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.
Aviation gasolines are dyed different colors primarily so that a mechanic can
ACS code: AM.II.J
Correct answer: identify the grade of fuel and detect mixing of different grades
Rationale: Different grades of avgas are dyed distinct colors (for example, blue and green for common grades) so the grade can be identified by sight and so contamination from mixing grades can be spotted, since a mixture produces an off or clear color. Color does not give a precise octane number, and dye is not an indicator of suspended water content.
What effect does water contamination have on the indication of a capacitance-type fuel-quantity system?
ACS code: AM.II.J
Correct answer: It causes the contents to over-read
Rationale: A capacitance fuel-quantity probe reads quantity from the dielectric between its plates, and water has a much higher dielectric constant than fuel. Water settling in the tank raises the measured capacitance more than an equal volume of fuel would, so the gauge over-reads the true fuel contents.
On what electrical principle does a capacitance-type fuel-quantity indicating system operate?
ACS code: AM.II.J
Correct answer: Capacitance
Rationale: A capacitance fuel-quantity system uses tank-mounted probes that act as capacitors whose dielectric is the fuel and air between the plates. As fuel level changes, the proportion of fuel to air alters the probe's capacitance, which is measured to indicate quantity by mass. Because fuel and air have different dielectric constants, the system reads on the principle of capacitance rather than simple resistance.
When checking a capacitance-type fuel quantity indicating system, how is the test set connected to the tank probes?
ACS code: AM.II.J
Correct answer: In parallel with the capacitance probes
Rationale: A capacitance fuel-quantity test set is connected in parallel with the tank probes so it can inject or simulate known capacitance values across the same circuit the indicator reads. Connecting in series or removing the probes would change the measured capacitance and prevent a valid calibration check of the system as installed.
The test set used to check a capacitance-type fuel quantity indicating system is built around a
ACS code: AM.II.J
Correct answer: capacitance bridge
Rationale: A capacitance-type fuel quantity system measures fuel by the change in tank-probe capacitance, so its test set is built around a capacitance bridge that simulates and checks those capacitance values. Resistance or inductance decade boxes would not match the capacitive sensing principle of the gauging system.
In a capacitance-type fuel quantity system, the compensator unit installed at the bottom of the tank measures the:
ACS code: AM.II.J
Correct answer: Dielectric constant (K value) of the fuel
Rationale: A capacitance fuel-gauging system depends on the dielectric constant (K value) of the fuel, which varies with fuel type and temperature. The compensator unit, always immersed at the bottom of the tank, measures this K value so the system can correct the main probes and read true fuel mass. It does not directly read quantity or specific gravity (FAA-H-8083-31, Aircraft Fuel Systems — capacitance fuel quantity indicating).
A float-type fuel quantity gauging system is best described as one that
ACS code: AM.II.J
Correct answer: Cannot be adjusted in service
Rationale: A float-type fuel gauge mechanically follows the fuel surface through a float and arm, and its calibration is fixed by the geometry of the float, tank, and sender. There is no in-service empty or full adjustment to make, so the system cannot be adjusted; faults are corrected by replacing components.
In a capacitance-type fuel quantity system, changes in fuel temperature are compensated by
ACS code: AM.II.J
Correct answer: placing a reference capacitor in parallel with the tank unit capacitance
Rationale: The dielectric constant of fuel changes with temperature, which would shift the capacitance reading. A compensating (reference) capacitor immersed in the fuel and connected in parallel with the tank-unit capacitance experiences the same change and cancels the temperature error. A simple series resistor or a potentiometer cannot track the dielectric variation.
A fuel quantity system that measures the specific gravity of the fuel displays the contents in
ACS code: AM.II.J
Correct answer: pounds or kilograms.
Rationale: Measuring specific gravity allows the fuel-quantity system to convert volume into a true mass, so the contents are displayed as a weight in pounds or kilograms. Volume units such as gallons or liters would not account for the change in fuel density that the specific-gravity measurement corrects for.
In an integral fuel tank, the final layer of sealing applied over the assembled joints is called the
ACS code: AM.II.J
Correct answer: brush coat.
