Beech A36 N7214Y accident near Grand Rapids, Minnesota

Probable cause

A loss of engine thrust due to the propeller separating from the aircraft as a result of fatigue failure of the propeller attachment bolts caused by the insufficient torque of the bolts during installation.

NTSB analysis

The pilot stated that about 40 minutes into the flight he heard a loud noise followed by a rapid deceleration of the engine, and then a "distinct bang." The windshield was immediately covered with a film of oil, completely obstructing forward visibility. He established a descent at best glide airspeed and established a course for the nearest suitable airport. During the descent, the engine continued to run rough and smoke entered the cockpit. It became apparent to the pilot that he did not have enough altitude to reach the airport. He attempted to increase engine power in order to slow the descent; however, the engine seized shortly afterward. He set up for a forced landing to a clearing adjacent to a set of power lines. During the forced landing, the airplane impacted trees and a utility pole, which separated the left tip tank and left aileron from the airframe. The airplane slid approximately 50 feet and came to rest upright. A postaccident inspection revealed that the propeller assembly had separated from the engine propeller flange. The propeller flange appeared intact. The propeller assembly was not recovered. Two propeller bolt fragments and one propeller alignment pin fragment were recovered from the engine cowling. Examination of the fracture surfaces revealed features consistent with reverse bending fatigue cracking. In addition, the washer faces of both nuts were worn and polished consistent with relative movement against the crankshaft flange. Maintenance records indicated that the propeller spinner bulkhead was replaced approximately one year prior to the accident in order to comply with a manufacturer's service letter. At the time of the accident, the propeller assembly had accumulated 1,906 hours total time in service, with 235 hours since replacement of the spinner bulkhead. About 531 hours had accumulated since overhaul of the propeller assembly.

Text from the NTSB. Open the NTSB docket for CHI08LA154 for the full record.