 This aircraft is flying through heavy cloud formations in temperatures at or below freezing. The success or failure of this mission may depend entirely upon the pilot's knowledge of the ice which is forming on his aircraft, and the steps he should take to avoid its hazards. Without this knowledge, he runs the risk not only of failing in his assigned mission, but even possible loss of aircraft and personnel. It is important to know where ice is likely to form on your aircraft and how it can interfere with normal flight procedures. The ice accumulations which are hazardous to flight can readily form on the leading edges of wings, on the tail assembly, on the windshield, on external radomes and antennas, on propeller blades, in the pitot tube, in carburetors and in jet engines. Ice accumulation on the wings and on the tail assembly is dangerous because it alters the shape of the airfoils. Airfoils are carefully designed to permit a smooth flow of air over the top and bottom surfaces. Any alteration in the shape of the airfoil will interfere with this smooth flow of air. When ice first appears, it has no serious effect, but gradually it builds forward and along the top and bottom surfaces. As the ice reaches the points of maximum camber, the shape of the airfoil has been drastically changed. Now the air flow is disrupted. Drag is increased and lift reduced. It is the shape rather than the weight of the ice which accounts for icing hazards. Aerodynamic capability is at a minimum. The stall speed of the aircraft is higher than normal and a dangerous condition exists. Ice may also form on the windshield. An accumulation here can reduce the pilot's visibility to a hazardous degree. It may form a heavy coating over radomes and seriously interfere with the operation of airborne radar. Ice will collect on external antennas and may not only cause malfunction of radio equipment but under extreme conditions may disrupt radio communications completely. It is impossible to see ice forming on spinning propeller blades, but ice can be seen on the hub dome and its presence here is usually an indication of propeller icing. Another indication, since propeller ice usually forms unevenly, is exaggerated vibration of the engine and the sound of ice being thrown from the propeller against the fuselage. Again it is not so much the weight of the ice that endangers flight but the manner in which the ice alters the shape of the blade. A heavy accumulation can change it from a blade into a rounded blunt object that is completely ineffective, unable to take hold and move the aircraft forward. Ice may form inside the pitot tube. The pitot tube activates the aircraft's airspeed indicator and the result of heavy icing here will be constriction of the flow of air, causing inaccurate airspeed readings for the pilot. Structural ice may form on external surfaces of the aircraft when the outside temperature is at or below freezing. However, even at temperatures above freezing, carburetor icing may occur. This may occur even when flying through clear air with temperatures as high as 25 degrees centigrade if there is sufficient moisture. As the air flows swiftly into the carburetor, the cooling effects of fuel vaporization and air expansion in the venturi reduce the temperature sufficiently to cause the moisture in the air to freeze on the interior surfaces. Carburetor ice has the same effect as gradually closing the throttle. It does not cause rough engine operation but as the ice slowly constricts the air passage, it does result in an even loss of power and in turn a drop in airspeed. When the outside air temperature is at or below freezing, a different icing condition occurs in the carburetor. The temperature inside the carburetor is well below freezing and there is in effect a harmless snowstorm taking place. However, in this situation, ice may still form on the throttle and it is advisable whenever structural icing is encountered to move the throttle occasionally to keep it from sticking. As a general rule, jet engine icing occurs under the same conditions as external structural icing. It is of particular concern in turbojet engines of the axial flow compressor type where the ice forms over the inlet guide veins causing a restriction in air flow. The result is reduced engine thrust, excessive turbine temperature and probable turbine failure. It should be noted of course that jet aircraft have the advantages of high speed to fly rapidly out of icing conditions and high ceilings to fly at altitudes where icing is rarely encountered. In general, it is important to remember that ice reduces the overall aerodynamic efficiency of the aircraft. It increases drag, reduces lift, stall speed is higher than normal, fuel consumption is greater and above all remember that maneuverability is greatly reduced. Avoid steep turns and fast climbs and when landing maintain the power needed to ensure sufficient airspeed to avoid stalling. Even though your aircraft carries de-icing equipment to help limit ice accumulation, your best defense against ice formation on the aircraft is to know how and where ice forms and how to avoid it whenever possible. We may find the two types of ice normally encountered in the structural icing of aircraft in an ordinary household refrigerator. The white granular ice that forms on the freezing compartment is rime ice. It is not very cohesive and can be flaked off easily. The hard glassy ice in the trays is clear ice. This type of ice is very cohesive and is difficult to break loose. All clouds contain moisture. Wherever this moisture is accompanied by temperatures below freezing, ice will form on aircraft. When clouds are formed in stable air at freezing temperatures and are composed of a comparatively small number of tiny water droplets, conditions are right for the formation of rime ice. Rime ice usually occurs at temperatures from 0 to minus 15 degree centigrade. As a general rule, at temperatures below minus 15 degree centigrade in stable clouds, all of the moisture will be frozen into ice crystals and will not adhere to flying aircraft. But rime icing has been observed with temperatures as low as minus 40 degree centigrade and should be anticipated anytime the outside air temperature is below 0 degree centigrade. When clouds are formed in turbulent air, both rime and clear ice may be encountered. In turbulent clouds, the moisture near the base is in the form of numerous large water droplets. With increased altitude and its colder temperatures, the water droplets become smaller and less numerous since many droplets freeze into ice. Near the top, the cloud is predominantly composed of tiny frozen ice crystals. The danger to the flying aircraft is not from the ice crystals, but from the water droplets which freeze upon contact. At temperatures below minus 15 degree centigrade, since the droplets are small and less numerous, rime ice can be expected. Where temperatures are between 0 and minus 15 degree centigrade, the droplets are large and numerous. In this area, dangerous accumulations of clear ice will form. Another condition that commonly gives rise to the formation of clear ice may be found in a situation where