Piper PA22 N2591A accident near Beckville, Texas

Probable cause

A stuck exhaust valve, which resulted in the failure of the valve and a loss of engine power. Contributing to the valve failure and loss of engine power was the extended time since the last overhaul.

NTSB analysis

The commercial pilot reported that, about 2 hours into the pipeline surveillance flight, the engine began running roughly. He initially established a course to a nearby airport; however, the engine was not producing enough power to maintain altitude. He subsequently executed a forced landing to a pasture, which resulted in substantial damage to the left wing.

A postaccident engine examination revealed that the No. 3 cylinder exhaust valve fractured at the stem seat. Metallurgical examination revealed that the fracture features were consistent with torsional loading at fracture. The stem was tapered adjacent to the fracture, and interconnected voids and surface oxidation were observed at the fracture surface. The presence of voids, tapering of the stem, and oxidation are consistent with high-temperature deformation under stress. Further examination revealed grain structure and subsurface oxidation consistent with exposure to elevated temperatures over an extended period of time, consistent with a stuck valve condition.

Factors leading to stuck exhaust valves include excessive engine temperature, dirty or contaminated engine oil, high lead content fuels, hot engine shut-downs, and poor engine baffling. Running engines past the recommended overhaul interval or at elevated temperatures risks creating an exhaust valve fatigue event. The investigation was unable to determine the exact combination of elements that led to the stuck valve. However, based on the available maintenance records, the most recent engine overhaul was completed in 1968, and it is likely that the extended 49-year interval contributed to the stuck valve. While this exceeded the engine manufacturer's 12-year or 1,500-hour time between overhaul interval, the operator was not required to comply under current regulations due to the type of operation.

Text from the NTSB. Open the NTSB docket for CEN17LA317 for the full record.