Mooney Aircraft Corp. M20K N9514R accident near Dawson, Georgia

Probable cause

Maintenance personnel’s failure to properly secure the No. 4 connecting rod bolt nut with a cotter pin, resulting in a total loss of engine power.

NTSB analysis

The pilot reported that, while en route to the destination airport, the airplane lost total engine power. The pilot declared an emergency with air traffic control and began to divert to the nearest airport. The pilot recognized that the airplane would not be able to glide to the airport, and he decided to land on a road with the landing gear and flaps retracted. The pilot and passenger sustained serious injuries, and the airplane was substantially damaged.

A postaccident teardown examination of the engine revealed that the No. 4 connecting rod was fractured midspan. One of the No. 4 connecting rod bolt nuts was found in the oil pan and exhibited no signs of pre- or postaccident damage. The cotter pin for that nut was not found; the other No. 4 connecting rod nut was found secured to its bolt with the cotter pin installed. Additionally, all other connecting rod bolt nuts remained attached to their respective bolts with cotter pins in place. Thus, it is likely that the nut found in the oil pan was not secured with a cotter pin and that the unsecured No. 4 connecting rod bolt nut became loose over time and backed out of the connecting rod bolt during the accident flight, resulting in the total loss of engine power.

Review of maintenance records revealed that the engine had accumulated about 1,240 hours of operation since major overhaul, which was completed more than 28 years before the accident. The engine manufacturer recommended that the engine be overhauled every 2,000 hours or 12 calendar years, whichever occurred first. The investigation could not determine when the No. 4 connecting rod nut was installed without a cotter pin.

Text from the NTSB. Open the NTSB docket for ERA21LA222 for the full record.