Rationale: Integral fuel tank sealing is built up in stages, and the final sealing layer applied over the assembled joints is the brush coat, painted on to give a continuous fuel-tight skin. Interfay sealant goes between mating faces and fillet sealant fills the joint angles, so neither is the final coat.
What is the primary purpose of a fuel boost pump in an aircraft fuel system?
ACS code: AM.II.J
Correct answer: To supply fuel under positive pressure to the engine-driven pump and prevent vapor lock
Rationale: Per FAA-H-8083-31, the boost (or auxiliary) pump provides positive fuel pressure to the engine-driven pump, prevents vapor lock by maintaining pressure on the fuel, supplies fuel for engine starting, and serves as a backup if the engine-driven pump fails.
Vapor lock in a fuel system is most likely to occur when
ACS code: AM.II.J
Correct answer: fuel vaporizes in the lines or pump and the vapor pockets block normal fuel flow
Rationale: Per FAA-H-8083-31, vapor lock results when fuel vaporizes in the lines or pump and the vapor bubbles block fuel flow. It is promoted by high temperatures, low fuel pressure (such as at high altitude), and rapid fuel flow through restrictions. A boost pump maintaining positive pressure helps prevent it.
In a gravity-feed fuel system, fuel is delivered to the engine by
ACS code: AM.II.J
Correct answer: the head of fuel created by tanks located above the carburetor
Rationale: Per FAA-H-8083-31, gravity-feed systems are used on high-wing aircraft where the fuel tanks are mounted above the carburetor. The weight (head) of the fuel provides the pressure to deliver fuel to the engine without a pump.
Why must fuel tanks be vented to the atmosphere?
ACS code: AM.II.J
Correct answer: To maintain atmospheric pressure in the tank and allow for fuel expansion as fuel is used
Rationale: Per FAA-H-8083-31, tank vents maintain atmospheric pressure inside the tank as fuel is consumed. Without venting, a vacuum would form as fuel is drawn out, collapsing the tank or stopping fuel flow; venting also allows for thermal expansion and contraction of the fuel.
The fuel selector valve in a typical light aircraft is used to
ACS code: AM.II.J
Correct answer: select the tank from which fuel is drawn or shut the fuel supply off
Rationale: Per FAA-H-8083-31, the fuel selector valve allows the pilot to select fuel from a particular tank, select 'both,' or shut the fuel supply off (OFF position). It controls which tank or tanks feed the engine.
A bladder-type fuel tank is best described as
ACS code: AM.II.J
Correct answer: a reinforced flexible rubberized cell fitted into a cavity in the aircraft structure
Rationale: Per FAA-H-8083-31, a bladder (flexible) fuel cell is a reinforced rubberized bag fitted into a cavity in the structure. It is snapped or laced into place and relies on the surrounding structure for support, offering a degree of self-sealing and weight savings.
An integral fuel tank is one in which
ACS code: AM.II.J
Correct answer: a portion of the aircraft structure itself is sealed to form the tank
Rationale: Per FAA-H-8083-31, an integral fuel tank uses a sealed-off portion of the aircraft structure (commonly called a 'wet wing') to hold fuel. The skin and structural members themselves form the tank walls, sealed at the seams and fasteners.
The primary purpose of a sump drain located at the lowest point of a fuel tank is to
ACS code: AM.II.J
Correct answer: drain off water and sediment that settle at the lowest point of the tank
Rationale: Per FAA-H-8083-31, water is heavier than fuel and settles to the lowest point of the tank. The sump drain (quick-drain valve) at this low point allows accumulated water and sediment to be drained off during preflight inspection.
When checking a fuel tank sump for water contamination, water is identified because it
ACS code: AM.II.J
Correct answer: settles to the bottom of the sample as a separate layer beneath the fuel
Rationale: Per FAA-H-8083-31, water is denser than aviation fuel and is immiscible with it, so it collects at the bottom of the sample. Water typically appears as clear globules or a separate layer beneath the colored fuel, and the fuel may appear cloudy when suspended water is present.
A fuel system crossfeed (cross-feed) arrangement on a multiengine aircraft is provided to
ACS code: AM.II.J
Correct answer: allow either engine to draw fuel from tanks on either side of the aircraft
Rationale: Per FAA-H-8083-31, a crossfeed system allows fuel from a tank on one side of the aircraft to be supplied to an engine on the opposite side. This helps maintain lateral balance and allows an engine to draw from any available tank if one engine or fuel supply is lost.
