Hoarfrost
Hoarfrost refers to the white ice crystals, deposited on the ground or exposed objects that form when the air is moist and surface is cold. Under clear frosty nights in winter soft ice crystals might form on aircraft or any object that has been chilled below freezing point by radiation cooling. This deposit of ice crystals is known as hoarfrost and may sometimes be so thick that it might look like snow. However frost on aircraft can disrupt aerodynamic performance and must be removed before flight. A typical method to remove hoarfrost is to melt it by pouring water over it, however when melting the frost make sure water does not penetrate control surface hinges where it might refreeze and cause problems. Another method is to use a brush or rag and wipe the frost off the aircraft surfaces.
A layer of frost the width of fine sandpaper can cause significant loss of lift.
Aircraft in flight that have flown from a very cold air-mass and enter a moist air-mass can experience hoarfrost forming on the aircraft and noticeably the windscreen, blocking out visibility. The best method to remove hoarfrost under these circumstances would be to find a cruise level of warm air to melt it off.
Clear and Rime Airframe Ice
One of the major weather hazards to aviation is icing. Pilots need to be aware of the icing process, under what conditions ice will form on an aircraft, the different forms it takes on an aircraft and its effects on the aircraft's flight characteristics.
Icing is most hazardous and most probable over mountainous regions than any other type of terrain. Rapid movements of upward air coupled with cooling support large water droplets. Each mountain region has its own areas that are prone to icing conditions. Pilots need to be familiar with these zones. The most dangerous icing takes place over the windward side of ridges and above the crests. This icing zone can extend 5,000 feet or higher above the top of the mountain.
Icing occurs when an aircraft flies through visible water and the temperature at the point where the moisture strikes the aircraft is 0° C or colder. Even though the air temperature around the aircraft may be a few degrees warmer than freezing, aerodynamic cooling can occur (due to the rapid movement of the aircraft through the air creating a wind chill effect) and lower the temperature of the aircraft's surface thus inducing icing. Supercooled water increases the rate of icing. As a supercooled water droplet hits the aircraft's surface a part of it freezes instantaneously. The manner in which the remaining portion of the water droplet freezes determines whether the ice formation is clear ice, rime ice or mixed ice.
- Rime Ice: formed from small supercooled droplets when the remaining liquefied portion after initial impact freezes rapidly before the drop has time to spread over the surface. This traps air between the droplets, and gives the ice an opaque white appearance. Rime ice forms in a temperature range of -10° C to -20° C. It has a lighter weight than clear ice. Its formation is irregular with a rough surface and brittle, making it easier to remove than clear ice.
- Clear Ice: After the initial impact of supercooled droplets from large raindrops strike the surface, the remaining liquefied portion flows out over the surface and freezes gradually. This freezes as a smooth sheet of sold ice. Clear ice forms in a temperature range of 0 to -15° C becoming heavier as sheets of ice build up and is difficult to remove.
- Mixed Ice: formed when supercooled water droplets are of various sizes or are intermingled with snow or ice particles. After initial impact, the remaining portion freezes rapidly and forms a mushroom shape on the leading edges of a wing. Ice particles are imbedded in clear ice and form a hard and rough-edged mass.
A pilot can avoid conditions in which icing occurs or upon encountering icing, react quickly by ascending or descending to a different altitude where the temperature is warm enough to melt the ice. Avoiding quick manoeuvres when icing occurs is recommended, as the aircraft is not operating at peak efficiency. If choosing to ascend to a warmer flight level, increasing airspeed to a faster than normal speed when climbing will avoid a stall. Should the ice continue to develop aerodynamic lift will be lost and normal fight cannot be sustained resulting in a loss of control of the aircraft.
Icing on the Ground
When ice forms on the wing before takeoff it is very similar to in-flight icing. Snow and rain that fall while the aircraft is on the ground freeze on the aircraft, sleeting conditions compound those effects even further. Before an aircraft takes off, it can receive other types of de-icing and anti-icing treatments.
Ground crew technicians spray hot chemicals to melt the ice. A thin stream of liquid removes any frozen precipitation (this is de-icing). A second chemical treatment made up of thicker chemicals clings to the surfaces to prevent ice from forming as the plane waits to leave the runway (this is anti-icing).
Starting a large prop engine on a cold day requires plenty of heat. This large tent and tarp engine cover moulds around the engine cowling and allows heater hoses to enter.
For melting ice off an aircraft’s leading edges, pilots can use electric de-icing boots.