gg. Pilot Report (PIREP). A report from a pilot of meteorological phenomena encountered by aircraft in flight, usually transmitted in a prescribed format. The letters “UA” identify the message as a routine PIREP while the letters “UUA” identify an urgent PIREP.
hh. Rime Ice. A rough, milky, opaque ice formed by the instantaneous freezing of small, supercooled water droplets.
ii. Runback Ice. Ice that forms from the freezing or refreezing of water leaving protected surfaces and running back to unprotected surfaces.
jj. Severe Icing. The rate of ice accumulation is such that ice protection systems fail to remove the accumulation of ice and ice accumulates in locations not normally prone to icing, such as areas aft of protected surfaces and any other areas identified by the manufacturer.
Immediate exit from the condition is necessary.9 kk. Significant Meteorological Information (SIGMET). Information about in-flight weather of operational significance to the safety of all aircraft. SIGMETs may include severe icing. (See CWA and AIRMET.)
ll. Stagnation Point. The point on a surface where the local air velocity is zero. The region of maximum icing collection efficiency is near this point.
mm. Stationary Front. A front that has little or no movement because the opposing forces of the two air masses are relatively balanced.
nn. Stratus Clouds. Clouds that form layers with a uniform base. Stratus clouds can appear in ragged patches and may produce drizzle, rain, or snow.
oo. Sublimation. A process where ice turns directly into water vapor without passing through a liquid state.
pp. Supercooled Large Drop (SLD). A supercooled droplet with a diameter greater than 50 micrometers (0.05 mm). SLD conditions include freezing drizzle drops and freezing raindrops.
qq. Supplemental Weather Service Location. An airport facility staffed with contract personnel who take weather observations and provide current local weather to pilots via telephone or radio.
9 Severe icing is aircraft dependent, as are the other categories of icing intensity. Severe icing may occur at any ice accumulation rate when the icing rate or ice accumulations exceed the tolerance of the aircraft. Icing certification implies an increased tolerance to icing intensities up through heavy.
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Page 8 Par 2-1 rr. Telephone Information Briefing Service (TIBS). A telephone recording of meteorological and/or aeronautical information obtained by calling an FSS.
ss. Total Air Temperature (TAT). Kinetic heating causes the Total (or Ram) Air Temperature (TAT) to be warmer than the Static Air Temperature (SAT). TAT is close to the temperature of the wing leading edge, which also experiences ram rise.
tt. Trace Icing. Ice becomes noticeable. The rate of accumulation is slightly greater than the rate of sublimation. A representative accretion rate for reference purposes is less than 1/4 inch (6 mm) per hour on the outer wing. The pilot should consider exiting the icing conditions before they become worse. Pilots should be aware that any ice, even in trace amounts, could be potentially hazardous.
uu. Warm Front. Any nonoccluded front that moves in such a way that warmer air replaces colder air.
vv. Weather Advisory. In standard aviation weather forecast terminology, a warning of hazardous weather conditions not predicted in the forecast area that may affect air traffic operations. These reports are prepared by the NWS.
CHAPTER 3. ATMOSPHERIC CONDITIONS ASSOCIATED WITH ICING
3-1. AIRCRAFT ICING CONDITIONS.
a. Nearly all aircraft icing occurs in supercooled clouds. Liquid droplets are present at outside air temperatures below 0°C (32°F) in these clouds. At outside air temperatures close to 0°C (32°F), the cloud may consist entirely of such droplets, with few or no ice particles present.
At decreasing temperatures, the probability increases that ice particles will be found in significant numbers along with the liquid droplets. In fact, as the ice water content increases, the liquid water content 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. The general rule is that the more ice particles and the fewer liquid droplets 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 droplets freeze and adhere. As a result, ice accumulation is often greatest at temperatures not too far below 0 °C (32°F), where liquid water content can be abundant, and is usually negligible at temperatures below about -20°C (-4°F).
c. An exception to the general rule just stated may be made for surfaces heated by a thermal ice protection system (or by aerodynamic heating near the stagnation point of an aircraft component at speeds in excess of perhaps 250 knots). For such surfaces, ice particles may melt upon impact and then run back to colder aft regions and refreeze.
d. The higher liquid water content associated with temperatures near freezing is not the only concern. Tests have shown that at outside air temperatures near freezing, the total air temperature may be above freezing. The result is no ice accretion near the stagnation, but the refreezing of the 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 total temperatures (TAT) between -5°C (23°F) and +2°C (35°F). Pilots should know whether the temperature instrument they are reading in the cockpit is outside air temperature (OAT) or TAT.
