What happened
The pilot and three passengers were conducting a cross-country sightseeing flight when the engine began to run rough and subsequently lost power. During the ensuing forced landing, the aircraft collided with terrain and struck a tree before coming to rest.
The investigation
Post-accident inspections of the engine revealed that while the engine would run, it was limited to idle speed. The engine was equipped with a Marvel-Schebler model HA-6 carburetor. During the removal of the carburetor for examination, the rotary mixture control (PN 242-541) fell out of the housing because the shaft retaining screw had broken.
Investigation into the maintenance history showed that the manufacturer, Marvel-Schebler/Tillotson, had issued Service Bulletin #A1-78 in September 1978. This bulletin recommended the installation of a spring retainer (PN 55-A239) designed to counteract end-play wear on the mixture control assembly and the retaining screw caused by vibration. The spring retainer is intended to provide an additional means of retaining the rotary mixture control even if the shaft retaining screw becomes worn or broken. This service bulletin had not been accomplished.
The engine had a total operating time of 2,568 hours, with 485 hours since its last overhaul and 4.7 hours since the previous 100-hour inspection.