On March 22, 1987, an Air Tractor AT301 (registration N3655G) operated by Jackson County Spray Co. was involved in an aviation accident near EL Dorado, OK. Investigators recorded the probable cause as: The pilot's failure to maintain control of the aircraft during takeoff due to a loss of engine power caused by a stuck carburetor flapper valve in the closed position, which was the result of a modification to the carburetor heat system. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A pilot experienced black smoke from the engine exhaust during takeoff from a county road, resulting in a collision with a tree and post that caused substantial damage to the aircraft.
What happened
The pilot reported that while initiating takeoff from an unpaved county road, the engine began emitting black smoke. This visual indicator of engine distress occurred early in the ground roll or initial climb phase. Shortly after the smoke appeared, the aircraft lost control or power and collided with a tree and a utility post. The impact resulted in substantial damage to the airframe.
The investigation
Post-accident examination of the aircraft revealed a critical modification to the carburetor heat system. Investigators found that the carburetor flapper valve was stuck in the closed position. This mechanical failure prevented proper airflow or fuel mixture regulation, leading to the observed black smoke and subsequent engine performance loss.
Findings
The primary factor contributing to the accident was the malfunction of the modified carburetor heat system. The stuck flapper valve directly caused the engine to produce black smoke, indicating a rich fuel mixture or combustion issue. This mechanical failure occurred during the critical takeoff phase from an unprepared surface, leaving no margin for error or recovery.
Probable cause
The pilot's failure to maintain control of the aircraft during takeoff due to a loss of engine power caused by a stuck carburetor flapper valve in the closed position, which was the result of a modification to the carburetor heat system.