Carburettor Icing: Possible Even on Warm Days

How the ice forms

The float-type carburettor is the most common design. It is prone to icing. The fuel vaporises, and in the venturi, a narrow throat in the carburettor, the air pressure falls. Both make the temperature inside the carburettor drop sharply. If water vapour in the air condenses when the carburettor temperature is at or below freezing, ice can form.

The ice usually sits at the throttle valve and in the venturi. It restricts the flow of the mixture and reduces power. If enough ice builds up, the engine can stop. The temperature inside the carburettor can be as much as 35 °C below the outside temperature.

In which weather conditions

Carburettor icing is not restricted to cold weather. It can occur on warm days if the humidity is high enough, especially at low power. The two sources on this page give different figures:

Source	Higher risk	Also possible
FAA handbook	below 21 °C and above 80% relative humidity	up to 38 °C and from 50% relative humidity
UK CAA (Safety Sense Leaflet 14)	between 0 °C and 10 °C with high relative humidity	in tests, serious icing at descent power above 25 °C and with relative humidity as low as 30%

Do not treat the figures as limits. The FAA handbook points out that carburettor icing is possible under conditions that its chart does not show. Signs of high humidity are: temperature and dew point close together, visible moisture on the ground, rain, fog or a low cloud base.

How you recognise icing

With a fixed-pitch propeller the first sign is a drop in rpm, which can be followed by rough running. With a constant-speed propeller, icing usually shows as falling manifold pressure without a drop in rpm, because the propeller pitch automatically compensates for the loss of power. Power is often lost gradually until a lot of ice has built up.

According to the UK CAA leaflet, if you open the throttle slightly to restore rpm with a fixed-pitch propeller, you disguise the early signs. If an EGT gauge is fitted, it often shows a drop in exhaust gas temperature before power falls.

What you do

Switching the heat on

Carburettor heat is an anti-icing system that preheats the air before it reaches the carburettor. If you recognise icing, you apply full carburettor heat at once and leave it on until you are certain that all the ice has gone. This is how the FAA handbook describes it; the flight manual for your type remains decisive. The engine may run rougher at first, because the melting ice passes through the engine. These symptoms can last from 30 seconds to several minutes.

Carburettor heat reduces power slightly, because warm air is less dense. Do not confuse this with icing. With ice, rpm on a fixed-pitch propeller first falls and then slowly rises again as the ice melts. Without ice, it falls and then stays constant.

When you do not use it

According to the FAA handbook, you do not use carburettor heat at full power, for example on take-off, or in normal operation, except to check for ice or to remove it. You check it during the run-up.

Descent and low power

Carburettor icing can occur in any phase of flight. It is particularly dangerous at reduced power in a descent, because ice can build up unnoticed until power is added. With the throttle closed, the engine cools quickly and is more susceptible to icing. So the UK CAA leaflet recommends switching on carburettor heat before you reduce power for the descent. During long periods of reduced power it should stay on, and you add power regularly to keep the engine warm.

Fuel-injected engines

Fuel-injected engines do not experience carburettor icing. They can, however, suffer impact ice at the air intake or ice in the fuel system.

What the EASA syllabus covers

The EASA syllabus for the PPL(A) theory lists, under piston engines, “Carburettor: design, operation, degraded modes of operation, indications and warnings” and “Icing”. In flight training, operating the carburettor heat is part of Exercise 4 (“Effects of controls”).