What Causes Carburetor Icing in Aviation?

Carburetor icing is a problem in small aircraft with piston engines that use a carburetor to mix air and fuel for the engine. Ice can build up inside, choking the engine and causing power loss. Here’s why it happens, explained simply:

1. Temperature Drop in the Carburetor

The carburetor has a narrow section called the venturi, like a pinched part of a straw. When air flows through it, two things cool the air down, making it cold enough for ice to form if moisture is present:

• Air Speeds Up, Pressure Drops, and Cools:

  • When air enters the venturi, it’s squeezed into a tight space, so it speeds up (like water shooting faster through a narrow hose).
  • According to the Bernoulli Principle, faster-moving air has lower pressure. Think of blowing through a straw: if you pinch it, the air speeds up but feels less forceful (lower pressure).
  • Lower pressure makes the air expand a bit, and expanding air gets colder because the molecules spread out and lose energy. This can drop the temperature by 20–30°C (36–54°F), even if it’s warm outside.

• Fuel Evaporation Cools More:

  • The carburetor sprays fuel into the air, and the fuel turns from liquid to vapor (like water evaporating off your skin). This process sucks heat from the air, cooling it further by about 5–10°C (9–18°F).
  • Together, these effects can make the air inside the carburetor cold enough to freeze, even if the outside temperature is as high as 20°C (68°F).

2. Moisture in the Air

For ice to form, there needs to be water in the air. This happens when:

• The air is humid (e.g., over 50–60% humidity).
• The plane flies through clouds, fog, or rain, where moisture is visible.
• If the air gets cold enough in the carburetor (below 0°C/32°F), this moisture can freeze into ice on the carburetor’s walls, throttle valve, or venturi.

3. Weather Conditions

Carburetor icing is most likely when:

• Outside temperatures are between -7°C and 21°C (20°F and 70°F), especially with high humidity.
• The temperature and dew point are close (less than 5°C apart), meaning the air is nearly saturated with moisture.
• For example, on a warm, humid day (15°C/59°F), the venturi can cool the air to below freezing, causing ice if moisture is present.

4. Throttle Position

• When the throttle is partially closed (like during descent or cruising), less air flows through, making it easier for ice to build up on the throttle valve.
• Full throttle lets more air through, which reduces the chance of icing but doesn’t eliminate it.

5. Aircraft Design and Operation

• Engines without a working carburetor heat system (which pulls warm air into the carburetor) are more at risk.
• Flying at low power settings for a long time, like during taxiing or descent, gives ice more time to form.

Why the Pressure Drop Happens

The pressure drop in the venturi is due to the Bernoulli Principle:

• Air has energy split between speed (kinetic energy) and pressure (pressure energy).
• In the narrow venturi, air speeds up to get through, using more of its energy for speed.
• This leaves less energy for pressure, so the pressure drops.
• Think of a crowd squeezing through a narrow doorway: people move faster but are more spread out, so there’s less “push” (pressure) among them.

This lower pressure causes the air to expand and cool, setting the stage for icing if moisture is present.

Symptoms of Carburetor Icing

• The engine loses power (RPM drops).
• The engine runs rough or backfires.
• In bad cases, the engine stops completely.

Prevention

• Carburetor Heat: Pilots turn on carb heat to send warm air (from around the engine) into the carburetor, melting or preventing ice.
• Check Weather: Pilots watch for humid conditions, clouds, or rain and apply carb heat proactively.
• Pre-Flight Checks: Make sure the carb heat system works before flying.

Note

Fuel-injected engines don’t have carburetors, so they avoid this type of icing, but they can still get impact icing (ice on air intakes) in similar conditions.