What happened
During the arrival phase of flight, the pilot reduced power on the base leg in preparation for landing. He noticed that his airspeed was decreasing at a rate faster than normal. In response, he advanced the throttle to restore power, but the engine did not respond as expected. The pilot then attempted to resolve the issue by switching fuel tanks and activating the boost pump while applying full throttle, yet the engine remained unresponsive.
Subsequently, while flaring for a forced landing, the aircraft struck a tree and crashed. Both occupants were wearing shoulder harnesses and were not injured.
The investigation
Post-accident examination revealed that the fuel primer was unlocked and extended approximately 1-1/2 inches. An operational check confirmed that the engine started and ran normally at 1500 RPM when the primer was closed. However, when the primer was unlocked and partially opened during the test, the engine began running roughly. Its speed decreased to 700 RPM, and black smoke was emitted through the exhaust.
The manufacturer confirmed that at low RPM with the primer open, fuel would siphon into the manifold, causing the engine to run too rich. The primer had been installed as optional equipment for starts in extremely cold weather and was not listed in the flight manual checklist.
Findings
The primary contributing factor was the partial opening of the fuel primer during flight, which caused the engine to run too rich due to fuel siphoning into the manifold. This condition led to a significant loss of power and rough operation. The pilot's attempt to switch fuel tanks and use the boost pump did not mitigate the issue because the root cause was mechanical configuration rather than fuel supply pressure or tank selection.