N8337XCessna 172C · 1961

On April 14, 2024, about 2008 eastern daylight time, a Cessna 172C, N8337X, was substantially damaged when it was involved in an accident near Manassas, Virginia. The private pilot was not injured, while the passenger sustained minor injury. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot was enroute to the destination airport at 5,500 ft mean sea level (msl). According to communications information from Federal Aviation Administration (FAA), the pilot was instructed by air traffic control to descend to, but remain at or above, 2,600 ft msl, and the pilot reported that he reduced throttle, but did not apply carburetor heat at any time during the descent. According to ADS-B data, the airplane began gradually descending from about 5,500 ft after 1959:09. The pilot was later cleared to descend to 2,000 ft msl, and while flying at that altitude with the destination airport in sight, he applied throttle to increase engine power, but the engine did not respond. About 2006, he informed the tower controller that the “motor was unresponsive” and the controller cleared the airplane to land on runway 16R. The airplane touched down in a grassy area on airport property short of the runway, collided with a drainage culvert, and nosed over, resulting in substantial damage to the vertical stabilizer.

Posataccident examination of the engine by a FAA inspector confirmed crankshaft, camshaft, and valvetrain continuity. Thumb suction and compression were noted in all cylinders, though it was low in two of the cylinders due to leakage past each intake valve. Examination of the air induction, ignition, exhaust, and fuel metering systems revealed no evidence of preimpact failure or malfunction. Impact damage to the carburetor precluded functional testing of the engine.

A High Resolution Rapid Refresh (HRRR) model sounding was created for the approximate time and location where the pilot initiated the descent. At 5,069 ft msl, the HRRR sounding indicated the temperature and dew point were about 16.8°C and about 0.4°C, respectively, with a relative humidity of 33 percent. At 6,076 ft msl, the HRRR sounding indicated the temperature and dew point were about 13.7°C and about -0.6°C, respectively, with a relative humidity of 37 percent.

Review of the icing probability chart contained within Federal Aviation Administration Special Airworthiness Information Bulletin CE-09-35 indicated that the atmospheric conditions at the time of the accident were “conducive to serious icing at glide [idle] power.”

FAA Advisory Circular 20-113 stated, "To prevent accident due to induction system icing, the pilot should regularly use [carburetor] heat under conditions known to be conducive to atmospheric icing and be alert at all times for indications of icing in the fuel system." The circular recommended that when operating in conditions where the relative humidity is greater than 50 percent, "…apply carburetor heat briefly immediately before takeoff, particularly with float type carburetors, to remove any ice which may have been accumulated during taxi and runup." It also stated, "Remain alert for indications of induction system icing during takeoff and climb-out, especially when the relative humidity is above 50 percent, or when visible moisture is present in the atmosphere."

A review of the airplane owner’s manual revealed that, although the “Let-Down” checklist did not specify the application of carburetor heat, the “Before landing” checklist did state to apply carburetor heat before closing the throttle.