Par 2-1 Page 9 CHAPTER 2. ATMOSPHERIC CONDITIONS ASSOCIATED WITH ICING
2-1. AIRCRAFT ICING CONDITIONS.
a. Supercooled Clouds. Nearly all aircraft icing occurs in supercooled clouds. Liquid drops are present at outside air temperatures (OAT) below 0 °C (32 °F) in these clouds. At OAT close to 0 °C (32 °F), the cloud may consist entirely of such drops, with few or no ice particles present. At decreasing temperatures, the probability increases that ice particles will exist in significant numbers along with the liquid drops. In fact, as the ice water content increases, the Liquid Water Content (LWC) tends to decrease since the ice particles grow at the expense of the water particles. At temperatures below about -20 °C (-4 °F), most clouds are made up entirely of ice particles.
b. Ice Accumulation. The general rule is that the more ice particles and the fewer liquid drops that are present, the less ice accumulation on the airframe. This is because the ice particles tend to bounce off an aircraft surface, while the supercooled drops freeze and adhere. As a result, ice accumulation is often greatest at temperatures not too far below 0 °C (32 °F), where LWC can be abundant. LWC is usually negligible at temperatures below about -20 °C (-4 °F).
c. Thermal Ice Protection. An exception to the general rule just stated may be made for surfaces heated by a thermal ice protection system (or from compressibility at higher speeds).
d. Runback Ice Accretion. Tests have shown that when outside air temperatures (OAT) are near freezing, the result is no ice accretion near the stagnation point, but the freezing or refreezing of water running back on the airfoil, causing runback ice accretions, possibly behind the protected areas. The formation of a ridge is possible. Pilots should be vigilant at OAT between -5 °C (23 °F) and +2 °C (35 °F).
e. Drop Adherence. The greater the LWC of the cloud, the more rapidly ice accumulates on aircraft surfaces. The size of the drops also is important. Larger drops have greater inertia and are less influenced by the airflow around the aircraft than smaller drops. The result is that larger drops will adhere to more of the aircraft surface than smaller drops.
f. Median Volume Diameter (MVD). Every supercooled cloud contains a broad range of drops, starting from between 1 and 10 micrometers (millionth of a meter) and usually not exceeding 50 micrometers (by comparison, the thickness of the average human hair is approximately 100 micrometers). A single drop size must be chosen as representative, and in icing terminology this is the MVD, the diameter such that half the liquid water is in smaller drops, and half in larger drops.
NOTE: Icing conditions can occur during operations in clouds with a significant amount of liquid water in drops with diameters larger than 100 micrometers. These conditions are referred to as freezing drizzle aloft in cloud or Supercooled Large Drop (SLD) in cloud. Paragraph 3-16 discusses some cues developed for aircraft with unpowered controls and pneumatic deicing boots, mainly relating to the location of the airframe ice, which the flightcrew can use in attempting to determine if such drops may be present in a cloud.
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g. SLD Conditions. An aircraft does not always have to be in a cloud to encounter SLD as conditions can exist in freezing precipitation below a cloud deck.
2-2. CLOUD TYPES AND AIRCRAFT ICING.
a. Formation. Air can rise because of many factors, including convection, orographic lifting (i.e., air forced up a mountain), or lifting at a weather front. As the air rises, it expands and cools adiabatically. If a parcel of air reaches its saturation point, the moisture within the parcel will condense and the resulting drops form a cloud. Cloud water drops are generally very small, averaging 20 micrometers in diameter, and are of such small mass that they can be held aloft by small air currents within clouds.
b. Extent of Icing. If rising air is moist (i.e., water vapor is plentiful) and lifting is vigorous, the result can be clouds with substantial LWC and, sometimes, large drops. The greater the LWC, the more rapid the icing; and the larger the drops, the greater the extent of icing. Tops of clouds often contain the most liquid water and largest drops, because the drops that reach the tops have undergone the most lifting. If the temperatures are cold enough at the tops (below or around -15 °C (5 °F)), ice particles will usually start to form that tend to deplete the liquid water.
c. Hazardous Conditions. Several types of clouds and the hazardous aircraft icing conditions that may be associated with them are discussed below.
(1) Stratus Clouds.
