Engine / induction icing

Engine Induction Icing

Engine induction icing is the formation of ice in the intake system of a piston engine, which blocks airflow and reduces power or can stop the engine.

Three main types:

1. Carburetor Icing

The most common and dangerous form for PPL aircraft with carbureted engines — see dedicated lesson Carburettor icing.

Mechanism:

• In the carburetor venturi air pressure and temperature drop adiabatically by up to 30 °C.
• Fuel evaporation cools further.
• In humid air and outside temperature +5 to +20 °C water vapour can condense on the carburetor wall and freeze — even when the outside air feels warm and "ice-free"!

Danger: ice accretion blocks the airflow → progressive power loss → engine runs weaker and can quit.

Detection:

• Sudden power drop without other reason,
• Difficulty restarting from idle,
• RPM fluctuations on fixed-pitch propellers,
• MAP drop on constant-speed (RPM constant, MAP falling).

Counter-measure: carb heat — see Carburettor icing lesson for details.

2. Throttle Icing

Mechanism:

• When the throttle valve is partially closed (power below 75%) a flow constriction forms downstream.
• Pressure and temperature drop at this point cause icing — similar to carb icing but downstream of the venturi.

Frequency: also possible on fuel-injected engines if a throttle valve is present (modern fuel injection often has a throttle for idle/cruise).

Symptoms: similar to carb icing.

Counter-measure: alternate air (see point 4).

3. Impact Icing

Mechanism:

• Heavy precipitation, very cold moist air or supercooled large droplets (SLD) strike the air intake scoop directly.
• Ice forms on the intake fairing or filter.

Conditions:

• OAT below 0 °C,
• Visible moisture (rain, snow, cloud).

Frequency: rare in PPL VFR (visible moisture below 0 °C means icing risk regardless — don't go there); relevant in IFR.

Symptoms:

• Very rapid power loss,
• MAP drop sudden.

Counter-measure: open alternate air, leave icing zone.

4. Alternate air vs carb heat

Carb heat:

• On carbureted engines: routes hot exhaust air through the carburetor venturi.
• Counters carburetor icing and throttle icing within the carburetor.

Alternate air:

• On fuel-injected engines: opens a bypass door in the cowling, drawing unfiltered, warm engine-compartment air instead of outside air.
• Counters impact icing and iced filter.
• On some types automatic (with a flapper valve), on others manual by the pilot.

Important: both systems reduce engine power (warm air = less O₂ = less combustion) — so use only when needed or in the pre-flight test.

Filter icing

In some aircraft (e.g. PA-28, Cessna 172):

• Air-intake filter ahead of the carburetor/throttle.
• The filter can become blocked by ice/snow buildup.
• Symptom: abrupt MAP drop without other explanation.
• Counter-measure: alternate air (bypasses the filter).

Prevention in flight

1. Test carb heat regularly in icing-prone conditions (see carb-ice lesson for susceptibility chart).
2. Weather briefing for SIGMET "SEV ICE" or "MOD ICE" — avoid icing conditions.
3. In humid conditions with OAT 0 to +25 °C: apply carb heat routinely, even without symptoms.
4. Maintain minimum altitude above cloud in cold air.

Rules of thumb

"If the engine stuttered and you were in cloud" → carb or impact icing. Carb heat or alternate air immediately.

"If MAP drops unexplained" → filter blockage or intake ice. Alternate air.

"At OAT between +5 and +15 °C in moist air" → highest carb-ice risk! Apply carb heat regularly.