The student pilot and flight instructor were conducting a personal flight in the Piper J-3 Cub.
The CFI reported that the pretakeoff checks were “normal” but that, during the takeoff, the engine felt “weak” and that it lost power during the initial climb.
The flight instructor conducted a forced landing to a field near Brenham, Texas, during which the plane hit the top of a tree and then the ground.
Examination of the airplane revealed that the exhaust valve for the No. 4 cylinder was stuck in the “open” position, possibly due to excessive deposits from the combustion process. It is likely that the stuck exhaust valve resulted in the partial loss of engine power reported by the flight instructor.
Probable cause: The reciprocating engine’s exhaust valve being stuck in the “open” position, which resulted in a partial loss of engine power during initial climb.
NTSB Identification: CEN17LA093
This January 2017 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.