Lake Tahoe, CA, USA
N6328D
Cessna 172
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.
On July 7, 2020, about 1523 Pacific daylight time, a Cessna 172N airplane, N6328D, was destroyed when it was involved in an accident near South Lake Tahoe, California. The pilot sustained fatal injuries; the passenger sustained serious injuries to which she succumbed to 5 days later. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The airplane departed Lake Tahoe Airport (TVL) South Lake Tahoe, California, shortly before the accident. A witness observed the airplane flying about 200 ft above ground level and heard the engine producing a sputtering sound consistent with losing and gaining power. The left wingtip struck a tree and the airplane aggressively yawed while continuing to fly low in a southerly direction until it disappeared from the witness's line of sight. The airplane wreckage was discovered about 2.5 nautical miles southwest of the departure end of runway 18. A postaccident examination of the airframe and engine revealed both propeller blades remained attached to the propeller hub, which was still attached to the engine. One propeller blade was straight with some nicks along the leading edge and the other blade displayed some tip bending with nicks and gouges along the outboard leading edge. When the engine was rotated at the propeller, valve train continuity was confirmed to the accessory section and thumb compression was obtained on all four cylinders; however, cylinder Nos. 3 and 4 had low compression. A borescope examination of the exhaust valves for cylinder Nos. 3 and 4 revealed carbon deposits around the exhaust valve seats at both cylinders. Thumb compression was subsequently achieved for these cylinders after the exhaust valves seats were staked. Examination of the interior components using a lighted borescope revealed no indications of catastrophic engine failure. Cylinder No. 4 was removed from the engine case. The piston face displayed significant carbon buildup. Both 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 cylinder bottom spark plug had a separated center electrode. The No. 4 cylinder top spark plug did not show a visible electrode gap. The remaining spark plugs were gray in appearance, consistent with normal wear. The fuel tanks were both breached during the impact sequence and no residual fuel was observed. Compressed air was blown through fuel lines located at the left and right wing roots and no obstructions were observed at any of the fuel selector positions nor did any debris exit the fuel line at the engine firewall or fuel tank return lines. The No. 4 cylinder bottom spark plug and separated center electrode tip were retained and sent to the NTSB Materials Laboratory for examination. The examination revealed that the firing head of the spark plug exhibited a thick layer of deposits and compounds, consistent with combustion products from inside the cylinder. The deposits were thick enough that they had begun to crack and spall from the spark plug surfaces that had been inside the combustion chamber. The deposit thickness was measured up to 0.01 inches. The loose and separated center electrode tip was almost completely covered in dark, reflective compounds, consistent with deposits from the engine combustion. Different areas of the tip were examined using energy dispersive x-ray spectroscopy (EDS) to determine the different chemical compositions exhibited during the examination. The identified composition was consistent with compounds produced from the combustion of brominated tetraethyl lead additives used in avgas. Examination of the maintenance records for the airplane and engine revealed 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. Several new parts were also installed during the overhaul, including eight new spark plugs. An entry dated May 7, 2020, stated all four cylinders were removed due to high oil consumption. They were returned to the repair station, overhauled, and reinstalled on the engine on June 13, 2020. The engine accumulated 26.3 hours since the cylinders were reinstalled. An autopsy was performed on the pilot by the El Dorado Pathology Medical Group, Placerville, California. The autopsy report listed the cause of death as “multiple blunt force injuries.” Toxicology testing performed 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.
The partial loss of engine power due to carbon buildup in the No. 4 cylinder resulting from the pilot’s inadequate mixture-leaning procedures.
Source: NTSB Aviation Accident Database
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