What happened
A certified flight instructor and a pilot under instruction were conducting a practice autorotation onto a taxiway. During the maneuver, the aircraft was at approximately 500 feet above ground level with an indicated airspeed of 65 knots and manifold pressure between 21 and 23 inches. Shortly after initiating the procedure, a low voltage warning light illuminated. Upon checking the operating gauges and tachometer, the crew identified a loss of engine power. The instructor took control of the aircraft, which subsequently performed a hard landing. The impact caused the tail boom to separate from the airframe.
The investigation
Post-accident examination of the engine showed that it ran rough at idle when the full mixture was applied, though it produced 2,000 revolutions per minute when the mixture was leaned. An inspection of the fuel servo revealed that the idle spring was missing from the mixture link and the idle was out of adjustment. While initial flow test results were outside of acceptable limits in the idle range, they were satisfactory at maximum throttle. After adjusting the idle link, test results returned to within specified limits across all ranges.
Findings
Investigators determined that the engine's mixture setting could vibrate between higher and lower settings because the missing idle spring failed to control the movement of the mixture adjustment. Maintenance records indicated that personnel had recently performed an adjustment to the idle setting.