Cessna 172G N3871L accident near Washington, California

Probable cause

A total loss of engine power during an enroute climb due to a stuck exhaust valve and multiple failed piston compression rings.

NTSB analysis

The pilot reported that he was climbing the airplane at full engine power and an airspeed of 80 mph, and he had progressively leaned the fuel/air mixture during the climb. About 6,000 ft mean sea level (msl), he noted that the the engine oil temperature was indicating its maximum limit of 240°F. The pilot lowered the nose to increase airspeed and reduced engine power to cool the engine. About 10 minutes later, the engine oil temperature decreased below redline and he resumed his climb with a slightly richer mixture setting. About 8,500 ft msl, the engine produced a “backfire” sound, the engine rpm decreased, and a loss of engine power ensued. The pilot declared an emergency and performed a forced landing to mountainous terrain, during which the airplane sustained substantial damage.

Examination of the engine revealed that the No. 2 cylinder’s piston exhibited a stuck top compression ring. The No. 3 cylinder exhibited a stuck exhaust valve, and the No. 3 piston’s top compression ring had separated into two. The No. 5 intake valve exhibited excessive clearance within its guide. The No. 6 cylinder’s piston top compression ring had separated into four pieces.

A review of the airframe and engine maintenance logbooks indicated that the last annual inspection was completed 6 months before the accident. At the time of the annual inspection, the engine had amassed a total time of 4,027 hours, and 1,896 hours since major overhaul. The engine tachometer indicated 2,277 hours during the last annual inspection and indicated 2,288.8 at the time of the accident. The engine-manufacturer recommended overhaul interval is 1,800 hours or 12 years, whichever occurs first.

Text from the NTSB. Open the NTSB docket for WPR21LA224 for the full record.