January 2009 Cover Story:
Icing and Flight

Story by Harris Teichman

Ice and icing conditions can greatly affect the way an aircraft performs. The ability to identify the forms of icing and what environmental conditions are most favorable for the formation of this potentially deadly flight hazard is an essential part of every pilot’s education.

There are two types of icing that may be associated with an aircraft. The first, structural icing, is ice that is picked up by the airframe and various components on the outside of the aircraft. Induction icing, the other form, occurs when ice is formed within the power plant, typically in the intake or venturi of the carburetor. The conditions under which these two types of ice form can be very different from one another.

Certain atmospheric conditions must exist for structural ice to form. The surface of the aircraft and the ambient temperature must both be at or below freezing. Also, the presence of super-cooled visible water droplets (water that stays in liquid form although its temperature is below freezing) is necessary or the humidity must be high. Structural icing can only occur in clouds or in freezing rain conditions. It may occur when the outside air temperature is as high as to 4?C, but at temperatures below -40?C, it is rare for ice to form. Most serious icing conditions are found in air temperatures that are between 0 and -10?C.

Induction icing includes carburetor icing and fuel and air induction system icing. Carburetor icing (essentially a combination of fuel and induction system icing) can form with an outside air temperature as high as 22?C or as low as -10?C. This is in sharp contrast to the temperatures most conducive to the development of structural icing. Carburetor ice forms within the carburetor passages, causing a blockage that can reduce fuel and air flow to the point that the engine will no longer operate. Fuel and induction system icing are most likely to occur with an air temperature at or below 10?C and when the relative humidity is high. Ice can form in the fuel system when water is present in the fuel and the temperature then drops. Ice in the induction system can reduce air supply to the engine, possibly causing the engine to fail.

Structural icing can have a profound effect on an aircraft. It reduces flight capability by increasing drag and weight and reducing thrust and lift. Additionally, it can render various components ineffective such as radios, flight instruments and control surfaces. It can also adversely affect visibility.

The airfoils, such as the wings and the tail surfaces, change shape as ice forms, and this alters the airflow over the wings and/or tail. The result is a loss of lift and an increase in drag. In experiments it has been discovered that one-half inch of ice on the leading edge of an airfoil can reduce lift and increase drag by up to 50 percent. Also, even though an aircraft may be able to maintain altitude, aircraft control can be impeded by ice formation on control surfaces such as ailerons, elevators, rudder and flaps.

Radio antennas can pick up a substantial amount of ice in a short time. Such an accumulation can interfere with radio communications and if a sufficient amount of ice is allowed to build up, the antennae may break off completely making communication with air traffic control difficult or impossible while limiting the performance of navigation radios, adding to the danger of the situation. Without communications, the pilot would be unable to make requests for course changes to get out of the icing area or even, in the extreme, declare an emergency.

Some other important elements on an aircraft that are affected by structural icing are the pitot tube and the static pressure ports. When these areas become iced, the altimeter, the vertical speed indicator, and the airspeed indicator may not read correctly or may fail completely.

Two other examples of structural icing are windshield and propeller ice. Windshield icing can cause decreased visibility, a dangerous situation, especially during landings. Propeller ice causes the propeller to become less effective, which in turn affects the airspeed. This resulting loss of power can be deadly if corrective action is not promptly taken. Essentially, anything on the exterior of an aircraft can potentially be a point of collection for ice, even something as small as a rivet. In fact, often the smaller a component is, the quicker it will accumulate ice.

There are three different kinds of ice. Rime ice is characterized by a milky, rough appearance. Clear ice usually looks like a smooth glaze. Mixed ice is a combination of both rime and clear. Each type of ice has properties that make it unique. Rime ice is the easiest form of ice to remove. It does deform an airfoil more than clear ice, however. Rime ice is usually found in stratiform clouds. It is usually formed when small super-cooled water droplets freeze as they come in contact with the surface of the aircraft. Rime will usually develop when the outside air temperature is between O?C and -20?C.

Clear ice, which is most often found in cumuliform clouds, can be very treacherous for an aircraft in flight. Clear ice sticks firmly to the surface of an aircraft and is difficult to remove. It also can build up quickly. The formation of this sort of ice, in contrast to rime ice, occurs as large super-cooled water droplets flow over the surface and freeze slowly. The severest of this kind of ice will usually form at outside air temperatures between O?C and -10?C.

Mixed ice is formed when the super-cooled water droplets are of different sizes. Additionally, mixed ice may be combined with hail or snow. Mixed ice will normally form odd shapes and is very solid. It is hard to remove.