(a) Stratus clouds, sometimes called layer clouds, form a stratified layer that may cover a wide area. The lifting processes that form them are usually gradual, and so they rarely have exceptionally high liquid water contents. Icing layers in stratus clouds with a vertical thickness in excess of 3,000 feet are rare, so a change of altitude of a few thousand feet may take the aircraft out of icing.
(b) Lake-effect stratus clouds are exceptional in that they may have very high LWC because of the moisture available when they form over lakes. In the continental United States, lake-effect stratus clouds are most common in the Great Lakes region, particularly in early winter when cold northwesterly winds blow over the unfrozen lakes.
(c) Drizzle-size drops occasionally occur in stratus clouds, and pilots should always be on the lookout for cues that might indicate the presence of these drops (see paragraph 3-16 for a list of cues developed for aircraft with unpowered controls and pneumatic deicing boots).
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(2) Cumulus Clouds.
(a) Cumulus clouds, which often form because of vigorous convection, can have high LWC. If an aircraft traverses them, the icing can be rapid. Because they tend to be of limited horizontal extent, it may be possible to avoid many of them. Because of the vertical development of cumulus clouds, icing conditions can be found in layers thousands of feet in depth, but with much less horizontal development than in stratus clouds.
(b) This class of clouds includes the cumulonimbus, or thunderstorm, clouds.
Updrafts in such clouds can be great and result in very large LWCs. Thus, a large icing threat can be added to the other excellent reasons to stay out of such clouds. The thunderhead anvil can spread out from the core for several miles and is composed mainly of ice crystals. These crystals will not adhere to unheated surfaces when they hit, but they may melt on a heated surface, run back, and refreeze. The ice content in the anvils can be high, and ingestion of the ice crystals has resulted in uncommanded thrust reductions.
(3) Orographic Clouds, Wave Clouds, and Cirrus Clouds.
(a) Orographic clouds form when moist air is lifted by flowing up the side of a mountain. As the parcel of air is lifted, it cools and forms a cloud. Such clouds can contain a large volume of water and, in some cases, large drops.
(b) Wave clouds, recognized by their wavy tops, can have high LWCs. Continued flight along a wave may result in airframe icing.
(c) Cirrus clouds, found at very high, cold altitudes, are composed entirely of ice particles. Flight through these clouds should not result in structural icing, although the possibility exists for runback icing from the refreezing of particles that melted on thermally or aerodynamically heated surfaces.
d. Freezing Rain and Freezing Drizzle.
(1) Freezing rain forms when rain becomes supercooled by falling through a subfreezing layer of air. Ordinarily, air temperatures decrease with increasing altitude, but freezing rain requires a temperature inversion, which can occur when a warmer air mass overlies a colder air mass. This situation can occur along a warm front, where a warm air mass overruns a cold air mass. When flying in freezing rain, normally there is warm air (above 0 °C (32 °F)) above.
(2) Freezing raindrops are defined as drops of 500 micrometers (0.5 mm) diameter or larger. A typical diameter is 2 mm, and the few that grow much larger than about 6 mm tend to break up. Using 20 micrometers (0.02 mm) as a typical diameter for a cloud drop, the diameters of rain and cloud drops differ by a factor of approximately 100, and the volume and mass differ by a factor of about 1,000,000. Drop mass affects how far aft of the stagnation point (leading edge surfaces) drops will strike the aircraft. Subsequently, freezing rain will result in ice forming in areas far aft of where it would normally form in icing conditions without freezing rain.
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(3) Drops of freezing drizzle consist of supercooled liquid water drops that have diameters smaller than 500 micrometers (0.5 mm) and greater than 50 micrometers (0.05 mm).
While smaller than freezing rain, drops of freezing drizzle are still larger than regular cloud drops, and can form through the same process. It consists of supercooled liquid water drops that have diameters smaller than 500 micrometers (0.5 mm) and greater than 50 micrometers
(0.05 mm). However, freezing drizzle is perhaps more commonly formed by a different process, known as the collision-coalescence process. When some drops in a cloud grow to approximately 30 micrometers in diameter through condensation, they begin to settle, falling fast enough so that they collide with some smaller drops. If the drops coalesce, the result is a larger drop, which now has an even better chance of capturing smaller drops. Under favorable conditions, this process can produce drizzle-size drops in a supercooled cloud, usually near the top, where the larger drops generally are found in any cloud. Statistics vary, but some studies have reported that freezing drizzle aloft forms more than 80 percent of the time by the collision-coalescence process in nonconvective clouds. Thus, in freezing drizzle, the pilot cannot assume that a warm layer (above 0 °C (32 °F)) exists above the aircraft. When drizzle drops are found within a supercooled cloud, they can result in accretions that cause very rapid and dangerous stall speed and drag increases for some aircraft and roll control anomalies for others. These situations may be caused by the roughness, shape, and extent of the accretion that forms. This is an instance of SLD icing as discussed earlier in this paragraph.
