The flight instructor reported that, after takeoff, when the Piper PA-28-140 was about 500 feet above ground level, the engine started to run roughly and lose power.
Engine rpm decreased to about 1,100 as he attempted to turn the airplane 180° back toward the airport in Granbury, Texas. However, he was unable to land at the airport, so he made a forced landing to a field short of the runway.
During a post-accident engine test run, the engine initially ran roughly, and black smoke was observed coming from the exhaust.
After a brief warmup, the engine could only produce about 1,600 rpm at full throttle, and the engine test run was stopped.
Examination of the No. 3 cylinder revealed that the exhaust valve was broken in several pieces and that some fragments remained in the cylinder, which produced gouging and scraping to the piston and cylinder head.
The broken exhaust valve, spring, exhaust valve keepers, and rotating cap exhibited carbon buildup.
Examination revealed a wear pattern on the top of the No. 3 cylinder exhaust valve rotator cap consistent with rocker arm wear. No rotational signatures were observed along the edge of the cap near the heavy carbon buildup, indicating that the rotator cap had stopped rotating, which subsequently led to a hot spot on the exhaust valve and caused the valve to fail.
The NTSB determined the probable cause as the partial loss of engine power due to the failure of the exhaust valve.
NTSB Identification: CEN14LA334
This July 2014 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.