Circumstances
While en route in conditions conducive to carburetor icing, the pilot observed a decrease in the aircraft's exhaust gas temperature (EGT). The pilot leaned the fuel mixture, but the EGT did not rise to the expected level. He then momentarily applied carburetor heat, checking for a decrease in engine rpm—a proper procedure for an aircraft with a fixed-pitch propeller. However, this aircraft was equipped with a constant-speed propeller, for which the correct check is a drop in manifold pressure. The pilot did not perform that check.
Power Loss and Forced Landing
Shortly thereafter, the engine lost all power. The pilot initiated a forced landing in a soft, wet field. The aircraft touched down with the landing gear retracted intentionally, and the landing was uneventful. As the aircraft slid across the field, it encountered a barbed wire fence.
Pilot Interview
In a telephone interview after the accident, the pilot stated that he was unaware that the proper procedure for checking carburetor ice accumulation in a constant-speed propeller aircraft is to monitor manifold pressure drop. He also did not know that as ice accumulates in the carburetor throat, the propeller governor maintains a constant rpm by flattening the propeller pitch, masking the power reduction. Additionally, he was unaware that a drop in EGT can indicate carburetor ice accumulation.