Aircraft Icing

Ice accretion during flight adversely affects aircraft aerodynamics by reducing lift while increasing weight and drag.
Icing increases the aircraft’s stall speed.
Differential icing can result in unequal loading on the aircraft.
Ice accumulation may jam flight control hinges, leading to loss of aircraft control.
Pressure instruments may become unreliable due to ice blockages.
Icing on aircraft antennas can cause communication problems.

Ice accretion refers to the accumulation of ice on an aircraft.
It is caused by supercooled water droplets present in the atmosphere when the temperature is below the freezing point.
Supercooled water droplets remain in the liquid state below 0°C because of the absence of sufficient freezing nuclei.
Freezing nuclei are microscopic dust particles that initiate the conversion of water into ice.
Large supercooled water droplets usually freeze between 0°C and –15°C, while smaller droplets freeze between –15°C and –40°C.

Opaque Rime Ice: White, opaque ice deposits that form on leading edges from small supercooled water droplets in clouds above the freezing level.
Rime ice causes relatively little aerodynamic distortion and can often be removed easily.
Clear Ice (Glaze Ice or Translucent Rime): Smooth, transparent ice formed by large supercooled water droplets.
Large droplets spread over the airframe before freezing, creating a dense layer of clear ice.
Clear ice is difficult to remove and may break away in large pieces that can damage the aircraft.
Mixed Ice: A combination of both clear ice and rime ice.

Freezing rain consists of supercooled raindrops occurring when surface temperatures are below the freezing point.
It is commonly associated with warm fronts, where a layer of warm air overlies cold surface air, creating a temperature inversion.
Freezing rain is extremely hazardous because it can cause rapid ice accumulation at low altitudes.

Hoar frost consists of feathery ice deposits that form both on the ground and on aircraft in clear air when temperatures are below the frost point.
It commonly forms when an aircraft descends rapidly from colder upper levels into warm, moist air.
The aircraft surface remains cold while the surrounding air is warm and moist, allowing frost to form.
Hoar frost can usually be removed easily and generally presents less danger than other forms of icing.

High clouds such as Cirrus (Ci), Cirrostratus (Cs) and Cirrocumulus (Cc) contain mainly ice crystals and therefore present very little icing hazard.
Medium-level clouds such as Altostratus (As) and Nimbostratus (Ns) contain supercooled water droplets that can cause moderate icing.
Altocumulus (Ac) clouds may also contain supercooled droplets capable of producing moderate icing, with severity increasing over mountainous regions.
Towering Cumulus (TCu) and Cumulonimbus (Cb) clouds contain abundant supercooled water droplets and can produce severe icing, especially down to temperatures of about –20°C.
Except for Nimbostratus, stratiform clouds generally present a lower icing risk than cumuliform clouds.

Clouds with higher bases generally present a greater icing risk because of lower temperatures and a higher concentration of supercooled water droplets.
Icing risk is greater in tropical regions during summer due to higher cloud bases.
Stratiform clouds over mountainous terrain present an increased icing risk because the 0°C isotherm is lowered.
During winter in North India, freezing levels are lower, increasing the likelihood of icing.

Engine Impact Icing occurs when supercooled water droplets strike and freeze on the engine air intake.
Impact icing restricts airflow into the engine, resulting in a loss of engine power.
Carburettor Icing occurs when air pressure decreases inside the carburettor, causing adiabatic cooling and the formation of ice.
Carburettor icing can occur even at temperatures as high as +30°C when the relative humidity exceeds 60%.