Textron Aviation Inc 172S N1817E accident near Erie, Colorado

Probable cause

A partial loss of engine power while maneuvering at a low altitude due to a stuck exhaust valve.

NTSB analysis

The flight instructor started the engine for the instructional flight, and it began to shake. The flight instructor shut down the engine, inspected it, and the alternator belt. Everything appeared to be normal, so he decided to restart the engine. After engine start, all indications were “in the green” and the engine no longer shook. All systems during the run-up appeared normal, and the instructor and student pilot departed.

After completing several maneuvers, the flight instructor gave the student pilot a simulated emergency. The student pilot addressed the simulation with the appropriate steps and the airplane descended to about 700 ft above ground level (agl). As the student pilot applied full power to recover from the simulated emergency, the engine started to shake again. The flight instructor took the flight controls and observed that the engine rpm started to drop. The flight instructor maneuvered the airplane to land in a field. During the landing, the airplane rolled nose-first into a culvert and sustained substantial damage to the right wing and engine firewall.

During the engine examination, no compression was observed at the No. 4 cylinder. Further examination revealed that the No. 4 exhaust valve was stuck open, and the No. 4 push rod was bent. No other anomalies were noted with the engine or systems that would have resulted in a loss of engine power.

A review of the maintenance records revealed that a service bulletin (SB) to examine the exhaust valve and guide condition was not completed, nor was it required to be. The SB suggested that an inspection should be performed at every 1,000 hours of operation for engines equipped with Hi-Chrome exhaust valve guides, as the accident engine was. The partial loss of engine power was likely due to the stuck exhaust valve. Compliance with the engine manufacturer’s SB, although not required, may have prevented the stuck valve.

Text from the NTSB. Open the NTSB docket for CEN24LA036 for the full record.