Mooney M20M N21890 accident near Tampa, Florida

Probable cause

The total loss of engine power due to the failure of maintenance personnel to properly tighten the fuel supply line b-nut at the manifold valve during installation.

NTSB analysis

While in cruise flight, the pilot noticed the turbine inlet temperature (TIT) was about 100° F above the maximum reading and that liquid was on the windshield. He reduced engine power, which lowered the TIT reading and then diverted to his home airport. While on the base leg of the airport traffic pattern at 1,200 ft above gound level, he advanced the throttle, but the engine did not respond. At 800 ft msl he noted he would be unable to reach the airport and ditched the airplane. Both occupants exited the airplane and were transported to land, and the airplane’s fuselage was substantially damaged during the ditching.

Following recovery of the airplane, the inlet fuel line at the manifold valve was 7.5 flats of the b-nut from being hand tight against the inlet fitting. There was no provision for safety wiring the b-nut and there was no torque stripe on the b-nut or inlet fitting. Fuel leakage was noted at the loose b-nut during pressure testing, but no leakage occurred after tightening the b-nut. There was no damage to the b-nut or fitting that would have precluded obtaining the proper torque. The manifold valve was overhauled and installed nearly 2 months earlier. The mechanic who signed off the installation indicated he normally would have applied a torque stripe on the b-nut. Based on this information, it is likely that the overheating of the engine and the subsequent loss of engine power was likely due to a fuel leak at the fuel manifold valve, resulting an excessively lean fuel to air mixture. It is also likely that the loose b-nut had not been properly tightened during the reinstallation of the manifold valve.

Text from the NTSB. Open the NTSB docket for ERA22LA064 for the full record.