Accident Sequence
Approximately five minutes into a cross-country instructional flight, the engine RPM began to drop. The RPM continued to decrease steadily until the engine lost total power. Attempts to restart the engine were unsuccessful. During the subsequent forced landing to a field, the nose wheel contacted soft ground, causing the airplane to nose over and come to rest inverted.
Investigation Findings
At the time of the accident, the airplane had accumulated 2 hours of total time since its last annual inspection. During that annual inspection, the carburetor had been removed for additional maintenance. Examination of the carburetor revealed that when the throttle control was actuated from inside the cockpit, the throttle cable and lever did not actuate the throttle shaft and throttle valve. The castellated lock nut securing the throttle lever to the throttle shaft on the carburetor was found not to be safetied via a required cotter pin. The lock nut had loosened, allowing the throttle cable and lever to move without corresponding movement of the throttle shaft and throttle valve.