APPENIDX F. OPERATIONAL FACTORS (CONTINUED)
considerations include evaluating the quantity and temperature of air available from the engine for ice protection and the airflow through the engine during the most critical operating mode.
For example, the engine’s flight-idle thrust may need to be increased to ensure the engine is able to provide the bleed air required by the ice protection system. These considerations are especially critical when evaluating thermal IPSs, where the heat source is the engine.
The engine air mass flow and aircraft flight speed determine the flow field around the engine inlet. Designers must know the inlet flow field to determine the heat transfer between the heated surfaces, the hot air needed for the engine installation ice protection, and the quantity of water impinging on the surfaces. During some operating modes, the inlet static pressure and temperature are below ambient. In marginal icing conditions, this drop in temperature may be enough to cause ice to form in the inlet at ambient temperatures above freezing. AC 20-147 discusses these factors, as well as scoop factor, in more detail.
Engine inlets, inlet air screens, and inlet lips are critical because the ice buildups on the surfaces are exposed to engine airflow. This ice buildup may shed and be ingested by the engine. Ice ingestion can cause serious damage to compressor or fan blades. Runback water can also refreeze on unprotected surfaces of the inlet. If this runback ice is excessive, it can reduce engine airflow or distort the flow pattern to excite compressor or fan blades to critical frequencies.
Various engine operating modes have an effect on propeller ice. Propeller surfaces are evaluated similarly to other surfaces in determining the extent and degree of required ice protection. The greatest quantity of ice normally collects on the spinner and inner radius of the propeller.
Propeller and spinner ice may be ingested into the engine. You may use inertial separation within the engine to avoid ice ingestion by the engine. You may also use anti-ice, rather than deice systems, to reduce the hazard of ice being shed into the engine. Airplane performance changes that may result from ice buildup on unprotected surfaces and on the propeller further emphasize the need to preserve acceptable aircraft power or thrust.
You should discuss details about approval of turbine engine induction system ice protection and turbine engine ice ingestion with the FAA Engine and Propeller Directorate. ACs 23-8B and 20 147 contain added guidance.
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APPENIDX G. ICING CONDITIONS EXPOSURE TIME To show an airplane’s tolerance to continuous icing, the applicant should perform analyses and consider flight tests of the airplane for a representative air traffic imposed “holding” condition in the continuous maximum icing conditions of 14 CFR part 25, Appendix C. You should show that recommended holding configurations and airspeeds are safe. Perform flight tests that include a continuous exposure to icing of at least 45 minutes with the IPSs working. Appendix R of this AC, paragraph R.4.1, provides added guidance on icing exposure times and critical ice shapes for these exposure times. Appendix R of this AC, paragraph R.3 contains guidance on icing exposure times following an IPS failure. AC 20-147 contains guidance on 14 CFR part 33 compliance icing conditions exposure times.
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APPENIDX H. ICE PROTECTION SYSTEMS H.1 ICE PROTECTION TECHNOLOGIES AND OPERATING MODES.
Applicants may use various ice protection technologies to comply with ice protection requirements. These technologies, discussed in section 7 of this AC, include:
x Thermal (hot air and electrothermal).
x Mechanical (pneumatic boots, electro-impulsive).
x Fluid (weeping systems).
x Icephobic.
These ice protection technologies use the following techniques to achieve protection against ice buildup:
x Heating the impinging supercooled water drops to above the freezing point.
x Using mechanical, thermal, or pneumatic energy to shed accreted ice.
x Lowering the freezing point of the impinging water below the local air temperature.
x Providing an aircraft component surface that prevents adhering ice.
You may use hybrid IPSs that combine two or more of the techniques for specific applications.
You may also use ice protection technologies either as anti-icers or as deicers.
Anti-icing IPSs prevent ice buildup on protection surfaces. Periodic cycling of deicing IPSs removes ice that collects on protection surfaces. You should address intercycle ice when certifying a deicer IPS. The IPS mode of operation used for certification should be the same as that used in service. For example, if you use the IPS as an anti-icer during certification, the AFM should recommend use of the IPS as an anti-icer.
Unless used in a deicing mode, evaporative anti-icing systems have no ice accretion on the protection surfaces or runback ice aft of the protection surfaces during normal operations in icing conditions. Therefore, there is less change in the aircraft’s aerodynamics caused by an evaporative anti-icing system (compared with the aerodynamics of the uncontaminated aircraft) than that caused by a deicing IPS.
