On July 27, 2022, about 0910 Alaska daylight time, a 5 Rivers LLC SQ2 airplane, N58SQ, was substantially damaged when it was involved in an accident near Soldotna, Alaska. The private pilot sustained serious injuries, and the passenger was uninjured. The airplane was operated by the pilot as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that, while in cruise flight at about 4,000 ft msl, he advanced the throttle to initiate a climb, but the engine began to lose power. In an effort to regain power, he further advanced the throttle and propeller levers and switched the fuel selector to the left tank to no avail. He subsequently made a forced landing in a small gully, which resulted in substantial damage to the wings and fuselage.

A postaccident examination revealed no preaccident mechanical malfunctions or anomalies with the airplane that would have precluded normal operation.

A Global Data Assimilation System (GDAS) model sounding was created for the accident time and location, with the modeled surface elevation about 2,415 ft msl. When the GDAS sounding data was interpolated for an elevation of 4,000 ft msl, the data indicated the temperature was about 4.9°C (40.8°F) and the dewpoint was about 1.6°C (34.9°F), with a relative humidity of about 79 percent.

Based on the carburetor icing probability chart, these conditions would result in “serious icing – cruise power.” When the pilot was asked if he applied carburetor heat after the loss of power, he stated that he did not, as there was no visible moisture in the area, and he thought carburetor ice would be unlikely.

Figure 1. Carburetor icing probability chart (Courtesy of FAA). The conditions present at the time of the power loss are indicated with a red circle.

A Lycoming Service Instruction states in part: "Flight Operation – During normal flight, leave the carburetor air heat control in the full cold position. On damp, cloudy, foggy or hazy days, regardless of the outside air temperature, be alert for loss of power. This will be evidenced by an unaccountable loss in manifold pressure or RPM or both, depending on whether a constant speed or fixed pitch propeller is installed on the aircraft. If this happens, apply full carburetor air heat, and open the throttle to limiting manifold pressure and RPM.”