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.
FIGURE 11. PROPELLER ICE ACCRETION DURING AN SLD ENCOUNTER WITH
THE PROPELLER ICE PROTECTION SYSTEM OPERATING
4-7. ANTENNA ICING. Because of their small size and shape, antennas that do not lay flush with the aircraft’s skin tend to accumulate ice rapidly. Furthermore, they often are devoid of an internal anti-icing or deicing capability for protection. During flight in icing conditions, ice accumulations on an antenna may cause it to begin to vibrate or cause radio signals to become distorted. Besides the distraction caused by vibration (pilots who have experienced the vibration describe it as a “howl”), it may cause damage to the antenna. If a frozen antenna breaks off, it can damage other areas of the aircraft in addition to causing a communication or navigation system failure.
4-8. COOLING INLET ICING. Some types of electronic equipment generate significant amounts of heat and require independent sources of cooling, which often use external air scoops.
These cooling inlets are susceptible to icing and may or may not be heated as part of the airplane’s icing protection system. Pilots should check their airplane’s AFM to determine if the cooling inlets are protected from ice.
4-9. EFFECTS OF ICING ON CRITICAL SYSTEMS.
a. Pitot Tube. The pitot tube is particularly vulnerable to icing because even light icing can block the entry hole of the pitot tube where ram air enters the system. This will affect the airspeed indicator and is the reason most airplanes are equipped with a pitot heating system. The pitot heater usually consists of coiled wire heating elements wrapped around the air entry tube. If the pitot tube becomes blocked, and its associated drain hole remains clear, ram air no longer is able to enter the pitot system. Air already in the system will vent through the drain hole, and the remaining will drop to ambient (outside) pressure. Under these circumstances, the airspeed indicator reading decreases to zero, because the airspeed indicator senses no difference between ram and static air pressure. If the pitot tube, drain hole, and static system all become blocked in flight changes in airspeed will not be indicated, due to the trapped pressures. However, if the static system remains clear, the airspeed indicator would display a higher-than actual airspeed.
At lower altitudes, the airspeed indicator would display a lower-than-actual airspeed.
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b. Static Port. Many aircraft also have a heating system to protect the static ports to ensure the entire pitot-static system is clear of ice. If the static port becomes blocked, the airspeed indicator would still function; however, it would be inaccurate. At altitudes above where the static port became blocked, the airspeed indicator would indicate a lower-than-actual airspeed. At lower altitudes, the airspeed indicator would display a higher-than-actual airspeed.
The trapped air in the static system would cause the altimeter to remain at the altitude where the blockage occurred. The vertical speed indicator would remain at zero. On some aircraft, an alternate static air source valve is used for emergencies. If the alternate source is vented inside the airplane, where static pressure is usually lower than outside static pressure, selection of the alternate source may result in the following erroneous instrument indications:
(1) The altimeter reads higher than normal,
(2) The indicated airspeed reads greater than normal,
(3) The vertical-speed indicator momentarily shows a climb.
c. Stall Warning Systems.
(1) Stall warning systems provide essential information to pilots. A loss of these systems can exacerbate an already hazardous situation. These systems range from a sophisticated stall warning vane to a simple stall warning switch. The stall-warning vane (also called an “AOA sensor” since it is a part of the stall warning system) can be found on many aircraft. The AOA provides flightcrews with an AOA display or feeds AOA data to computers that interpret this information and provide stall warning to the crew when the AOA becomes excessive. These devices consist of a vane, which is wedge-like in shape and has freedom to rotate about a horizontal axis, and is connected to a transducer that converts the vane’s movements into electrical signals transmitted to the airplane’s flight data computer. Normally, the vane is heated electrically to prevent ice formation. The transducer is also heated to prevent moisture from condensing on it when the vane heater is operating. If the vane collects ice, it may send erroneous signals to such equipment as stick shakers or stall warning devices. Aircraft that use a stall horn may not give any indication of stall if the stall indicator opening or switch becomes frozen.
(2) Because contamination of the wing reduces lift, even an operational, ice-free stall warning system may be ineffective because the wing will stall at a lower angle of attack due to ice on the airfoil. The stall onset will therefore occur prior to activation of the stall warning devices, leading to a potential pitch or roll upset. It is imperative that pilots monitor airspeed closely when in icing conditions. On airplanes recently certificated for icing conditions, stall warning systems may have been demonstrated to provide adequate warning within the icing conditions required for certification (part 25, appendix C) once certain ice protection systems were activated. However, the adequacy of stall warning systems in conditions outside appendix C, such as SLD, has not been demonstrated.
d. Windshields.
(1) Anti-icing generally is provided to enable the flightcrew to see outside the aircraft in case icing is encountered in flight. On high-performance aircraft that require complex Par 4-9 Page 23
Page 24 Par 4-9 windshields to protect against bird strikes and withstand pressurization loads, the heating element often is a layer of conductive film or thin wire strands through which electric current is run to heat the windshield and prevent ice from forming.
(2) Aircraft that operate at lower altitudes and lower speeds generally have other systems of window anti-icing/deicing. One system consists of an electrically heated plate installed onto the airplane’s windshield to give the pilot a narrow band of clear visibility.
Another system uses a bar at the lower end of the windshield to spray deicing fluid onto it and prevent ice from forming.
e. EPR Probe (turbine engines).
(1) Ice crystals can clog and freeze over turbine engine pressure ratio (EPR) probes as well, resulting in unreliable and misleading power indications. These indications may lead a pilot to believe that an engine is producing more or less power than it actually is, and may result in improper throttle adjustments.
