Icing Conditions - ATPL 050 — Meteorology

The thermodynamics module promised that supercooled water would matter, and the clouds module showed it arriving as freezing rain. This lesson makes good on both: where and when ice actually accretes on an aircraft — in the air, on the ground, and in a place with no cloud in sight at all, the carburettor of a piston engine on a warm, humid day. The next lesson takes what kind of ice grows; this one takes the conditions.

What ice needs

Strip every icing story to its skeleton and the same requirements appear:

an aircraft surface at or below 0 °C — the airframe's own temperature, not the gauge reading, is the real criterion;
outside air cold enough to carry supercooled water, from 0 °C down to about −40 °C, with everything serious concentrated in the warmer half of that range;
liquid water to freeze on impact: supercooled droplets in cloud, fog, rain or drizzle; or, with no liquid at all,
water vapour sublimating directly onto a sub-zero surface as hoar frost — the one route that needs neither cloud nor precipitation.

Most of the list should feel familiar. The thermodynamics module built the physics: freezing needs freezing nuclei, and freezing nuclei are rare — so cloud droplets cooled below 0 °C simply stay liquid, ready to freeze the instant they touch a wing, and only near −40 °C do they finally freeze spontaneously without one. The clouds module added the two ways liquid water arrives already below zero outside cloud: freezing rain from a warm nose aloft, and freezing drizzle grown by coalescence inside sub-zero stratus.

The supercooled-water supply

How much supercooled water the air offers depends on temperature, and the relationship is the exam's favourite curve: the greatest amounts sit just below 0 °C, and the amount falls steadily as the air gets colder — partly because more and more droplets have found nuclei and frozen, partly because colder air holds less water in the first place. By −40 °C the cloud is essentially all ice crystals, and crystals mostly bounce off a cold, dry airframe rather than stick. That is why the band the thermodynamics module flagged, 0 °C to about −20 °C, where supercooled water is common, is where most airframe icing happens, and why icing reported at −35 °C is rare and usually light.

Cloud type, droplet size

The other half of the supply question is what kind of cloud you are in:

Cumuliform clouds — large droplets and high water contents: their strong updraughts can support big drops, and keep condensing fresh water faster than it falls out;
Stratiform clouds — small droplets, more numerous, but with less total water per cubic metre.

Hold that pairing: the next lesson turns it directly into ice types, because large droplets build clear ice and small droplets build rime — which is why the clouds module called deep clouds of large supercooled droplets the worst icing clouds of all.

Icing on the ground

An aircraft parked overnight can be iced before it ever moves. The circumstances the syllabus wants:

a clear, calm night with air temperature near or below 0 °C and moist air — radiative cooling takes the skin below the air temperature and below the frost point, and hoar frost grows by sublimation, sometimes with the thermometer still reading above zero;
freezing rain, freezing drizzle or wet snow falling on the airframe — the fastest and heaviest ground icing, sheeting surfaces in glaze;
freezing fog — supercooled fog droplets riming every surface they touch, exactly as the fog lesson described;
positive air temperatures over cold-soaked fuel tanks — the special case of the next section.

Icing in flight and outside cloud

In flight there are three places to collect ice, and the third is the one that surprises people:

inside cloud — supercooled droplets from 0 °C down to about −40 °C, with the serious accretion between 0 and −20 °C;
in precipitation — freezing rain and freezing drizzle below or ahead of cloud, the heaviest icing of all, glazing an aircraft in air that may be completely clear;
outside cloud and precipitation entirely — a cold-soaked airframe descending into warmer, moist air grows hoar frost by sublimation: the vapour meets a surface far below frost point and deposits straight to ice, fogging windshields and frosting wings.

The cold-soaked wing

Why is the airframe ever colder than the air around it? Three reasons the syllabus names. First, fuel temperature: after hours of cruise at −50 °C outside, the fuel in the wing tanks is soaked far below zero, and it warms back up much more slowly than the descent happens, so the wing skin above the tanks stays sub-zero through approach, landing and turnaround, and clear ice or frost can form there in rain or humid air at clearly positive outside temperatures. Second, radiative cooling on the ground at night, which the previous section already used. Third, the mirror image: the surface temperature the aircraft brings with it — a machine still warm from a low-level sector resists icing for a while, and a cold-soaked one invites it. All three say the same thing: icing follows the temperature of the surface, not the OAT gauge.

Carburettor icing: the warm-day trap

One icing habitat needs no supercooled water and no sub-zero day: the venturi of a piston engine's carburettor. Two coolers run in series there. The air accelerating through the throat expands, and expansion cools it; the fuel sprayed into it evaporates, and evaporation steals latent heat and cools it again: together the throat can run 25 °C or more colder than the intake air. Bring moist air in at +20 °C and the throat is below freezing with water vapour to spare: ice builds on the throat walls and the throttle butterfly, chokes the airflow, and the engine runs rough or quits — in clear air, on a warm day. The standard statement: carburettor icing is possible in moist air at outside temperatures up to about +30 °C, and it is most likely at high humidity and low power settings, when the partly closed butterfly makes the constriction worst.

Intensity: what makes it worse

Given that ice will form, five factors set how fast it builds:

air temperature — via the supply curve: the closer to 0 °C (from below), the more supercooled water;
the amount of supercooled water in the cloud or precipitation — cumuliform beats stratiform, freezing rain beats both;
the amount of ice crystals in the air — crystals alone mostly bounce off, but striking a wet, iced surface they stick and build the accretion faster;
speed — faster flight sweeps up more water per minute, though at high true airspeeds kinetic heating warms the leading edges and can lift them above 0 °C, stopping accretion altogether;
the shape and thickness of the part: thin parts collect ice fastest — a thick, blunt section pushes the air aside well ahead of itself and small droplets swing around it with the flow; a thin edge disturbs the flow so late that the droplets fly straight in and hit.

That last factor is why aerials, probes, struts and the thin tailplane leading edge ice up first and worst — and why the tailplane can be badly iced while the thick wing ahead of the pilot still looks almost clean.

Mountains make everything worse

Terrain is an icing amplifier. Air forced up a windward slope condenses water faster than the cloud can rain it out, so orographic lifting raises the liquid-water content: icing over mountains and windward slopes is more intense, and lasts longer, than in the same cloud type over flat country. The stratiform case is the syllabus's special point: in a stratiform cloud sitting on high ground, the steady forced ascent keeps feeding new condensed water in faster than fallout removes it, so the droplet concentration is markedly higher than in the same cloud away from the hill — hill stratus can ice like a far more serious cloud. Add the clouds module's nimbostratus, deep and supercooled on a frontal slope, and the freezing level sitting lower over cold high ground, and the mountain crossing collects every factor this lesson has named.

What the exam asks

This lesson's questions are conditions questions: which temperatures, which clouds, which situations. The overview, following the learning objectives:

Summarise the general conditions for ice accretion: air temperature, airframe temperature, supercooled water, sublimation

Explain the formation of supercooled water and relate the air temperature to the amount available

Relate cloud type to the size and number of droplets in cumuliform and stratiform clouds

Indicate the circumstances for ice on an aircraft on the ground

Explain the circumstances for ice in flight: in cloud, in precipitation, and outside both

Explain the influence of fuel temperature, radiative cooling and the aircraft's surface temperature

Explain the weather conditions for carburettor icing

Describe the factors that set the intensity of icing, from water content to speed and the shape of the part

Explain the effects of topography, including the higher drop concentration in stratiform orographic clouds