20
Icing
Ice accretion on an airplane structure or within the engine induction system can significantly reduce flight safety by causing:
- adverse aerodynamic effects — ice buildup on the airframe structure can modify the airflow pattern around airfoils (wings and propeller blades), leading to a serious loss of lift and an increase in drag; ice/snow or frost has a thickness and/or roughness similar to medium or coarse sandpaper, and on the leading edge and upper surface of a wing it can reduce lift by as much as 30%, and increase drag also by as much as 40%;
- a loss of engine power, or complete stoppage, if ice blocks the engine air intake or carburetor ice forms;
- a weight increase and a change in the CG position of the airplane, as well as unbalancing of the various control surfaces and the propeller, perhaps causing severe vibration and/or control difficulties;
- blockage of the pitot tube and/or static vent, producing errors in the cockpit pressure instruments (airspeed indicator, altimeter, vertical speed indicator);
- degradation or loss of communication and navigation (if ice forms on the antennas); and
- loss of visibility (if ice forms on the windshield).
The possibility of icing conditions can be determined from weather forecasts and prognostic charts, but the most accurate information on icing conditions, both current and forecast, can be obtained from PIREPs (pilot reports), SIGMETs (weather advisories that warn of conditions that could be dangerous to all aircraft) and AIRMETs (that warn of hazards primarily for small aircraft).
Structural Icing
For ice to form on the aircraft structure, two conditions must be satisfied:
- there must be visible moisture; and
- the temperature must be at or below freezing (0°C).
However, aerodynamic cooling can lower the temperature of the airplane structure below that of the surrounding air by a few degrees, making it possible for ice to form on the structure even though the ambient air temperature is still a few degrees above freezing. So be on the watch for structural icing when the air temperature is below about +5°C and you are flying in visible moisture.
Temperature usually decreases in the atmosphere as you climb. The altitude where the temperature has fallen to 0°C is known as the freezing level, and it is possible to estimate this level, at least approximately.
The rate at which temperature falls with altitude (known as the lapse rate) depends on a number of variables, but the standard (average) lapse rate is a temperature decrease of 2°C for every 1,000 feet of altitude gained. For instance, if the air temperature is +8°C at 5,000 feet MSL, then you would need to climb approximately 4,000 feet for the temperature to fall to 0°C, and so the freezing level in this case is at 9,000 feet MSL.
In general terms, the worst continuous icing conditions are usually found near the cloud tops above the freezing level in heavy stratified clouds or in freezing rain. Icing can occur up to 5,000 feet above the freezing level, but rarely above this where the droplets in the clouds are usually already frozen. In cumuliform clouds with strong updrafts, however, large water droplets may be carried to high altitudes making structural icing a possibility up to high altitudes.
Clear Ice
Clear ice is the most dangerous form of structural icing. Liquid water drops exist in the atmosphere at temperatures well below the normal freezing point of water (0°C), possibly at -20°C or even lower. These are known as supercooled drops, and can occur when rain falls from air warmer than 0°C into a subzero layer of air beneath or when the rate of creation of liquid water in a subzero cloud due to adiabatic lifting exceeds the rate at which the creation of ice crystals by glaciation can occur. Supercooled drops are in an unstable state, and will freeze on contact with a subzero surface — the skin of an airplane, or the propeller blades, for example.
Each drop will freeze gradually because of the latent heat released in the freezing process, which allows part of the water drop to spread backward before it freezes. The slower the freezing process, the greater the spread-back of the water before it freezes. The spread-back is greatest at temperatures just below freezing. The result is a sheet of solid, clear, glazed ice with very little air enclosed.
The surface of clear ice is smooth, usually with undulations and lumps. It is quite tenacious but, if it does break off, it could be in large chunks capable of doing damage.
A good indication to a pilot that freezing rain may exist at higher altitudes is the presence of ice pellets, formed by rain falling from warmer air and freezing on the way down through colder air. Wet snow, however, indicates subzero temperatures at some higher altitude, and warmer air at your level. The snow that formed in the subzero air above is now melting to form wet snow as it passes through your level.
