Accident Overview
On July 7, 2020, at approximately 1523 Pacific daylight time, a Cessna 172N airplane, registration N6328D, was destroyed after colliding with a tree near South Lake Tahoe, California. The pilot sustained fatal injuries; the passenger sustained serious injuries and succumbed to them five days later. The airplane was operating under Title 14 Code of Federal Regulations Part 91 as a personal flight.
Flight and Impact
The flight departed Lake Tahoe Airport (TVL) in South Lake Tahoe shortly before the accident. A witness observed the airplane flying about 200 ft above ground level and heard the engine making a sputtering sound consistent with losing and gaining power. The left wingtip struck a tree, causing the airplane to yaw aggressively as it continued flying low in a southerly direction until it disappeared from the witness's view. The wreckage was found about 2.5 nautical miles southwest of the departure end of runway 18.
Airframe and Engine Examination
Postaccident examination showed both propeller blades attached to the hub, which remained on the engine. One blade was straight with nicks along the leading edge; the other had tip bending with nicks and gouges on the outboard leading edge. Rotating the engine by the propeller confirmed valve train continuity and thumb compression on all four cylinders, though cylinders No. 3 and 4 had low compression. Borescope inspection revealed carbon deposits around the exhaust valve seats on those cylinders. Thumb compression was achieved after staking the exhaust valve seats. No signs of catastrophic engine failure were observed internally. Cylinder No. 4 was removed; its piston face showed significant carbon buildup. The top and bottom spark plugs for cylinder No. 4 were dark with heavy carbon soot, consistent with a rich fuel/air mixture. The No. 4 bottom spark plug had a separated center electrode, and the top spark plug had no visible electrode gap. The remaining spark plugs appeared gray, consistent with normal wear.
Fuel System Examination
Both fuel tanks were breached during impact, with no residual fuel found. Compressed air blown through fuel lines at the left and right wing roots showed no obstructions at any fuel selector positions, and no debris exited the fuel line at the engine firewall or fuel tank return lines.
Spark Plug Analysis
The No. 4 cylinder bottom spark plug and its separated center electrode tip were sent to the NTSB Materials Laboratory. Examination revealed a thick layer of deposits on the firing head, up to 0.01 inches thick, consistent with combustion products. The deposits had begun to crack and spall. The loose center electrode tip was almost entirely covered in dark, reflective compounds. Energy dispersive x-ray spectroscopy identified compounds consistent with those from the combustion of brominated tetraethyl lead additives used in avgas.
Maintenance History
The most recent annual inspection was completed on March 19, 2020, at a tachometer time of 4,251.4 hours. During this inspection, the engine underwent a major overhaul, and all four cylinders were replaced with overhauled cylinders. Eight new spark plugs were also installed. On May 7, 2020, all four cylinders were removed due to high oil consumption, then sent to a repair station, overhauled, and reinstalled on June 13, 2020. The engine had accumulated 26.3 hours since the cylinders were reinstalled.
Autopsy and Toxicology
An autopsy performed on the pilot by the El Dorado Pathology Medical Group listed the cause of death as "multiple blunt force injuries." Toxicology testing by the FAA's Forensic Services Laboratory detected quinine in the pilot's blood and urine and oxymetazoline in his urine; these medications are generally not considered impairing.