N669WH

On March 5, 2018, about 1430 mountain standard time, an experimental MacDonald Craig Mac Cub airplane, N669WH, was involved in an accident near Ogden, Utah. The pilot and pilot-rated passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that, while the airplane was maneuvering over a marshland, the engine began to run roughly, and a partial loss of power followed. Because the airplane was unable to maintain altitude, the pilot initiated a forced landing onto a 10-ft-wide muddy road. During the landing roll, the airplane sunk in the mud, nosed over, and came to rest inverted. The left wing and vertical stabilizer were substantially damaged.

The pilot reported that the carburetor heat control was not activated at the time of the partial loss of engine power. The pilot was in the rear airplane seat, and the pilot-rated passenger was in the front seat. Although the pilot had access to both sets of flight controls, the rear seat compartment was not equipped with a carburetor heat control handle.

The Ogden-Hinckley Airport (OGD) automated weather observing station was located about 6 nautical miles northwest of the accident site The 1353 observation reported that the temperature was 35.6°F and that the dew point was 19.4oF.

An engine examination and test run were performed. The results revealed no preaccident mechanical failures or malfunctions that would have precluded normal operation.

The Federal Aviation Administration Special Airworthiness Information Bulletin CE-09-35, Carburetor Icing Prevention, stated the following:

Pilots should be aware that carburetor icing doesn't just occur in freezing conditions, it can occur at temperatures well above freezing temperatures when there is visible moisture or high humidity. Icing can occur in the carburetor at temperatures above freezing because vaporization of fuel, combined with the expansion of air as it flows through the carburetor, (Venturi Effect) causes sudden cooling, sometimes by a significant amount within a fraction of a second. Carburetor ice can be detected by a drop in rpm in fixed pitch propeller airplanes and a drop in manifold pressure in constant speed propeller airplanes. In both types, usually there will be a roughness in engine operation.

The special airworthiness information bulletin included a chart that showed the probability of carburetor icing for various temperature and relative humidity conditions. According to that chart, the weather conditions at the time of the accident were conducive to carburetor icing at glide and cruise power.