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
Par 4-3 Page 19 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.
b. Deiced Wing. The FAA recommends that the deicing system be activated at the first indication of icing. Because some residual ice continues to adhere between pneumatic boot system cycles, the wing is never entirely “clean.” The amount of residual ice increases as airspeed or temperature decreases. At airspeeds typical of small airplanes (demonstrated in tunnel tests at airspeeds below 145 knots calibrated airspeed (KCAS)), it may take many boot cycles to effectively shed the ice. It may appear that the boots are not having any effect at all until shedding occurs.
(1) Use of ice adhesion inhibitors on pneumatic deicing boots is highly recommended; consult the AFM or maintenance manual for the recommended product and application interval. The ice that can build between boot cycles, called intercycle ice, can be significant. On many icing accidents and incidents, loss of airspeed and stall can occur in a span of minutes. Ice accretion will decrease CLmax, which will translate into an increase in stall speed.
(2) The increase in stall speed becomes more of a concern at the higher AOAs characteristic of approach and landing because the aircraft is operating closer to CLmax. Thus the pilot should consider continuing activation of the deicing system for a period after exiting the icing conditions so that the wing will be as clean as possible and any effect on stall speed minimized. If the icing conditions cannot be exited until late in the approach or significant icing appears to remain on the wing after activating the system, an increase in the aircraft’s stall speed is a possibility and adjustment of the approach speed may be appropriate. Consult the AFM or POH for guidance.
(3) Intercycle ice will also accrete on airplanes with electrothermal deicing systems.
It is typical for these systems also to produce runback ice behind the protected area.
c. Anti-Iced Wing.
(1) An anti-icing system is designed to keep a surface entirely free of ice throughout an icing encounter. Anti-icing protection for wings is normally provided by ducting hot bleed air from the engines into the inner surface of the wing’s leading edge and thus is found mainly on transport turbojets and business jets, but not on turbopropeller or piston airplanes. Even on transport and business jets, there are often sections along the span of the wing that are not protected. An important part of icing certification for these planes is checking that the protected sections are extensive enough and properly chosen so that ice on the unprotected areas will not affect the safety of flight.
(2) Anti-icing systems can be evaporative or running wet. On newer designs, the wing anti-ice system may be running wet by design, forming runback ice accretions. The effects of these accretions are evaluated during certification, but only in part 25, appendix C icing conditions. Runback ice can serve as accretion sites for additional accumulations.
4-4. EFFECTS OF ICING ON ROLL CONTROL.
a. This chapter is in effect a continuation of the previous one, since ice on the wings forward of the ailerons can affect roll control. The ailerons are generally close to the tip of the wing, and wings are designed so that stall starts near the root of the wing and progresses outward. In this way, the onset of stall does not interfere with roll control of the ailerons.
However, the tips are usually thinner than the rest of the wing, and so they most efficiently collect ice. This can lead to a partial stall of the wings at the tips, which can affect the ailerons and thus roll control.
b. If ice accumulates in a ridge aft of the boots, but forward of the ailerons, possibly due to flight in SLD conditions, this can affect the airflow and interfere with the proper functioning of the ailerons, even without a partial wing stall at the tip.
(1) This is the phenomenon that the National Transportation Safety Board (NTSB) found to be responsible for the accident of an ATR-72 turbopropeller aircraft in Roselawn, Indiana in October 1994 (see Appendix 1, paragraph 1). Flight test investigations following the accident suggested two ways in which the ailerons might be affected by ice in front of them.
(2) One has been termed “aileron snatch,” in which an imbalance of forces at the aileron is felt by the pilot of an aircraft without powered controls as a sudden change in the aileron control force. Provided the pilot is able to adjust for the unusual forces, the ailerons may still be substantially effective when they are deflected. The other is that ailerons may be affected in a substantial degradation in control effectiveness, although without the need for excessive control forces.
4-5. TAILPLANE ICING.
a. Most aircraft have a nose-down pitching moment from the wings because the center of gravity (CG) is ahead of the center of pressure. It is the role of the tailplane to counteract this moment by providing “downward” lift (see Figure 9). The result of this configuration is that actions that move the wing away from stall, such as deployment of flaps or increasing speed, may increase the negative AOA of the tail. With ice on the tailplane, it may stall after full or partial deployment of flaps (see Figure 10).
FIGURE 9. TAIL DOWN MOMENT Tail Download C of L CG Weight Page 20 Par 4-4
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b. Since the tailplane is ordinarily thinner than the wing, it is a more efficient collector of ice. On most aircraft, the tailplane is not visible to the pilot, who therefore cannot observe how well it has been cleared of ice by any deicing system. Thus, it is important that the pilot be alert to the possibility of tailplane stall, particularly on approach and landing. A no-flap landing should be considered to avoid a tailplane stall, consistent with AFM procedures. Tailplane stall is discussed in detail in Chapter 7, paragraph 7-9.
FIGURE 10. PITCHOVER DUE TO TAIL STALL Icing CG Aircraft nose pitches down Weight
c. On some transport turbojets, the tailplane has no ice protection. However, the tailplanes on these aircraft are usually quite thick and therefore, are a less efficient collector of ice. Furthermore, these aircraft are subjected to extensive certification testing and analysis to ensure that the tailplane will not be placed at such an extreme angle in actual operations to experience a stall, even with a large ice accretion.
4-6. PROPELLER ICING. Ice buildup on propeller blades reduces thrust for the same aerodynamic reasons that wings tend to lose lift and increase drag when ice accumulates on them. The greatest quantity of ice normally collects on the spinner and inner radius of the propeller. However, in one suspected large drop icing during a flight test (Figure 11), ice was experienced along the entire span of the propeller blades. This resulted in a 50-knot loss of airspeed in one minute, 25 seconds. There was little airframe ice and no indication of propeller icing. As ice accretes on the propeller blades increasing blade drag, the propeller governor of the constant speed propeller flattens the blade pitch to maintain revolutions per minute (RPM). In the cockpit, the pilot sees no change in RPM or torque.