Cessna 150M N714PW accident near Ritzville, Washington

Probable cause

Maintenance personnel’s improper installation of the No. 2 cylinder exhaust valve, which allowed the valve to drop into the cylinder and resulted in a total loss of engine power.

NTSB analysis

The pilot was conducting a cross-country flight when he heard a “loud metal sound” followed by a partial loss of engine power. While troubleshooting the rough-running engine, he diverted to a local airport. The airplane then had a total loss of engine power, and the pilot initiated an off-airport landing to an open field. During the landing roll, the nose landing gear collapsed and the airplane impacted terrain, which resulted in substantial damage to the fuselage and both wings.

Postaccident examination of the engine revealed mechanical damage to the No. 2 cylinder’s combustion dome and piston face. The keeper halves for the exhaust valve were missing, and the keeper shoulder on the valve stem showed fretting contact damage consistent with relative movement between the keeper halves and the valve stem. Contact wear patterns were also observed on the exhaust valve spring seat. The keepers likely became dislodged during the accident flight, allowing the valve to drop into the combustion chamber, where it was repeatedly impacted by the piston. The valve was then pushed back into the exhaust cavity, where the valve stem impacted the head adjacent to the exhaust valve guide. The fragmentation of the piston and exhaust valve likely led to the loss of engine power.

Possible causes for dislodged keepers include improper installation or weak valve springs. The height of the intake and exhaust valve springs were similar, so weak springs are unlikely. Thus, the keepers were likely not installed properly before the accident. With improper installation, failure can be expected relatively soon after installation. Although the last record of work that would require disassembly of the valve to complete (a top overhaul) was dated about 36 years before the accident flight, the engine was reported to have only accrued about 320 hours since repair of the cylinders. It is likely that the keeper halves were not properly seated on the exhaust valve shoulder when the valve was installed. This incorrect installation allowed the keepers to eventually work their way out of position, allowing the valve to drop into the combustion chamber.

Text from the NTSB. Open the NTSB docket for WPR24LA248 for the full record.