When ice forms on an aircraft
Icing is an aerodynamic hazard before it is a weight problem. It damages the smooth leading edge that produces lift, raises drag and stall speed, and can affect instruments, controls and engines.
The three conditions for airframe icing
For ice to accrete in flight there must be liquid water, usually supercooled droplets, an ambient temperature below zero, and an airframe surface below zero. A supercooled droplet remains liquid below its normal freezing point because it lacks an effective freezing nucleus. It freezes when it strikes the aircraft.
Where the droplets are
Large supercooled droplets are most likely in cumulus, cumulonimbus and nimbostratus from 0°C to about −20°C. Smaller droplets occur in layer cloud and at colder levels. Below about −40°C liquid droplets are very small and icing is usually negligible.
| Cloud or setting | Water and temperature | Likely hazard |
|---|---|---|
| Cu, Cb, Ns | Large droplets from 0°C to −20°C. | Moderate to severe clear or mixed ice. |
| St, Sc, As, Ac | Small droplets, often through a wide cold range. | Light to moderate rime ice. |
| Ci, Cs, Cc | Mostly ice crystals. | Nil or trace airframe icing. |
| Mountain wave or lift | Enhanced liquid water in orographic cloud. | Severity can be greater than cloud type suggests. |
Clear, rime and mixed ice
The appearance of the ice tells you how the impacting droplet froze. That mechanism explains both its shape and its danger.
Clear or glaze ice
Large supercooled droplets freeze relatively slowly. Latent heat delays complete freezing, so water spreads backwards from the impact point before it freezes. The result is glossy, transparent or translucent clear ice. It adheres strongly, changes the aerofoil shape and may build rapidly in Ns, Cu and Cb between 0°C and −20°C. It is the most dangerous common type.
Rime ice
Small droplets freeze almost instantly where they strike. Air becomes trapped between the frozen droplets, producing white, opaque, porous rime ice. There is little flowback. It is often lighter than clear ice, but it can still spoil leading edges and block intakes.
Mixed, rain ice and frost
In cloud from 0°C to −20°C, a mixture of droplet sizes produces mixed ice, combining leading edge rime with clear ice flowback. Rain ice occurs when rain becomes supercooled after falling through an inversion into subzero air, often ahead of a warm front or occlusion. It can build so fast that an immediate turn onto a reciprocal heading is needed.
Hoar frost is different. It is a feathery ice crystal deposit that forms in clear air by sublimation of water vapour onto a subzero airframe. It can form on the ground on a cold night or after a rapid descent into warm moist air.
Severity, effects and avoidance
Severity depends on droplet size and concentration, cloud type, temperature, aircraft shape and speed. A thin wing, a pitot head, an antenna or a tailplane can collect ice faster than a broad wing, so a small amount of visible wing ice can understate the threat elsewhere.
Why even a small deposit matters
| Effect | What ice does | Operational consequence |
|---|---|---|
| Aerodynamic | Destroys a smooth leading edge. | Less lift, more drag and higher stall speed. |
| Weight and balance | Adds uneven mass to wings or propellers. | Loss of stability and vibration. |
| Controls and instruments | Can jam hinges or block pressure heads. | Control restriction and erroneous ASI, VSI or altimeter indications. |
| General systems | Obscures screens and affects aerials or gear wells. | Reduced visibility, radio interference or mechanical problems. |
Oxford notes that a deposit with the roughness of coarse sandpaper can reduce lift by 30 percent and increase drag by 40 percent. The correct response to significant icing is not to wait for a perfect forecast: change altitude, heading or both before protection equipment is overwhelmed.
Forecast and report language
Forecast icing is light, moderate or severe, based on cloud and its vertical extent. Reported intensity is linked to the rate of accumulation and the aircraft’s protection equipment. In severe icing, the equipment fails to control the hazard and an immediate change of heading or altitude is essential.
Engine icing and practical decisions
Engine icing has mechanisms distinct from airframe icing. It can occur at temperatures where a pilot might not expect a wing to accrete ice.
Piston engine induction icing
Impact icing is ice on intake areas from snow, rain or supercooled droplets. Carburettor icing results from the pressure drop through the venturi and the latent heat absorbed as fuel evaporates. Those two cooling processes can lower the carburettor temperature sharply.
The serious carburettor icing range is broadly −10°C to +25°C in cloud, fog or precipitation, and Joshi emphasises that it can occur even in clear air at temperatures up to +30°C when humidity is high. It is unlikely when relative humidity is below 60 percent.
Jet engine icing
Ice may form on intake lips, inlet guide vanes or early inlet stages. A shed piece can damage compressor blades. In freezing precipitation, engine anti icing is selected on when indicated outside air temperature is +10°C or below. The approach is particularly relevant because high engine speed and low forward speed can increase intake cooling.
Before take off
Frost, snow and clear ice must be removed. Hoar frost may seem light, but any rough contamination changes the aerofoil and can increase the take off run. Cold soaked fuel can keep upper wing skin at or below zero even when ambient air is above zero, allowing clear ice to form in humid air, drizzle, rain or fog.