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Carburettor icing: why it can stop your engine on a summer day

It is July, 24 degrees, a shower has just passed and you are descending towards the circuit with the engine almost at idle. Nobody is thinking about ice. Yet right now there may be enough ice building in the carburettor to stop the engine. Here is why, how to read the risk from the METAR and what to do with the carb heat.

  • Student pilot
  • EASA 021 · Aircraft General Knowledge
  • EASA 050 · Meteorology
  • 10 min read
  • 19 October 2026

The problem

METAR

METAR LROP 191230Z 25008KT 9999 SCT030 24/19 Q1014

A training aircraft with a carburettor engine descends from 3,000 ft at reduced power. After a few minutes the RPM has dropped a little, the engine starts to shake, and when the pilot opens the throttle the power does not come back. Outside it is 24 °C. But carburettor icing does not care how warm it is outside. What matters is how humid the air is and how much it cools inside the carburettor.

Carburettor icing is one of the classic causes of power loss in light aircraft. EASA has a dedicated safety leaflet on it, and it shows up in two PPL subjects: 021, with the engine, and 050, with meteorology.

What happens inside the carburettor

In the carburettor the air passes through a narrowing, the venturi, where it speeds up and its pressure drops. That is exactly where fuel is sprayed in, and as it evaporates it takes heat from the air. The two effects add up: the air in the carburettor can end up more than 20 °C colder than the air outside. If the air is humid, its water freezes on the walls and on the throttle valve.

TO THE ENGINE WARM, HUMID AIR venturi: pressure drops here: up to 20 °C colder throttle valve ICE fuel evaporates float chamber
The air cools for two reasons: pressure drops in the venturi, and the evaporating fuel takes heat. Ice (light blue) sticks to the walls and the throttle valve.

The ice narrows the passage, the engine gets less mixture and loses power. If you do nothing, the ice can block the passage completely and the engine stops. And an engine stopped by ice is hard to restart, because both the carburettor and the engine have cooled down.

Why mostly at low power

With the throttle closed, the valve is almost shut and ice blocks it more easily. The engine also gives off less heat, so the hot air for carb heat is less hot. Descent, approach and taxiing are the riskiest moments.

Read the risk from temperature and dew point

The 24/19 group in the METAR gives you all you need: temperature and dew point. The closer they are, the more humid the air. From them you work out relative humidity, and a risk chart published by EASA and the UK CAA shows how likely icing is, at cruise and at reduced power.

The risk chart from the EASA leaflet EGAST GA 5, redrawn. The magenta dot is the day in our story: 24 °C, dew point 19 °C, relative humidity 74 %. The sloping lines are relative humidity, drawn straight as in the leaflet (100 % = saturated air, mist or cloud).

For 24/19 the relative humidity is about 74 %. On the chart that means moderate icing at cruise and serious icing at reduced power: exactly the situation in our story. Icing is possible over a wide range of temperatures, from below freezing to around +30 °C and even higher, and in the flight tests quoted by the EASA leaflet serious icing at descent power appeared above +25 °C with relative humidity as low as 30 %, a point the chart itself shows only as light icing. So treat the chart as a guide. The highest risk is in warm, very humid air: after rain, in mist, near water or in cloud.

In very cold air the moisture is already frozen into crystals that pass through the carburettor without sticking, so the risk falls. On a dry, freezing winter day carburettor icing is less likely than on a humid summer day.

The calculation, as pseudocode

The official chart is drawn directly on temperature (T) and dew point (Td), so the program only has to find which zone holds the point. Relative humidity, from the Magnus formula, is just for the readout.

Pseudocodehumidity and risk zone
function vapour_pressure(t):             # hPa, Magnus formula
    return 6.112 * exp(17.62 * t / (243.12 + t))

function humidity(t, td):                  # only for the readout
    return vapour_pressure(td) / vapour_pressure(t)

# the four zones of the EASA chart, outermost first; each is drawn
# on the temperature / dew point grid, under the 100 % line
ZONES = [ yellow: "light, cruise or descent",
          orange: "serious at descent power",
          green:  "moderate cruise, serious descent",
          blue:   "serious at any power" ]

function risk(t, td):
    result = "low risk on the chart"
    for zone in ZONES:                  # the innermost zone that holds the point wins
        if zone.contains(t, td): result = zone.label
    return result

risk(24, 19)   # humidity 0.74, green zone → moderate at cruise, serious in the descent

The zones are traced from the chart in the EASA leaflet EGAST GA 5, “Piston engine icing”, the one you will meet in the PPL exam. The chart is a guide, not a guarantee, and your aircraft's flight manual comes first.

