SEA01LA063 · Piper PA-24

While en route in conditions conducive to carburetor icing, the pilot noticed a reduction in the aircraft's exhaust gas temperature (EGT). Although he leaned the fuel mixture, this did not seem to increase the EGT to where he thought it should be. He therefore momentarily applied the carburetor heat and checked for a decrease in engine rpm, as is the proper procedure in an aircraft equipped with a fixed-pitch propeller. But since this aircraft was equipped with a constant-speed propeller, he should have checked for a drop in manifold pressure, which he did not do. Soon thereafter, the engine lost all power and the pilot descended to a forced landing in a soft wet field. Although the intentional gear-up touch-down was uneventful, as the aircraft slid across the field, it encountered a barbed wire fence. In a telephone interview after the accident, the pilot stated that he had been unaware that the proper procedure to use in checking for the accumulation of carburetor icing with a constant-speed propeller was to check manifold pressure drop. He was also not aware that as ice accumulated in the carburetor throat, resulting in a gradual reduction of power, that the propeller governor would keep the rpm constant as long as it was able to flatten the pitch of the propeller. He was also unaware that a drop in EGT may also be an indication of ice accumulating in the carburetor throat.

Verbatim NTSB analysis and probable cause from the NTSB dataset