The pilot and the passenger were attempting to take off when surveillance video captured, and a witness heard, the sound of the airplane’s engine backfiring followed by a sudden decrease and increase in engine power. Dark smoke was also seen trailing under the belly of the airplane. Data downloaded from the airplane’s onboard engine monitor recorded an rpm drop from 2,600 to 1,500 rpm then back to 2,500 rpm shortly after the airplane became airborne. Fuel flow, manifold pressure, and horsepower readings uniformly tracked with the decrease/increase of rpm, consistent with the pilot quickly reducing the throttle to idle then back to full power.
Postaccident examination of the engine revealed the No. 2 cylinder bottom spark plug was loose, and stains/residue from combustion exhaust gas leakage was visible around the spark plug port consistent that it had been loose prior to the accident. Review of the airplane’s engine maintenance logbook revealed that the spark plugs were, “cleaned, gapped, tested, and rotated” at the last annual inspection, about 37 hours before the accident.
Based on the available information, it is most likely that the backfiring heard coming from the engine was due to the loose No. 2 cylinder spark plug, which was not properly torqued at the last annual inspection. Despite the fact that the engine was still producing close to full power following the backfire, the pilot entered a steep turn, likely in an attempt to return to the airport, after which the airplane most likely stalled from an altitude that was too low to recover.
NTSB probable cause: The pilot’s failure to maintain control of the airplane while maneuvering to return to the airport which resulted in an aerodynamic stall. Contributing to the accident was the improperly installed No. 2 cylinder bottom spark plug by maintenance personnel, which came loose and resulted in an engine backfire on takeoff.
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