Cessna 120 accident near Livermore, California, August 21, 2019

The NTSB's narrative final · quoted from the NTSB record

The accident flight was the first flight following the airplane’s annual inspection. The pilot reported that, during the takeoff roll, the airplane took longer than expected to accelerate, and after becoming airborne, did not seem to be producing full power. Witnesses stated that the airplane made a sharp left turn about 100 feet above the ground and the left wing impacted the ground; the airplane cartwheeled before coming to rest, resulting in substantial damage. A postaccident examination revealed that the upper and lower spark plugs from the No. 4 cylinder and the lower spark plug from the No. 3 cylinder exhibited oil and soot bridging the gap between the electrodes. All other spark plugs exhibited black soot build-up on the electrodes and insulators. The reason for the fouled spark plugs was not determined based on the examination. The mechanic who completed the annual inspection reported that he cleaned, gapped, and functionally tested all of the spark plugs. After the inspection, he conducted an engine run-up and found that one of the plugs had become fouled. He cleaned the plugs a second time and reinstalled them using new gaskets. He ran the engine up a second time, and it ran smoothly. Based on the condition of the No. 4 cylinder spark plugs, it is likely that the engine was not producing nominal power and that its takeoff and climb performance was degraded. The witness observations of a steep left turn and descent to impact is consistent with an aerodynamic stall due to the pilot exceeding the airplane’s critical angle of attack during the initial climb.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.