What is the primary purpose of the idle cutoff (ICO) position on the mixture control of a float-type carburetor?
ACS code: AM.III.I
Correct answer: To shut off fuel flow to the engine for normal shutdown
Rationale: Stopping a reciprocating engine by moving the mixture to idle cutoff cuts off fuel flow entirely, clearing the cylinders and induction system of fuel and reducing the chance of an inadvertent start (kickback). The handbook recommends idle cutoff over closing the throttle because the cylinders are left free of raw fuel, preventing fouling and fire hazards. ICO does not enrich the mixture (that is the full-rich position) and has no effect on venturi airflow.
In a float-type carburetor, the venturi is used primarily to
ACS code: AM.III.I
Correct answer: create a pressure drop that draws fuel from the discharge nozzle
Rationale: As air accelerates through the restricted throat of the venturi, its velocity increases and its static pressure decreases (Bernoulli's principle). This low pressure at the venturi throat, compared to the higher pressure acting on the fuel in the bowl, draws fuel out of the discharge nozzle. The venturi does not itself vaporize fuel or control float level; the float and needle valve maintain bowl level.
What is the function of the economizer (power enrichment) system in a carburetor?
ACS code: AM.III.I
Correct answer: To supply additional fuel at high power settings
Rationale: The economizer (also called the power enrichment system) provides a richer mixture only at high power output settings, where the additional fuel is needed for cooling and to develop maximum power. At cruise and lower settings the system is closed so the engine runs on the leaner, more economical main metering mixture. It is not a leaning device and does not act at idle.
In a typical continuous-flow fuel injection system, the fuel/air metering unit meters fuel in proportion to
ACS code: AM.III.I
Correct answer: the airflow through the throttle body
Rationale: The fuel/air control unit meters fuel based on the volume of air entering the engine, sensed at the throttle and air metering section, so that fuel delivery tracks airflow as the throttle is moved. Oil pressure and exhaust gas temperature are not the metering reference for this system; they are unrelated engine parameters used for monitoring rather than fuel scheduling.
Compared with a float-type carburetor, a key advantage of a fuel injection system is
ACS code: AM.III.I
Correct answer: freedom from carburetor (fuel-evaporation) icing
Rationale: Because fuel injection delivers fuel at or near the intake port rather than evaporating it in a venturi, there is no significant pressure-and-temperature drop in the induction airstream, so fuel-evaporation (carburetor) icing does not occur. Injection systems are generally more, not less, complex, and many still require a boost pump for starting and as a backup, so those are not its advantages.
The needle valve and float assembly in a float-type carburetor are used to
ACS code: AM.III.I
Correct answer: maintain a constant fuel level in the float bowl
Rationale: As fuel is drawn from the bowl, the float drops and opens the needle valve to admit more fuel; as the level rises, the float closes the valve. This maintains an essentially constant fuel level, which is necessary for proper metering at the discharge nozzle. The mixture ratio is set by the metering jets and mixture control, and airflow is governed by the throttle and venturi, not by the float.
The main metering system of a float carburetor controls fuel flow primarily during
ACS code: AM.III.I
Correct answer: normal cruise and higher power operation
Rationale: The main metering system supplies and meters fuel through the main discharge nozzle across the cruise and higher power range, where venturi suction is strong enough to draw fuel. At very low airflow (idle), venturi suction is insufficient, so a separate idle system supplies fuel; rapid throttle movement is handled by the accelerating system. Thus the main system governs the normal operating range, not idle or transient enrichment.
An engine equipped with a carburetor experiences a gradual loss of power and a drop in RPM that is corrected by applying carburetor heat. The most likely cause was
ACS code: AM.III.I
Correct answer: ice forming in the carburetor and restricting airflow
Rationale: A gradual RPM and power loss that clears after applying carburetor heat is the classic indication of carburetor ice; the heat melts the ice and restores normal airflow, often with a brief further roughness as the melted water passes through. A stuck economizer or a flooded bowl would produce a rich condition that carburetor heat would not relieve, so those are inconsistent with the symptom described.