NOTE: Running-wet IPSs may accrete ice aft of the protection surface when used as either an anti-icer or as a deicer. Runback ice may change the aircraft aerodynamics.
Anti-icing IPSs require greater energy than deicing IPSs. The greater energy required by antiicing IPSs could result in significant decreases in available airplane thrust and airplane performance if the engines provide energy for ice protection. On the other hand, sometimes the thrust levels required for evaporative anti-ice protection may not allow the aircraft to descend using normal procedures. Without enough heat from the engines, the evaporative IPS may run wet and require runback ice evaluation.
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APPENDIX H. ICE PROTECTION SYSTEMS (CONTINUED)
Adverse aerodynamic effects may result from ice accretion roughness on the protected and unprotected surfaces before IPS activation and its full effectiveness. (Figures R-1 through R-3 in appendix R of this AC show examples of ice accretion roughness that may exist before the IPS becomes fully effective.) Preactivation ice accretion roughness on these surfaces may be significant. You should address preactivation ice accretion roughness aerodynamic effects during the airworthiness approval of IPSs. You should also evaluate intercycle ice roughness and shapes for deicing IPSs during certification.
Detailed descriptions of ice protection methods and systems are in Chapter III of the FAA Aircraft Icing Handbook, DOT/FAA/CT-88/8-1, dated March 1991 (Reference H1). Most of this material is also available at the Electronic Aircraft Icing Handbook (EAIHB), web site, http://aar400.tc.faa.gov/Programs/FlightSafety/icing/eaihbk.htm The next section provides a review of aircraft IPSs. This review includes comments relevant to their compliance with regulatory requirements.
H.1.1 Ice Protection Technologies.
H.1.1.1 Thermal Ice Protection Systems.
Thermal IPSs may operate as either anti-icers or deicers. The protection surface of a thermal IPS is typically heated to temperatures above freezing. Thermal IPS designs that heat the protection surface enough to evaporate the impinging liquid water drops for all required icing conditions are called “evaporative” thermal IPSs. Thermal IPSs that heat the protection surface only to temperatures above freezing are called “running wet” thermal IPSs. This IPS technology allows the impinging drops to flow on the surface without freezing. Running-wet thermal IPSs may evaporate impinging liquid water drops for portions of the required icing conditions.
Applicants often select evaporative IPSs over running wet IPSs since they do not need to consider frozen run-back water with evaporative systems. However, evaporative IPSs require greater thermal energy. You may provide the thermal energy required electrically or by heated air. Because of the high heat energy required by thermal ice protection, electric thermal ice protection is typically used for small surfaces (propellers, air data probes, control surface horns, windshields, carburetor components, and turboprop engine inlets). It is also used for other surface areas that are not easily protected with heated air (helicopter rotor blades). Heated air for hot air thermal IPSs is typically bled from turbine engines. You may salvage the hot air or fluids from other heat sources, including oil coolers and heat exchangers. High by-pass ratio turbofan engines with their small turbines provide limited hot air. This scarcity of thermal energy is leading to an increased use of running-wet thermal IPSs and to a decrease in the extent of the airframe ice protection. For example, on many recent transport airplane designs there is no protection on the horizontal and vertical stabilizers and wing leading edges may have limited protection.
You should determine the energy required for safe flight of the aircraft. You may do this through flight tests in critical icing conditions. This approach is costly, time-consuming, and runs the risk of not finding critical icing conditions. Alternatively, you may use thermal H-2
APPENDIX H. ICE PROTECTION SYSTEMS (CONTINUED)
analyses, verified by dry air and natural icing flight tests with instrumented aircraft. These analyses are complex since the heat required is a function of many parameters, including:
x Evaporation of the impinging water loading.
x The target surface temperature distribution for a running-wet IPS.
x External airflow speed and temperature.
x Heat distribution on the protection surfaces.
x Heat transfer between the freestream and heated surfaces.
x Radiant heat transfer.
x Internal heat transfer. (For heated air systems, internal heat transfer may be a function of engine power settings, altitude, ambient temperature, and system losses.)
x The heat conduction and convection coefficients of the protection surface material.
You may use computer codes, analytical methods, or previous experience in the analyses to estimate the required heat. SAE ARP5903 (Reference H2) contains guidance on using computer codes. (References H3 through H5 contain more information on determining the required heat.)