e. The greater the liquid water content of the cloud, the more rapidly ice accumulates on aircraft surfaces. The size of the droplets also is important. Larger droplets have greater inertia and are less influenced by the airflow around the aircraft than smaller droplets. The result is that larger droplets will impinge on more of the aircraft surface than smaller droplets.
f. Every supercooled cloud contains a broad range of droplets, 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 droplet 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. An icing-certificated aircraft is certificated for flight in stratus-type clouds with MVDs up to 40 micrometers and for cumulus-type clouds with MVDs up to 50 micrometers. The MVD distribution is such that aircraft that undergo icing certification are not evaluated for operations in clouds with a significant amount of liquid water in droplets with diameters larger than 100 micrometers. Such conditions are sometimes encountered, and accidents and incidents in Par 3-1 Page 9
Page 10 Par 3-1 such conditions have been documented. These conditions are referred to as freezing drizzle aloft in cloud or SLD in cloud. Table 1, Evolution of Icing Certification Standards, 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 droplets may be present in a cloud.
g. An aircraft can also encounter SLD conditions in freezing drizzle (droplets with a diameter of 50 to 500 micrometers) or freezing rain (droplets with a diameter of 500 micrometers and larger) below a cloud deck. These droplets are, by definition, larger than those for which any aircraft is certificated, and accidents and incidents have occurred following flight in freezing drizzle or freezing rain. The larger drops can cause ice accretion behind the protected leading edge.
3-2. CLOUD TYPES AND AIRCRAFT ICING.
a. 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 droplets form a cloud. Cloud water droplets 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. If rising air is moist (water vapor plentiful) and lifting is vigorous, the result can be clouds with substantial liquid water content and, sometimes, large droplets. The greater the liquid water content, the more rapid the icing; and the larger the droplets, the greater the extent of icing.
Tops of clouds often contain the most liquid water and largest droplets, because the droplets that reach the tops have undergone the most lifting. If the temperatures are cold enough at the tops (below around -15 °C (5 °F)), however, ice particles will usually start to form, which tends to deplete the liquid water.
c. 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 liquid water content 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 Table 1 for a list of cues developed for aircraft with unpowered controls and pneumatic deicing boots).
(2) Cumulus Clouds.
(a) Cumulus clouds, which often form because of vigorous convection, can have high liquid water content. 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 liquid water contents. 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 droplets.
(b) Wave clouds, recognized by their “wavy” tops, can have high liquid water contents. 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 droplet, 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. The size difference is shown in Figure 1. Droplet Par 3-2 Page 11
Page 12 Par 3-2 mass affects how far aft of the stagnation point (leading edge surfaces) droplets will strike the aircraft. The mass of freezing rain is typically 1,000,000 times that of cloud droplets. Because of this, freezing rain will result in ice forming in areas far aft of where it would form in ordinary supercooled clouds.
FIGURE 1. DROPLET SIZE COMPARISON Typical raindrop 2 millimeters Typical cloud droplet
0.02 millimeters
(3) Freezing drizzle also 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).
(4) However, freezing drizzle is perhaps more commonly formed by a different process, known as the collision-coalescence process. When, through condensation, some droplets in a cloud grow to approximately 30 micrometers in diameter, they begin to settle, falling fast enough so that they collide with some smaller droplets. If the droplets coalesce, the result is a larger droplet, which now has an even better chance of capturing smaller droplets. Under favorable conditions, this process can produce drizzle-size drops in a supercooled cloud, usually near the top, where the larger droplets 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 non-convective clouds. Thus, in freezing drizzle, the pilot cannot assume that a warm layer (above 0°C (32°F)) exists above the aircraft.
(5) The diameters of representative cloud and drizzle drops differ by a factor of about 10 and the volume and mass by a factor of about 1,000. The greater inertia and impingement efficiency of the drizzle drops will result in icing beyond the usual icing limits for typical cloud droplets. 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. (See Table 1 for a list of cues developed for aircraft with unpowered controls and pneumatic deicing boots.)
3-3. FRONTS.
a. 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. Along a warm front, the warmer air tends to slide gradually over the cold front, forming stratus clouds conducive to icing (see Figure 2). 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 liquid water content if the lifted air is moist (see Figure 3). SLD in the form of freezing rain and freezing drizzle are sometimes found near fronts, as explained above.
FIGURE 2. WARM FRONT
FIGURE 3. COLD FRONT
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c. Because of the icing and other hazards associated with fronts, they are best avoided, if possible. When flying through a front, the shortest route through the front should be taken, instead of flying along the front, to reduce the time spent in potential icing conditions.
