Understanding Aircraft Icing Hazards: Snow, Ice Crystals, and Hoar Frost

Snow, Ice Crystals, and Evaporation Cooling

Unlike super-cooled droplets, dry snow and ice crystals do not typically adhere to an aircraft and therefore do not cause standard structural icing. However, they present a unique hazard through temperature manipulation.

If an aircraft strikes a pocket of snow where the skin temperature is above freezing, the snow may melt. As this meltwater flows back over colder, below-freezing portions of the airframe, it can freeze into a hard layer of ice. Furthermore, "wet snow" presents a significant danger during ground operations or immediately after takeoff. If wet snow is left on the aircraft, it can freeze hard through evaporation cooling once the aircraft is in motion. This can occur even when the ambient temperature is slightly above freezing.

A similar phenomenon occurs with runway contaminants. Slush or water splashed onto the undercarriage or wheels can freeze via evaporation cooling, potentially jamming the landing gear or interfering with brake performance.

Hoar Frost and Sublimation

One of the most dangerous "clear air" icing threats is hoar frost. Unlike the ice formed in clouds, hoar frost is a white, feathery, crystalline formation that can cover the entire surface of an aircraft.

Hoar frost forms through sublimation, a process where water vapor changes directly into ice crystals without ever passing through a liquid stage. This typically occurs when an aircraft is parked outside on clear, cold winter nights. The mechanism is driven by radiation cooling: the upper surfaces of the aircraft lose heat by radiating it to the sky, causing the surface temperature to drop below that of the surrounding air.

Hoar frost can also form during flight if an aircraft flying in freezing temperatures descends suddenly into warmer, moist air. In these instances, the frost often accumulates more rapidly and persists longer near the integral fuel tanks, because the fuel inside the tanks warms much more slowly than the surrounding aircraft structure.

Impact on Aircraft Performance

The danger of these light accumulations—whether they are hoar frost or frozen wet snow—lies in their aerodynamic impact. Even a light, seemingly insignificant coating of frost can drastically alter the wing's profile.

The primary aerodynamic consequence is a significant increase in stall speed. By disrupting the laminar flow and changing the camber of the airfoil, frost reduces the maximum lift coefficient the wing can produce. This can lead to catastrophic results if a pilot attempts a takeoff with a frosted wing, as the aircraft may fail to achieve sufficient lift at the calculated rotation speed.