Typically, you perform thermal analyses for IPS components; however, you should integrate the component analyses into a single analysis for the complete thermal IPS. This should include all duct heat losses (for hot air IPSs) to determine the available and required IPS heat. Analysis should also show that temperatures resulting from the IPS, including ducting immediately downstream of the engine bleed ports, do not cause fire hazards or adversely affect the integrity of the structure.
Because you may need estimates in the analyses, you can determine empirical corrections for the estimates using icing wind tunnels and icing tankers (SAE ARP5905 (Reference H6) and ARP5904 (Reference H7)). You should use dry and wet air flight tests to verify the heat required to achieve the needed IPS surface temperatures.
Because of the influence of the water loading on the energy required for thermal IPSs, you may consider the icing conditions defined by appendix C more important for determining the design of thermal IPSs than mechanical IPSs.
Typically, dedicated energy sources provide the energy for thermal IPSs. The energy may be provided by engine-driven electric generators or alternators, or by extraction of hot air from the turbine engine compressor. The needed thermal energy may result in a significant loss of engine thrust and a significant increase in fuel flow. You should verify the bleed air extraction required by the IPS against the engine installation handbook for compatibility. While most engines incorporate flow-limiting valves in the bleed ports, these valves are usually designed to choke at H-3
APPENDIX H. ICE PROTECTION SYSTEMS (CONTINUED)
higher flows than those expected for normal operation. You must show the aircraft performance losses resulting from the engine-bleed air in the AFM. Also, ensure the needed bleed air is available at low thrust levels. This may require the engine to run at higher thrust levels during some phases of flight (such as during descent). Also, ensure the needed bleed air is available during one-engine-inoperative operations. This may result in IPS design or procedure changes.
For some type designs, there may be a requirement to use the autothrottle to keep the necessary engine thrust and bleed airflow. This will ensure acceptable ice protection during descent.
Thermal IPSs may operate sequentially or periodically as deicing systems to reduce energy requirements. You must show the adequacy of the heat source to provide satisfactory ice protection for safe aircraft operation in the icing conditions of 14 CFR parts 25, Appendix C and 29, Appendix C. Also include the required IPS procedures in the AFM.
“Running-wet” thermal airframe IPSs may result in runback ice accretions. You should consider runback-ice accretions as part of the surface roughness expected during normal operations in icing conditions. Ice accretion shapes resulting from runback water are difficult to predict analytically. You may need to test in natural icing conditions, simulated icing conditions, or in an icing wind tunnel to determine the extent and characteristics of runback ice. Section H.2 of this appendix, and References H3 through H5 contain more information on design methods that will reduce runback icing.
Perform similar thermal analyses to show the adequacy of electrical energy sources. These sources should provide enough thermal energy to protect surfaces during inflight icing. Also pay attention to showing compliance for related regulations, like pilot visibility, air data probes, propellers, and induction systems.
You should consider safe aircraft operation in icing conditions when failures of electric generators occur. You should also consider failures of the engine bleed air valves and controls, the cross-over duct valve and control, and IPS sequencing equipment to ensure symmetric ice protection on both sides of the aircraft during one-engine-inoperative operations. Also, review the reliability of the protection surface temperature controls to ensure the protection surface’s structural integrity.
You should evaluate preactivation surface roughness—resulting from ice accretion that collects before the IPS becomes fully effective after activating the IPS following AFM procedure (paragraph 6.2 of this AC). Appendix R, section R.1.1 of this AC, contains added information about preactivation ice shapes. Section 8.2, of this AC, contains means to evaluate aircraft safe flight with preactivation ice accretion.
You may use airframe thermal IPSs as deicers. This reduces the ice protection energy requirements. As with any deicer, intercycle ice accretion on the protected surfaces will occur.
You should evaluate the effects of the intercycle ice accretion roughness. Assess intercycle surface roughness when showing safe inflight icing operation of the aircraft. You should pay attention to adverse aerodynamic effects caused by sequential deicing and asymmetric ice buildups. Consider guidance presented in appendix Q of this AC for the presence and shedding of the intercycle ice when using thermal IPSs as deicers.
