N4375U

On August 25, 2021 about 0945 Pacific daylight time, a Cessna 150D, N4375U, was substantially damaged when it was involved in an accident near Ontario, California. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot stated that, before departure, she added 16 gallons of fuel to the fuel tank, which brought the total fuel aboard to 28 gallons. The pilot maneuvered in the local area for about 2 hours 45 minutes before returning to the airport. During the descent to 2,500 ft mean sea level (msl), the engine began to surge; in response, the pilot added carburetor heat. The engine sounded normal for about 3 to 4 minutes and then began to surge again.

As the airplane approached the airport, the pilot noticed that a partial loss of engine power had occurred, and that the engine was running between 1,000 and 1,200 rpm, equivalent to a near-idle setting. The pilot attempted to troubleshoot the partial loss of power by verifying that the mixture was rich, the fuel selector was positioned to “BOTH,” and the magnetos were both selected. The pilot further stated that, despite these attempts, the airplane could not maintain level altitude, so she selected an area for an off-airport landing. About 20 seconds before touchdown, the engine lost total power, and the airplane collided with a fence, causing damage to the airplane wings. The pilot noted that, after the accident, fuel was draining from the airplane, and moisture (haze) was visible.

Postaccident examination at the accident site found that the fuel tanks were intact, and only a small drip from the left fuel vent was observed. The airplane owner stated that he recovered the airplane and that about 3.5 to 4 gallons of fuel from the left wing and about 0.5 gallons from the right wing were drained. The owner added that both the carburetor bowl and gascolator contained fuel.

A representative from Cessna Aircraft stated that the airplane was equipped with longrange fuel tanks that held a total of 38 gallons of fuel. The representative estimated that the airplane had a range of 800 miles (or 7.1 hours) at 5,000 ft msl, with the engine at 2,500 rpm, and at a cruise speed of 113 mph.

The airplane was equipped with an on/off fuel selector valve, and the two fuel tanks were interconnected before the valve. A check valve in the left fuel tank vent line was designed to prevent fuel from leaking out, but the valve contained a small (0.04inch) hole so that pressure in a full tank could bleed out. The Cessna representative stated that this hole would allow a small drip of fuel to leak out of the tank when the airplane was in a leftwinglow position. The amount of fuel that leaked out would be based on the amount of fuel that was in the tank (head pressure) and the length of time that the airplane was in that position. Fuel could escape from the system if damage had occurred forward of the fuel valve. The Cessna representative estimated that, on the basis of the accident flight track, the airplane should have had more than 10 gallons of fuel when the engine failure occurred.

In a postaccident statement, the pilot indicated that, given the haze outside, she should have applied carburetor heat as soon as she began descending the airplane. The pilot further stated that she should have had a greater reserve of fuel and had planned for a 20% discrepancy in the airplane’s fuel performance numbers.

The meteorological aerodrome report for Chino, California, that was issued at 0953 stated that the temperature was 79°F and the dew point was 61°F. The temperatures were applied to the FAA Carburetor Icing Probability Chart, which revealed that both recorded temperatures were within the "serious icing at glide power" portion of the chart.

The FAA published Special Airworthiness Information Bulletin (SAIB) CE-09-35 in June 2009, which discussed carburetor ice prevention. The SAIB noted that carburetor icing does not only occur in freezing conditions; it can occur at temperatures well above freezing when visible moisture or high humidity is present. Vaporization of fuel combined with the expansion of air as it flows through the carburetor (the Venturi effect) causes sudden cooling, and a significant amount of ice can build up within a fraction of a second.