Figure 20-1 Clear ice formed from large, supercooled water drops.
Rime Ice
Rime ice occurs when tiny, supercooled liquid water droplets freeze nearly instantaneously on contact with a surface of subzero temperature. Because the drops are small, and there is little or no runback during the quick freezing process, the amount of water remaining after the initial freezing is insufficient to coalesce into a continuous sheet before freezing. The result is a mixture of tiny ice particles and trapped air, giving a rough, opaque, crystalline deposit that is fairly brittle.
Rime ice often forms on leading edges and can affect the aerodynamic qualities of an airfoil or the airflow into the engine intake. It does cause a significant increase in weight.
Mixed (or Cloudy) Ice
Cloud and rain falling from clouds may consist of drops of many sizes. A mixture of clear ice (from large drops) and rime ice (from small drops) may result. This is known as mixed ice (referred to in some countries as cloudy ice).
Frost
Frost forms when moist air comes in contact with a subzero-temperature surface. The water vapor, rather than condensing to form “liquid” water, changes directly to ice in the form of frost. This is a white crystalline coating that can usually be scraped off.
Frost can form in clear air when the airplane is parked in subzero temperatures or when the airplane flies from subzero temperatures into warmer moist air — for example, on descent, or when climbing through a temperature inversion (where temperature increases with altitude).
Although frost is not as dangerous as clear ice, it can obscure vision through a cockpit window and can possibly affect the lifting characteristics of the wings, which can be extremely serious. Although frost does not alter the basic aerodynamic shape of the wing (like clear ice does), frost can disrupt the smooth airflow over the wing, causing early separation of the airflow from the upper surface of the wing and a consequent loss of lift.
Frost on the wings during takeoff may disturb the airflow sufficiently to prevent the airplane from becoming airborne at its normal takeoff speed, or prevent it from becoming airborne at all.
Cold Soaking
Another phenomenon pilots need to be wary of is “cold soaking.” The wings of aircraft are said to be “cold-soaked” when they contain very cold fuel as a result of having just landed after a flight at high altitude or from having been refueled with very cold fuel. Whenever precipitation falls on a cold-soaked aircraft when on the ground, clear icing may occur. Even in ambient temperatures between -2°C and +15°C, ice or frost can form in the presence of visible moisture or high humidity if the aircraft structure remains at 0°C or below. Clear ice is very difficult to detect visually and may break loose during or after takeoff. The following factors contribute to cold-soaking: temperature and quantity of fuel in fuel cells, type and location of fuel cells, length of time at high altitude flights, the temperature of the new fuel added when refueling, and time lapsed since refueling.
When obtaining or making icing reports, take into consideration the aircraft associated with the report. Aircraft that cruise at speeds higher than 250 knots will have less of a problem than slower aircraft, due to aerodynamic heating.
Structural Icing and Cloud Type
Cumulus-Type Clouds
Cumulus-type clouds nearly always consist predominantly of liquid water droplets at temperatures down to about -20°C, below which either liquid-drops or ice-crystals may predominate. Newly formed parts of the clouds will tend to contain more liquid drops than in mature parts. The risk of airframe icing is high in these clouds in the range 0°C to -20°C, and medium to high in the range -20° to -40°C, with only a small chance of structural icing below -40°C.
Since there is a lot of vertical motion in convective clouds, the composition of the clouds may vary considerably at the one level, and the risk of icing may exist throughout a wide altitude band in (and under) the clouds. Updrafts will tend to carry the water droplets higher and increase their size. If significant structural icing does occur, it may be necessary to descend into warmer air. If descent to warmer air is not possible, turn around and return the way you came, leaving the icing conditions. If aircraft power is sufficient and the aircraft is equipped with either anti- or deicing equipment, climbing into warmer air in a winter warm front inversion or into very cold air where the cloud is glaciated is also an option.
