What are the three types of induction system icing and how do they differ?

The three types are impact ice, fuel evaporation ice, and throttle ice. Impact ice forms when supercooled water droplets or wet snow freeze on the air intake or filter upon contact, blocking airflow. Fuel evaporation ice (also called refrigeration ice) forms in carbureted engines when the vaporization of fuel absorbs heat and drops the temperature sharply, potentially below freezing even on warm days. Throttle ice forms at the throttle valve itself, where the pressure drop across a partially closed throttle causes adiabatic cooling that can ice over the valve and restrict power.

Why is carburetor icing dangerous even on warm, sunny days?

As explained in the Pilot's Handbook of Aeronautical Knowledge, the combination of fuel evaporation and pressure drop across the throttle can reduce local air temperature by as much as 60-70°F, meaning icing conditions can exist even when the outside air temperature is as high as 100°F with relative humidity as low as about 50%. This makes carburetor icing particularly insidious because pilots may not suspect it in seemingly benign weather. The first indication in a fixed-pitch propeller aircraft is typically an unexplained drop in engine RPM, while a constant-speed propeller aircraft may show a drop in manifold pressure.

How do you use carburetor heat to eliminate induction system icing, and are there any risks?

Applying full carburetor heat diverts air that has been warmed by an exhaust heat muff into the induction system, raising the temperature above freezing and melting any ice that has formed. According to the Pilot's Handbook of Aeronautical Knowledge, the pilot should expect a further, temporary drop in RPM and possibly rough engine operation as the ice melts before power gradually recovers. A key risk is that carburetor heat introduces unfiltered air, which can allow dust and debris into the engine, and it also reduces air density, slightly decreasing power output, so it should not be used routinely during takeoff unless carburetor ice is already confirmed.

Induction System Icing Types Impact Ice Fuel Ice and Throttle Ice

An aircraft engine is essentially an air pump. If the induction system that supplies air to that pump becomes blocked by ice, power drops—sometimes without warning and sometimes with catastrophic speed. Unlike airframe icing, which is visible to the crew, induction icing can form inside ducts and at the carburetor throat where no one can see it. For the Aviation Maintenance Technician (AMT) preparing for the FAA Powerplant knowledge test, understanding the three distinct types of induction system icing—impact ice, fuel evaporation ice (also called refrigeration ice), and throttle ice—is not only a frequent exam topic but a genuine safety responsibility.

Each type of ice forms through a different physical mechanism, occurs under different atmospheric conditions, and demands a different design or procedural response. A technician who can identify which type is occurring—or which type a specific design feature is intended to prevent—will troubleshoot more effectively and serve pilots far better than one who treats "carburetor ice" as a single undifferentiated phenomenon.

The Induction System and Why It Is Vulnerable

The induction system encompasses everything from the air intake opening on the cowling, through the air filter, into the carburetor or fuel-injection throttle body, through the intake manifold, and ultimately to the intake valves. Along this path there are multiple locations where temperature, pressure, and moisture content conspire to freeze water out of the air. The FAA Aviation Maintenance Handbook (FAA-H-8083-32) and the Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25) both discuss induction icing extensively because it bridges airframe, powerplant, and flight operations knowledge.

Impact Ice

Impact ice is the most straightforward of the three types. It forms when supercooled water droplets—liquid water that exists below 0 °C (32 °F) because it has not yet had a nucleation surface—strike a surface and freeze on contact. In the induction system, the most vulnerable locations are the air intake opening itself and the air filter element.

Because the ice builds up at the entrance to the induction path, impact ice is most hazardous in visible moisture (clouds, rain, freezing drizzle) when outside air temperatures are at or below freezing. As the opening or filter clogs, the engine experiences a progressively richer mixture and reduced manifold pressure as air flow is restricted. On many certified aircraft, the air filter is bypassed automatically or through a pilot-operated alternate air valve precisely because a completely plugged filter will kill the engine if no alternate air source exists.

From a maintenance standpoint, the technician must verify that alternate air doors open freely and seal properly in the normal position, that the air box is structurally sound, and that any heated air induction plumbing is intact. Many turbocharged and turbine installations use engine bleed air or exhaust heat to warm intake air continuously, eliminating impact ice risk at the cost of a small power and efficiency penalty.

Fuel Evaporation Ice (Refrigeration Ice)

Fuel evaporation ice is unique to float-type carburetors and any induction system where liquid fuel is atomized and evaporated inside the airstream before combustion. When fuel evaporates, it absorbs a substantial amount of heat from the surrounding air—this is the latent heat of vaporization. Aviation gasoline evaporating inside the carburetor venturi, combined with the pressure drop across the venturi and throttle, can cause a local temperature drop of as much as 60–70 °F (about 15–21 °C) below the incoming air temperature.

This dramatic cooling can freeze moisture out of the air even when the outside air temperature is well above 0 °C. In fact, fuel evaporation ice is most insidious on warm, humid days—conditions a pilot might never associate with icing. The FAA's carburetor icing probability chart shows that icing is possible at ambient temperatures as high as 100 °F (38 °C) with relative humidity as low as about 50%. The ice accumulates on the venturi walls, the throttle plate, and the discharge nozzle, progressively leaning and strangling the mixture.

