Piper PA28 N7213W accident near St. Agatha, Maine

Probable cause

The failure of the crankshaft gear alignment dowel due to fatigue crack propagation and overstress, which resulted in a partial loss of engine power.

NTSB analysis

During an instructional flight while maneuvering, the student pilot moved to increase the throttle, which was followed by a pop that was felt and heard from the engine. Subsequently, the power became stuck at idle and would not produce additional power, even with several troubleshooting actions performed by the flight instructor. The flight instructor completed a forced landing in a field, where the landing gear collapsed, and the fuselage was substantially damaged.

Examination of the engine revealed that the crankshaft gear would not engage and, as a result, the engine exhibited no compression and all components to the engine’s accessory gear box would not rotate. The crankshaft gear bolt was found loose about 2 to 3 threads, the lock plate holding the crankshaft gear bolt was loose, and the alignment dowel had sheared.

Further forensic review of the dowel revealed that it exhibited signatures consistent with fatigue crack propagation and overstress fractures. It is likely that this crankshaft alignment dowel failure resulted in the loss of engine crankshaft continuity and the inability of the engine to produce power when commanded.

The engine manufacturer had published a service bulletin that likely would have required replacement of the alignment dowel; however, the bulletin was only required to be completed during overhaul, after a propeller strike, or whenever the crankshaft gear was removed. There was no record in the maintenance records that the service bulletin was complied with and the most recent overhaul inspection was completed prior to the publication of the service bulletin.

Text from the NTSB. Open the NTSB docket for ERA20LA277 for the full record.