Piper PA28 N4139A accident near Tat Momoli, Arizona

Probable cause

A total loss of engine power due to a fatigue crack in the connecting rod bushing end.

NTSB analysis

The pilot reported that during the cross-country flight, the engine experienced a total loss of power. The pilot initiated a forced landing to a nearby road where, during the landing roll, the airplane impacted a barbed wire fence, resulting in substantial damage to the right wing.

Postaccident examination revealed a large hole in the engine case at the No. 4 cylinder that exposed the crankshaft and connecting rod. The No. 4 cylinder was seized, and the connecting rod was fractured at the piston end. Further examination revealed that the connecting rod fractured from a fatigue crack in the bore on the piston end, which likely initiated at an internal corner of the bore. The fatigue crack progressed through most of the pin bore cross-section. Following the fracture of the connecting rod, the continued movement of the broken part led to subsequent overstress fractures and damage to nearby components, which resulted in catastrophic engine failure and a complete seizure of the engine.

The cause of the fatigue crack initiation could not be determined, as both the initiation site (or sites) and the termination of the fatigue crack were destroyed by postfracture impact damage. The general location of the initiation, the internal corner of the pin bore, suggested the presence of concentrated stress in that location, which could have been the result of a material defect, such as a pore, inclusion, or corrosion pit that may have occurred when the connecting rod was rebushed. A crack initiation could also have been the result of improper sizing of the internal bushing, improper installation of the connecting rod, or use of the wrong parts. The engine was overhauled 42 flight hours before the accident.

Text from the NTSB. Open the NTSB docket for WPR18LA248 for the full record.