Flight Icing Considerations for General Aviation Aircraft
For many pilots the term “In-Flight Icing” conjures up thoughts of terror in the cockpit when being caught off-guard by unexpected icing. In reality, in-flight icing can be a very severe threat to any aircraft, especially general aviation aircraft. Eventually, even pilots who embark under ideal conditions each flight will eventually find themselves in an icing situation. There are steps a pilot can take to minimize the risk associated with icing both before, and during flight. This paper will use the SHEL Model to examine various aspects of in-flight icing. Consideration will be given to the types of ice and their associated weather (Environment), detection & protection equipment (Hardware), procedures for dealing with icing conditions (Software), and modifications of pilot behavior which will result in a better approach to minimizing icing risks (Liveware). (Nasa, 2002)
Types of Ice (Environment)
In-flight icing can be categorized into two main types based on the conditions required to produce the ice. These two types are structural icing and carburetor icing. Carburetor icing refers to the buildup of ice inside the carburetor venturi due to the condensation and freezing of moisture contained in air as it undergoes evaporative cooling while vaporizing fuel. Severe carburetor icing can result in engine failure due to blockage of the air/fuel mixture into the engine. Structural ice refers to adhesion of ice to the aircraft due to an aircraft surface at or below freezing combined with flight into conditions with visible moisture. (Craig, 1997)
Although carburetor ice is a fairly narrow topic, structural icing can accrete on and affect a broad range of aircraft systems and components. These include wings and control surfaces, air intakes, pitot static instrument sensors, windshields and propellers. Structural ice can produce a variety of problems including decreased wing efficiency and lifting, decreased functionality of control surfaces, blockage of air to the engines, malfunction of airspeed indicator, and reduced visibility through windshield. (Craig 1997)
There are several classifications of structural ice; clear, rime and mixed. The most insidious form of structural ice is clear ice. Clear ice results when large super-cooled liquid water droplets strike an aircraft surface which is at or below freezing. This results in the water creating a coating of ice where it came in contact with the aircraft. With clear ice the freezing of the water occurs slowly, and the super-cooled water will smear towards the rear of the aircraft as the water progressively freezes to the surface. Clear ice is considered the most dangerous form of ice because of the speed at which it can accrete on an aircraft combined with the weight the ice can add as well as the fact that the ice accretes in areas where de-ice or anti-ice equipment cannot provide protection such as the top and bottom of the wing. (Padfield 1994) (Lester, 1997)
Rime ice is a much more common form of ice, and is much less dangerous. Rime ice forms when the aircraft is flying through visible moisture and small particles of snow or water adhere to the aircraft. Rime ice will generally add little weight to the aircraft, and usually accretes in areas protected by de-ice and anti-ice equipment. Rime ice also generally accumulates at a slower rate than clear ice and thus allows pilots time to escape. (Padfield 1994)
The third form of ice is mixed ice, and that is exactly what it is, rime ice mixed with clear ice. The type of ice an aircraft may encounter is predominately a function of temperature and the size of the water droplets in the atmosphere. Clear ice is generally found in temperatures of 0c to -10c and in an atmosphere where the droplets are large. Rime ice is generally found in temperatures between -10c and -40c in atmospheres where water droplets are small or the moisture is in snow form. Between these ranges of temperatures and ranges of moisture particle sizes there is a transition zone where the combination of clear ice and rime ice can coexist forming mixed ice. (Padfield, 1994)
The conditions necessary to produce icing cannot be observed from the ground, and therefore icing in general is very difficult to predict. The meteorologists at the National Weather Service use a series of computer models to forecast icing based on temperature, relative humidity, the dewpoint and dewpoint spread. Based on the specific scenario an icing forecast may be issued with an indication of the probability of icing. The various models used from region to region error on the side of safety by generally over forecasting icing. Because forecasting icing is difficult the National Weather Service heavily relies upon pilot reports to supplement their forecasts. Pilot reports of icing is the factor that changes mere “Forecast Icing Conditions” to “Known Icing Conditions.” This distinction will become important when we discuss aircraft capability and certification. (Nasa 2002)
As stated earlier, the droplet size and the temperature have the greatest impact on what type and severity of ice we can expect. By knowing the conditions ideal for each type of ice to form we can apply these criteria to specific weather scenarios. Clear ice requires large drops of super-cooled water, and is most commonly found in thunderstorms and cumulus clouds, especially near the tops. Rime ice is very common and is usually found in stratus clouds. It is important to note that the previous two examples are stereotypical, and that ice can and will be encountered in many other types of conditions such as temperature inversions, ground fog, even in severe clear VFR provided the moisture content is high enough and the temperature is right. (Gardner, 1999)
The weather conditions necessary for carburetor ice to develop are much more frequently encountered than the conditions for structural icing. As the air enters the carburetor the temperature of the air may drop as much as 70F. For this reason, carburetor ice can form when the outside temperature is as high as 90F. For carburetor ice to develop the air must contain a considerable amount of moisture, as a result the most common condition for producing carburetor ice is a temperature 60F or below and high humidity. (Eichenberger, 1995)
