled 4. Although this likely did not contribute to the accident, it is another indication of improper maintenance practices.

An editorial "what led to it / how to avoid it" analysis for this accident is generated separately and will appear here.