What is the function of a fuel pressure relief valve on a positive-displacement, engine-driven fuel pump?
ACS code: AM.II.J
Correct answer: It bypasses excess fuel to regulate the pump's output pressure
Rationale: Per FAA-H-8083-31, an engine-driven vane (positive-displacement) pump produces more pressure and volume than the engine needs. A built-in relief valve bypasses excess fuel back to the pump inlet to regulate the output to the desired pressure.
Why is an electric boost pump frequently a centrifugal type rather than a positive-displacement type?
ACS code: AM.II.J
Correct answer: Fuel can flow through a centrifugal pump even when it is not operating
Rationale: Per FAA-H-8083-31, the centrifugal boost pump is non-positive-displacement, so it does not need a separate relief valve and fuel can flow through it (bypass) even when it is not operating. This allows the engine-driven pump or gravity to draw fuel through an inactive boost pump.
The main fuel strainer (gascolator) in a typical reciprocating-engine fuel system is normally located
ACS code: AM.II.J
Correct answer: at a low point in the system between the tank and the fuel metering device
Rationale: Per FAA-H-8083-31, the main strainer (gascolator) is placed at the lowest point in the fuel system, between the tank and the carburetor/metering device, so that water and sediment can collect in its bowl and be drained, protecting the engine fuel metering components.
Which fuel-quantity indicating system measures the actual mass (weight) of fuel rather than its volume?
ACS code: AM.II.J
Correct answer: The capacitance (dielectric) type system
Rationale: Per FAA-H-8083-31, the capacitance-type (dielectric) fuel-quantity system uses tank units that act as capacitors. Because fuel and air have different dielectric constants, the system can be calibrated to read fuel mass (pounds), which is more accurate than volume because fuel density changes with temperature.
In a capacitance-type fuel-quantity indicating system, the tank units function as
ACS code: AM.II.J
Correct answer: capacitors whose capacitance varies with the level of fuel between their plates
Rationale: Per FAA-H-8083-31, the tank units (probes) are capacitors whose dielectric is the fuel and air between their plates. As fuel level changes, the dielectric changes, altering capacitance. The bridge circuit converts this change into a fuel-quantity indication.
A fuel flowmeter in a fuel-injected reciprocating engine system actually indicates fuel flow by sensing
ACS code: AM.II.J
Correct answer: the fuel pressure at or across the injector nozzles, calibrated as flow
Rationale: Per FAA-H-8083-31, in many fuel-injected reciprocating-engine installations the 'fuel flow' gauge is actually a pressure gauge that measures the pressure drop across the injector nozzles. Because flow and that pressure are related, the gauge is calibrated in gallons per hour or fuel flow.
Why is microbial growth a concern in aircraft fuel tanks, particularly with turbine fuels?
ACS code: AM.II.J
Correct answer: Microbes live at the fuel-water interface and form slime that clogs filters and corrodes tanks
Rationale: Per FAA-H-8083-31, microbial contaminants (fungi and bacteria) live at the fuel-water interface where free water is present. They form a slime that can clog filters and fuel system components and can cause corrosion of tank structure. Keeping water drained off helps control growth.
During fueling operations, why must the aircraft and the fueling source be electrically bonded together?
ACS code: AM.II.J
Correct answer: To equalize electrical potential and prevent a static spark from igniting fuel vapors
Rationale: Per FAA-H-8083-31, fuel flowing through hoses and nozzles generates a static electrical charge. Bonding the aircraft to the fuel source (and grounding) equalizes the electrical potential so that a static spark cannot occur at the fuel opening and ignite fuel vapors.
A pressure-fueling (single-point) system on a large aircraft offers the advantage of
ACS code: AM.II.J
Correct answer: fueling all tanks rapidly from one connection while reducing contamination and static hazards
Rationale: Per FAA-H-8083-31, single-point pressure fueling fuels all tanks from one ground connection under pressure. It reduces fueling time, minimizes the chance of fuel contamination and static buildup associated with open over-wing fueling, and reduces the hazard of personnel working on top of the wing.
