Engine Power Loss Leads to Forced Landing and Fence Collision (N3936D)

Accident Overview

An airplane sustained a total loss of engine power while approaching the vicinity of its destination airport. The pilot executed a forced landing on a highway approximately 2 miles from the airport. During the landing roll, the airplane impacted a fence.

Engine Examination

A post-accident examination of the engine revealed that rotation of the crankshaft did not generate rotation of the camshaft. Removal of the starter drive gear assembly disclosed missing gear teeth: four teeth were missing from the camshaft sprocket and one tooth was missing from the crankshaft sprocket. The retrieved gear teeth were sent to the Safety Board Materials Laboratory for examination.

Metallurgical Findings

Laboratory examination determined that the camshaft gear sprocket failed due to fatigue initiating at the root of one of the teeth. Several other teeth exhibited fatigue consistent with secondary initiation due to load shedding from the original failed tooth. The crankshaft gear failure was also secondary. Microstructural examination and hardness testing of a section of a camshaft tooth indicated that it met drawing specifications. A thin layer of decarburization was observed on the surface.