Ice, Ice, Baby!

Published Nov. 19, 2014
By MSgt Julie Meintel
445 Airlift Wing

Winter 2014 -- The holidays will be here before we know it, and with them comes Old Man Winter, bringing along his whipping winds, blowing snow, and freezing ice. All of these things mean changes in the way we do business, both in our day-to-day lives and as flyers.

Some things never change, and Mother Nature is one. She is always going to be in charge of the weather. She makes the rules, and it is up to us to keep up. Seasons are always going to change, and we always need to be prepared and know how to adjust. Heading into winter, one of the main things we need to pay attention to as flyers is aircraft icing. That one is guaranteed to ruin your day if you neglect it. So where do we go for information? That's right, Air Force Handbook 11-203, Vol II, Weather for Aircrews, the authoritative guide on all things weather related. Chapter 11 deals exclusively with icing issues in depth. It's a great book to keep handy in your pubs kit so you can refer to it again and again.

Let's just start at the beginning. Icing in any of its forms is hazardous to safe flight for a multitude of reasons. One, it can add weight to the aircraft, and that's bad for a few reasons. Additional weight decreases lift and increases drag by a significant margin, and that will definitely affect your flight. Ice can cause vibrations on rotors and propellers, and it can decrease the aircraft's overall efficiency, requiring more power to maintain flight. Also, if it builds up on the outside of the aircraft, it will have a negative effect on your ability to control your aircraft. Your brakes and your landing gear may not function properly, you can lose radio communication, you may have false instrument readings, and your outside vision will probably be reduced and may be completely lost.

There are two main categories of aircraft icing: structural and induction, and they are just what they sound like. Structural icing builds up on the aircraft surfaces, or structure, and induction icing appears in the air induction systems, where air is taken into the engines. This type of icing may also show up in the fuel systems. Within these main categories, there are subcategories, but we'll get to those in a bit.

The required ingredients for structural icing are air temperatures, as well as the aircraft's surfaces at or below freezing, and visible liquid droplets of water or high humidity. When these things combine, ice begins to form on the outer surfaces of an aircraft. Clouds are the most common form of moisture in the air, but freezing rain and drizzle are other easily visible forms of moisture that can cause icing. Freezing precipitation is actually the most dangerous of all the various icing conditions because of how quickly it can build up and how hard it is to remove.

Now we'll get to some of those subcategories I mentioned earlier. Within structural icing, there are three sub-categories of ice: clear, rime, and mixed. Clear ice is the most dangerous of the three types; it is hard and shiny, and it's very difficult to remove with de-ice equipment. You will find this most often where you find high moisture content in the clouds and temperatures slightly below freezing. It adheres well to the aircraft's surfaces and can build to a dangerous level in a short time. It will be smooth if it is made up only of freezing rain, but if there is snow, ice pellets, or small hail mixed in, it will be rough. Rime ice is milky and granular in appearance, making it more brittle and easier to remove than clear ice. It is made up of small water droplets that freeze when they strike the surface of the aircraft, and a lot of air usually gets trapped in with the water. That is what gives this type of ice its rough and opaque appearance. It is not as heavy as clear ice either, so its weight is not really an issue.

Mixed ice is just what it sounds like: a mix of small and large water droplets, sometimes with snow or ice particles mixed in. It builds rapidly and ice particles can become embedded in clear ice, making a rough surface. Additionally, when we think about structural icing, we have to consider frost, that thin layer of crystalline ice that forms on external surfaces. It decreases your lift to drag ratio and obstructs your vision. It's not included in the icing categories and is generally more of an annoyance than a real risk. However, it can make your flight more difficult and possibly create a hazard during takeoff, so make sure it is all removed from your jet before you go.

Induction icing can occur in a wide range of weather conditions, and it can affect the entire power plant. It's most common in the air induction system, which takes air into the aircraft engines, and it will be found in the fuel system sometimes. The subcategory within induction icing is called carburetor icing, and it is actually a combination of icing both systems. Carburetor icing is extremely dangerous and often results in total engine failure. This type of icing forms during fuel vaporization combined with the expansion of the air as it passes through the carburetor. The bigger problem with carburetor icing is not so much preventing ice from forming but clearing out ice that is already there. Ice forms in the induction system when conditions are favorable for structural icing as well, but it can also form when temperatures are above freezing. Be aware that this might happen, and watch for it.

Most of the time, your ground de-icing and your anti-icing procedures will keep you out of trouble, but icing could sneak up on you if you are not paying attention. Always remove ice and snow before takeoff, and use your anti-ice equipment. Avoid clouds when the temperature is between 0 and -20 degrees C, but if that's not possible, climb or descend to an altitude where you are out of that range. Make sure to give PIREPS so other aircraft near your vicinity know what to expect.

You may not always be able to avoid cold weather and ice, but you can take a few minutes to refresh your memory before winter hits so you are not caught unprepared. Fly safe!

Type of Icing	Effects	Conditions
Structural Clear Rime Mixed	Decreased lift/increased drag Rotor/propeller vibration More power consumption Less control of brakes/landing gear Radio communication impairment False instrument readings Impaired outside vision	Temps at or below freezing, with humidity, clouds, freezing rain, or drizzle present
Induction Air Induction Fuel System	Reduced power plant performance Possible engine failure	Temps above, at, or below freezing

KC-135R Stratotanker aircraft