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APPENDIX H. ICE PROTECTION SYSTEMS (CONTINUED)
Electrothermal ice protection is typically used for ice protection of air data sensors. Ice bridging may occur with local thermal ice protection. Also, for running-wet thermal IPSs, runback ice may grow forward and bridge the heated surface. Applicants should address this issue during the design of thermal IPSs Reference H8 provides information about rotor blade ice protection and rotorcraft IPS design.
H.1.1.2 Mechanical Ice Protection Systems.
Mechanical IPSs use mechanical energy to shed ice from protection surfaces. Inflating, deflating, and flexing of elastic tubes apply mechanical energy to break the adhering ice.
Applicants can also use mechanical force transferred through aircraft surfaces to break the adhering ice. The freestream shear force and the mechanical energy transferred to the ice then cause the ice to shed. Since the IPSs allow ice to build up before shedding, these IPSs are deicers.
Pneumatic deicing boots use rapid inflation, deflation, and flexing of elastic tubes to break and eject adhering ice. The elastic tubes are installed streamwise or chordwise.
The mechanical energy may be delivered electrodynamically by dynamically charging ribbonlike coils rigidly supported beneath the protection surface. The strong, short-period electromagnetic field formed by the charged coils induces eddy currents in the aircraft surface above the coil. The eddy current causes the aircraft skin to respond to the strong repulsive electromagnetic force field. The small but rapid skin deflection of the electro-impulse deicing (EIDI) IPSs provides a sharp force to break and eject adhering ice.
Alternatively, impulsive movements of the protection surface can deliver the debonding and ejecting mechanical force of an electroexpulsive deicing system (EEDS). This impulsive surface movement is caused by pulsing an electrical current in opposite directions through closely spaced parallel conductors, or conductive layers, embedded in a nonconducting elastomeric blanket beneath the protection surface. An eddy current deicing system (ECDS) provides an impulsive movement of the protection surface by a reaction to electrical current pulsed through planar coils embedded within and spanwise along the leading edge of the protected surface.
Deicing IPSs, such as EIDI, EEDS, and ECDS, may produce strong electromagnetic fields.
Consider the effects of these electromagnetic fields on the operation of other electrical or magnetic equipment, such as high-lift devices’ magnetic near-far sensors, magnetic compasses, and shielding of wiring and electrical controls (14 CFR §§ 23.1327, 23.1351, and 23.1547 and 14 CFR §§ 25.1327, 25.1351, and 25.1547). Also, these types of IPSs typically operate with brief, but high levels of energy. These high-energy levels could produce a significant ignition source during failures of either the IPS or in combination with external events (for example, bird strikes). You should evaluate aircraft level flammable fluid and system safety requirements when installing these types of IPSs close to flammable fluids (14 CFR §§ 23.853, 23.863, and
23.1309; 14 CFR §§ 25.853, 25.863, and 25.1309; 14 CFR §§ 27.853, 27.863, and 27.1309;
14 CFR §§ 29.853, 29.863, and 29.1309; 14 CFR § 25.981; and SFAR 88).
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APPENDIX H. ICE PROTECTION SYSTEMS (CONTINUED)
An emerging technology uses hydrogen and oxygen pressure produced by electrolysis of water.
Electrolysis of ice melted at the boundary of the ice accretion and the protection surface debonds and ejects the remaining ice accretion. This technology is not mature enough to be considered for an aircraft ice protection system. Of all these mechanical ice protection technologies, pneumatic deicing boots are most commonly used.
The ice shedding efficiency of mechanical IPSs depends on many factors, including:
x The strength of the bond between the ice and the protection surface.
x The shearing force of the local airflow.
x The fracture strength and modulus of elasticity of the adhering ice.
x The ability of the IPS to break the ice into small enough particles so the local airflow shears them from the protective surface.
x The ability of the mechanical force and elasticity of the surface material to peel and eject the broken ice.
x The characteristic dimensions of the IPS components.
x The variations of the protection surface’s modulus of elasticity and pliability with temperature.
x Characteristics of the applied mechanical force.
x The surface condition of the protection surface.
x The intercycle interval and thickness of the adhering ice.
x Characteristics of the applied air pressures (inflation and deflation, and inflated dwell time).
Because of the many variables involved, the ice shedding efficiency of mechanical IPSs depends on the aircraft design.