2-3. FRONTS.
a. Formation. When air masses of differing temperatures, pressures, or relative humidity meet, a front is formed. If the front moves so that warmer air replaces colder air, it is called a warm front; if it moves so that colder air replaces warmer air, it is called a cold front. An occluded front forms when an air mass is trapped between two colder air masses and is forced to higher and higher levels. In all three cases, significant lifting occurs. If sufficient moisture and subfreezing temperatures are present, icing conditions are created.
b. Warm and Cold Fronts. Along a warm front, the warmer air tends to slide gradually over the cold front, forming stratus clouds conducive to icing (see Figure 2-1). In a cold front, the cold air plows under the warm air, lifting it more rapidly and resulting in the formation of cumulus clouds with high LWC if the lifted air is moist (see Figure 2-2). SLD in the form of freezing rain and freezing drizzle are sometimes found near fronts, as explained above.
FIGURE 2-1. WARM FRONT CIRRUS
CIRROSTRATUS
WARM AIR ALTOSTRATUS
NIMBOSTRATUS COLD AIR NEL EN St. Louis Indianapolis 200 miles Columbus 400 miles Pittsburgh 600 miles 1008 1011 1014 METAR KSTL 1950Z 21018KT 1SM -RA OVC010 18/18 A2960 METAR KIND 1950Z 16012KT 3SM RA BKN020 15/15 A2973 o St. Louis 1002 METAR KCMH 19502 13018KT 6SM HZ OVC060 14/10 A2990 1005 METAR SCT150 KPIT 1950Z 12/01 13012KT 10SM A3002 1017 FIGURE 2-2. COLD FRONT
COLD AIR WARM AIR
CUMULONIMBUS mali St. Louis Indianapolis 200 miles Columbus 400 miles Pittsburgh 600 miles METAR KSTL 1950Z 30018KT 10SM SCT010 08/02 A2979 10192 METAR KIND OVC010 1950Z 20024KT 3SM 24/23 A2974 +TSRA St. Louis dianapolis Columbus METAR KCMH 1950Z 20012KT 6SM HZ BKN025 25/24 A2983 METAR KPIT 1950Z 20022KT 220 FU SCT035 A2989 Par 2-3 Page 13
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c. Navigating a Front. Because of the icing and other hazards associated with some fronts, exposure to icing conditions of varying severity is possible. When flying through a front, the pilot should take the shortest route through the front, instead of flying along the front, to reduce the time spent in potential icing conditions.
2-4. CONVECTIVE WEATHER AND ICE CRYSTALS.
a. Convective Weather Systems. Convective weather systems, especially those associated with tropical weather fronts, can pump large quantities of moisture to high altitudes that freezes into ice crystals that can remain aloft. These ice crystals can remain as a cloud well after the convective system has decayed. Clouds and temperatures less than 10 °C are better indicators of the possible presence of ice crystals when near convective weather.
b. Hazards. Above flight level (FL) 250, clouds contain little liquid water and mostly contain ice particles. These clouds with no liquid water have about 20 times less radar reflectivity than rain drops, and therefore are difficult to detect. Airborne weather radar will receive little to no returns at these altitudes unless it is tilted down to lower altitudes near or below the freezing level. Strong returns from the lower altitudes indicate the possibility of hail, severe turbulence, or large quantities of ice crystals that could be encountered above and accrete inside turbine engines when overflying these areas. Large deposits may ultimately result in engine upset, engine damage from ice shedding, power loss, or engine shutdown.
