J-2 N16667 accident near Lock Haven, Pennsylvania

Probable cause

The airplane owner’s failure to clean and lubricate the engine valve guides and stems, which resulted in a partial loss of engine power during the initial climb due to an exhaust valve failure.

NTSB analysis

The pilot reported that, after departure and while in the traffic pattern, the airplane was operating normally but that the engine then suddenly lost partial power. Due to the airplane's low altitude at the time of the power loss, the pilot decided to land between several houses in a clearing. During the forced landing, the airplane impacted trees.

Examination of the engine revealed that the exhaust valve on the No. 1 cylinder remained in the "open" position and would not move when the engine was rotated by hand. Further examination revealed that the exhaust valve was stuck in the valve guide. Although automotive fuel was approved for use in this engine, 100LL aviation fuel was used for the accident flight. According to manufacturer guidance, continued operation with automotive fuel can lead to sticking. However, it could not be determined if automotive fuel had been used during previous flights and resulted in the valve sticking or if the use of the 100LL fuel resulted in the valve sticking. An entry in the engine maintenance records revealed that the valve guides and stems were to be lubricated every 5 hours and cleaned every 10 hours; however, no entry was found in the records indicating that the valve guides and stems had been lubricated since the airplane's most recent inspection about 15 flight hours before the accident. Due to the exhaust valve being stuck, the exhaust valve head separated from its stem during the initial climb, which subsequently resulted in the failure of the exhaust valve's associated cylinder components and the partial loss of engine power.

Text from the NTSB. Open the NTSB docket for ERA14LA308 for the full record.