It is a warm, muggy summer afternoon. The outside air temperature is 25 °C, the sky is hazy, and there is not a cloud in sight, let alone a snowflake. Somewhere inside the engine of a small training aeroplane, ice is quietly growing on a metal plate the size of a coin.

Carburetor icing is one of the most counter-intuitive hazards in light aviation. It does not need freezing weather or visible moisture, and it often strikes when pilots least expect it: in a gentle, low-power descent on a pleasant day. The FAA’s Pilot’s Handbook of Aeronautical Knowledge states that it can occur in outside air as warm as 38 °C (100 °F).

The cure has been fitted to carburetted aircraft engines for decades. It is a simple control, usually labelled “CARB HEAT”. The trick is knowing when to pull it, and having the nerve to leave it on.

A refrigerator in the intake

Most traditional light-aircraft piston engines use a float-type carburetor. Air flows through a narrow throat, the venturi, where its pressure drops and fuel is drawn in through a jet. The fuel-air mixture then passes a throttle valve, a pivoting plate, on its way to the cylinders.

Two things in that process steal heat. The drop in pressure in the venturi cools the air, and the evaporating fuel absorbs heat as it vaporises, just as sweat cools skin. According to the FAA, the temperature drop inside the carburetor can be as much as 60 to 70 Fahrenheit degrees, which is roughly 33 to 39 Celsius degrees.

That is the “aha” that explains everything. On a 38 °C day, a 39-degree drop takes the air inside the carburetor to about −1 °C. If that air is humid, its water vapour condenses and freezes onto the venturi walls and the throttle valve, choking the flow of mixture to the engine.

Warm and humid is the danger zone

The FAA says carburetor ice is most likely when the temperature is below 21 °C (70 °F) and the relative humidity is above 80 percent. Because of the sudden cooling inside the carburetor, it can also form at temperatures up to 38 °C (100 °F) and humidity as low as 50 percent.

Its chart of icing probability is worth studying because it overturns the instinct that ice belongs to winter. The high-risk box is not centred on freezing conditions. It spans roughly −7 °C to 21 °C with high humidity: a damp spring morning, a summer day near a lake, flying just below a cloud base.

The FAA’s 2009 Special Airworthiness Information Bulletin on the subject, SAIB CE-09-35, put numbers to the problem, attributing 212 accidents between 1998 and 2007 to carburetor icing. EASA endorsed that bulletin for fixed-wing aircraft in its own Safety Information Bulletin 2010-03 the following year.

Low power is when it bites

Ice can form in any phase of flight, but the FAA says it is particularly dangerous at reduced power in a descent. With the throttle nearly closed, the throttle valve restricts the flow and creates even more cooling, while a cooler engine vaporises its fuel less completely.

Worse, the ice can build unnoticed while the engine is idling, only to reveal itself when the pilot opens the throttle for a go-around or to stretch an approach, and the power is not there. For this reason the FAA recommends that, when icing conditions are suspected and a closed-throttle descent is planned, full carb heat is selected before closing the throttle and left on throughout.

King Schools on the carburetor icing lesson every private pilot needs to remember.

Know the symptoms

The warning signs depend on the propeller. In an aircraft with a fixed-pitch propeller, such as most basic trainers, the first indication is a drop in rpm, which may be followed by rough running. In an aircraft with a constant-speed propeller, the governor holds rpm constant, so the clue is a fall in manifold pressure instead.

The FAA adds the broader symptoms: loss of power, altitude or airspeed, sometimes with vibration or engine roughness. Some aircraft have a carburetor air temperature gauge, whose yellow arc, typically −15 °C to +5 °C, marks the range where icing may occur.

Carb heat, and the nerve to leave it on

Carburetor heat is an anti-icing system that preheats the air before it reaches the carburetor. Applied early, it prevents ice; applied late, it melts ice that has already formed, as long as there is not too much of it. The FAA is clear that prevention is the better option.

Here is the part that catches pilots out. Hot air is less dense, so selecting carb heat reduces power, sometimes by up to 15 percent according to the FAA, and enriches the mixture. If ice is present, the meltwater then goes through the engine, which can run rough for anything from 30 seconds to several minutes. Everything sounds worse before it gets better.

That is exactly why the FAA tells pilots to resist the temptation to reduce carb heat and to keep it fully hot until normal power returns. With a fixed-pitch propeller, the classic sign of ice is a drop in rpm when heat is applied, followed by a gradual rise as the ice melts. If there is no ice, the rpm drops and stays put.

Real carb ice in flight, filmed by Cubonaut875: listen to the engine as carb heat begins melting the ice, from about the four-minute mark.

There are also times not to use it. Because carb heat reduces power and raises engine temperatures, the FAA says it should not be used when full power is needed, as on take-off, or in normal operation, except to check for or remove ice. It should be checked during the engine run-up before every flight.

Fuel injection, and helicopters

Fuel-injected engines, which spray fuel directly into the cylinders or just ahead of the intake valves, largely avoid the evaporative cooling in a carburetor; the FAA lists reduced evaporative icing as one of their advantages. They can still suffer impact icing, where ice blocks the air intake, which is why they have an alternate air source.

For carburetted helicopters, EASA’s bulletin stresses that the consequences are more severe. An engine stoppage means an immediate autorotation, with little time to diagnose the problem, and it recommends applying full carb heat before lowering the collective to start a descent.

FLY8MA.com explains carb ice in a Cessna, including how to recognise it and use carb heat.

The lesson is simple and worth tattooing on every student pilot’s kneeboard. Ice does not need winter. Warm, humid air and a closed throttle are enough, and the knob that fixes it only works if you pull it early and leave it alone.

Sources: FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25, Chapter 7), FAA SAIB CE-09-35, EASA Safety Information Bulletin 2010-03, Boldmethod, NASA Glenn Research Center