The pilot was performing “practice” aerial application operations at 30′ above the ground and the Cessna 188’s engine slowly lost power.
He made a left turn and landed in a bean field near What Cheer, Iowa, which resulted in substantial damage to the right wing and left elevator.
During post-accident interviews with the pilot, he reported that immediately before the loss of engine power he was flying wings level, the mixture was forward (full rich), and the power was set to a cruise setting.
He further reported that the carburetor heat was off for the entirety of the flight.
He said the loss of engine power was gradual and there was no visible oil spray over the windscreen. He reported that immediately after the accident he checked the oil level and found no anomalies.
During a post-accident examination, an airframe and powerplant mechanic reported that he drained 13 gallons of fuel from the single fuel tank. He further reported that he observed the float carburetor and gascolater, and both were unremarkable. He also reported that the propeller could be rotated in a normal manner.
The FAA Pilot’s Handbook of Aeronautical Knowledge (PHAK) states in part: “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 PHAK further states: “If enough ice builds up, the engine may cease to operate. Carburetor ice is most likely to occur when temperatures are below 70° Fahrenheit (°F) or 21° Celsius (°C) and the relative humidity is above 80%. 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%. This temperature drop can be as much as 60 to 70 absolute (versus relative) Fahrenheit degrees (70 x 100/180 = 38.89 Celsius degrees).”
An automated weather observing system about the time of the accident 13 nautical miles northeast of the accident site reported a temperature of 81° Fahrenheit (27 Celsius) and a dew point 66° Fahrenheit (19 Celsius).
According to a carburetor-icing-probability chart, the engine was at risk of “moderate icing” at cruise power.
The airplane manufacturer owner’s manual in part states: “In atmospheric conditions that are conducive to carburetor icing, select the minimum amount of carburetor heat for normal operation that will keep ice cleared from the carburetor.”
It is likely that the loss of engine power was due to carburetor ice, however, the pilot, who was also the airplane owner, did not make the airframe or engine available for examination by the National Transportation Safety Board.
Probable cause: The loss of engine power for undetermined reasons due to the pilot not making the airplane or engine available for examination, which resulted in a forced landing and an impact with terrain.
NTSB Identification: GAA16CA393
This July 2016 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.