Piper J3C-65 N98550 accident near Egremont, Massachusetts

Probable cause

A partial loss of engine power due to the No. 4 intake and exhaust valves becoming lodged in the valve guides, which resulted from a lack of use, improper engine storage preparation, and improper return to service from indefinite storage.

NTSB analysis

The pilot reported that the accident flight was the first flight in the airplane in 6 months. During the initial climb, the engine experienced a partial loss of power. The pilot checked the magnetos and carburetor heat but was unsuccessful in his attempts to regain full engine power. Subsequently, he chose to land the airplane between two houses. A postaccident engine compression test revealed that the Nos. 1 and 4 cylinders had low compression. The No. 1 rocker cover was removed, and the rocker arm was tapped with a mallet, which brought its compression to within normal range. Disassembly of the No. 4 cylinder revealed that it was leaking past the exhaust valve and through the exhaust system. In addition, disassembly revealed a scored intake valve stem and valve guide. The intake valve would not operate and had to be forcefully removed from the valve guide. According to the engine overhaul manual, scored valve stems can cause the engine to produce low power. The pilot did not report performing any of the procedures to return the engine to service from indefinite storage (that is, more than 90 days). Therefore, it is also likely that the engine was never prepared for indefinite storage and, thus, was not stored improperly for the 6 months it was not used before the accident flight. During storage, the No. 4 intake and exhaust valves most likely became lodged in the valve guides, which resulted in a partial loss of engine power and subsequent forced landing to a field. It is likely that, if the engine had been properly stored, the recommended lubrication would have prevented the valves from sticking and scoring and that, if the engine had been properly removed from storage, the scored valves may have been detected and replaced, either of which action could have prevented the accident.

Text from the NTSB. Open the NTSB docket for ERA11LA379 for the full record.