Aircraft Structural Icing Types

When I was driving home today through freezing rain and so much ice was forming on my windshield that I could barely see a thing, I thought to myself about how glad I was to not be flying at that time. At least in my car I could slow down or pull over and wait for the weather to pass; if I was in the air right then though, I would probably be heading very quickly into an emergency situation and “test pilot” territory! Thankfully, I’ve been lucky enough to avoid a hazardous icing situation so far, but I still think it’s important to occasionally remind ourselves of the types of aircraft icing we could encounter, the hazards they pose, and how we could mitigate or avoid those hazards.

Structural Icing

Structural ice is the stuff that sticks to the outside of the aircraft; it could be on the propeller, windows, wings, empennage (tail), fuselage, or even the landing gear. For structural ice to form it is necessary for the aircraft to be travelling through areas of visible moisture, e.g. clouds or rain, and for the airframe and/or the surrounding air to be below freezing temperatures. Almost all icing tends to occur in the temperature interval between 0 °C and -20 °C, with about half of all reports occurring between -8 °C and -12 °C (AC 00-6B).  There are four main types of structural ice: clear, rime, mixed, and frost.

Clear Ice

Clear ice, also called glaze ice, is a glossy or translucent ice formed by the relatively slow freezing of large supercooled drops of water. Because the water droplets freeze gradually, they can run along the surface of the aircraft forming dense layers of ice and spreading beyond the reach of any anti-icing or de-icing equipment installed on the aircraft. Clear ice also has a tendency to form horns near the top and bottom of the aerofoil’s leading edge, which would greatly affect the airflow.

Have a look at the three pictures below of the same aerofoil and you can see how quickly large amounts of clear ice can form on the leading edge; just imagine what that would do to the flight characteristics! (Image source: NASA)

Clear ice build-up on the wing leading edge. Source: weather.gov

Rime Ice

When small supercooled water droplets hit the surface of an aircraft and freeze instantaneously, they form rime ice. It is less dense than clear ice, and has a milky, opaque appearance resulting from air being trapped when it strikes the surface and freezes. Usually, rime ice accumulates along the stagnation point of an aerofoil and is more regular in shape and conformal to the aerofoil than clear ice, resulting in less disruption in airflow by comparison.

The same aerofoil used in the clear ice test was also used to test the accumulation of rime ice in the pictures below. As you can see, the rime ice build-up is not quite as dramatic as clear ice, but it would still certainly affect your aircraft.

Rime ice accumulation on the nose cone. Source: weather.gov

Mixed Ice

Mixed icing, as the name implies, is a combination of both rime and clear ice. It forms when the air contains both small and large supercooled water droplets and can result in a very rough accumulation of ice. The only time I’ve really seen this myself for certain was when I was on descent to land to avoid thunderstorms heading my way; as I was coming down through the clouds it looked almost like wet snow was sticking to the aircraft. It definitely made me a bit nervous, but thankfully the air was much warmer down below and the ice fell off quickly.

Again, we can see an example of mixed ice accumulation on the same aerofoil over a short period of time below.

Frost

Frost forms when the surface temperature of the aircraft is below freezing, and colder than the dew point of the air surrounding it. It’s very similar to how dew is formed, except the water vapour in the air skips the liquid phase and deposits directly into ice on the surface. Generally, this is something that you will only see form when the aircraft is on the ground, such as on a cold morning. It is possible though for frost to form on an aircraft when it is descending from high altitude into clear humid air; the aircraft skin can remain at a temperature below freezing for some time, even if the OAT is indicating above 0 °C.

Frost

Hazards

Ice, even in small amounts, will have a negative effect on your aircraft by reducing thrust, increasing drag, decreasing lift, and increasing weight. As more ice accumulates, these negative effects increase in intensity; resulting in an increase in stall speed and an overall decrease in aircraft performance.

“It takes but 1/2 inch of ice to reduce the lifting power of some aircraft by 50 percent and increases the frictional drag by an equal percentage.”

AIM 7-1-21

Other than effects on aircraft performance, ice can also affect other systems such as:

  • Pitot static system – ice could block air inlets and render the altimeter, airspeed indicator, and VSI unusable.
  • Control surfaces – movement may be impeded.
  • Landing gear – retraction or extension may not operate correctly.
  • Antennas – ice could block transmissions or break more fragile antennas off.
  • Gyroscopic instruments – if run by a venturi vacuum pump (uncommon now), that could become clogged by ice.

How to Avoid a Hazardous Icing Situation

Larger aircraft are usually equipped with anti-icing and/or de-icing capabilities, which is something we’ll cover in another post, but even with these will not necessarily be sufficient in severe icing. For those of us flying unprotected aircraft, here are some tips to help you avoid icing problems:

  • Avoid flight through known icing conditions, check the forecast and look at the PIREPs close to your intended flight path.
  • Always clean off any ice, snow, or frost on your aircraft before your take-off.
  • Do not fly through rain or wet snow when the outside air temperature is near 0 °C or below.
  • If you do encounter icing, then leave the ice forming layer as quickly as possible. This may require turning around, descending into warmer air, or climbing above the layer (if possible). Declare an emergency if you need to!
  • If your aircraft is contaminated by ice, then assume the stall speed has increased also. Avoid abrupt manoeuvring or aggressive climbs; on approach and landing, increase speed and use partially extended flaps.

Further Information

For a more in-depth study about icing then check out these resources from NASA:

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