Protection from Ice, Anti-Ice versus De-Ice (Hardware)
In-flight icing can produce a variety of problems of different levels of severity for a variety of aircraft systems. For this reason, most general aviation aircraft are equipped with some form of anti-ice or de-ice equipment. Smaller, lower performance aircraft are commonly equipped with only the most basic equipment to address the most severe risks. The certification of these aircraft usually indicates flight into “Known Icing Conditions” is prohibited. Larger more complex aircraft are commonly equipped with a suite of ice protection devices, and depending upon the capabilities they may be certified for flight into “Known Icing Conditions.” (Lombardo, 1993)
There are two categories of ice protection devices, anti-ice and de-ice. An anti-ice device is intended to be activated before flight into icing, and is designed around the premise of preventing ice accretion. A de-ice device is designed to remove ice from the airplane after the ice has accumulated, and therefore does not require operation until ice is present. Some devices may operate as both a de-ice and an anti-ice protection system. (Lombardo, 1993)
Probably the most common form of ice protection is the carburetor heat. All airplanes with carbureted engines will have a carburetor heat control. Although the specifics of each airplane may differ, generally this control will change the routing of air to the engine. Normally denser air would be pulled from outside the airplane to run the engine. When carburetor heat is turned on, the air will be redirected through ducting adjacent to the exhaust stack. The heat from the exhaust stack will heat the intake air to a very high temperature at which point carburetor ice cannot form. The heating of intake air will also reduce density and therefore reduce performance. (Nasa 2002)
Another common form of ice protection found on most aircraft is the pitot-static heat. Generally pitot-static heat is considered anti-ice in that it needs to be turned on at the first indication of ice to prevent a problem. Pitot-static heat uses electrical current to heat the pitot-head and in some cases the static ports of the pitot static system. This anti-ice provision ensures the airspeed indicator, altimeter, and vertical speed indicator will not fail due to ice clogging the sensor ports. (Gardner 1999)
The larger more complex aircraft for which certification into “Known Icing Conditions” is permitted generally have a collection of anti-ice or de-ice devices that allow the aircraft to carry out a mission into icing conditions. These devices use a variety of designs that may include the use electrical current or heated air to heat surfaces, fluids to melt ice on surfaces, or pneumatic devices that expand to crack ice off from surfaces. Generally the areas that are protected include the wings, elevator surface, windshield, propellers, or even alternate air induction pathways to the engine. (Padfield, 1994)
Regardless of what type of ice protection an airplane has or what premise it operates on, the most important aspect is that the pilot fully understands the limitations, operations, and flaws of that equipment and its design. For example, if your airplane has pneumatic inflatable boots on the wings it’s important to understand where that air pressure comes from. In the event of the loss of a vacuum pump does that also mean loss of wing de-ice. In most cases, ice protection gear simply buys the pilot time to escape the conditions. Very few aircraft are certified for continuous flight in icing conditions, and even those that are don’t stand a chance against clear ice since it will commonly accrete on the top and bottom of the wing where ice protection doesn’t operate. (Nasa 2002)
Procedures for Dealing with Ice (Software)
Determining a procedure for dealing with icing conditions starts on the ground prior to flight. The greatest tool a pilot can have to combat ice is understanding and knowledge of the source of the ice and the nature of the weather system producing it. For example, if the aircraft is picking up rime ice from snow at 10,000 feet, the temperature is -1C, and the pilot knows the temperature at 8,000 is +1C, then a 2,000 foot descent will allow the pilot to exit the icing conditions.
Generally the best policy is to exit the icing conditions and understand and follow anti-ice and de-ice procedures for your aircraft. For this reason it’s important to have a plan B and a plan C already determined before you take-off. Know where alternate airports and VFR conditions are should discontinuance of the flight be necessary. Be prepared to make a 180 to immediately exit the icing conditions. Be knowledgeable in the use of and activate any ice protection gear your airplane is equipped with. Know and fly minimum ice penetration speeds to prevent icing on the underside of the wing. Disengage the auto-pilot so that you can observe any flight handling characteristic changes that may result from buildup of ice. (Eichenberger, 1995)
Complacency versus Skepticism (Liveware)
Managing and minimizing the risks of icing requires the pilot maintain a healthy degree of skepticism. This includes skepticism about forecasts and weather products, equipment performance/capabilities, and pilot skill. Complacency has no place in the cockpit of an airplane. There is no such thing as a routine fight. Even pilots who continually begin flights under perfect conditions will eventually be faced with unplanned and adverse circumstances or situations. By maintaining a healthy degree of skepticism and expecting and anticipating problems(not just icing) the pilot will be better prepared to deal with an emergency.
Padfield, R. R., (1994). Flying In Adverse Conditions. New York, NY: TAB Books.
Eichenberger, J, A., (1995). Handling In-Flight Emergencies. New York, NY: TAB Books.
Lombardo, D., (1993). Advanced Aircraft Systems. New York, NY: McGraw Hill.
Gardner, B., (1999). The Complete Multi-Engine Pilot. New Castle, WA: ASA.
Craig, P. A. (1997). MultiEngine Flying, Second Edition. New York, NY: McGraw Hill.
Lester P. F. (1997). Aviation Weather. Englewood, CO: Jeppesen.
Nasa Glenn Research Center. (2002). Pilot’s Guide to In-Flight Icing Interactive Training CD-ROM. Cleveland, Ohio: National Aeronautics and Space Administration.