A form of precipitation that causes structural ice is freezing rain. If the outside air temperature drops below O?C, drizzle and larger liquid droplets (rain) become super-cooled and as they come into contact with the surface of the aircraft, they become ice. The different kinds of ice are further classified by the intensity with which they form. There are four categories of icing intensity: trace, light, moderate and severe. Trace ice is present when ice just becomes perceptible. This kind of icing should not cause problems and the use of deicing equipment is not necessary unless an hour or more is spent in these conditions. Light ice can be a detriment to flight if deicing equipment is not used after an hour or unless the pilot takes steps to exit the area of icing. The third kind of ice intensity, moderate, calls for the pilot to change course to exit the area of icing at once and the use of deicing or anti-icing equipment is needed. Severe icing is an emergent condition. In this type of icing environment, all deicing and anti-icing equipment is rendered useless and it becomes imperative for survival that the pilot change course immediately in an effort to melt the ice, either by climbing to an area of lower temperature where the ice will sublimate, or by descending to a lower altitude where the temperature may be higher and ice can melt off of the airframe.

Different aircraft will be affected in various ways by ice. What is considered light ice to one pilot and his aircraft may well be a case of moderate ice for another. The rate of ice formation on an aircraft is determined, in part, by the particular features of that aircraft. The speed of the aircraft as well as the size and shape of its airfoil have an impact on ice accumulation. Another factor is the angle the airfoil has in relation to the relative air flow (angle of attack).

The larger the airframe component, and particularly the larger its leading-edge radius, the less ice it will collect. This is true because the larger objects give the air ahead more warning that they are coming and allow the air to get out of the way with more gentle turns which carry more water along, causing more water to miss the component. The effect of speed is just the opposite. The faster the component is moving through the air the less warning the air has that it is going to have to scramble out of the way. Do not draw the conclusion that slowing down is the thing to do when ice is encountered… the increase in angle of attack will make matters worse … (Newton, 1983).

There are a number of devices which can aid a pilot in the prevention and removal of ice. These different deicing tools may be used to work in an anti-ice capacity, helping to prevent the formation of ice or they may be used to remove ice after a certain amount has formed. The equipment is best used in the anti-ice capacity.

Propeller ice can be of great significance. Consider the comment “…the propellers are most important and if I could have either wing deicers or propeller deicers I’d take the propellers. If they are doing their work efficiently, you can pull a lot of ice-covered airplane around the sky.” Propeller deicers are available in two different forms. One has heat in the leading edge of the blade while the other type sprays alcohol on the leading edge of the propeller. Occasionally, when attempting to deice the propeller, pieces of ice may come off unevenly which can cause serious vibrations. Ice flying off of a propeller can also damage the fuselage or other parts of the aircraft. In this kind of situation, one pilot suggests, “If there is ice on a propeller and the deicing method used is having trouble getting it off, try running the RPM up and down in surges to give extra, and irregular, centrifugal force, to help sling the stuff off.

Of course, it is important to have effective wing deice equipment as well. Wing deicers come in two different kinds. There are wings that have heated leading edges, a very effective tool for ice-prevention. Jet aircraft use hot bleed air routed to the wings and tail surfaces. Piston and turbo-prop aircraft use a combination of electro-thermal wing heating along with rubber boots. Heated surfaces provide anti-ice rather than de-icing protection.

Rubber wing de-ice boots are a de-icing tool. When a sufficient amount of ice has formed on the wing, the boots can be inflated, theoretically breaking the ice off the wing. Sometimes the ice breaks unevenly, however, leaving rough pieces of ice sticking up from the wing. If they are not used correctly, boots can actually do more harm than good. When boots are left in the inflated position too long they can accumulate ice in large lumps on the wing, effectively freezing them in place and rendering them useless. A third type of de-ice/anti-ice protection is the “weeping wing”. De-icing fluid is delivered to the leading edge of the wing and seeps out of small holes in the wing. Depending when it is deployed the weeping wing system can be both anti-ice and de-ice capable.

Windshields can also be equipped with de-ice equipment, either in the form of a heated windshield (or part of the windshield) or an alcohol spray, similar to the propeller de-ice systems. Protection of the windshield is essential if the pilot is to have good visibility.
The pitot-static system will usually accumulate ice faster than other portions of the aircraft. There are heating elements in the pitot tube that will melt ice or prevent it from forming at all.

Finally, carburetor heat is a vital anti-ice tool. Since carburetor ice can form in fairly warm outside temperatures, it is important that the use of carburetor heat is fully understood.

A pilot should expect carburetor ice in typical icing conditions, such as rain and snow. If, when flying in warmer temperatures when the relative humidity is high, the pilot notices a drop in RPM (with a fixed pitch propeller), he can assume he has developed carburetor ice. This is easily remedied by applying full carburetor heat to melt the ice.

Aircraft icing, whether airframe or induction, does not have to be a deadly situation. Not all icing situations can be avoided, but if pilots take the time to learn the various conditions that are conducive to ice formation and know their aircraft’s de-ice/anti-ice systems well and the proper way to use them, most ice emergencies can be handled with minimal damage to the aircraft and, most importantly, maximum flight safety.