Accident Overview
On August 21, 2019, at about 1150 Pacific daylight time, a Cessna 120 airplane, registration N4237N, sustained substantial damage during an accident near Livermore, California. The pilot, who was the sole occupant, was seriously injured. The flight was conducted as a personal operation under Title 14 Code of Federal Regulations Part 91.
Pilot Report
According to the pilot, the airplane had experienced an issue with the fuel leads to the engine in 2018. After repairs, she flew the airplane back to her home airport and parked it in a hangar until its annual inspection, which occurred the month of the accident. The accident flight was the first flight after that inspection. The engine started normally, and the pre-takeoff runup revealed no anomalies. During the takeoff roll, the airplane took longer than expected to accelerate. After becoming airborne and beginning a shallow climb, the engine seemed not to be operating at full power, although it sounded normal. The airplane stopped climbing, and the pilot initiated a left turn for a forced landing. During the emergency descent, the left wing struck either brush or the ground, and the airplane tumbled before coming to rest in brush-covered terrain.
Witness Accounts
A witness reported that the engine was not sputtering but did not sound like it was producing full power. At about 100 feet above ground level, the airplane made a sharp left turn and began to descend. The left wing impacted the ground, and the airplane cartwheeled before coming to rest.
The mechanic who performed the annual inspection also witnessed the accident. He observed the airplane low over the runway after takeoff, appearing not to climb. Shortly after, it banked steeply left and struck the ground.
Maintenance History
The mechanic reported that during the annual inspection, he cleaned, gapped, and functionally tested all spark plugs. Afterward, during an engine run, he found a rough-running magneto. Removing all spark plugs, he found one fouled. He cleaned the plugs a second time and reinstalled them with new gaskets. A second engine run was smooth.
Postaccident Examination
A postaccident engine examination revealed that the upper and lower spark plugs from the No. 4 cylinder and the lower spark plug from the No. 3 cylinder had oil and soot bridging the gap between the electrodes. All other spark plugs showed black soot buildup on the electrodes and insulators. The ignition harness was impact-damaged and could not be functionally tested. The magnetos were removed and disassembled, with no anomalies noted in their internal components. The oil filter was opened and found free of particulates. The propeller assembly and valve covers were removed. The engine was rotated by hand, establishing thumb compression on all cylinders, and valves moved consistently. A borescope examination of the cylinders revealed no anomalies.
Additional Information
The pilot did not complete and submit the National Transportation Safety Board form 6120.1, nor did she provide copies of the accident airplane's most recent maintenance logbook entries.