Cessna 172N N5944J accident near Dade City, Florida

Probable cause

The in-flight loosening of the No. 3 cylinder exhaust valve rocker arm stud, which resulted in a complete loss of engine power.

NTSB analysis

During the accident flight, the exhaust gas temperature on the No. 3 cylinder began as the coldest and then transitioned to the hottest. Subsequently, the cylinder head temperature on the No. 3 cylinder also transitioned to the hottest. According to the pilot, the engine then began to run roughly, and he adjusted the throttle and mixture positions to gain control of the malfunctioning engine. During emergency communications with air traffic control, the engine lost all power, and the pilot conducted an emergency landing to a field.

Examination of the engine revealed that the No. 3 cylinder exhaust valve rocker arm stud had backed out of the cylinder, which prevented the exhaust valve from opening properly. Specifically, the loose rocker arm stud prevented the pushrod from depressing the exhaust valve far enough in to release exhaust from the cylinder, which damaged the intake valve pushrod and caused a complete loss of engine power. The No. 3 cylinder had been replaced about 14 months before the accident and had accumulated 480.9 hours since installation. It is likely that the rocker arm and nut assembly became loose over time; the initial torque value could not be determined.

Text from the NTSB. Open the NTSB docket for ERA13FA217 for the full record.