Cessna 172P N54872 accident near Conroe, Texas

Probable cause

Failure of the rear crankshaft gear dowel pin due to improper installation, which resulted in a total loss of engine power.

NTSB analysis

The airplane was at 1,600 feet mean sea level with the engine power set at 2,300 rpm. When the airplane was about 6 miles from the airport, the pilot heard a loud “boom” and the engine lost all power. The pilot did not have sufficient altitude to glide to the airport, so he landed the airplane on a street. When the airplane was on final approach to land, it collided with power lines. The pilot was able to land the airplane, but was forced to swerve to the right to avoid oncoming traffic. The right wing struck a utility pole, which resulted in substantial damage.

A postaccident engine examination revealed that when the crankshaft was turned, there was no movement of the intake or exhaust valves or magneto gears. The rear crank gear bolt was loose and the gear dowel pin was sheared, which would cause the camshaft and rear accessory gears to stop turning. Metallurgical examination revealed beach markings at the aft end of the crankshaft approximately in plane with the aft face of the crankshaft where it mated to the crankshaft gear. There was fretting damage on the crankshaft gear, wear on the dowel pins, and wear on the lock washer, indicative of movement as a result of insufficient clamping force from the attachment bolt at the time of installation. The engine had accrued 752 hours since it was overhauled in August 2007.

Text from the NTSB. Open the NTSB docket for CEN12TA122 for the full record.