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
Par 4-5 Page 21
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
g. How can I tell how my airplane was certificated for icing?
(1) The airplane was certificated to 14 CFR § 23.1419 at Amendment 23-14 or later if your AFM or POH references “part 25, appendix C” icing conditions, or “14 CFR § 23.1419” at Amendment 23-14 or later.
(2) The “Certification Basis” section of your airplane’s type certification data sheet (TCDS) may reference “14 CFR § 23.1419” at Amendment 23-14 or higher, or “SFAR 23.” The TCDS can be found in the FAA’s on-line Regulatory and Guidance Library at http://rgl.faa.gov.
(3) If there is only a minimum equipment list (MEL) for icing conditions in the AFM or POH, the certification basis of your airplane is prior to Amendment 23-14 (1973). Some pilots mistakenly believe IFR certificated airplanes are analyzed or tested for inadvertent icing encounters—they are not.
h. How is certification related to the operating rules?
(1) Section 34 of Special Federal Aviation Regulation (SFAR) 23, and § 34 of part 135, appendix A.
(2) These are referenced in the part 91 and part 135 operating rules, respectively, and are equivalent to 14 CFR § 23.1419 at Amendment 23-14.
4-12. ICE PROTECTION EQUIPMENT ON AIRPLANES NOT CERTIFICATED FOR ICING.
a. All aircraft are required to have ice protection for their propulsion systems in case of an inadvertent icing encounter, and nearly all aircraft have pitot heat and an alternate source of static air.
b. Some general aviation aircraft, that are not certificated for flight in icing conditions, also have ice protection systems on their wings and tailplane, providing an additional safety margin should an inadvertent encounter with icing occur. These systems are for emergency use only.
c. The FAA recommends that aircraft not certificated for flight in icing conditions, but that are equipped with these “non-hazard” deicing/anti-icing equipment exit those conditions as expeditiously as possible, coordinating with ATC as necessary.
d. The differences between these systems and fully certified systems are significant.
Airplane performance is unknown, stall warning in icing conditions most likely will not activate prior to stall, controls may jam due to ice accretion, and system features required for known icing may not be present in these “non-hazard” systems.
4-13. ANTI-ICING SYSTEMS. Anti-icing systems operate on the principle that ice should not be allowed to accumulate on the aircraft or certain aircraft systems when flying through icing conditions. Usually, anti-icing is accomplished using electric heat, hot air, or chemicals. While an aircraft’s AFM or POH is the ultimate authority on the operation of anti-icing systems, a good Par 4-11 Page 27
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.
a. Electric Systems. Electric systems normally are used for smaller areas such as antennas, static ports, air temperature probes, pitot tubes, and windshields.
b. Hot Air Systems.
(1) Hot 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.
(2) One drawback of a hot air system is that tapping air from the engine to anti-ice large surfaces reduces the amount of available thrust, which 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.
(3) 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.)
(4) 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.
c. Chemical Systems.
(1) 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.
(2) 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 conductive to icing. This will prevent the buildup of any appreciable amounts of ice.
4-14. DEICING SYSTEMS.
a. The operating philosophy behind deicing systems differ from that of anti-icing systems because deicing systems permit a certain amount of ice accumulation before they can be activated. Because ice is permitted to accrete between cycles, and because after each cycle there Page 28 Par 4-13
Par 4-14 Page 29 is some residual ice, the wing or the tailplane is never entirely “clean.” If the systems are operated properly, however, the intercycle ice buildup on the airfoil should be limited, along with the accompanying drag increases.
b. 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.
(1) At the AOA typical of cruise, this ice should have very little effect on lift. At the higher AOA characteristic of approach and landing, the decrease in CLmax will translate into an increase in stall speed. Thus, the pilot should cycle the deicing system on approach after exiting the icing conditions so that the wing will be as clean as possible and any effect on stall speed minimized.
(2) If the icing conditions cannot be exited until late in the approach, or if significant icing appears to remain on the wing after cycling the system, the pilot should assume an increase in the aircraft’s stall speed and adjust the approach speed accordingly. Pilots should consult the AFM or POH for guidance. Usually, deicing is accomplished using an electroimpact system, an electrothermal system, or a pneumatic boot system.
c. Electroimpact System.
(1) 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.
(2) The shattered ice is then carried away by the airflow.
d. Electrothermal System.
(1) 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, the cycle being repeated. Such systems are common on propellers and helicopter main rotors, and have been recently introduced on wing and tail leading edges.
(2) 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.
e. Pneumatic Boots.
(1) Pneumatic boots, pictured in Figure 12, 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.
(2) 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 is sufficiently plastic to deform to the shape of an expanded 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. Traditional advice on avoiding this problem has been to wait for a layer of ice of a predetermined thickness to form before cycling the boot. This thickness has been variously prescribed as 1/4 inch, 1/2 inch, and even 1 inch.
(3) Studies done in the late 1990’s 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 quarter century or more. Furthermore, it is recognized that a layer of ice 1/2 inch thick, especially if rough, 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. More recent studies in 2005 have shown that at airspeeds typical of general aviation airplanes with modern boot designs, ice will not shed at every boot inflation.
(4) Depending on the icing conditions and airspeed, it could take anywhere from 4 to 25 minutes to shed ice (perhaps longer at the extremely cold temperature of -30°C (-22°F), even when boots were activated at the first sign of icing and cycled every minute thereafter. Cycling early and often provided a small improvement over waiting until 1/4 or 1/2 inch of ice has accreted before activating the boots. Residual and intercycle ice are 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. It is essential that the pilot consult the AFM or POH (the POH must be consistent with the operating limitations section of the AFM) for guidance on proper use of the system. The 2005 testing did show that the proper application of ice adhesion inhibitors improved ice shedding at colder temperatures. The FAA strongly encourages the use of the manufacturer’s recommended ice adhesion inhibitors.
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