Because of a lack of acceptable analysis methods and our inability to test full-size aircraft in icing wind tunnels, you may need to perform full-scale flight tests in natural or simulated icing conditions, or icing tunnel tests with full-scale wing sections or engine inlets. SAE ARP5904 (Reference H7) provides useful information about the use of icing tankers for evaluating deicing efficiency. You should evaluate changes to the IPS that change the IPS’ ice shedding efficiency.
Appendix Q of this AC provides more information on adverse effects that may result from ice shedding.
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APPENDIX H. ICE PROTECTION SYSTEMS (CONTINUED)
Applying icephobic material can significantly reduce ice adhesion to the deicing boot surface.
However, operators need to reapply these materials periodically to ensure their effectiveness. Do not use icephobic material on deicing boots for certification. This is because of the difficulty of ensuring proper use of these materials during normal, in-service operations. Another reason is the variability of material’s icephobic effectiveness as the material erodes from the boot surface during normal operations.
The protection surface roughness of mechanical IPSs depends on the residual ice that remains on the surface after cycling of the IPS and the intercycle ice that collects between cycles. You should consider that this protection surface roughness is present during normal operation of the aircraft in icing conditions. You should consider effects of the residual ice that remains on the aircraft after exiting icing conditions and cycling of the deicing IPS on the aircraft’s flying qualities and stall protection. You should describe the characteristics of the normal-operations protection surface roughness. You should determine these ice buildup characteristics by icing wind tunnel tests, or during natural or simulated icing flight tests. The character of the normaloperation protection surface roughness varies with the mechanical IPS’s design, how it is operated, the aircraft configuration, how the aircraft is operated, and the icing conditions.
Appendix R, section R.1.2, of this AC, has more intercycle ice information.
You must ensure that the pneumatic deicing boots are effective during continuous flight in the icing conditions defined by 14 CFR parts 25 and 29. The horizontal extent of icing clouds do exceed the arbitrary standard horizontal extents of 17.4 nm and 2.6 nm defined in Appendix C of 14 CFR parts 25, Appendix C and 29, Appendix C. Showing the effectiveness of the IPS for only the number of cycles required to transit those distances is not enough for complying with the intent of § .1419 of 14 CFR parts 23, 25, 27, and 29.
Deicing boot ice bridging and the ice shedding characteristics of early deicing boot designs have led to the recommended procedure of cycling deicing boots after collecting ¼ to 1½ inches of ice on the boot surface. This practice originates from the belief that a bridge of ice could form if the boots are operated prematurely. Classical deicing boot ice bridging occurs when a thin layer of ice is plastic enough to deform to the shape of the inflated deicing boot tube without breaking or shedding following tube deflation. As the deformed ice shape hardens and builds up added ice, the deicing boot becomes ineffective in shedding the “sheath” of ice. Icing tunnel and natural icing flight tests have shown the concern over ice bridging is considered unnecessary with modern boots. Modern boots are defined as those that use small diameter tubes (up to
1.75 inches), nominal working pressures of 15 psig and higher, and fast inflation and deflation times.
It is difficult for the flightcrew to determine the thickness of the ice accurately by visual observation. Also, constant checking of the ice thickness may unacceptably increase the pilot’s workload. Therefore, the recommended AFM procedure for boot operation should be to operate the boots at the first sign of ice accretion and not wait for a specific amount of ice to collect.
The critical intercycle ice roughness may be sensitive to temperature and altitude. At colder temperatures the adhesive characteristics of rime ice are high and the ice is rough. An “alligator skin-like” surface roughness may occur as fractures of the residual ice act as “seeds” for added H-7
APPENDIX H. ICE PROTECTION SYSTEMS (CONTINUED)
ice accretion between deicing boot cycles. Also, temperature may affect the flexibility of boot material. Near freezing temperatures often result in aerodynamically adverse ice ridges that develop from residual ice along the trailing edges of individual deicing boot tubes. These ice ridges result from residual ice that remains on the aft portion of individual deicing tubes if the ice is not shed by the lower local airspeed (and shear) when the tube inflates. The localized ice accretion acts as an efficient ice collector for the impinging and runback water during the periods of tube deflation and inflation. You should show the effectiveness of the boots in flight throughout the altitude range of the required icing conditions. Operate the boots at the coldest temperature of the required icing conditions to show proper operation of the boots and to verify boot structural integrity.