Par 3-1 Page 15 CHAPTER 3. ICING EFFECTS, PROTECTION, AND DETECTION
3-1. FORMS OF ICING. Aircraft icing in flight is usually classified as being either structural icing or induction icing. Structural icing refers to the ice that forms on aircraft surfaces and components, and induction icing refers to ice in the engine’s induction system.
a. Structural Icing. Ice forms on aircraft structures and surfaces when supercooled droplets adhere to them and freeze. Small and/or narrow objects are the best collectors of drops and ice up most rapidly. This is why a small protuberance within sight of the pilot can be used as an ice evidence probe. It will generally be one of the first parts of the airplane on which an appreciable amount of ice will form. An aircraft’s tailplane will be a better collector than its wings, because the tailplane presents a thinner surface to the airstream. The type of ice that forms can be classified as clear, rime, or mixed, based on the structure and appearance of the ice. The type of ice that forms varies depending on the atmospheric and flight conditions in which it forms.
(1) Clear Ice. A glossy, transparent ice formed by the relatively slow freezing of supercooled water (see Figure 3-1, Clear Ice). The terms “clear” and “glaze” have been used for essentially the same type of ice accretion. This type of ice is denser, harder, and sometimes more transparent than rime ice. With larger accretions, clear ice may form “horns” (see Figure 3-2, Clear Ice Buildup with Horns). Temperatures close to the freezing point, large amounts of liquid water, high aircraft velocities, and large drops are conducive to the formation of clear ice.
FIGURE 3-1. CLEAR ICE
FIGURE 3-2. CLEAR ICE BUILDUP WITH HORNS
(2) Rime Ice. A rough, milky, opaque ice formed by the instantaneous or very rapid freezing of supercooled drops as they strike the aircraft (see Figure 3-3, Rime Ice). The rapid freezing results in the formation of air pockets in the ice, giving it an opaque and rough appearance, making it porous and brittle. For larger accretions, rime ice may form a streamlined
Par 3-1 Page 16 extension of the wing. Low temperatures, lesser amounts of liquid water, low velocities, and small drops favor formation of rime ice.
FIGURE 3-3. RIME ICE
(3) Mixed Ice. Mixed ice is a combination of clear and rime ice formed on the same surface. Because there is a difference in rates of ice accumulation, ice that builds up quickly traps air pockets that give the ice a cloudy appearance (see Figure 3-4, Mixed Ice). Hence, mixed ice is sometimes called cloudy ice. It is the location, size, shape, and roughness of the ice that is most important from an aerodynamic point of view. This is discussed in paragraph 3-2.
FIGURE 3-4. MIXED ICE
b. Induction Icing.
(1) Ice in the induction system can reduce the amount of air available for combustion.
The most common example of reciprocating engine induction icing is carburetor ice. Most pilots are familiar with this phenomenon, which occurs when moist air passes through a carburetor venturi and is cooled. As a result of this process, ice may form on the venturi walls and throttle plate, restricting airflow to the engine. This may occur at temperatures between 20 °F (-7 °C) and 70 °F (21 °C). The problem is remedied by applying carburetor heat, which uses the engine’s own exhaust as a heat source to melt the ice or prevent its formation. Fuel-injected aircraft engines usually are less vulnerable to icing, but still can be affected if the engine’s air source becomes blocked with ice. Manufacturers provide an alternate air source that may be selected in case the normal system malfunctions.
(2) In turbine-engine-powered aircraft, air that is drawn into the engines creates an area of reduced pressure at the inlet, which lowers the temperature below that of the surrounding air.
In marginal icing conditions (i.e., conditions where icing is possible), this reduction in temperature may be sufficient to cause ice to form on the engine inlet, disrupting the airflow into the engine. Another hazard occurs when ice breaks off and is ingested into a running engine,
Par 3-1 Page 17 which can cause damage to fan blades, engine compressor stall, or combustor flameout. When anti-icing systems are used, runback water also can refreeze on unprotected surfaces of the inlet and, if excessive, reduce airflow into the engine or distort the airflow pattern in such a manner as to cause compressor or fan blades to vibrate, possibly damaging the engine. Another problem in turbine engines is ice, particularly snow and ice crystals accumulating on the engine probes used to set power levels (e.g., engine inlet temperature or Engine Pressure Ratio (EPR) probes), which can lead to erroneous readings of engine instrumentation (e.g., Air Florida B-737 accident National Transportation Safety Board (NTSB) accident report NTSB/AAR-82-08).