What is the purpose of an air shroud placed around the fuel nozzles in a gas turbine engine?
ACS code: AM.III.I
Correct answer: It prevents carbon buildup on the nozzle tip.
Rationale: A shroud fitted around the fuel nozzle directs cooling air across the nozzle tip, keeping it cool and sweeping the spray clear so carbon cannot bake onto the orifice. By preventing carbon buildup, it maintains the correct spray pattern and angle.
Carbon deposits forming on the fuel spray nozzles of a gas turbine engine will have the effect of
ACS code: AM.III.I
Correct answer: changing the angle of the fuel spray pattern.
Rationale: Carbon deposits on a spray nozzle distort the orifice and disturb the discharge, changing the angle and shape of the fuel spray cone. A poor spray pattern can cause uneven burning, hot streaks, and flame impingement on the liner. It does not raise the chamber pressure ratio, which is set by the compressor.
In a vaporizing-type combustion system, the fuel vapor is discharged from the vaporizer tubes
ACS code: AM.III.I
Correct answer: against the direction of the incoming airflow.
Rationale: In a vaporizing combustion system the fuel is fed into heated vaporizer tubes and discharged back upstream, against the direction of the primary airflow entering the chamber. This counterflow promotes thorough mixing and gives the rich vapor time to blend with primary air before it turns and burns, producing a clean flame.
A duplex fuel nozzle in a gas turbine engine has two orifices that provide
ACS code: AM.III.I
Correct answer: one for low-speed conditions and the other for high-speed conditions.
Rationale: A duplex nozzle has two orifices: a small primary one giving good atomization at the low fuel flows of low-speed operation, and a larger main one that opens at higher fuel pressure for high-speed, high-flow conditions. Both orifices flow fuel; neither is for water injection.
In the primary combustion zone of a turbine engine, the chemically correct air-fuel ratio for efficient burning of kerosene-type fuel is approximately
ACS code: AM.III.I
Correct answer: 15:1 by mass.
Rationale: For efficient combustion, turbine fuel must burn close to its stoichiometric air-fuel ratio of about 15:1 by mass. Although the overall engine ratio is far leaner (around 60:1 or more), airflow is split so the primary combustion zone stays near this 15:1 chemically correct ratio.
When a fuel-system icing inhibitor or biocidal additive is mixed into turbine fuel, the additive
ACS code: AM.III.I
Correct answer: remains in the fuel and is burned with it in the combustion chamber.
Rationale: Fuel additives such as icing inhibitors and biocides are blended into the fuel in small, metered amounts and remain in the fuel, being consumed when the fuel is burned in the combustion chamber. They are not flushed out or diluted with water; they pass through and are burned with the fuel.
What action is taken to protect integral fuel tanks from corrosion caused by microbiological contamination?
ACS code: AM.III.I
Correct answer: A biocidal additive is added to the fuel.
Rationale: Microbiological organisms grow at the fuel/water interface in integral tanks and produce corrosive by-products. Adding a biocidal additive to the fuel kills these organisms, preventing the sludge and corrosion they cause. Liners and chromate coatings address mechanical or galvanic protection, not biological growth.
If an alternate fuel is to be used, what must be checked or adjusted to keep a turbine engine performance check valid?
ACS code: AM.III.I
Correct answer: The fuel specific gravity setting on the fuel control.
Rationale: A turbine engine performance check assumes a particular fuel energy content, which is tied to the fuel's specific gravity. When an alternate fuel is used, the fuel specific gravity setting must be checked or adjusted so that computed fuel flow and thrust remain valid. RPM and EPR calibrations are not fuel-dependent in this way.
Kerosene-type fuel (such as Jet A) is used in turbine engines primarily because it
ACS code: AM.III.I
Correct answer: has high heat energy per gallon and lubricates fuel system components.