The purpose of fuel dump (jettison) valves installed on some large aircraft is to
ACS code: AM.II.J
Correct answer: discharge fuel overboard in flight to reduce aircraft weight to the maximum landing weight
Rationale: Per FAA-H-8083-31, fuel jettison (dump) systems allow fuel to be discharged overboard in flight to reduce the aircraft's weight quickly to or below the maximum landing weight, for example after a takeoff that requires an emergency return.
Before repairing a fuel tank that has contained fuel, the most important safety step is to
ACS code: AM.II.J
Correct answer: purge the tank of all fuel vapors so it is free of combustible vapor
Rationale: Per FAA-H-8083-31, a fuel tank must be purged of all fuel vapors before any repair involving heat or sparks. Even an empty tank holds explosive vapors; purging and ensuring the tank is free of combustible vapors prevents an explosion during welding or other hot work.
Aviation gasoline (avgas) grades are identified primarily by
ACS code: AM.II.J
Correct answer: their octane/performance number indicating antiknock characteristics
Rationale: Per FAA-H-8083-31, avgas grades (such as 80, 100, and 100LL) are designated by their octane or performance numbers, which indicate the fuel's antiknock (detonation-resistance) characteristics. Each grade is also dyed a distinctive color for identification.
If the wrong grade of fuel must be identified after a tank is filled, one reliable indicator is that each avgas grade is
ACS code: AM.II.J
Correct answer: dyed a distinctive color associated with that grade
Rationale: Per FAA-H-8083-31, aviation gasoline grades are dyed distinctive colors (for example, 80 is red, 100LL is blue, 100 is green) so they can be visually distinguished. Turbine fuel (Jet A) is colorless to straw-colored, which also helps detect contamination or misfueling.
Why is it hazardous to service a turbine-powered aircraft with aviation gasoline when turbine (jet) fuel is required?
ACS code: AM.II.J
Correct answer: The fuels have different combustion and lubricity properties that can cause engine damage or failure
Rationale: Per FAA-H-8083-31, misfueling a turbine engine with avgas (or fueling a reciprocating engine with jet fuel) can cause engine damage and failure because the fuels have different combustion, lubricity, and volatility characteristics. Proper fuel grade and correct nozzles/placards are critical to prevent misfueling.
Fuel lines in an aircraft fuel system that are subject to vibration are most commonly fabricated from
ACS code: AM.II.J
Correct answer: flexible hose, to absorb vibration and relative movement between components
Rationale: Per FAA-H-8083-31, flexible hose is used where there is relative movement or vibration between connected components (for example, between airframe-mounted lines and the engine), while rigid metal tubing is used in fixed areas of the airframe. Flexible hose absorbs vibration and accommodates engine movement.
A fuel heater is installed in some turbine-engine fuel systems primarily to
ACS code: AM.II.J
Correct answer: prevent ice crystals in the fuel from blocking the fuel filter
Rationale: Per FAA-H-8083-31, a fuel heater warms the fuel before it reaches the fuel filter to prevent ice crystals (formed from dissolved/free water at low temperatures) from accumulating and blocking the filter. It protects the filter from icing rather than improving combustion.
What does a fuel filter bypass (differential pressure) warning indicate?
ACS code: AM.II.J
Correct answer: The filter element is becoming clogged and the bypass valve is about to open
Rationale: Per FAA-H-8083-31, when a fuel filter becomes clogged, the differential pressure across it increases. A bypass valve opens to allow unfiltered fuel to continue to the engine, and an impending-bypass (differential-pressure) indicator warns that the filter element is dirty and needs servicing.
On a low-wing aircraft equipped with a fuel selector that has 'LEFT,' 'RIGHT,' and 'OFF' positions but no 'BOTH' position, the pilot manages fuel by
ACS code: AM.II.J
Correct answer: drawing from one tank at a time and switching periodically to keep the load balanced
Rationale: Per FAA-H-8083-31, low-wing aircraft typically use pump-fed systems and a selector without a BOTH position because fuel cannot reliably gravity-feed and unported feed from both could be uneven. The pilot selects one tank at a time and switches periodically to keep the tanks balanced.