What happened
While flying in the cruise phase of flight, the pilot had leveled off and was in the process of leaning the fuel mixture. During this maneuver, the mixture control ceased to have any effect on the engine. The pilot reported that the engine subsequently began to lose power before eventually quitting entirely.
In response to the engine failure, the pilot performed a forced landing in tundra terrain. During the landing sequence, the airplane nosed over.
The investigation
A postaccident inspection of the aircraft revealed that the mixture control cable had unscrewed from the mixture control arm. Records indicated that this cable had been replaced 238 hours before the accident. Additionally, the cable had undergone inspections 142 hours and 44 hours prior to the event.
The investigation found that the service manual required the jam nut, which locks the cable to the arm, to be torqued to 15 +/- 2 inch-pounds. There was no requirement for safety wire on this specific linkage. It was noted that if the connection is disconnected, gravity can allow the mixture control arm to fail to reach the idle-cutoff position.