Rationale: Kerosene-type jet fuel has a high heat-energy content per gallon, giving good range, and its oily nature lubricates fuel system components such as pumps and the fuel control. Its low volatility also makes it safer to handle than highly volatile gasoline.
The heat-energy (calorific) value of an aviation fuel is best described as the
ACS code: AM.III.I
Correct answer: amount of heat energy released by burning one pound of the fuel.
Rationale: The heat-energy or calorific value of a fuel is the quantity of heat energy released when a unit mass of fuel is completely burned, commonly expressed as the energy contained in one pound of fuel. It is a measure of energy content, not of vaporization point or boiling temperature.
The specific gravity of a turbine fuel primarily affects
ACS code: AM.III.I
Correct answer: the range of the aircraft for a given tank volume.
Rationale: Specific gravity sets how much fuel mass, and therefore how much energy, a given tank volume holds. A higher specific gravity means more energy carried for the same tank size, increasing aircraft range. Thrust rating and engine efficiency depend on the combustion process, not on fuel density.
Successful ignition of the fuel/air mixture in a turbine engine depends upon
ACS code: AM.III.I
Correct answer: both the volatility and the atomization of the fuel.
Rationale: Successful ignition needs the fuel both to vaporize readily, which is governed by volatility, and to be broken into a fine spray, which is governed by atomization. Good atomization increases the surface area available for vaporization, so both factors together determine whether the mixture will ignite.
Kerosene-type jet fuel is used instead of gasoline in turbine engines because it is
ACS code: AM.III.I
Correct answer: less volatile and provides good lubricating properties.
Rationale: Kerosene-type jet fuel is chosen over gasoline because it is less volatile, making it safer to store and handle, and because its oily nature lubricates fuel system components. The choices stating that kerosene is highly or more volatile are incorrect, since its lower volatility is precisely the advantage.
If the specific gravity of a fuel is increased, the weight of a full tank of that fuel will
ACS code: AM.III.I
Correct answer: increase.
Rationale: Specific gravity is a measure of mass per unit volume, so for a fixed tank volume a higher specific gravity means more fuel mass. The weight of a full tank therefore increases as the fuel specific gravity rises.
Reid vapor pressure is the vapor pressure exerted by a fuel when heated to the standardized reference temperature of
ACS code: AM.III.I
Correct answer: 100 degrees F (38 degrees C).
Rationale: Reid vapor pressure is the vapor pressure of a fuel measured at the standardized reference temperature of 100 degrees F (38 degrees C). It is a measure of a fuel's tendency to vaporize and form vapor lock at that reference condition.
On a standard day, the metered fuel flow to the burners of a gas turbine engine is greatest at
ACS code: AM.III.I
Correct answer: standard-day sea level.
Rationale: Burner fuel flow must match the mass of air entering the engine. Air density is greatest at standard-day sea level, so the most air—and therefore the maximum metered fuel flow—occurs there. Per FAA-H-8083-32, as altitude or temperature rises, air density falls and the fuel control reduces fuel flow to maintain the correct fuel/air ratio.
Why is an acceleration control unit incorporated in a gas turbine engine fuel control system?
ACS code: AM.III.I
Correct answer: It limits the rate of increase in fuel flow during rapid acceleration.
Rationale: The acceleration control schedules fuel during rapid throttle advances so that the extra fuel is added gradually. Per FAA-H-8083-32, limiting the rate of fuel increase keeps the fuel/air ratio within limits, preventing over-fueling that would cause compressor surge, over-temperature, or flameout during acceleration.
After a fuel control unit has been replaced on a turbine engine, what is normally required?
ACS code: AM.III.I
Correct answer: The engine must be re-trimmed.
Rationale: A new fuel control unit will not deliver exactly the same schedule as the one removed, so the engine must be re-trimmed to restore the correct relationship between throttle position, fuel flow, and rated thrust/rpm. Per FAA-H-8083-32, trimming is performed after the unit is installed and the engine is stabilized.
