Cessna 172 N6328D accident near Lake Tahoe, California

Probable cause

The partial loss of engine power due to carbon buildup in the No. 4 cylinder resulting from the pilot’s inadequate mixture-leaning procedures.

NTSB analysis

Shortly after takeoff, a witness observed the airplane flying about 200 ft above ground level and heard the engine power fluctuating. The left wingtip then struck a tree and the airplane aggressively yawed while continuing to fly low. The airplane impacted the ground about 2.5 nautical miles southwest of the departure airport.

A postaccident examination of the engine revealed low compression on cylinder Nos. 3 and 4. Further examination of the No. 4 cylinder revealed significant carbon buildup on the piston face and the top and bottom spark plugs. This buildup would likely have resulted in the No. 4 cylinder ignition system misfiring and a partial loss of engine power.

The four-cylinder engine was overhauled about 4 months before the accident, during which four overhauled cylinders and eight new spark plugs were installed. Additionally, about 2 months (and about 26 hours) before the accident, all four cylinders were overhauled again due to high oil consumption.

Carbon deposits can form when excessive heating occurs in the combustion chamber of the engine. While the high amount of carbon buildup is unusual for a cylinder with only 26 hours of operation since overhaul, the reason for the buildup was likely due to the pilot’s inadequate mixture-leaning procedures.

Text from the NTSB. Open the NTSB docket for WPR20LA212 for the full record.