Amateur-built RV-10 N917VA accident near Sun Lakes, Arizona

Probable cause

The fatigue fractures of the upper and lower alternator attachment bolts, which decreased the common drive belt tension and resulted in the subsequent in-flight separation of an alternator drive belt, the overheating of the engine, and the subsequent total loss of engine power.

NTSB analysis

The private pilot reported that, during the en route climb, he smelled antifreeze and realized that the engine was overheating. He then reduced power to idle and turned the airplane back toward the departure airport. As the airplane descended, the pilot added power, but the engine did not respond. He decided to land at a nearby closed airport; however, when he realized that the airplane would be unable to reach it, he performed a forced landing to a desert. During the landing roll, the airplane struck several bushes, which resulted in the nosewheel and left main landing gear collapsing and the left wing and fuselage sustaining substantial damage.

The airplane was powered by a modified automobile engine, which was equipped with two alternators and a water pump to circulate the engine coolant. The common drive belt that connected the two alternators and water pump was found detached. In addition, the upper and lower attachment bolts that secured one of the alternators were fractured, and the alternator was partially displaced. Examination of the fracture surfaces of both bolts revealed that they exhibited rachet marks, consistent with fatigue fractures. It is likely that, once the alternator attachment bolts fractured, the common drive belt tension decreased, which resulted in the separation of the belt. The belt separation would have precluded the water pump from operating and led to the engine overheating and eventually losing total power.

Text from the NTSB. Open the NTSB docket for WPR18LA030 for the full record.