N6090QMooney M20C · 1965

On September 21, 2010, about 0940 Pacific daylight time, a Mooney M20C, N6090Q, experienced a partial loss of engine power during takeoff from Visalia Municipal Airport, Visalia, California. During a low altitude course reversal turn back to the runway, the airplane impacted the ground hard and was substantially damaged. The commercial pilot owned, operated, and maintained the airplane. He was not injured during the personal flight. Visual meteorological conditions prevailed, and the flight was performed under the provisions of 14 Code of Federal Regulations Part 91. No flight plan was filed. The flight was originating at the time of the accident.

The pilot reported that, in preparation for takeoff, he had performed an engine run-up, and no anomalies were noted. Thereafter, he applied full engine power and commenced taking off. At 300 feet above the runway he heard a loud pop noise, and the engine’s rpm decreased to 1,500. Unable to maintain flight, the pilot initiated a course reversal turn for a forced landing back on the airport. However, the airplane’s altitude was insufficient to complete the turn, and he impacted the ground adjacent to the departure runway. The impact was sufficiently hard to activate the emergency locator transmitter, break off the nose gear, and bend the outboard 6 feet of the right wing.

Federal Aviation Administration (FAA) personnel subsequently examined the airplane’s engine. The FAA reported that a bottom spark plug of the Lycoming engine's number three cylinder had blown out of its cylinder.

FAA personnel additionally reported that the spark plug had been installed in the cylinder hole with a Heli-Coil insert, which surrounds the plug's threads in the cylinder. The blown out spark plug was found, and it appeared to have failed due to wear from repeated insertions and removals, while the Heli-Coil gradually loosened. FAA personnel further opined that the spark plug had not been loose in the cylinder. Rather, the Heli-Coil showed evidence of loosening over time. Evidence of the Heli-Coil’s loosening would not, necessarily, have been apparent to the mechanic during routine maintenance, according to the FAA.

The engine had over 1,600 hours of total time since its last major overhaul. The number three cylinder had not been replaced since its 1987 overhaul, according to maintenance records.