SEA01LA063

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.

On March 8, 2001, approximately 1230 Pacific standard time, a Piper PA-24, N6337P, collided with a fence while sliding across a field during an intentional gear-up landing after the aircraft experienced a total loss of engine power while in cruise flight near Saint Helens, Oregon. The private pilot and his passenger where not injured, but the aircraft, which was owned and operated by Walker's Crushing and Grading, sustained substantial damage. The 14 CFR Part 91 personal flight, which departed Portland-Hillsboro Airport about 20 minutes prior to the accident, was en route to Paine Field, Everett, Washington. The aircraft descended through instrument meteorological conditions after loosing power, but the pilot executed the forced landing in visual meteorological conditions. The pilot had filed and activated an IFR flight plan. There was no report of an ELT activation.

According to the pilot, because he was flying in conditions that were conducive to the accumulation of carburetor ice, he applied carburetor heat a couple of times while he was climbing through the clouds to his assigned IFR altitude. Soon after he leveled off at 8,000 feet (above the clouds), he applied the carburetor heat again and noted no significant change in engine rpm. About five minutes later, while in cruise flight, he noticed that the aircraft's exhaust gas temperature (EGT) had decreased about 20 degrees. Not being able to determined why the exhaust temperature had dropped, he leaned the fuel mixture, but this action did not seem to result in the desired increase in EGT. Since leaning did not seem to effect the exhaust temperature, the pilot pulled the carburetor heat on momentarily. As he applied the carburetor heat, he checked for an RPM drop, which did not occur. He did not check to see if there was any effect on the manifold pressure, or if the pressure had dropped from what it was at in the preceding few minutes. About one minute after this last brief application of carburetor heat, the engine suddenly quit. The pilot then descended through the overcast and executed a forced landing in an open field located in densely forested terrain. Because the field looked soft and wet, the pilot elected to keep the gear retracted. Although the touchdown was uneventful, as the aircraft was sliding to a stop, it encountered a barbed wire fence.

After the accident, the engine's air induction, fuel delivery, and ignition systems where inspected, and no indication was found of any anomaly or malfunction the would have contributed to a power loss. After the inspection, the engine was started and successfully run on the fuel that remained in the fuel gasculator bowl.

In a post-accident phone interview, the pilot said that he had not been aware that on an aircraft equipped with a constant-speed propeller, such as the one he was flying, a buildup of ice in the carburetor throat would be indicated by a decrease in manifold pressure. He had thought that carburetor icing would cause a decrease in engine rpm (as it does with a fixed-pitch propeller), and therefore had been concentrating on that instead of checking for a drop in manifold pressure. He said that he also had been unaware that as ice built up in the carburetor throat, resulting in a gradual reduction in power, that the propeller governor would maintain a constant engine rpm as long as it was able to continue flattening (reducing) the pitch of the propeller. He also stated that he was unaware that a drop in EGT is also an indication of possible icing of the carburetor throat.