The primary defense is the carburetor heat system, which draws warm air from a muff surrounding the exhaust system and routes it into the carburetor air box, bypassing the filtered cold-air inlet. This raises carburetor temperature above freezing, melting existing ice and preventing new formation. The technician's responsibilities include ensuring the carburetor heat door closes the filtered-air path completely when heat is selected (partial closure causes partial heating and may actually worsen icing), that the exhaust heat muff is free of cracks (which could introduce carbon monoxide-laden exhaust gases into the induction air), and that carburetor air temperature (CAT) gauges—when installed—read accurately.

On fuel-injected engines, fuel is introduced much closer to or directly into the intake ports, so the cooling effect of evaporation does not occur inside the throttle body in the same way. This is why fuel-injected engines are far less susceptible to fuel evaporation ice—but they are not entirely immune to the other two types.

Throttle Ice

Throttle ice is the subtlest and least well-understood of the three types. It forms specifically at and around the throttle valve (butterfly valve) in both carbureted and fuel-injected engines. When the throttle is partially closed, air accelerates through the restricted gap, causing a local pressure drop. By Bernoulli's principle, this pressure drop produces a corresponding temperature drop—sometimes enough to freeze moisture out of the passing airstream onto the throttle plate and the adjacent walls of the throttle bore.

Throttle ice is therefore most pronounced at reduced power settings—approach, descent, or any extended idle operation. A fully open throttle produces minimal restriction and minimal pressure differential, so little or no throttle ice forms at full power. The practical danger is that a pilot who has been running at reduced power during a long descent may advance the throttle for a go-around or balked landing and find the throttle physically restricted by ice, unable to deliver the expected power increase precisely when it is most needed.

Because throttle ice can occur even in fuel-injected systems (which have throttle bodies with butterfly valves), pilots of injected engines must not assume they are immune to all induction icing. Applying carburetor heat—or, on injected engines, the alternate air—before reducing power for descent is a standard precautionary technique. The technician should ensure throttle linkage moves freely through its full range and that any ice-detection or alternate-air systems associated with the throttle body are functional.

Why It Matters: Overlap and Combined Icing

In real-world conditions, more than one type of induction icing can occur simultaneously. A flight into IMC on a cool, humid day may produce impact ice at the filter while fuel evaporation ice builds in the venturi and throttle ice forms at the partially closed butterfly. The total restriction can be severe and rapid. This is why the FAA mandates that most certified aircraft have an alternate induction air source that bypasses the primary filtered, cooled path entirely—ensuring the engine can continue to receive some airflow regardless of which section has iced over.

Key Numbers and Rules

Temperature range for carburetor (fuel evaporation) icing: possible at outside air temperatures from below freezing up to approximately 100 °F (38 °C), with relative humidity as low as about 50%.
Temperature drop due to fuel evaporation and pressure drop: as much as 60–70 °F (~15–21 °C) below incoming air temperature inside the carburetor venturi.
Impact ice conditions: visible moisture and OAT at or below 0 °C (32 °F).
Throttle ice hazard: greatest at partial or idle power settings; can occur in both carbureted and fuel-injected engines.
Carburetor heat check: application of full carburetor heat should cause a brief RPM drop (if ice was present, melting ice briefly richens the mixture further before it clears, then RPM gradually rises as the ice clears); if no ice, RPM drops slightly and stays lower due to the density reduction of warm air.
Cracked heat muff: a cracked or leaking exhaust heat muff is an immediate airworthiness concern because exhaust gases (containing carbon monoxide) can enter the induction air supply.
Alternate air doors: must seal fully in the normal position to prevent unfiltered air entry in non-icing conditions, and must open fully in the alternate position to ensure adequate airflow.

Common Test Traps

"Only happens in cold weather": Fuel evaporation ice is the trap here—the exam frequently presents a scenario with temperatures in the 60s °F and high humidity. Students who associate icing only with freezing temperatures will miss this.
"Fuel injection is immune to all induction icing": Fuel-injected engines do not suffer from fuel evaporation ice inside the throttle body, but they absolutely can experience impact ice and throttle ice. The exam may test whether a student knows this distinction.
Carburetor heat RPM response: The exam may ask what happens to RPM when carb heat is applied with ice present versus without. With ice present, RPM first drops (warm, less dense air plus extra water richening the mixture) then gradually rises as ice clears. Without ice, RPM simply drops and stays lower.
Alternate air versus carburetor heat: On fuel-injected engines, the system is called "alternate air," not carburetor heat. Confusing the terminology on a mixed question can cost points.
Location of each ice type: Impact ice forms at the intake opening and filter; fuel evaporation ice forms at the venturi and discharge nozzle; throttle ice forms at the throttle valve. The exam may present a symptom or location and ask which type is implicated.

Mastering these distinctions—where each type forms, what atmospheric conditions favor it, how each affects engine operation, and what the maintenance technician must inspect or verify—gives the AMT powerplant candidate a significant advantage on exam day and a genuine foundation for keeping aircraft engines running safely in the real world.