3-4. CONVECTIVE WEATHER AND ICE CRYSTALS.
a. Some turbine engine designs have shown a susceptibility to ice crystals that form in the atmosphere because of convective weather activity. Deep lifting (thousands of feet) and condensation of water vapor in an unstable atmosphere characterize convective weather. Some or all of the following can be found in areas of convection: strong wind shear, turbulence, lightning, and high-condensed water or hail. Convective weather can range from isolated thunderstorms or cumulonimbus clouds, to convective complexes or squall lines to tropical storms or hurricanes. It can extend hundreds of miles laterally and above 50,000 feet vertically. 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.
b. These clouds of ice crystals may not be detectable on conventional airborne radar.
Although these ice crystals do not typically accrete on airframe surfaces or trigger ice detectors, when present in high concentrations they can still accrete on warm surfaces within the flow-path of turbine engines. This accretion can ultimately result in engine upset, engine damage from shedding, or power loss.
c. Dry ice crystals, particularly those found above 25,000 feet altitude, do not provide good radar reflectivity. Dry ice particles have about 20 times less radar reflectivity than rain droplets, and therefore are difficult to detect. Clouds and temperatures less than ten degrees centigrade are better indicators of the possible presence of ice crystals when near convective weather. Follow AFM procedures.
CHAPTER 4. ICING EFFECTS, PROTECTION, AND DETECTION
4-1. 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 impinge on them and freeze. Small and/or narrow objects are the best collectors of droplets 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 4). 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 5). Temperatures close to the freezing point, large amounts of liquid water, high aircraft velocities, and large droplets are conducive to the formation of clear ice.
FIGURE 4. CLEAR ICE
FIGURE 5. CLEAR ICE BUILDUP WITH HORNS
(2) Rime Ice. A rough, milky, opaque ice formed by the instantaneous or very rapid freezing of supercooled droplets as they strike the aircraft (see Figure 6). The rapid freezing results in the formation of air pockets in the ice, giving it an opaque appearance and making it porous and brittle. For larger accretions, rime ice may form a streamlined extension of the wing.
Low temperatures, lesser amounts of liquid water, low velocities, and small droplets favor formation of rime ice.
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FIGURE 6. RIME ICE
(3) Mixed Ice. Mixed ice is a combination of clear and rime ice formed on the same surface. It is the location, size, shape, and roughness of the ice that is most important from an aerodynamic point of view. This is discussed later in this chapter, on effects of icing.
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, 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 (for example, engine inlet temperature or engine pressure ratio (EPR) probes), which can lead to erroneous readings of engine instrumentation (see the description of the Air Florida B-737 accident in Appendix 1, paragraph 2).
(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 (that is, straight-line-of-sight inlet design). However, on turboprop engines that include an inlet section with sharp turns or bird-catchers, however, 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 AFM is very important. Supercooled water droplets 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 Page 16 Par 4-1
Par 4-1 Page 17 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.
4-2. GENERAL EFFECTS OF ICING ON AIRFOILS. The two figures in this chapter depict important information on the effects of ice contamination on an airfoil. (For this AC, an airfoil is a cross-section of a wing or tailplane.)
a. Figure 7 shows how ice often affects the coefficient of lift for an airfoil. Note that at very low angles of attack, there may be little or no effect of the ice on the coefficient of lift. Thus when cruising at a low angle of attack (AOA), ice on the wing may have little effect on the lift.
However, note that the maximum coefficient of lift (CLmax) is significantly reduced by the ice, and the AOA at which it occurs (the stall angle) is much lower. Thus when slowing down and increasing the AOA for approach, therefore, the pilot may find that ice on the wing that had little effect on lift in cruise now causes stall to occur at a lower AOA and higher speed. 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. For large ice shapes, especially those with horns, the lift may also be reduced at a lower AOA as well. Depending on the airfoil section, the lift loss may even be larger if ice accretes behind areas normally protected, such as due to large drop impingement and runback.
FIGURE 7. LIFT CURVE Clean Airfoil Airfoil With Ice Angle of Attack CL (coefficient of lift)
b. Figure 8 shows how ice affects the coefficient of drag 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 8. DRAG CURVE Clean Airfoil Airfoil With Ice Angle of Attack CD (coefficient of drag)
(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. Furthermore, on takeoff, approach, and landing, the wings of many aircraft are multi-element airfoils with three or more elements. Ice may affect the different elements in different ways and affect the way in which the flows over the elements interact.
4-3. EFFECTS OF ICING ON WINGS. The effect of icing on a wing depends on whether the wing is protected and the kind and extent of protection provided.
a. Unprotected Wing. An aircraft with a completely unprotected wing will not be certificated for flight in icing conditions, but it 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.
An increase in power is 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. The ice on the wing also causes a decrease in CLmax, possibly about 30 percent, for an extended encounter. The rule of thumb is that the percentage increase in stall speed is approximately half the decrease in CLmax, so the stall speed may go up by about 15 percent. 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 very Page 18 Par 4-2