CAP 231 N231X accident near Lebanon, Pennsylvania

Probable cause

The partial separation of one counterweight and the subsequent seizure of the engine due to mechanical interference. Contributing to the partial separation of the counterweight was the overstress fracture of a retaining ring and the fatigue fracture of the counterweight washer for reasons that could not be determined based on the available evidence.

NTSB analysis

The private pilot reported that he was practicing aerobatics near the departure airport and that the airplane was in a 45° nose-up maneuver when he heard a "loud knock" from the engine, followed by a significant "shake." He leveled the airplane, reduced the manifold pressure, and then turned the airplane back toward the airport. Subsequently, the engine rattled and began to shake more, followed by a total loss of power. The pilot's attempts to restart the engine were unsuccessful. He planned a high base leg and sideslip onto the final approach leg of the airport traffic pattern to ensure the airplane would reach the runway. During the landing flare, the airplane floated, touched down beyond the midpoint of the grass runway, and bounced. The pilot applied hard braking, but the airplane rolled off the end of the runway, down an embankment, and nosed over.

Postaccident examination of the engine revealed that it was seized and that one counterweight, which was normally secured to the crankshaft at two places, was separated at one location. The retaining ring for one side of one counterweight, which was properly installed, fractured due to overstress, and its associated washer fractured due to reverse bending fatigue. The fracture of these components allowed the roller to separate and jam between the crankcase and crankshaft, which resulted in mechanical interference. The roller's contact with the counterweight bushings and/or counterweight washers could have been caused by a several things; however, because the fatigue crack initiation sites were destroyed by postfracture damage, the root cause for the overstress fracture of the retaining ring and fatigue fracture of the washer associated with the counterweight could not be determined.

Text from the NTSB. Open the NTSB docket for ERA17LA194 for the full record.