Piper PA-28-181 N38696 accident near Watertown, South Dakota

Probable cause

The stuck No. 2 cylinder exhaust valve, which resulted in a partial loss of engine power and a forced landing due to an undersized exhaust valve guide. Contributing to the accident were the improper manufacture of the engine cylinders, and the operation of the engine in an overly rich fuel condition.

NTSB analysis

During an instructional flight, the airplane had just taken off when the engine lost partial power. The flight instructor took control of the airplane and conducted a forced landing on the remaining runway. During the forced landing, the airplane departed the runway surface and struck a fence and trees, which resulted in substantial damage to both wings.

Engine monitoring data showed that during the takeoff, engine power increased to a maximum of 2,505 rpm. About five seconds after rotation, engine power decreased to 1,446 rpm. Postaccident examination of the engine revealed a partially open, stuck exhaust valve on the No. 2 cylinder. The valve stem had a metallic debris smeared at mid span of the stem, and the valve guide had numerous metallic deposits present. The metallic debris and deposits contained lead, carbon, and bromine, consistent with combustion products, and the valve guide was below the manufacturer-specified minimum guide-to-stem clearance from the interior through mid-span of the guide. Relatively thick deposits were also noted on the piston crown and exhaust port.

Based on the composition of the deposits, the No. 2 cylinder exhaust valve likely became stuck due to cumulative engine operations at an overly rich fuel condition, leading to incomplete fuel burn and an accumulation of combustion deposits on the valve stem and guide. The presence of the deposits suggests that the engine was operated at relatively low cylinder head temperatures (CHT). This is consistent with the accident flight data showing that the No. 2 cylinder temperature peaked at 336°F. Additionally, a review of 109 previous flights totaling about 165 hours showed a maximum No. 2 cylinder temperature of 364°F and a median temperature of 293°F.

According to the engine manufacturer’s guidance, continued operations at much lower temperatures could be detrimental to engine operation and likely contributed to the build up of combustion products on the valve.

The valve guides on all four cylinders were found to be below the minimum inner diameter specification. The cylinders were factory-new when installed on the engine at overhaul by the engine manufacturer on January 11, 2023. Based on the measurements, the exhaust valve guides were not correctly sized during manufacture of the cylinders. It is likely that the undersized valve guide contributed to the valve sticking with the combustion deposits.

Given the CHTs values recorded in the accident flight and accumulation of combustion deposits observed in the examined cylinder, it is likely that combustion deposits accumulated in the valve guide and led to valve sticking in addition to the undersizing of the valve guide at factory overhaul.

Text from the NTSB. Open the NTSB docket for CEN24LA121 for the full record.