Many inflight icing accidents and incidents occur during near-freezing icing conditions. If you base the boot air supply on gage air pressure, you should evaluate ice shedding performance and intercycle ice roughness throughout the temperatures and altitudes defined by 14 CFR parts 25, Appendix C and 29, Appendix C or as limited by the aircraft’s flight envelope. Because boot pressure is dependent on bleed air mass flow, you should use dry air flight-testing to show boot operation and pressures at minimum engine power settings. You should perform this demonstration at the altitudes defined by 14 CFR parts 25, Appendix C and 29, Appendix C or as limited by the aircraft’s operational envelope. You must provide the minimum engine power setting, if determined, in the AFM. You should address deicing boot air pressure variation resulting from air pressure regulator tolerances when you show the effectiveness of the deicing boots. Also, some cockpit annunciation lights are set below these tolerances. You should determine ice shedding performance and intercycle ice roughness at these lower pressures.
You should consider the potential for liquid water collection in the pneumatic deicing boots.
This water may freeze and prevent proper operation of the boots. Examine the pneumatic boot arrangement for low points, which may collect water. You should also consider installing water drains. Provide periodic inspection and drainage procedure instructions in the proper manuals.
Drainage or “weep” holes may become blocked on some aircraft. Therefore, you should provide water/air separators or heaters to prevent blockage. You should evaluate the effectiveness of water/air separators and drainage holes by flying through rain, followed by flight at altitudes with temperatures below the freezing point.
System failure analyses of mechanical IPSs should include an assessment of the deicing boots’ pressure control valves, and the annunciation of IPS failures to the flight crew.
The boots should not result in hazardous effects on aircraft performance and handling qualities.
Perform tests to evaluate possible hazardous changes to handling qualities and performance when the boots are operated. This is necessary because inflated leading-edge pneumatic deicing boots distort the surface’s contour and change surface flow conditions. You should evaluate these effects on the aircraft’s handling qualities by running the boots at speeds ranging from stall speed to (VNE+VD)/2 or (VMO+VD)/2. Include any deicing boot limits, such as altitude, temperature, airspeed, aircraft configuration, or flight phase, in the AFM. Also include information about anomalous aircraft behavior associated with operation of the deicing boots.
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APPENDIX H. ICE PROTECTION SYSTEMS (CONTINUED) H.1.1.3 Fluid Ice Protection Systems.
Fluid (freezing point depressant) IPSs introduce a freezing point depressant agent, typically a glycol compound, on the surface at the leading edge. The freezing point depressant causes the impinging water to have a freezing point lower than the local airflow temperature. The impinging water mixture then evaporates or sheds from the surface. The freezing point depressant fluid may be introduced on the protection surface by being:
x Pumped through a porous surface;
x Channeled along the protection surface in grooves by centrifugal force;
x Sprayed from external spray bars; or x Contained in compounds (such as grease) that have been applied to the protection surfaces, such as propellers.
This ice protection technology is associated with products developed during World War II by TKS Ltd., a conglomerate of Tecalemit Ltd., Kilfrost Ltd., and Sheepbridge Stokes in Great Britain. However, systems made by other companies are in current use. These systems work by pumping a liquid freezing point depressant through the porous skin of the protection surfaces.
Applicants can use the IPS as an anti-icer or a deicer. These systems also have the potential of being simple and reliable; however, the supply of fluid limits the protection. You should ensure an acceptable supply of fluid and evaluate the effectiveness of the IPS within the required icing conditions, for all phases of flight, and for the needed aircraft configurations. Also consider fire hazards and possible fire extinguisher requirements associated with the ice protection fluid (14 CFR §§ 23.863 and 23.1199(b)).
H.1.1.4 Icephobic Ice Protection Systems.
Icephobic materials are occasionally considered for use as an aircraft IPS. Typically the hydrophobic material is applied to the protection surface. The objective of the icephobic material is to prevent supercooled water and ice from adhering to aircraft surfaces. This ice protection technology is not commercially available. As discussed in paragraph H.1.1.2, of this appendix, appliances may use icephobic silicon compounds to supplement the ice shedding effectiveness of mechanical IPSs. However you may not take credit for its use during approval of the IPS since preserving its effective use is difficult to control.
H.1.2 Ice Protection System Operation.
All IPSs are activated by some means to ensure safe flight in icing conditions. This may be accomplished by:
The flight crew seeing the presence of icing conditions or ice buildup on monitored surfaces and then activating the IPS.
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