Does fuel injection eliminate induction icing?

It eliminates carburetor ice, the refrigeration ice that forms when fuel vaporizes in a venturi, because injected fuel is discharged at the cylinder intake port rather than upstream of it. It does not eliminate impact ice on the air filter or inlet screen, which is why injected engines still carry an alternate air source.

What does the idle cutoff position actually do?

It moves the mixture control past full lean into a detent that shuts fuel flow off entirely. The engine stops because it is starved of fuel, not because ignition was removed, so no combustible charge is left sitting in the cylinders. That is why shutting down with idle cutoff, rather than the magneto switch, is standard practice.

How does a continuous-flow fuel injection system meter fuel?

Airflow through the throttle body is metered first. The fuel/air control unit then meters fuel in proportion to that airflow and to the mixture setting, and sends it to a flow divider, which splits it among individual lines. Each nozzle sprays continuously into its own intake port just ahead of the intake valve.

What is the difference between a float carburetor and a pressure carburetor?

A float carburetor holds fuel at atmospheric pressure in a vented chamber and relies on the venturi pressure drop to move it, which makes it icing-prone and sensitive to flight attitude. A pressure carburetor delivers fuel under pressure and meters it using air and fuel pressure differentials across a regulator, so it is less icing-prone and tolerates attitude changes.

Why lean the mixture at cruise but enrich it at high power?

Fuel that does not burn still absorbs heat. At high power the economizer enriches the mixture to cool the charge and buy detonation margin, which costs fuel but protects the engine. At cruise the power setting is low enough that the engine can be leaned toward best economy without driving cylinder temperatures out of limits.

Engine Fuel and Fuel Metering Systems — FAA A&P Test Questions (ACS AM.III.I)

Engine Fuel and Fuel Metering Systems is one of the largest Powerplant subject areas, and it is mostly about how an engine decides how much fuel to add to a given mass of air. You work through the float-type carburetor system by system, then the pressure carburetor, then continuous-flow fuel injection, then turbine fuel control basics. Many questions are scenario-based: a symptom is described and you name the system responsible. Carburetor icing shows up constantly, because it is the symptom candidates most often diagnose backwards.

What ACS AM.III.I covers

Start with the float carburetor, because the FAA tests it the hardest. Air accelerating through the venturi loses pressure, and that pressure difference — atmospheric pressure in the vented float chamber pushing against low pressure at the discharge nozzle — is what meters the fuel. The float and needle valve do nothing but hold the fuel level constant so that difference stays meaningful. From there FAA-H-8083-32 breaks the carburetor into the systems the exam names directly: the main metering system, which controls flow from off-idle through full throttle; the idling system, which supplies fuel when airflow through the venturi is too weak to meter; the mixture control, including the idle cutoff detent that stops fuel flow so the engine shuts down with clean cylinders; the accelerating system, which adds a momentary shot of fuel when the throttle is opened quickly; and the economizer, or power enrichment system, which enriches the mixture at high power for detonation protection while still letting you lean at cruise. Fuel injection is examined by contrast: a fuel/air control unit meters fuel in proportion to metered airflow, a flow divider distributes it, and nozzles discharge continuously into each intake port ahead of the intake valve, giving better cylinder-to-cylinder distribution, quicker throttle response, and freedom from carburetor ice. Expect carburetor heat scenarios, mixture-ratio questions covering best power against best economy, float and needle-valve troubleshooting, and the fuel control unit on turbines. Work you sign for on any of it falls under 14 CFR Part 43.

Where this sits on the test

ACS AM.III.I is tested on the FAA Powerplant written test, one of 1,355 ACS-tagged questions in the Powerplant bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

FAA-H-8083-32

Three traps candidates fall into

Carburetor ice gets filed away as a cold-weather problem. Fuel vaporization plus the venturi pressure drop can cool the mixture far below outside air temperature, so ice forms on warm, humid days with the thermometer well above freezing — and that is exactly the scenario the exam likes to describe.
Applying carburetor heat is expected to raise rpm immediately. With ice present, rpm first drops further and the engine runs rough as melted water passes through, then recovers above where it started. On an ice-free engine, heat simply drops rpm, because warm air is less dense and the mixture goes richer.
The economizer and the accelerating system get swapped. The accelerating system delivers a brief extra charge of fuel when the throttle is opened rapidly. The economizer supplies additional fuel at high power settings so the mixture stays rich enough to cool the charge and resist detonation, and it is inactive at cruise.

7 free sample questions from ACS AM.III.I

AM.III.ITap an answer
What is the primary purpose of the idle cutoff (ICO) position on the mixture control of a float-type carburetor?
AM.III.ITap an answer
In a float-type carburetor, the venturi is used primarily to
AM.III.ITap an answer
What is the function of the economizer (power enrichment) system in a carburetor?
AM.III.ITap an answer
In a typical continuous-flow fuel injection system, the fuel/air metering unit meters fuel in proportion to
AM.III.ITap an answer
Compared with a float-type carburetor, a key advantage of a fuel injection system is
AM.III.ITap an answer
The needle valve and float assembly in a float-type carburetor are used to
AM.III.ITap an answer
The main metering system of a float carburetor controls fuel flow primarily during

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.

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Frequently asked questions

Does fuel injection eliminate induction icing?: It eliminates carburetor ice, the refrigeration ice that forms when fuel vaporizes in a venturi, because injected fuel is discharged at the cylinder intake port rather than upstream of it. It does not eliminate impact ice on the air filter or inlet screen, which is why injected engines still carry an alternate air source.
What does the idle cutoff position actually do?: It moves the mixture control past full lean into a detent that shuts fuel flow off entirely. The engine stops because it is starved of fuel, not because ignition was removed, so no combustible charge is left sitting in the cylinders. That is why shutting down with idle cutoff, rather than the magneto switch, is standard practice.
How does a continuous-flow fuel injection system meter fuel?: Airflow through the throttle body is metered first. The fuel/air control unit then meters fuel in proportion to that airflow and to the mixture setting, and sends it to a flow divider, which splits it among individual lines. Each nozzle sprays continuously into its own intake port just ahead of the intake valve.
What is the difference between a float carburetor and a pressure carburetor?: A float carburetor holds fuel at atmospheric pressure in a vented chamber and relies on the venturi pressure drop to move it, which makes it icing-prone and sensitive to flight attitude. A pressure carburetor delivers fuel under pressure and meters it using air and fuel pressure differentials across a regulator, so it is less icing-prone and tolerates attitude changes.
Why lean the mixture at cruise but enrich it at high power?: Fuel that does not burn still absorbs heat. At high power the economizer enriches the mixture to cool the charge and buy detonation margin, which costs fuel but protects the engine. At cruise the power setting is low enough that the engine can be leaned toward best economy without driving cylinder temperatures out of limits.

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