Behind the Forecast: How an icy flight can get dicey

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.

Science Behind the Forecast: How an icy flight can get dicey

LOUISVILLE, Ky. (WAVE) - Icing is a significant issue for airplanes during Winter.

Ice accumulates on planes when tiny droplets hit and freeze on a plane's front surfaces. Ice collecting on a plane can reduce lift, which helps keep the plane in the air, and even cause an aerodynamic stall (this is an issue that can lead to a temporary loss of control).

Ice can build on a plane on the ground and in the air. Ice accumulation on the ground occurs when precipitation falls onto the upper surfaces, like the wing and tail, and freezes.

Ground icing is mitigated by de-icing aircraft while at an airport.

The following are needed for icing to occur, according to the National Weather Service:

  • Air temperatures 0°C or colder:
    • If an aircraft spent time in subfreezing temperatures and then travels to above freezing temperatures, the aircraft's surface can stay below freezing for a while. This continues the icing threat even in above freezing temperatures.
  • Wet snowflakes or supercooled liquid water droplets
    • Supercooled liquid water droplets mostly found at temperatures between 0°C (32°F) and -20°C (-4°F). Small amounts of supercooled water droplets can be found in air as cold as -40°C; this is, however, quite rare.
    • When a supercooled droplet hits a surface, the collision destroys the droplet's internal stability and increases its freezing temperature; this is known as aerodynamic heating.

In clouds, the water is pure. This means it can remain a liquid well below 32°. When aircraft fly through the supercooled droplets, the droplets freeze on contact with the plane's surfaces. This type of ice builds up on the leading edge of the aircraft like the nose, tail, and leading edge of the wings.

The impact of the weight of ice is less significant than the airflow disruption it causes, according to the Air Safety Foundation. Ice is dangerous because it alters the wings' and tail's shape; this increases drag on a plane and reduces the lift.

Multiple types of ice can affect flight:

  • Rime ice: This appears bumpy, opaque, and generally follows the contour of a plane's surface. This typically forms when flying through stratiform clouds; an aircraft's surface must be below freezing for rime to form. It can be removed by de-icing systems or prevented by anti-ice.
  • Clear (or glaze) ice: It is sometimes clear and smooth; it usually contains air pockets that cause a bumpy, translucent appearance. It occurs when planes fly through large supercooled water droplets in convective (cumulus) clouds and freezing rain. Clear ice is typically more transparent, denser, and harder, than rime ice, and is commonly difficult to break.
  • Mixed ice is a composite of rime and clear ice.

Tests have shown that snow ice and frost accumulation on a wing just as thin and rough as sandpaper can reduce lift by 30% and increase drag by 40%. A NASA study found that nearly 30% of total drag associated with accumulating ice persisted even after the surfaces were cleared.

So how do you keep the ice off of planes? Deicing is one solution. It is the process of removing ice from an aircraft. Anti-icing is another step that helps keep ice from reforming after it is cleared.

Deicing compounds are typically comprised of glycol and water. Glycol works by lowering water's freezing point. Deicing agents are heated and sprayed over an aircraft to remove ice, snow or frost. There are protocols set by the Federal Aviation Administration (FAA) that detail how and when deicing occurs.

It's important to work quickly and thoroughly when deicing an aircraft. Holdover time, the amount of time after deicing before the plane is iced-over is key. Deicing typically is done after all passengers have boarded the plane and the aircraft is ready to take off, because holdover time may only be a few minutes.

If precipitation such as sleet, snow or freezing rain is falling, then the plane may also be anti-iced. Anti-icing fluids are made of a higher concentration of glycol to water as well as a thickening agent so it sticks to the plane. Per the FAA's recommendations, anti-icing fluids should be applied within three minutes of deicing and/or not at all if the deicing fluid is frozen or if ice has reformed. If this does occur, the deicing must be done again.

According to the FAA, the parts of the aircraft that may need deicing and anti-icing are:

  • Wings
  • Vertical and horizontal tail surfaces
  • Fuselage
  • Engine inlets and fan blades
  • Control surfaces and gaps
  • Landing gear and landing gear door
  • Antennas and sensors
  • Propellers

Copyright 2019 WAVE 3 News. All rights reserved.