(2) There have been several instances where EPR probes became clogged with ice crystals during climb or cruise (see Air Florida accident in appendix 1. Pilots of turbojet aircraft should calculate a backup N1 setting for takeoff/go-around in icing conditions as a crosscheck for EPR. In flight actuating engine nacelle anti-ice when in heavy clouds usually prevents ice blockage.
f. TAT Probe.
(1) Ice crystals can clog and freeze over the TAT probe on some aircraft. This tendency to freeze over appears to be sensitive to the location of the probe on the airframe. If the TAT freezes over, the indicated temperature will erroneously rise to zero degrees C and hold. In this situation, some aircraft systems will alert the flightcrew that there is a disagreement between various ambient temperature sensors, thus indicating the presence of ice crystals.
(2) Freezing of the TAT probe has been a precursor in many of the turbine engine power loss events occurring in the area of convective weather systems.
4-10. CERTIFICATION FOR FLIGHT IN ICING CONDITIONS. An aircraft which is “certificated for flight in icing conditions” goes through an extensive procedure intended to ensure that it can safely operate throughout those icing conditions encompassed by the icing envelopes specified in 14 CFR part 25, appendix C. This process typically includes extensive analysis (done today with sophisticated computer modeling), tunnel testing, dry-air testing, testing behind an icing tanker, and flight in natural icing conditions. Its objective is not only to verify that the aircraft has functioning ice protection, but also to verify that the aircraft will have acceptable performance and handling qualities in all the environmental conditions covered by the icing envelopes.
a. What is Covered.
(1) The icing envelopes are mainly based on various types of stratus and cumulus supercooled clouds. The envelopes specify maximum amounts of liquid water and drop sizes
expected at certain temperatures and altitude ranges. The envelopes were formulated during the 1950s, based on contemporary research, and research that is more recent generally is consistent with the envelopes.
(2) It has been estimated that these envelopes encompass 99.9 percent of all conditions encountered in research programs in stratus and cumulus clouds.
b. What is Not Covered.
(1) First, the pilot should bear in mind that 99.9 percent is not 100 percent, and so vigilance in exceptional conditions is always wise.
(2) Second, the cloud measurements on which the envelopes were based generally did not include SLD conditions. Recent research shows that SLD (in particular, freezing drizzle aloft within cloud) is more common in supercooled clouds than had been thought (see Table 1 for some cues of SLD conditions).
(3) Third, freezing rain or freezing drizzle may be encountered beneath the clouds.
Neither of these icing conditions is included in the icing envelopes.
(4) Finally, ice crystals may be encountered in high concentrations at higher altitudes in the area of convective weather systems. Currently these conditions are not included in the icing envelopes, although new envelopes have been preliminarily developed.
4-11. SIGNIFICANCE OF ICING CERTIFICATION.
a. Icing certification is an extensive process. It includes testing and analysis to check that aircraft can operate safely for extended periods in the conditions covered by the icing envelopes. For example, certification includes testing and analysis to show that an aircraft can hold in significant icing conditions for up to 45 minutes. Nonetheless, pilots of certificated aircraft should not be casual about operations in icing conditions, particularly extended operations. It is always possible to encounter an unusual condition for which the aircraft has not been certificated, such as liquid water content outside the envelopes, which is sometimes indicated by a very rapid rate of accumulation. This can result in runback and ice accumulation aft of protected surfaces.
b. SLD may result in droplets impinging aft of protected surfaces and causing ice accumulation. These surfaces may be very effective ice collectors, and ice accumulations may persist as long as the aircraft remains in icing conditions. Note also that icing conditions can develop very quickly and may not be immediately recognized. For example, even though the rate of accumulation may be quite gradual, a thin, extremely rough accretion can develop on a critical surface in minutes. This can be very hazardous, particularly on approach and landing.
NOTE: Not all icing certification is the same.
c. Prior to 1973, small airplanes were approved for flight in icing if they were equipped with a minimum suite of ice protection equipment. For example, see Bureau of Flight Standards Par 4-10 Page 25
Page 26 Par 4-11 Release 434. No analysis or testing to show safe operation in the part 25, appendix C icing envelopes was required.
d. Many small airplanes flying today fall in this category. In 1973, at Amendment 23-14, 14 CFR part 23, § 23.1419 was amended to require analysis and testing to demonstrate that the airplane can operate safely in the part 25, appendix C icing envelopes.
e. In 1993, at Amendment 23-43, § 23.1419 was amended to require small airplanes, certificated for known icing, to comply with performance, stability, controllability, and maneuverability regulations in icing. There were no quantitative criteria for performance prior to Amendment 23-43.
f. Keep in mind, even though the small airplane icing certification regulations changed in 1973 and 1993, the means of showing compliance have changed continuously, as shown by the example in Table 1. Table 1 compares the current means of compliance in AC 23.1419-2C (2004) to the guidance that existed prior to 1993.
TABLE 1. EVOLUTION OF ICING CERTIFICATION STANDARDS Criteria prior to 1993 Current Criteria Simulated ice shapes tested Unprotected areas only Unprotected and protected (e.g., intercycle ice on boots) Performance criteria Qualitative Must meet Subpart B minimum climb gradients Stall warning Can use different type in icing
(e.g., buffet versus stall warning system) Stall warning type in icing should be same as in non-icing and margin should meet Subpart B Maneuver margin No policy There should be no buffet or stall warning during maneuvering at operating airspeeds Autopilot No policy Evaluate operation, disconnect at stall warning Propeller icing performance No policy Evaluate performance in natural icing Use of ice adhesion inhibitors No policy Cannot use for certification