Supercooled water droplets
freezing point is 0 degrees although supercooled water droplets have a freezing temp of up to -40 degrees.
water in its liquid form at tempretatures below freezing are known as “supercooled water droplets”
Rime ice
When very cold supercooled droplets collide with the leading edges of the wing, they freeze so quickly that they don’t have time to splash.
causing it to freeze as a droplet.
Rime ice progression
A small supercooled water droplet approaches the the wing surface
Because it is so small, it freezes almost instatnly upon impact, maintaining its spherical shape.
As other droplets freeze upon impact, a granular deposit of rime ice is built up.
How it affects and aircraft in flight.
Over time the ice deposits will cause a mushroom shape of water droplets which will then affect aircraft performance.
The disruption of airflow approaching the wing will cause a reduce in lift and an increase in drag.
Rime ice and pitot tube
Rime can form on the pitot tubes which will impair airspeed indication.
Clear ice.
Larger supercooled droplets are found at warmer (Still below zero) temperatures. because the larger droplets are not so cold and have more latent heat to lose, they take longer to freeze
Clear ice progression
Large supercooled water droplet approaches the wing.
because it is bigger and the temp is warmer the water droplet has time to spread across the leading edge of the wing.
A layer of clear ice forms over the surface. it has almost no air trapped within it. it is heavy, strong and clings to the surface.
Conditions necessary for the formation of airframe icing.
Sub zero temps
supercooled droplets
droplet size.
Sub zero temps
Ice melts when its temperature is above 0 degrees. therefore airframe icing is impossible unless the skin of the aircraft is at sub zero temps.
Supercooled droplets
Liquid water must be present to form ice. If ice is also ready present in the atmosphere it cant stick to the airframe
Droplet size.
If supercooled water is small then rime ice is likely.
If supercooled droplets are larger than clear ice is likely.
Favourable temps for icing (rime and clear)
From 0 degrees to -15 will cause large supercooled droplets (more chance of clear ice)
From -10 to -30 smaller supercooled water droplets will be smaller (more chance of rime ice.
Droplet size is the biggest indicator for the type of ice that forms.
Catch efficiency meaning
The rate at which supercooled water droplets freeze is called catch efficiency
Thicker areofoil and catch effciency
The thicker the leading edge of an areofoil causes airflow to seperate well before the areofoil.
This means better catch effciency and less chance of icing.
Thinner areofoil.
The thinner the areofoil means that icing is more prevalent because the airflow seperates a lot more closer to the leading edge.
The intertia carries more droplets through to impact with the surface and freeze.
Cloud and airframe icing.
Aircrafts in cloud (more moisture) and above the freezing level can expect to encounter airframe icing.
Large droplets can’t be supported in cloud unless there is a strong updraft due to gravity causing them to fall to the ground.
Cloud types clear ice
Clear ice is most likely to form in clouds with great vertical devolopment
Thick altostratus or above freezing level in large cumulus, cumulonimbus and nimbostratus.
Cloud types rime ice
Rime ice is more likely to develop in shallower clouds, or near the tops of large clouds.
Thin altostratus or altocumulus or near the tops of large cumulus.
Possible for stratocumulus, stratus or small cumulus but only above the freezing level.
Freezing rain
Cloud is not always necessary for airframe ice.
Supercooled rain drops fall from a cloud base onto an aircraft cruising below the cloud but above the freezing level.
(Aircraft skin temp below freezing)
Frost
Frost is frozen dew. it forms on the top surfaces of the aircraft left out over night.
if it’s not removed before flight, it can disrupt the airflow over the wing causing a loss of lift and increase in drag