Stratiform Clouds
Stratiform clouds can consist entirely or predominantly of liquid water drops down to about -15°C, with a risk of structural icing. If significant icing is a possibility, it may be advisable to fly at a lower level where the temperature is above 0°C, or at a higher level where the temperature is colder than -15°C. In certain conditions, such as stratiform clouds associated with an active front or with orographic uplift, the risk of icing is increased at temperatures lower than usual; continuous upward motion of air generally means a greater retention of liquid water in the clouds. The most serious icing in stratiform clouds is generally found near the cloud tops, where the creation of liquid water by adiabatic lifting is at its maximum.
Raindrops and Drizzle
Raindrops and drizzle from any type of clouds will freeze if they meet an airplane whose surface is below 0°C, with a higher risk of clear ice forming the bigger the water droplets are. You need to be cautious when flying in rain at freezing temperatures. This could occur for instance when flying in the cool sector underlying the warmer air of a warm front from which rain is falling.
High-Level Clouds
High-level clouds, such as cirrus, with their bases above 20,000 feet, are usually composed of ice crystals which will not freeze onto the airplane, and so the risk of structural icing is only slight in these clouds.
Structural icing is most likely to accumulate rapidly on an airplane in conditions of freezing rain, for instance when flying in below-freezing air underneath the surface of a warm front from which rain is falling.
Figure 20-2 Danger area beneath a warm front.
Induction Icing
Carburetor Icing
Ice can form in the carburetor and induction system of an engine in moist air with outside air temperatures as high as +25°C (or even higher). It will disturb or prevent the flow of air and fuel into the engine, causing it to lose power, run roughly and perhaps even stop.
Cooling occurs when the induction air expands as it passes through the venturi in the carburetor (adiabatic cooling), and occurs also as the fuel vaporizes (absorbing the latent heat of vaporization). This can easily reduce what was initially quite warm air to a temperature well below zero and, if the air is moist, ice will form.
Throttle icing is more likely to occur at lower power settings when the partially closed butterfly creates a greater venturi cooling effect, compared with high power settings when the butterfly is more open and the venturi effect is less.
Most airplanes whose engines have carburetors are fitted with a carburetor heat control that can direct hot air from around the engine into the carburetor, instead of the ambient air. Being hot, the air should be able to melt the ice and prevent further ice from forming. The correct method of using carburetor heat for your airplane is found in the Pilot Operating Handbook.
Engine Intake Icing
Structural icing near the engine air intake at subzero temperatures can restrict the airflow into the induction system and cause problems. Some aircraft have an alternate air system in case this occurs.
Figure 20-3 Carburetor ice.
Instrument Icing
Icing of the pitot-static system can affect the readings of the pressure-operated flight instruments (the airspeed indicator, the altimeter, and the VSI). If the airplane has a pitot heater, then use it when appropriate.
Hints on Flying in Icing Conditions
Use all available information, such as forecasts and PIREPs, to plan your flight so that you avoid areas of icing, unless your airplane is equipped with deicing or anti-icing equipment. Flight into known icing conditions is not authorized if the aircraft is not certified specifically to do so. Check that all the aircraft’s airfoils are clean prior to takeoff. Frost, and indeed any contamination, should be removed from the wings and other lifting surfaces prior to flight if they are to produce lift efficiently.
If taxiing or taking off in below-freezing temperatures, avoid splashing water or slush onto the airplane, since it could freeze onto the structure. Always check full-and-free movement of the controls prior to commencing the takeoff roll.
Use deicing and anti-icing equipment as recommended in icing conditions. If they are not adequate, then change course or altitude to fly out of icing conditions as quickly as possible. Consider making a 180° turn. Carry a little extra airspeed to give an added margin over what could be an increased stalling speed, and avoid abrupt maneuvers.
Be alert for incorrect readings from the pressure instruments (airspeed indicator, altimeter, vertical speed indicator) if pitot heat is not available in your airplane.
Avoid cumuliform clouds if possible, since clear ice may occur at any altitude above the freezing level. Avoid flying in or near the tops of stratiform clouds. If icing-up in stratiform clouds, either descend to warmer air above freezing, or climb out of the cloud deck or to colder air well below freezing, say -10°C or less. If descending, think carefully of the terrain below, and how far you will have to descend to fly into warmer air.
