Accident Overview
The pilot and three passengers were on a cross-country sightseeing flight when the engine began to run rough and subsequently lost power. The aircraft collided with terrain during a forced landing and struck a tree before coming to rest.
Engine Examination
After the accident, the engine was checked and found to run only at idle power. The engine was equipped with a Marvel-Schebler model HA-6 carburetor. During removal of the carburetor for examination, the rotary mixture control (part number 242-541) fell out of the carburetor housing because the shaft retaining screw had broken.
Service Bulletin
In September 1978, Marvel-Schebler/Tillotson, the original manufacturer of the carburetor, issued Service Bulletin #A1-78. This bulletin provided for the installation of a spring retainer (part number 55-A239) to counteract end-play wear on the mixture control assembly and retaining screw due to vibration. The spring retainer also served as an additional means of retaining the rotary mixture control, even if the existing shaft retaining screw was worn or broken. The service bulletin had not been accomplished on the accident aircraft.
Engine History
The engine had a total operating time of 2,568 hours, with 485 hours since overhaul and 4.7 hours since the previous 100-hour inspection.