What happened
During a daylight flight near Prince George, British Columbia, a Schweizer 269C helicopter was performing a training maneuver when the engine lost power. The aircraft, operated by an owner-pilot and a flight instructor, was conducting a simulated engine-out approach into a logging cut-block. After the pilot completed the autorotation and applied power to climb, the engine failed, causing the helicopter to shake and yaw to the left. The instructor took control and performed an emergency autorotation into a treeline. Although the aircraft struck trees during the maneuver, the crew managed to reach a nearby clearing to land. The two occupants escaped without injury, but the Schwezer 269C, registration not provided, sustained substantial damage to the landing gear, chin bubble, and main rotor blades.
The investigation
Investigators examined the Lycoming HIO-360-D1A engine and its fuel system components. While a post-accident inspection of the exhaust valves initially suggested clearances were within limits, subsequent engine testing revealed that the fuel/air mixture was excessively lean. This lean mixture was attributed to a late-opening power enrichment valve in the Precision Airmotive Corporation fuel servo and a restricted fuel injector nozzle. During a test run, the engine exhibited smoke and flame from the exhaust and lost power. Further inspection of the cylinders revealed that the exhaust valve-to-guide clearances were actually below the allowable minimum, which had not been detected by the inspection methods prescribed in existing service bulletins.
Findings
- A distorted fuel spray pattern and a lean fuel mixture caused the engine's exhaust valves to overheat.
- The overheating resulted in sticking exhaust valves, which led to the loss of engine power.
- Existing service bulletins were insufficient to mitigate the known risks of power loss associated with this engine and fuel servo combination.
- The engine was likely subjected to high temperatures during the transition from low power during autorotation to high power during the climb, triggering the valve failure.