What is the purpose of an attenuator installed between the high-pressure fuel pump and the fuel control unit in a turbine engine fuel system?
ACS code: AM.III.I
Correct answer: It damps out pulsations in the fuel delivery to the fuel control unit.
Rationale: A gear or plunger high-pressure pump delivers fuel with pressure ripples. An attenuator acts as a damper that smooths these pulsations so the fuel control senses a steady pressure and meters fuel accurately, while reducing pulsation-induced wear (FAA-H-8083-32, Engine Fuel and Fuel Metering Systems).
A barometric pressure controller in a turbine fuel system primarily controls
ACS code: AM.III.I
Correct answer: fuel flow to suit atmospheric pressure changes.
Rationale: Despite its name, the barometric pressure controller does not control barometric pressure; it senses ambient/inlet air pressure and adjusts fuel flow accordingly. As atmospheric pressure changes with altitude, it trims fuel delivery so the fuel/air ratio remains correct (FAA-H-8083-32, Engine Fuel and Fuel Metering Systems).
On a FADEC-equipped turbine engine, how are the two channels of the electronic engine control (EEC) powered?
ACS code: AM.III.I
Correct answer: Each channel uses a separate winding of the dedicated control alternator.
Rationale: A FADEC's EEC has two independent channels for redundancy. Each channel is powered by its own separate winding of the engine-driven permanent-magnet control alternator, so the engine self-powers its control electronics and a fault in one supply does not disable both channels (FAA-H-8083-32, Engine Fuel and Fuel Metering Systems).
The chemically correct (stoichiometric) fuel/air ratio by weight for the burning zone of a turbine combustor is approximately
ACS code: AM.III.I
Correct answer: 15:1.
Rationale: The chemically correct fuel/air ratio for kerosene-type jet fuel is about 15 parts air to 1 part fuel by weight, giving complete, efficient combustion. The overall ratio through the engine is much leaner, but primary air keeps the burning zone near 15:1 (FAA-H-8083-32, Engine Fuel and Fuel Metering Systems).
Which parameter is a primary scheduling input to a hydromechanical fuel control unit on a turbojet engine?
ACS code: AM.III.I
Correct answer: Compressor discharge (burner) pressure.
Rationale: A hydromechanical fuel control schedules fuel as a function of engine operating parameters; compressor discharge (burner) pressure is a key signal because it represents the mass of air being handled, letting the unit set a fuel flow that keeps the fuel/air ratio correct. Mixture controls belong to reciprocating engines, not turbines (FAA-H-8083-32, Engine Fuel and Fuel Metering Systems).
What is the purpose of the low-pressure (boost) fuel pump in a turbine engine fuel system?
ACS code: AM.III.I
Correct answer: To prevent cavitation of the high-pressure fuel pump.
Rationale: The low-pressure pump raises fuel pressure above its vapor pressure before the fuel reaches the high-pressure pump. This steady positive supply prevents the fuel from vaporizing at the high-pressure pump inlet, avoiding cavitation that would damage the pump and disrupt fuel delivery (FAA-H-8083-32, Engine Fuel and Fuel Metering Systems).
The plungers and bearing surfaces of a turbine engine high-pressure fuel pump are lubricated by
ACS code: AM.III.I
Correct answer: the fuel passing through the pump.
Rationale: A high-pressure fuel pump is lubricated and cooled by the fuel it pumps, which films the plungers and bearing surfaces as it passes through. This is why fuel cleanliness and lubricity matter, and why dry-running an unprimed pump can quickly destroy it (FAA-H-8083-32, Engine Fuel and Fuel Metering Systems).
Why is shroud (cooling) air directed over the head of a duplex fuel nozzle through holes in the nozzle body?
ACS code: AM.III.I
Correct answer: To minimize carbon formation on the nozzle face.
