What happened
A pilot and 4 fare-paying passengers were enroute to the Grand Canyon National Park for aerial sightseeing when the aircraft experienced a total loss of engine power. The engine failure was caused by an internal failure of the left magneto. Following the loss of power, the aircraft collided with terrain. The impact resulted in the destruction of the aircraft by ground fire.
The investigation
Investigators examined the left magneto and determined it was not in compliance with AD 78-09-07 R3, which addressed impulse coupling failure. The investigation revealed that the air taxi company employed only two people: an owner/president/pilot and a director of maintenance/mechanic. The mechanic acknowledged failing to follow the correct guidelines specified in the applicable Airworthiness Directive.
Technical analysis showed that the impulse coupling jammed when the clearance between the flyweight and the stop pin reached zero. This mechanical failure caused the teeth on the engine's crankshaft gear and the dowel pin to shear off.