icing

Icing refers to the accumulation of ice on an aircraft's external surfaces during flight, particularly on wings, tail surfaces, windscreens, and engine inlets. Unlike ground icing, which is static, flight icing forms when an aircraft passes through clouds or precipitation containing supercooled water droplets, droplets that remain liquid below freezing point. When these droplets strike the aircraft, they freeze on contact, building up layers of ice that degrade aerodynamic performance and add weight.

The severity of icing depends on liquid water content, air temperature, droplet size, and aircraft speed. Rime ice forms when droplets freeze quickly after impact; it is opaque, brittle, and lighter. Glaze ice forms when droplets partially evaporate or when larger droplets freeze slowly after hitting the surface; it is clear, dense, and adheres strongly. Mixed icing combines both types. Even thin ice accumulations can reduce lift by 20 percent or more and increase drag significantly. Thick buildup can shift the center of pressure and make the aircraft difficult or impossible to control.

Detection and prevention

Pilots detect icing through visible signs, instrument readings, or automatic ice detectors mounted on the wing or fuselage that sense changes in surface texture. Aircraft avoid or escape icing by requesting different altitudes from air traffic control, since icing layers are typically confined to specific temperature bands. Modern transport aircraft are certified for flight into known icing conditions if equipped with ice protection systems.

Ice protection systems fall into two categories. Deicing systems remove ice after it forms, typically using heated elements on leading edges or boots that expand to crack and shed ice. Anti-icing systems prevent ice formation altogether by heating surfaces above the freezing point. Piston-engine aircraft often use alcohol-based anti-icing fluid sprayed onto windscreens. Turbine engines take hot air from the compressor to heat engine inlets and, sometimes, wing leading edges. Proper maintenance of these systems is critical; a failed heater element or a missing inlet cover can allow ice to form in protected areas.

The term originated from meteorological observation of icing conditions rather than from the ice itself. Maintenance crews inspect for ice damage after any flight through suspected icing, looking for brittle failures, stress cracking, or loose rivets caused by the expansion and contraction of ice. Accident investigations often reveal that icing contributed to loss of control when ice protection systems were disabled, inadequate, or not actively used during flight.