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.
(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 extension of the wing. Low temperatures, lesser amounts of liquid water, low velocities, and small drops favor formation of 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.
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 (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.
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.
(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.
(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 of any available deicing system, including pneumatic boots. Proper operation of the boots is necessary to minimize the effect of this ice. The amount of ice increases as airspeed or temperature decreases. At airspeeds typical of small airplanes, it may take many boot cycles to effectively shed layers of ice. It may appear that the boots are not having any effect at all until shedding occurs. In many icing accidents and incidents, loss of airspeed and stall can occur in a span of minutes. Any remaining ice accretion will increase in the stall speed.
(2) A layer of ice that is rough at any thickness on a wing’s leading edge can have a significant effect on aircraft performance, stability, and control. Consequently, some manufacturers now advise that the boots be cycled as soon as icing is encountered, rather than waiting for a prescribed thickness to accrete. The FAA recommends that the deicing system be activated at the first indication of icing as activating the boots early and often never results in having more ice on the wing than waiting for a late activation. It is essential that the pilot consult the AFM or pilot’s operating handbook (POH) (the POH must be consistent with the operating limitations section of the AFM) for guidance on proper use of the system.
(3) At the AOA typical of cruise, this ice should have very little effect on lift. An increase in stall speed becomes more of a concern at 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 pilot cannot exit the icing conditions 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.
(4) A traditional concern in the operation of pneumatic boots has been ice bridging. This is attributed to the formation of a thin layer of ice which forms to the shape of an expanding deicing boot without being fractured or shed during the ensuing tube deflation. As the deformed ice hardens and accretes additional ice, the boot may be ineffective in shedding the bridge of ice. Studies done in the late 1990s have established that there are few, if any, documented cases of ice bridging on modern boot designs. In addition, several icing tunnel tests sponsored by the FAA since 1999 showed no ice bridging on modern boot designs. Known cases are confined to boots of designs dating back a half century or more.
b. Electroimpact System. The electroimpact system deices a surface using pulses of energy to produce rapid flexing movements of the airplane’s skin surface, which break the bond of accumulated ice and the shattered ice is then carried away by the airflow. This system is the least commonly used.
c. Electrothermal System. The electrothermal system deices a surface by heating the surface to a temperature above freezing to break the bond of accumulated ice. The shattered ice is then carried away by the airflow. The surface is allowed to cool to allow ice to form, and the heat is activated again to shed the ice, thus repeating the cycle. Such systems are common on propellers and helicopter main rotors, and have been recently introduced on wing and tail leading edges.
(1) Propellers are deiced using rubber boots with embedded heater wires to break the adhesion of ice to the propeller blades. Sometimes the blades are heated alternately in sections due to limits of available electrical power. The alternate sections are heated symmetrically to avoid an imbalance of the propeller while sections of ice are being removed and dislodged from the propeller by centrifugal force. Often, on aircraft that have such systems, the skin surrounding the airframe is reinforced with doublers to strengthen the skin where ice is most likely to be flung from the propellers. However, the initial imbalance caused by ice accumulation and the loud noise created by ice shedding and hitting the airframe can be unsettling to passengers and distracting to flightcrews.
(2) Intercycle and residual ice can accrete on airplanes with electrothermal deicing systems. It is typical for these systems also to produce runback ice behind a protected area. Because other parts of the aircraft, including part of the span of the wing, are not protected from ice, a drag increase from those areas will still be present. This is accounted for in the icing certification process, and the pilot can fly the aircraft safely by following the operating procedures in the AFM or POH. Residual ice and the ice that accumulates between deicing cycles can be expected to have some effect on CLmax, but note that this effect is significant only at higher AOA.
3-5. ANTI-ICING SYSTEMS. 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 or through an evaporative or running wet system. Anti-icing systems are designed to be turned on prior to encountering icing conditions. Operating them as deicing equipment may result in a system failure or damage.
a. Bleed Air Systems. Bleed air systems are used for larger areas of the aircraft, such as engine nacelles and wing leading edges. Bleed air from turbine engines is the most common type of anti-icing protection for engine nacelles and wings of transport and business turbojets. Hot air is distributed to piccolo tubes, which consist of a perforated pipe installed directly behind the airplane’s skin. Such hot air systems are quite effective in preventing the formation of ice.
NOTE: One drawback of a hot air system is that tapping air from the engine to anti-ice large surfaces affects engine temperature limits that require reduced power settings, which reduce the performance of the engine(s). This may have a significant effect on climb performance (especially one engine inoperative climb performance in multiengine turboprops, turbojets, and turbofans). This performance loss is the reason this system is not common on smaller turbine powered airplanes. Pilots should keep in mind that while cruising or descending with anti-ice systems on, higher-than-normal power settings may be required to ensure sufficient bleed air is being supplied to the anti-ice system, and to prevent engine surges/stalls (see the particular AFM for the appropriate settings.) In addition, in some situations on some airplanes, the hot air system may not fully evaporate all impinging water drops, resulting in runback ice. This may occur with an inoperative engine, and may be the reason your AFM requires a minimum engine power setting on descent.
b. Evaporative/Running Wet Systems. Evaporative or running wet systems are newer designs used on smaller jet aircraft, turbopropeller, and piston aircraft. These chemical systems apply a chemical agent that lowers the freezing point of water found on aircraft surfaces and decreases the friction coefficient of those surfaces to prevent ice from adhering to the surfaces. Examples of such chemical agents are isopropyl alcohol and ethylene glycol. This 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 14 CFR part 25 appendix C icing conditions.
NOTE: While an aircraft’s AFM or POH is the ultimate authority on the operation of anti-icing systems, a good rule of thumb is to activate anti-icing systems at the first signs of visible moisture encountered during conditions conducive to icing. This will prevent the buildup of any appreciable amounts of ice.
3-6. EFFECTS OF ICING ON ROLL CONTROL.
a. Ailerons. This paragraph 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 generally a 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. Airflow. 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 NTSB found to be responsible for the accident of an ATR-72 turbopropeller aircraft in Roselawn, Indiana in October, 1994. 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.
3-7. TAILPLANE ICING.
a. Downward Lift. Most aircraft have a nose-down pitching moment from the wings because the center of gravity (CG) is ahead of the center of lift. It is the role of the tailplane to counteract this moment by providing downward lift (see Figure 3-8, Tail Down Moment). 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 deployment of flaps (see Figure 3-9, Pitchover Due to Tail Stall).
b. Tailplane Stall. 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 after full flap deflection, on airplanes not evaluated for susceptibility. A no-flap landing should be considered to avoid a tailplane stall, consistent with AFM procedures. Tailplane stall is discussed in detail in Chapter 6, paragraph 5-12.
c. Testing and Analysis. On many 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.