N4922GCessna 172N · 1979

On July 8, 2023, at 2201 Pacific daylight time, a Cessna C172N, N4922G, was substantially damaged when it was involved in an accident near San Rafael, California. The pilot was seriously injured and the passenger was fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The airplane departed San Rafael Airport (CA35), San Rafael, California, about 1000 and arrived at Silver Springs Airport (SPZ), Silver Springs, Nevada, at 1150. According to the pilot, he intended on refueling the airplane at SPZ before returning to CA35. Fuel was available at SPZ; however, airport personnel were not available to operate the necessary equipment to refuel the airplane. The pilot subsequently departed from SPZ about 1900 and flew to Minden-Tahoe Airport (MEV), Minden, Nevada, about 1930, where the pilot purchased 22.63 gallons of 100 Low Lead (LL) fuel using the self-service fuel pump.

According to the pilot and ADS-B flight track data, the airplane departed from MEV at 2000 and circled over MEV as it climbed from 4,600 ft mean sea level (msl) to about 12,500 ft msl in order to clear rising terrain before proceeding direct to CA35. The pilot stated he remained at altitude as long as possible before starting an enroute descent about 65 miles northeast of CA35. The airplane maintained 90-110 knots during the descent on a heading of about 240° until about 2159. During the remainder of the flight the airplane continue to slow and descend, with the airplane slowing to 43 knots at 500 ft at 2200:17. The airplane continued to descend, and its ground speed increased until the last ADS-B data return at 2200:48, which indicated 70 kts at 0 ft about 0.4 miles northeast of the approach end of runway 22 at CA35. The accident site was located about 0.2 miles northeast of the approach end of runway 22 at CA35.

The pilot did not have strong memories of the accident flight, but recalled the engine did not respond when he advanced the throttle as he attempted to level off during the descent. He checked the fuel selector, magnetos, and selected the carburetor heat, but the engine did not respond. He then attempted to maintain the best glide airspeed of 70 knots but could not recall what happened during the remainder of the flight. The pilot reported that he would not normally select the carburetor heat until below 1500 ft and entering the traffic pattern.

Figure 1 – ADS-B Flight Profile View (Google Earth)

Figure 2 – ADS-B Final Approach Profile (Google Earth)

According to the carburetor icing probability chart in the FAA Special Airworthiness Information Bulletin CE-95-35, Carburetor Icing Prevention, the temperature and dew point at the time of the accident were conducive to the formation of serious [carburetor] icing (at cruise power).

The airplane wreckage was mostly intact and located on the edge of a small waterway about 0.2 miles northeast of the approach end of runway 22 at CA35. The wreckage was partially submerged in water, partially inverted, and oriented with the nose pointed in an easterly direction. High tension power lines, about 30 ft tall about 175 ft northeast of the wreckage, were found broken.

A postaccident examination of the wreckage was conducted. Flight control continuity was verified from the cockpit to all flight control surfaces, with the flaps found in the UP position. Both wings remained partially attached to the fuselage by their respective control cables and both fuel tanks’ pick-up lines were separated from the fuel tanks. The fuel selector valve was found in the “Both” position. All fuel selector valve positions were tested with compressed air and the valve operated as designed; no fuel was present in the lines. A small amount of blue in color liquid consistent with fuel was found in the strainer bowl. A large amount of water was also present in the strainer bowl (Note: The engine was submerged in water at the accident scene). A fuel smell was noticed when the right tank fuel cap was opened.

Arcing signatures were observed on the right-wing tip as well as the wing struts of both wings. Powerline cable impact signatures were observed on both wing struts and on both sides of the empennage. An undetermined length of an approximately 1-inch diameter cable was wrapped around the rudder, both horizontal stabilizers, and the elevator. Both wing struts were severed at their fuselage attachment points consistent with impact with a cable. The carburetor heat selector was found on-scene in the ON position. No preimpact anomalies were noted with the airframe during examination.

Postaccident examination of the engine revealed no evidence of any preaccident mechanical malfunctions or failures that would have precluded normal operation. The metal two-bladed fixed-pitch propeller remained attached at the crankshaft flange and the propeller blades remained attached to the propeller hub. Both propeller blades were straight and no chordwise scratches or leading-edge gouges were noted. One propeller blade exhibited witness marks (about midspan) and arcing signatures consistent with contacting an energized powerline. The subject propeller blade tip was missing a corner section of material and exhibited arcing signatures.

FAA Special Airworthiness Information Bulletin CE-09-35, Carburetor Icing Prevention, stated that:

…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.

Step 3 of the normal Descent checklist contained in the Pilot’s Operating Handbook states “Carburetor Heat – AS REQUIRED (to prevent carburetor icing).”

Step 4 of the Before Landing checklist in the Pilot’s Operating Handbook states “Carburetor Heat – ON (apply full heat before closing the throttle).”