Mooney M20F N9573M accident near Beverly, Massachusetts

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.

NTSB analysis

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. During the takeoff, the pilot advised the air traffic controller that the airplane was having an engine problem and that he planned to “head up and turn back.” The airplane subsequently made a right turn toward the intersecting runway shortly after the engine backfired and it’s possible the pilot reduced power in an attempt to land but changed his mind and decided to circle back to another runway. The airplane then entered a steep left turn and descended, in what a witness described as a stall-like state. The last radar return indicated the airplane was about 110 ft above the ground at a groundspeed near its published stall speed. According to data downloaded from the airplane’s engine monitor, the engine was operating near full power as the airplane descended to ground contact.

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. There were no other entries in the engine maintenance logbook regarding removal of the spark plugs after the annual inspection. According to the engine manufacturer’s service instruction on how to install spark plugs, the spark plugs should have been torqued to a specific torque value to prevent them from coming loose.

Examination of the fuel system and its components revealed evidence of water/corrosion and debris; however, testing found no indication that would have precluded fuel from reaching the engine. While the data showed that the cylinder head temperature was low on the No. 1 cylinder, there was no evidence of a stuck valve, and the ignition system tested normally. An erratic exhaust gas temperature (EGT) reading for the No. 1 cylinder was also recorded for the entire flight. The EGT data had been erratic since the engine’s last annual inspection. The No.1 cylinder’s EGT connector was found undamaged and the yellow (positive) wire for the EGT probe was not connected (the red wire was connected). As such, the probe had most likely not been working properly since the last annual inspection.

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.

Text from the NTSB. Open the NTSB docket for ERA25FA230 for the full record.