Accident Summary
On June 2, 2021, at approximately 1000 Pacific daylight time, a Cessna 172G, registration N3871L, was substantially damaged during a forced landing near Washington, California. The pilot and the passenger were not injured. The flight was conducted as a personal flight under Title 14 Code of Federal Regulations Part 91.
Pilot Report
The pilot reported that the airplane was climbing at full power with an indicated airspeed of 80 mph. He progressively leaned the mixture during the climb. About 30 minutes into the climb, at an altitude near 6,000 ft mean sea level (msl), the engine oil temperature gauge reached its maximum limit of 240°F. The pilot lowered the nose to increase airspeed and reduced engine power. Approximately 10 minutes later, the oil temperature decreased below 240°F, and the pilot resumed the climb to 9,500 ft msl using a richer mixture. At about 8,500 ft msl, the pilot heard a "backfire" sound from the engine and observed a gradual decrease in engine rpm, confirming a loss of engine power.
The pilot attempted to troubleshoot but could not restore engine power. Unable to maintain altitude, he declared an emergency and maneuvered the airplane toward a clear area in the mountainous terrain. During the forced landing, the airplane sustained substantial damage to both wings and the lower fuselage.
Maintenance History
A review of the airframe and engine maintenance logbooks indicated that the last annual inspection was completed on December 6, 2020, six months before the accident. At that inspection, the engine had accumulated 4,027 total hours and 1,896 hours since major overhaul. The engine tachometer indicated 2,277 hours at the last annual inspection and 2,288.8 hours at the time of the accident. The engine manufacturer recommended an overhaul interval of 1,800 hours or 12 years, whichever occurs first.
Engine Examination
Examination of the engine revealed that the magneto-to-engine timing was set at 33° before top dead center (BTDC) for the left magneto and 32° BTDC for the right magneto. The manufacturer specification was 28° BTDC for the left and 26° BTDC for the right. About 6 quarts of engine oil were extracted from the oil sump.
The No. 2 cylinder exhibited low compression. All cylinders were removed for further examination. Cylinders Nos. 1 and 4 showed no anomalies. The No. 2 cylinder had a seized top compression ring on the piston ring land with flat spots observed on the ring. The No. 3 cylinder had a broken top compression ring (two pieces) and a stuck exhaust valve in the valve guide. The No. 5 cylinder showed excessive key-holing of the intake valve within its guide. The No. 6 cylinder top compression ring was broken into four pieces.