Accident Overview
A certified flight instructor and a pilot under instruction were conducting a practice autorotation maneuver onto a taxiway. The helicopter was at an airspeed of 65 knots indicated, with manifold pressure between 21 and 23 inches of mercury, and an altitude of approximately 500 feet above ground level. Shortly after initiating the maneuver, the low voltage warning light illuminated. The pilots scanned the operating gauges and tachometer, which indicated a loss of engine power. The certified flight instructor took control; the helicopter subsequently landed hard, and the tail boom separated from the fuselage.
Post-Accident Examination
Post-accident examination of the engine revealed that it would run rough at idle with full mixture applied. When the mixture was leaned, the engine produced 2,000 revolutions per minute. Examination of the fuel servo disclosed no anomaly other than the idle spring missing from the mixture link and that the idle was out of adjustment. Initial flow test results were out of limits in the idle range but satisfactory at maximum throttle. After proper adjustment of the idle link, test results fell within specified limits in all ranges.
Maintenance Context
Maintenance records indicated that maintenance personnel had recently made an adjustment to the idle setting.
Purpose of the Idle Mixture Spring
The idle mixture spring is designed to control the movement of the mixture adjustment. Without this spring installed, the mixture setting can vibrate to either higher or lower settings, potentially affecting engine performance.