Rationale: Air bled through holes in the nozzle body forms a thin film over the nozzle head. This air shields the atomizer face from the flame and sweeps it clean, minimizing the carbon (coke) deposits that would otherwise build up and distort the spray pattern (FAA-H-8083-32, Engine Fuel and Fuel Metering Systems).
A fuel heater (fuel/oil heat exchanger) in a turbine fuel system is installed primarily to prevent
ACS code: AM.III.I
Correct answer: low-pressure fuel filter icing.
Rationale: Water dissolved in fuel can freeze and block the low-pressure fuel filter at altitude. The fuel heater warms the fuel upstream of the low-pressure filter so any ice melts or does not form, preventing filter icing and the fuel starvation that would follow (FAA-H-8083-32, Engine Fuel and Fuel Metering Systems).
On a FADEC engine, the electronic engine control (EEC) commands the hydromechanical fuel metering unit
ACS code: AM.III.I
Correct answer: in all modes of operation.
Rationale: On a full-authority digital engine control (FADEC) engine, the EEC has electronic control of the hydromechanical fuel metering unit in all modes of operation, with no purely manual mechanical mode. The metering unit is the muscle that the EEC commands; the EEC always sets the fuel schedule (FAA-H-8083-32, Engine Fuel and Fuel Metering Systems).
During engine acceleration, fuel flow is increased at a controlled rate primarily to
ACS code: AM.III.I
Correct answer: prevent compressor surge and the risk of flameout.
Rationale: If fuel were added too quickly during acceleration, the mixture would over-richen before the compressor speeds up, disturbing airflow and over-temperaturing the turbine. Controlling the rate of fuel increase keeps the fuel/air ratio within limits, preventing compressor surge and the risk of flameout or over-temperature (FAA-H-8083-32, Engine Fuel and Fuel Metering Systems).
A barometric pressure controller modifies the output of the fuel control unit by means of
ACS code: AM.III.I
Correct answer: pressure sensing.
Rationale: The barometric pressure controller senses air pressure (compressor delivery or ambient) with a capsule or bellows and uses that pressure signal to modify the fuel control unit's output, so fuel flow tracks changes in air mass. It is a pressure-sensing device, not a temperature- or density-sensing one (FAA-H-8083-32, Engine Fuel and Fuel Metering Systems).
In a positive-displacement, axial-piston (swash-plate) fuel pump, if the swash plate is perpendicular to the axis of the pump, the fuel delivery will be
ACS code: AM.III.I
Correct answer: zero, because the pistons produce no stroke
Rationale: Per FAA-H-8083-32, delivery from a swash-plate (axial-piston) pump depends on the swash-plate angle. With the plate perpendicular to the pump axis there is no angle, the pistons do not stroke, and no fuel is displaced, so delivery is zero. Tilting the plate produces piston stroke and flow.
In a turbine engine, burner fuel flow reaches its maximum value at
ACS code: AM.III.I
Correct answer: standard-atmosphere sea level conditions
Rationale: Per FAA-H-8083-32, maximum burner fuel flow occurs where the engine ingests the greatest mass of air. Air density is highest at standard-day sea level, so the most air—and therefore the most metered fuel—is required there. With increasing altitude or temperature, density and fuel flow both decrease.
The type of fuel control unit most commonly used in modern turbine engines is
ACS code: AM.III.I
Correct answer: hydromechanical, using fuel pressure and mechanical sensing
Rationale: Per FAA-H-8083-32, most turbine-engine fuel controls are hydromechanical: they use fuel pressures and mechanical sensing elements (governors, capsules, cams) to schedule fuel. Even on FADEC engines the metering unit itself is hydromechanical, commanded electronically. Purely mechanical or purely electrical metering is not used for the basic fuel control.
How is the servo pressure that controls fuel-pump swash-plate angle obtained?
