On August 17, 2020, about 1715 eastern daylight time, a Cessna 150 airplane, N45083, was substantially damaged when it was involved in an accident near Surry, Maine. The private pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The private pilot reported that the cross-country flight was uneventful until about 5 miles from the destination airport. During the descent into the traffic pattern, the engine sputtered and lost power about 1,500 ft above ground level (agl). He reported that he pumped the throttle and rocked the wings; however, the engine did not regain power. He added 30° of flaps and navigated toward a beach. About 500 ft agl, he saw that the beach was very rocky and elected to ditch in the shallow water in a bay. The airplane landed in the water and the pilot and passenger were able to egress and swim to shore. Review of radar data provided by the Federal Aviation Administration (FAA), revealed that most of the flight was captured on radar data. The flight track began about 5 nautical miles west of the departure airport at 1521 and proceeded on a mostly direct route of flight toward the destination airport. About 1715 the flight track turns toward the eventual accident site in the bay and the airplane slows and descends. The airplane was last identified at 1715:50, at 89 knots groundspeed, 975 ft mean sea level, about 1.5 nautical miles from the accident site. An FAA inspector reported that the airplane was buoyed by first responders after the accident; however, the airplane subsequently nosed over and became mostly submerged in the saltwater bay. Upon the airplane’s recovery to land, days after the accident, the inspector reported that it sustained substantial damage to the wings and fuselage. The engine displayed continuity when the propeller was rotated by hand. The ignition switch was found on BOTH. About 1 gallon of fuel was drained from the gascolator, the left fuel tank contained no fuel or salt water, and the right tank was about half full with diluted saltwater and an undetermined amount of fuel. The throttle and mixture control levers were found full forward and the fuel selector was found ON. The carburetor heat lever was found in the OFF position. During a postaccident interview, the pilot reported that he had about 3 hours of fuel onboard, and he believed the accident flight was about 2 hours and 10 minutes. He recalled that the master fuel ON/OFF lever was on. He could not recall whether the carburetor heat was applied during the descent to land. He added that applying carburetor heat during flight was something he was not normally accustomed to, given that most of his flight experience was with fuel-injected engines, which do not require carburetor heat. Review of an FAA carburetor icing probability chart for the given temperature and dew point near the accident site revealed that the conditions were conducive for serious icing at glide power.

According to the FAA Pilot’s Handbook of Aeronautical Knowledge, carburetor ice occurs due to the effect of fuel vaporization and the decrease in air pressure in the venturi, which causes a sharp temperature drop in the carburetor. If water vapor in the air condenses when the carburetor temperature is at or below freezing, ice may form on internal surfaces of the carburetor, including the throttle valve.

The reduced air pressure, as well as the vaporization of fuel, contributes to the temperature decrease in the carburetor. Ice generally forms in the vicinity of the throttle valve and in the venturi throat. This restricts the flow of the fuel-air mixture and reduces power. If enough ice builds up, the engine may cease to operate. Carburetor ice is most likely to occur when temperatures are below 70°F (21°C) and the relative humidity is above 80 percent. Due to the sudden cooling that takes place in the carburetor, icing can occur even in outside air temperatures as high as 100°F (38°C) and humidity as low as 50 percent.