Event
A pilot conducting a banner-towing flight returned to a field to release the banner. During the descent to the field, the pilot reduced engine power. Just before releasing the banner, the pilot increased the throttle to arrest the descent and initiate a climb. However, the pilot reported "little or no positive reaction from the engine." After releasing the banner, the aircraft continued to descend. The pilot prepared for a forced landing to the field, during which the aircraft impacted a tree and caught fire.
Investigation
Examination of the airplane and engine revealed no mechanical anomalies. The carburetor heat was found in the "off" position. The closest weather reporting station, approximately 6 miles north of the accident site, recorded a temperature of 34 degrees Celsius and a dew point of 22 degrees Celsius. Interpolation of an FAA carburetor icing probability chart indicated that the conditions were conducive to carburetor ice at glide and cruise power settings.
Carburetor Icing Conditions
According to the FAA Pilot's Handbook of Aeronautical Knowledge, during closed-throttle operations, carburetor heat should be adjusted to the full ON position before closing the throttle and left on during the closed-throttle operation. The handbook also states that after a power loss is noticed, immediate action should be taken to eliminate ice already formed in the carburetor and to prevent further ice formation.