What happened
While flying in conditions conducive to the formation of carburetor ice, the pilot observed a reduction in the exhaust gas temperature (EGT). In an attempt to rectify the situation, the pilot leaned the fuel mixture, but the EGT did not increase as expected. The pilot then briefly applied carburetor heat and monitored engine RPM for a decrease.
Shortly after this procedure, the engine lost all power. The pilot subsequently performed a forced landing in a soft, wet field. The aircraft made an intentional gear-up touchdown that was otherwise uneventful; however, as the aircraft slid across the terrain, it struck a barbed wire fence.
The investigation
The investigation revealed that because the aircraft was equipped with a constant-speed propeller, checking for a decrease in RPM was not the correct method to identify ice accumulation. Instead, the pilot should have monitored for a drop in manifold pressure.
Further examination of the pilot's actions indicated several points of unawareness regarding engine monitoring:
- The pilot did not realize that a drop in EGT can serve as an indication of ice accumulating in the carburetor throat.
- The pilot was unaware that the propeller governor would maintain a constant RPM as long as it could flatten the propeller pitch, even as ice accumulation caused a gradual reduction in power.
- The pilot was unaware that checking manifold pressure is the proper procedure for detecting carburetor icing when using a constant-speed propeller.