N53725Cessna 172P · 1981

On December 23, 2023, about 1000 Hawaii-Aleutian standard time, a Cessna 172P, N53725, was substantially damaged when it was involved in an accident near Kahului Airport (PHOG), Kahului, Hawaii. The pilot and two passengers were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that, after five touch-and-go landings, while on the base leg of the traffic pattern, the engine lost partial power. The pilot applied carburetor heat, switched the fuel tanks, and pumped the throttle, but engine power remained at idle. Unable to reach the runway, the pilot performed a forced landing short of the runway. Upon touchdown, the airplane nosed over and came to rest inverted, which resulted in substantial damage to the fuselage and empennage. About 20 gallons of automotive fuel remained in the fuel tanks at the accident site.

A postaccident engine examination revealed the presence of fuel in the strainer and carburetor bowl. The fuel was drained from the strainer and carburetor bowl, and a trace amount of water was found. Continuity was established on all linkages to the carburetor and airbox; all linkages functioned normally, and the butterfly valves were found to operate normally. The spark plugs were removed and the plug electrodes appeared normal. The magnetos produced spark on all ignition leads. The airplane was started normally and accelerated smoothly from idle to 1,700 rpm with no anomalies noted.

A review of the local meteorological data and the Carburetor Icing Probability Chart located in the Federal Aviation Administration’s (FAA) Special Airworthiness Information Bulletin CE-09-35, Carburetor Icing Prevention, dated June 30, 2009, revealed that the weather conditions at the time of the accident were conducive to serious icing at glide power.

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.”

According to Transport Canada TP 10737 (Use of Automotive Gasoline [Mogas] in Aviation), Mogas is generally higher in volatility than Avgas and will thus absorb more heat from the mixing air when vaporizing, resulting in ice accumulation at higher ambient temperatures. It goes on to say that “the likelihood of carb icing while flying on Mogas is higher,” and advises that, “[a]lthough the severity of the carb icing and the methods to deal with it are similar for both Avgas and Mogas, its ONSET is likely to occur at HIGHER AMBIENT TEMPERATURES and at LOWER HUMIDITY with Mogas. In other words, conditions under which a pilot may feel there is only a slight risk for carb icing on Avgas may in fact be ideal for the formation of ice while using more volatile Mogas. This will result in the need to select ‘carb heat on’ in less severe icing conditions and for a longer duration while using Mogas.”