In a typical aircraft reciprocating-engine fuel system, what is the primary purpose of the engine-driven fuel pump?
ACS code: AM.II.J
Correct answer: To supply fuel under pressure to the carburetor or fuel injection system during normal engine operation
Rationale: The engine-driven fuel pump is the main pump in a gravity-independent system; it delivers fuel under pressure to the metering device (carburetor or injection unit) whenever the engine is running. Venting is handled by the tank vent system, and metering of the air-fuel ratio is the job of the carburetor or fuel injection system, not the pump.
What is the purpose of the fuel tank sump and its drain valve?
ACS code: AM.II.J
Correct answer: To collect water and sediment at the lowest point of the tank so they can be drained off
Rationale: The sump is a low point in the tank where water (heavier than fuel) and sediment settle and collect. A drain valve at the sump lets the mechanic or pilot drain off the contaminants during preflight or maintenance. It is not a pressure-relief device, and the vent connects to the top of the tank, not the sump.
During a fuel system pre-flight check, water contamination in a tank is most reliably detected by
ACS code: AM.II.J
Correct answer: draining a fuel sample from the lowest sump and checking for separated water
Rationale: Water is heavier than aviation fuel and settles to the lowest point in the tank, the sump. Draining a sample from the sump drain lets the mechanic see water as a separate layer or clear globules at the bottom of the sample. Fuel quantity gauges and line temperature are not reliable indicators of water contamination.
In a gravity-feed fuel system, fuel flows to the engine because
ACS code: AM.II.J
Correct answer: the fuel tanks are mounted above the carburetor and gravity moves the fuel downward
Rationale: A gravity-feed system relies on the tanks being located higher than the carburetor, typically in a high-wing aircraft, so fuel flows downhill to the engine without a pump. Suction-lift from a pump describes a pressure-feed system, and tank pressurization is not used in simple gravity-feed designs.
What is the purpose of a strainer or filter in an aircraft fuel system?
ACS code: AM.II.J
Correct answer: To trap dirt, water, and other contaminants before fuel reaches the metering system
Rationale: The main fuel strainer or filter removes solid contaminants and helps separate water before fuel reaches the carburetor or injection unit, protecting downstream components. Filters do not raise fuel pressure, and vaporizing fuel is a function of the carburetor venturi and intake, not the strainer.
Vapor lock in a fuel system is best described as a condition in which
ACS code: AM.II.J
Correct answer: fuel vaporizes in the lines and the resulting vapor bubbles interrupt the flow of liquid fuel
Rationale: Vapor lock occurs when fuel in the lines vaporizes, usually from excessive heat or low pressure, forming vapor bubbles that block or interrupt the flow of liquid fuel to the engine. Ice in the filter is a separate icing problem, and a stuck selector valve is a mechanical malfunction, not vapor lock.
When servicing a fuel system, the use of a boost pump to clear vapor from the lines helps prevent vapor lock primarily because it
ACS code: AM.II.J
Correct answer: raises the pressure in the lines, which keeps the fuel in a liquid state
Rationale: Increasing line pressure raises the temperature at which the fuel will vaporize, so a boost pump that pressurizes the lines keeps fuel in liquid form and suppresses vapor lock. The pump does not freeze the fuel, and removing dissolved water is a function of filters and sumps, not vapor-lock prevention.