Amateur-built DRAGONFLY N85TL accident near Mesa, Arizona

Probable cause

A partial loss of engine power due to a reduction in the No. 3 cylinder’s compression as a result of the exhaust valve not having consistent uniform contact with the cylinder head valve seat.

NTSB analysis

The pilot reported that, shortly after takeoff, the engine power decreased from about 3,100 to 2,500 rpm. The airplane continued to barely climb until it reached an altitude of about 150 ft above the ground. At that point, the airplane was unable to continue the climb or maintain altitude, and it began to descend. The pilot landed the airplane onto a field where it struck vegetation and came to rest in a bush about 1/2 mile from the departure end of the runway.

During postaccident examination of the engine, the No. 3 cylinder had noticeably less compression, and a hissing noise was heard coming from the exhaust pipe. The cylinder was removed from the engine, and the piston head had a tan, sandy-colored appearance. Also, a light yellow band was observed around one-quarter of the circumference of the exhaust valve. Additional examination of the valve and valve seat revealed no defects; however, it did not appear that the exhaust valve was seated with consistent uniform contact; inconsistent uniform contact between the exhaust valve and the cylinder head valve seat would decrease engine performance.

Text from the NTSB. Open the NTSB docket for WPR14LA242 for the full record.