Try it yourself

Enter the temperature and dew point, or paste a whole METAR: the chart picks out the temperature group by itself.

The signs in the cockpit

  • Fixed-pitch propeller (most training aircraft): the RPM drops slowly although you have not touched the throttle. Rough running and vibration may follow.
  • Constant-speed propeller: the RPM stays constant, so the sign is a drop in manifold pressure or, in level flight, a drop in airspeed.

The drop is slow and easy to blame on something else. So the rule is simple: RPM that falls on its own means ice until proven otherwise.

What to do with carb heat

The carb heat control sends air warmed around the exhaust into the carburettor. The warm air melts the ice and the water passes through the engine.

  1. Full heat, not half. Partial heat can bring the air right into the range where it ices most easily. Use it only if the aircraft has a carburettor air temperature gauge.
  2. Leave it on long enough. The ice can take a minute or longer to melt, depending on how much has built up.
  3. Do not be put off by what happens next. The RPM first drops further and the engine may run rough. Then the RPM rises as the ice clears.
  4. In the descent, apply heat before you reduce power, while the engine is still hot, and on long descents add a little power from time to time so the engine does not cool too much.
2400 2200 RPM time → ice builds carb heat ON ice melts COLD: normal RPM
What the RPM gauge shows: a slow drop (ice), a bigger drop when heat goes on (warm air, water through the engine), then a climb as the ice clears. Back to cold, it returns to normal.

Why the RPM drops first

Warm air is less dense, so the engine gets less air and the mixture becomes richer: carb heat on its own costs power (PPL manuals quote around 15 %). On top of that, water from the melting ice passes through the cylinders and the engine runs rough for a few seconds. Many pilots get a fright and push the heat back to cold just when it had started to work. Wait: the RPM comes back.

The pre-take-off check

During the power check, at the RPM in your checklist, you select carb heat hot. The RPM should drop a little (PPL manuals quote about 100 RPM, but the exact figure is in your flight manual), then come back when you select cold. If the RPM is higher at the end of the check than at the start, there was already ice in the carburettor and the hot air melted it. Do another check before you take off.

Common traps

"It's summer, it can't freeze"

Outside temperature on its own means little. A humid summer day after rain is riskier than a dry winter day.

Carb heat on take-off

For take-off the carb heat stays cold, unless your flight manual says otherwise. It costs power exactly when you need all of it. At high power, hot air increases the risk of detonation, and icing is unlikely anyway with the throttle wide open.

Lots of hot air on the ground

The hot air bypasses the air filter, so on the ground you use it as little as possible to avoid drawing dust into the engine.

Fuel-injected engines

A fuel-injected engine has no carburettor, so it does not get carburettor ice. But ice can still form on the air intake or the filter if you fly through freezing precipitation. That is what the alternate air source is for.

Check yourself

1. The METAR gives 15/14. What is the risk in the descent?

Humidity is about 94 %, and at 15 °C that falls in the most dangerous zone: serious icing at any power. Carb heat on before you reduce power, and check often.

2. In the cruise, without touching the throttle, the RPM slowly drops by 100. What do you do?

Suspect ice and apply full carb heat. The RPM will first drop further and the engine may shake, then it recovers. Leave it hot for at least a minute, then select cold and watch the RPM.

3. In the power check, after returning the heat to cold, the RPM is higher than before. What does it mean?

There was already ice in the carburettor, and the hot air melted it. Repeat the check and keep the risk in mind for the whole flight.

4. Why not take off with carb heat on?

It costs power just when you need all of it, the air is not filtered, and at high power the risk of detonation rises. With the throttle wide open, icing is unlikely anyway.

5. A January morning, M15/M20. High risk?

No: the air is very cold and dry, and its moisture is already frozen. The point falls outside the chart, with low risk. That does not excuse you from the pre-take-off check.

Read the METAR before you fly

Temperature and dew point come from the METAR. Our METAR and TAF article has a decoder: paste the report and see every group explained.

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FlightDesk Aero articles are training material aligned with the EASA PPL syllabus. For real flights, use your aircraft's flight manual, the EASA safety leaflet on piston engine icing and your instructor's guidance.