ACS code: AM.III.I
Correct answer: from pump delivery pressure through variable restrictions
Rationale: Per FAA-H-8083-32, servo pressure is tapped from the pump's own high delivery pressure and passed through variable restrictions—the governor, barometric, and throttle-valve orifices. By varying these restrictions the controls set the servo pressure acting on the swash plate, and hence the pump stroke and delivery.
Why is a barometric pressure control included in a turboshaft engine fuel system?
ACS code: AM.III.I
Correct answer: to vary pump output in relation to the pressure variation at the intake
Rationale: Per FAA-H-8083-32, the barometric pressure control senses intake/compressor air pressure, which changes with altitude and conditions. It varies pump output so the fuel delivered matches the changing air mass, keeping the fuel/air ratio correct and preventing over-fueling as ambient pressure falls.
When a turbine fuel pump is static (engine stopped), the swash plate is positioned
ACS code: AM.III.I
Correct answer: at maximum stroke, driven by the spring
Rationale: Per FAA-H-8083-32, when the engine is stopped there is no servo pressure to restrain the swash plate, so the spring drives it to its maximum-stroke position. This provides full pump delivery for starting; as the engine runs up, servo pressure then moves the plate to the required running stroke.
Why must the fuel supply to a turbine engine be limited during rapid acceleration?
ACS code: AM.III.I
Correct answer: to prevent excessively high EGT and possible compressor surge
Rationale: Per FAA-H-8083-32, during rapid acceleration the compressor airflow lags the fuel increase, so unrestricted fuel would over-richen the mixture. Limiting fuel keeps the fuel/air ratio within limits, preventing the excessive exhaust gas (turbine) temperature and the airflow breakdown that causes compressor surge.
Which component corrects for air-density effects on the fuel/air mixture in a gas turbine engine?
ACS code: AM.III.I
Correct answer: the barometric pressure control unit
Rationale: Per FAA-H-8083-32, air density varies with altitude and temperature, changing the air mass for a given volume. The barometric pressure control unit senses this and adjusts fuel flow accordingly, correcting the fuel/air mixture so the engine is not over- or under-fueled as density changes. A simple throttle valve cannot do this automatically.
What is the primary purpose of the high-pressure fuel pump in a gas turbine engine?
ACS code: AM.III.I
Correct answer: to provide the majority of the fuel pressure delivered to the engine
Rationale: Per FAA-H-8083-32, the high-pressure fuel pump generates the high pressure needed to meter fuel and force it through the nozzles for proper atomization. The low-pressure (boost) pump only provides a modest pressure rise to feed the HP pump; the bulk of system pressure is produced by the HP pump.
When a turbine engine is shut down, what are the positions of the pressurizing valve and the dump valve in the fuel system?
ACS code: AM.III.I
Correct answer: pressurizing valve closed, dump valve open
Rationale: Per FAA-H-8083-32, when the engine is shut down there is no fuel pressure to hold the valves over, so the pressurizing valve is spring-closed to seal the system, while the dump valve opens to drain residual fuel from the manifold and prevent post-shutdown burning or coking in the nozzles.
Air density affects the fuel/air ratio in a turbine engine. For equal total parts, which of the following weighs the most?
ACS code: AM.III.I
Correct answer: 100 parts of dry air with no water vapor
Rationale: Per FAA-H-8083-32, water vapor is less dense than dry air, so replacing part of the air with water vapor lightens the mixture. For the same number of parts, 100 parts of dry air weighs the most. This is why high humidity reduces air density and affects fuel/air metering and engine trimming.
A fuel/air mixture ratio expressed as 11:1 normally refers to
ACS code: AM.III.I
Correct answer: 1 part fuel to 11 parts air, by weight
Rationale: Per FAA-H-8083-32, fuel/air mixture ratios are quoted by weight as air-to-fuel, so 11:1 means 11 parts air to 1 part fuel—that is, 1 part fuel to 11 parts air. This is rich relative to the roughly 15:1 stoichiometric ratio.