(3) Ice also may accumulate on both the engine inlet section and on the first or second stage of the engine’s low-pressure compressor stages. This normally is not a concern with pitot-style engine airflow inlets (i.e., straight-line-of-sight inlet design). However, on turboprop engines that include an inlet section with sharp turns, ice can accumulate in the aerodynamic stagnation points at the bends in the inlet duct. If ice does accumulate in these areas, it can shed into the engine, possibly resulting in engine operational difficulties or total power loss.
Therefore, with these types of engine configurations, the use of anti-icing or deicing systems per the Airplane Flight Manual (AFM) is very important. Supercooled water drops tend to form ice on the turbine engine inlet, fan, and first few stages of the compressor. Ice crystals, when present in high concentrations, tend to form ice deeper in the turbine engine’s compressor section. Ice accretions can ultimately shed and damage the compressor, or cause engine surge or flameout.
These conditions are analyzed and tested during original engine airworthiness approvals. These tests are conducted to demonstrate the turbine engine’s tolerance to these conditions.
3-2. GENERAL EFFECTS OF ICING ON AIRFOILS. Figure 3-5, Lift Curve, and Figure 3-6, Drag Curve, below, depict important information about the effects of ice contamination on an airfoil. (For this AC, an airfoil is a cross-section of a wing or tailplane).
a. Stall. Figure 3-5 shows how ice affects the lift coefficient for an airfoil. Note that the Maximum Coefficient of Lift (CLmax) is significantly reduced by the ice, and the Angle of Attack (AOA) at which a stall occurs (the stall angle) is much lower with ice than without ice.
When slowing down and increasing the AOA for an approach, the pilot may find that ice on the wing that had little effect on lift in cruise now induces a stall at a higher AOA associated with a lower airspeed. Even a thin layer of ice at the leading edge of a wing, especially if it is rough, can have a significant effect in increasing stall speed. This effect may be even larger if ice accretes behind areas normally protected.
Par 3-2 Page 18 FIGURE 3-5. LIFT CURVE
b. Drag. Figure 3-6 shows how ice affects the drag coefficient of the airfoil. Note that the effect is significant even at very small AOAs.
(1) A significant reduction in CLmax and a reduction in the AOA where stall occurs can result from a relatively small ice accretion. A reduction of CLmax by 30 percent is not unusual, and a large-horn ice accretion can result in reductions of 40 percent to 50 percent. Drag tends to increase steadily as ice accretes. An airfoil drag increase of 100 percent is not unusual, and, for large-horn ice accretions, the increase can be 200 percent or even higher.
FIGURE 3-6. DRAG CURVE
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(2) Ice on an airfoil can have other effects not depicted in these curves. Even before airfoil stall, there can be changes in the pressure over the airfoil that may affect a control surface at the trailing edge.
3-3. EFFECTS OF ICING ON UNPROTECTED WINGS. An aircraft with a completely unprotected wing is unlikely to be certificated for flight in icing conditions, but may inadvertently encounter icing conditions. Since a cross-section of a wing is an airfoil, the remarks above on airfoils apply to a wing with ice along its span. The ice causes an increase in drag, which the pilot detects as a loss in airspeed or an increase in the power required to maintain the same airspeed. (The drag increase is also due to ice on other parts of the aircraft). The longer the encounter, the greater the drag increase; even with increased power, it may not be possible to maintain airspeed. If the aircraft has relatively limited power (as is the case with many aircraft with no ice protection), it may soon approach stall speed and a dangerous situation. A similar scenario applies to aircraft that are certificated for flight in icing conditions if the wing ice protection system fails in icing conditions.
3-4. DEICING SYSTEMS. The operating philosophy behind deicing systems differs from that of anti-icing systems because deicing systems are activated after encountering icing conditions, permitting a certain amount of ice accumulation.
a. Pneumatic Boots. Pneumatic boots, pictured in Figure 3-7, consist of rubber tubes attached to critical aircraft surfaces, such as the leading edges of wings and horizontal and vertical stabilizers. The tubes may be either chordwise or spanwise. The pneumatic boots are collapsed during normal operations, with suction provided by a vacuum pump to avoid disruption of airflow over the wings. When the system is activated in flight, a timer-operated valve selectively inflates all tubes or half of the tubes intermittently to crack the ice and then allow the airflow over the wings to blow off the broken ice.
FIGURE 3-7. WING BOOT
(1) Because ice is permitted to accrete between cycles (called intercycle or residual ice) the wing or the tailplane is never entirely clean. Residual and intercycle ice is inherent in the use