N10495

On October 24, 2024, about 1535 Hawaii-Aleutian standard time, a Cessna 150 airplane, N10495, sustained substantial damage when it was involved in an accident near Lanai, Hawaii. The pilot and passenger onboard were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that, after climbing to about 800 ft agl, the engine suddenly stopped producing power. The pilot reported applying full carburetor heat and the engine began to make some power. The pilot then reduced carburetor heat and, again, the engine stopped producing power. Subsequently, the pilot applied full carburetor heat, and the engine began to produce some power; however, the engine was not producing enough power to maintain altitude. The pilot made an emergency landing on a dirt road about three miles north of the airfield. The airplane sustained substantial damage to the left wing.

The automotive gas (Mogas) was drained from the carburetor bowl; no particulates or water was observed. Continuity was established between the throttle, mixture, and carburetor heat controls in the cockpit and the engine.

The spark plug electrodes were inspected and found to have normal wear. The No. 4 cylinder spark plugs were observed to be white in color while the spark plugs from the other cylinders were observed to have a smoky dark gray appearance. Ignition was verified on all spark plugs. Thumb compression was observed on all cylinders. The cylinders were examined with a borescope and the No. 4 cylinder was observed to have light longitudinal scratches from top of ring travel to the bottom of the piston’s stroke. All cylinders were in serviceable condition.

The airplane engine was started and idled within normal limits. The rpm was increased to 1,700 and the magnetos and carburetor heat both functioned normally and the engine ran smoothly. The rpm was increased to 2,400 and the engine ran a little rough; rpm was reduced to 2,000 rpm and the engine ran a little worse. The engine idled smoothly when warm at 700 rpm. The oil filter was removed; there were small flakes of metal observed in the oil filter media, but no glitter. The Marvel Schebler MA-3SPA 10-4894-1 carburetor was removed for further inspection. At the time of the accident the carburetor had 708.6 hours since overhaul. The carburetor was reported to be primarily run on unleaded Autogas.

On December 5, 2024, the FAA and Marvel Schebler conducted a flow test and disassembled the carburetor. The flow test was slightly rich but within normal parameters. The carburetor was disassembled; a small amount of debris was noted on the internal filter but nothing likely to cause operational issues.

On the day of the accident, at 1535, the weather reported at Lanai Airport (LNY), about 3 miles to the south of the accident site, included a temperature of 24°C and a dew point of 19°C. The calculated relative humidity at this temperature and dewpoint was 70%. Review of the icing probability chart contained within FAA Special Airworthiness Information Bulletin (CE-09-35) – Carburetor Icing Prevention, revealed the atmospheric conditions at the time of the accident were “conducive to serious icing at glide [idle] power.”

The bulletin further 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 FAA Advisory Circular 20-113, "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."

According to Transport Canada publication 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 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.