The freezing rain environment is likely to cause the highest rate of structural ice accumulation. If icing-up in freezing rain, either climbing or descending may take you into warmer or clear air. Act quickly and decisively before the build-up of ice is so great that it causes a significant deterioration in the airplane’s performance.
Icing on the tail surface can cause a tail stall on approach or restrict elevator movement. If you suspect ice on the tail carry extra power on the approach and consider keeping flaps up. Recovery from a tail stall is opposite from a wing stall — the nose of the aircraft will pitch down but at a higher airspeed: increase pitch and reduce power (remember, the tail works as an inverted airfoil).
Warning!
Ice of any type on the airframe or propeller, or in the carburetor and induction system, deserves the pilot’s immediate attention and removal. Wings which are contaminated by ice prior to takeoff will lengthen the takeoff run because of the higher speed needed to fly — a dangerous situation! An ice-laden airplane may even be incapable of flight. Ice or frost on the leading edge and upper forward area of the wings (where the majority of the lift is generated) is especially dangerous. Anti-icing and deicing equipment is intended for and used to provide the pilot time to safely exit icing conditions. This kind of equipment should not be viewed as a way to safely fly in icing conditions over long periods of time.
Most training airplanes are not fitted with airframe de-icers (removal) or anti-icers (preventive), so pilots of these airplanes should avoid flying in icing conditions (that is, in rain or moist air at any time the airframe is likely to be at subzero temperatures). If a pitot heater is fitted, use it to avoid ice forming over the pitot tube and depriving you of airspeed information.
Review 20
Icing
Structural Icing
1. What two conditions must be met for structural icing to occur on an airplane?
2. What sort of ice is likely to form from large, supercooled droplets striking a subzero airplane?
3. What sort of ice is likely to form from small, supercooled droplets striking a subzero airplane?
4. If the air temperature is +6°C at 1,500 feet MSL, and a standard temperature lapse rate exists, what will be the approximate freezing level (feet MSL)?
5. If the air temperature is +12°C at 1,500 feet MSL, and a standard temperature lapse rate exits, what will be the approximate freezing level (feet MSL)?
6. In which conditions is the formation of clear ice on the airplane structure most likely?
7. What does freezing rain consist of?
8. Under what conditions can freezing rain occur?
9. Rain drops falling from warm air into subzero air may remain in liquid form as supercooled water drops. What do these form? What do they form when frozen?
10. What will be the result if supercooled water drops in freezing rain strike a subzero aircraft structure?
11. How is the possibility of freezing rain at higher altitudes indicated?
12. What are ice pellets falling at ground level evidence of?
13. If you fly through rain which freezes on impact, what does this indicate about the temperature at higher altitudes?
14. If you fly through wet snow, what does this indicate about the temperature of higher altitudes? What does this indicate about the temperature at your altitude?
15. Can clear ice alter the basic aerodynamic shape of the wing?
16. Can frost cause a loss of lift from the wing? If so, how?
17. Should you remove frost, ice or any other contaminant from the wings prior to flight?
18. The risk of clear ice forming on the airplane’s structure is greater when flying in which type of clouds?
19. Clouds of which height level are least likely to contribute to structural icing on an airplane?
20. In which conditions is structural icing most likely to have the highest rate of accumulation?
21. What three weather sources reflect the most accurate information on icing conditions, both current and forecast?
Induction Icing
22. Does the outside air temperature need to be below freezing for carburetor icing to occur?
23. Can ice form in the carburetor when the ambient air temperature is above freezing?
24. What happens to air entering the carburetor and induction system as it expands through the venturi?
25. What control in the cockpit is used to protect against carburetor icing?
26. Can carburetor ice form when you are flying in moist air at +10°C?
Instrument Icing
27. Which instrument(s) is/are likely to give faulty indications if ice forms over the pitot tube?
28. Which instrument(s) is/are likely to give faulty indications if ice forms over the static vents?