ERA23LA120 · Mooney M20C

The pilot reported that the right magneto dropped 150 rpm when he and the flight instructor performed the engine runup. They leaned the fuel mixture and ran the engine up to about 1,700 rpm a couple of times until both magnetos dropped 125 rpm during the magneto checks. Once the magneto checks were normal, they initiated the takeoff. The pilot retracted the landing gear and a few moments later, while they were about 70 ft above the runway, there was a popping noise, the engine rpm went to nearly zero, and they began to lose altitude. They lowered the landing gear and touched down near the end of the runway. The airplane skidded off the end of the runway into the grass and slid into an earth embankment. The airplane then became airborne, rotated 90º to the left, and came to rest after a few hundred feet. The flight instructor then advised the pilot of flames near the pilot’s feet and they both exited the airplane. The airplane was destroyed by the postimpact fire. Examination of the engine revealed that the No. 3 bottom spark plug was not in the spark plug hole and was hanging from the ignition cable. The spark plug hole thread insert (Helicoil) was partially pulled from the spark plug hole. Examination of the spark plug revealed the lower three threads of the spark plug displayed absence of rust, but the threads located above them were rusty, and the bottom portion of the Helicoil was still intact. No other anomalies were discovered with the engine that would have precluded normal operation. Review of maintenance records indicated the engine had last been overhauled about 30 years ago, but this most probably did not contribute to the accident. The most recent annual inspection was completed about 2 months before the accident. Review of the logbook entry for the annual inspection indicated that during the annual, the spark plugs had been “removed, cleaned, inspected and installed.” The engine time in service between the last annual inspection and the last recorded entry in the engine logbook, which stated that the No. 2 cylinder spark plug was replaced was 1 hour. The investigation was not able to determine if the No. 3 cylinder spark plug was removed and reinstalled at that time. The accident occurred 14 days later. Therefore, it is likely that the No. 3 bottom spark plug was either not inserted all the way or it wasn’t properly torqued when it was installed during the annual inspection or most recent maintenance work.

Verbatim NTSB analysis and probable cause from the NTSB dataset