warm moist air is forced to rise over a colder air mass. Rain or drizzle falling from the above freezing air into air well below freezing becomes freezing rain. Areas of freezing rain produce the most hazardous of all icing conditions, clear ice. When a flying aircraft strikes water droplets, the leading edges of its structures become covered by a film of water. If cooled sufficiently, this film freezes from the inside out, forming a clear, dense, cohesive glaze of clear ice, the most dangerous form of structural ice. Its form is a rounded, blunt shape on the leading edge and may spread over upper and lower surfaces, drastically disrupting the flow of air and causing a considerable loss of efficiency. Clear ice can accumulate rapidly and will adhere firmly when it forms. Rime ice is somewhat less dangerous than clear ice since it accumulates more slowly and is easier to dislodge. It forms at lower temperatures where the water droplets are smaller and less numerous. Each droplet that strikes the aircraft freezes before another strikes on top of it. The resulting accumulation will be in the form of tiny pellets frozen together in a spongy mass. Rime ice usually forms a sharp, protruding ridge which does not affect the flow of air as extremely as a clear ice formation does. Very often the ice that forms will be a combination of rime and clear, an irregular conglomerate, forming a rough, blunt mass which can build up to serious proportions. Before any flight, the pilot should obtain from his weather officer a complete picture of any icing conditions along his route. In addition, there are several basic flight rules that should be observed. Never fly into a region where icing conditions are known to exist or are suspected. In any cloud where temperatures are zero degrees centigrade or below, icing conditions should be suspected. Fly over or around such clouds whenever possible. When penetration of the cloud is unavoidable, select an altitude where the temperature is such that icing is least probable. If temperature distribution data are not available, you can apply the temperature lapse rate rule. Let us assume that you are flying at 9,000 feet and the outside temperature is minus 4 degrees centigrade. You can determine the approximate temperature at any other altitude since the temperature changes 2 degrees centigrade with each 1,000 foot change of altitude in moist air. As we have seen, the area of most dangerous icing is between zero and minus 15 degrees centigrade. Applying the lapse rate rule, you can determine that approximately 5,000 feet below your present altitude, you are likely to find temperatures about 6 degrees above freezing. At this level, structural icing is unlikely. When landing at an airport which is overcast and where icing conditions exist, the pilot who uses the normal slow descent will break out underneath the clouds with his aircraft heavily iced. Under these conditions, the landing descent or takeoff climb should be accomplished as rapidly as possible to shorten the time spent in the clouds. Icing is often severe in upslope conditions where vertical air currents cause turbulence in clouds above mountains, particularly on the windward side. Regions of clear icing may extend 4,000 feet or higher above the tops of the mountains, with rhyme icing above the area of clear icing. A pilot taking off on the windward side of a mountain must avoid the danger zone which lies across his normal flight path. Conduct your ascent away from the mountains until sufficient altitude is gained to fly over the danger zone. Accordingly, when landing at the same airport, fly well to windward of the mountain before descending. As we have seen, areas of freezing rain are the most hazardous for structural icing. This pilot is flying through rain at freezing temperatures. If he continues at this level, the aircraft will quickly accumulate a dangerous amount of ice. In such a situation, you must immediately find warmer air with temperatures above freezing or turn back. Judging from the information gained in your pre-flight weather briefing and knowledge of the standard temperature lapse rate, you may find there is above freezing air below. With ample terrain clearance, descend as rapidly as possible to the lower altitude. However, descent to warmer air is not always possible. This pilot is flying through freezing rain at a minimum altitude. He is becoming badly iced, but does not have adequate terrain clearance to descend safely. In this case, if from your pre-flight briefing you know the location of the warmer air from which the rain is falling and can reach it quickly, you may climb out of the freezing rain. As soon as you reach above freezing temperatures, you should level off and proceed at this altitude. Because applying the temperature lapse rate rule, you know that higher in the cloud icing conditions will again be encountered. Sleet is an accumulation of small water particles that have frozen during their fall through cold air. Since it is already frozen, sleet does not adhere to your aircraft and presents no serious problem. Realize, though, that sleet indicates liquid moisture aloft and proceed without changing altitude. It is important to remember, however, that freezing rain may be mixed with sleet. Whenever liquid moisture is present at freezing temperatures, a dangerous situation exists and the corrective measures previously discussed should be considered. Falling snow indicates that temperatures aloft are well below freezing. As long as the temperature of the air remains cold enough, the snow will stay dry and well frozen. But at temperatures closer to the freezing level, the snow will begin to melt and become wet. This pilot is encountering falling snow along his flight path. Since he is flying in temperatures near the freezing point, the snow is wet. It melts when it hits his aircraft and refreezes, building up a rough ice formation. Upon encountering wet snow, climb toward colder temperatures where the snow is drier and will not stick to your aircraft. Even an aircraft stationary on the ground may collect a coating of ice, frost, or snow. Deposits of this kind, which reduce aerodynamic efficiency and may cause flight surfaces to bind, should be thoroughly cleaned from the aircraft before takeoff. As we have seen, when flying through any form of liquid moisture at temperatures of zero degrees centigrade or lower, ice formation on aircraft can be expected. It can reduce the aerodynamic efficiency of wings and tail assembly. Seriously affect visibility through the windshield and the operation of radar and radio equipment. It can impair the efficiency of the propellers and constrict air passages in the pitot tube, in carburetors, and in jet engine air inlets. Before any flight, make a thorough study with your weather officer of icing conditions which may exist along your route. Never continue a flight into icing conditions. Fly over or around such conditions whenever possible. While the de-icing equipment on your aircraft will help minimize icing in many situations, remember your best defense against icing hazards is to know where you may encounter them and to avoid them whenever possible.