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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FIGURE 12. WING BOOT All Tubes Deflated
All Tubes Inflated
4-15. MAINTENANCE CONSIDERATIONS. Some anti-icing and deicing systems are known to be very reliable, while others may require a lot of maintenance to remain effective.
Pneumatic boots, for example, are known for their susceptibility to damage from many sources and should be inspected carefully. The rubber used for the boots is subject to degradation from atmospheric pollution, which results in the rubber cracking and losing some of its elastic properties. An ice adhesion inhibitor should be applied to pneumatic deicing boots in accordance with the maintenance manual. Any product that is not recommended by the airplane or boot manufacturer should be approved by the FAA. Other problems are defects, delaminations, or tears in the rubber caused by the impact of objects, such as foreign matter found on airport ramps. Pinholes or tears in pneumatic deicing boots will draw in moisture when system vacuum is supplied, and subsequent freezing of this moisture can render the system ineffective. Pilots should have maintenance personnel evaluate any defects in the boots when they are found.
(1) Flightcrews always should ensure their airplane’s anti-icing and deicing equipment is in a serviceable condition.
(2) The airplane’s MEL should be consulted for details on what is permitted to be inoperable and what equipment deficiencies constitute “no go” items.
4-16. ICE DETECTION.
a. Electronic.
(1) Many modern aircraft come equipped with electronic ice detectors. A common in-flight ice detector consists of a probe that vibrates at a specific frequency. When ice begins to form on the probe, the frequency of the probe’s vibration will change because of the increased mass of ice on the probe, and an indicator will light in the cockpit. These detectors are activated for a short time period, generally one minute, after which the probe is heated electrically to melt the accreted ice. The process is then repeated. If the aircraft is flying in continued icing conditions, ice will continue to form on the probe, and the light in the cockpit will remain on.
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(2) Pilots should consult their AFM or POH to determine if their ice detection system is an “Advisory” or “Primary” system. The difference between the systems is the redundancy of the system and testing required for certification. The majority of airplanes have an “Advisory” system, which means the pilot is responsible for detecting ice and ensuring ice protection systems are activated. This is true even for ice protection systems that are automatically activated when the ice detection system detects ice.
(3) Currently, there are no electronic detection systems that can reliably detect ice crystals, although new systems are under development.
b. Visual. Strategically located protuberances also may serve as ice indicators. There is usually some unprotected protuberance or surface visible to the crew, such as windshield wipers, pod pylons, or landing lights that can serve as an icing reference because they tend to build up ice first. If there is no such protuberance visible to the crew, manufacturers may provide one for this purpose. These ice detectors, referred to as “ice evidence probes,” are typically in plain view of the cockpit and normally collect ice more readily than larger components or surfaces. If ice begins to accumulate on such an ice detector, the flightcrew should assume the rest of the aircraft also is accumulating ice and take appropriate action. These detectors only serve their purpose if pilots include them in their scan during flight in potential icing conditions. Pilots should monitor critical surfaces at temperatures near freezing, since ice may form on critical surfaces prior to forming on visual ice indicators.
4-17. VISUAL CLUES OF SLD CONDITIONS. If SLD is known to be present, most aircraft with unpowered controls and airframe deicing systems should request a route or altitude change to exit the conditions. This action may be prudent for other aircraft as well. The cues listed below were developed for aircraft with unpowered controls and pneumatic deicing boots as indicative of SLD conditions. Of most concern is the accretion of ice in areas aft of where it would usually be found. Such “aft accretions” could sometimes be the result of runback due to high liquid water rather than SLD. Excessive runback icing, however, may have effects similar to SLD, so similar pilot action may be appropriate. The cues are:
a. Ice may become visible on the upper or lower surface of the wing, aft of the active part of the deicing boots. Pilots should look for irregular or jagged lines of ice or for pieces of ice shedding off the airplane. During night operations, adequate illumination should be used to observe all areas. On most airplanes, the last inch of the deicing boot is inactive.
b. The aft limit of ice accumulation on a propeller spinner that is not heated will reveal ice extending beyond normal limits, typically back to the blades.
c. Unheated portions of side windows may begin to accumulate granular dispersed ice crystals or a translucent or opaque coating over the entire window. This icing may be accompanied by other ice patterns on the windows, such as ridges. These patterns may occur from within a few seconds to half a minute after exposure to SLD conditions.
d. Ice coverage may become unusually extensive, with visible ice fingers or feathers on parts of the airframe that normally would not be covered by ice. The aircraft’s performance may Page 32 Par 4-16
Par 4-17 Page 33 (and 34) degrade. Pilots should remain vigilant when icing conditions are present, and any alteration of the aircraft’s performance should be monitored closely as a sign of icing on the airplane.
NOTE: Pilots should be vigilant for the ice accretions listed above when the following are observed:
• Visible rain or drizzle at temperatures below +5°C outside air temperature (OAT).
• Droplets that splash or splatter on impact at temperatures below +5°C OAT.
(1) Vigilance for SLD ice accretions should also be exercised when flying into or over areas reporting precipitation at the surface, such as rain, freezing rain, sleet, ice pellets, drizzle, freezing drizzle, or snow, where temperatures are near freezing. However, pilots should be aware that SLD could occur aloft without any SLD precipitation on the surface. Current weather information can miss SLD, so it is important to know and watch for cues on the airplane.
(2) While the pilot should be aware of these general cues, there may be specific cues that are characteristic of SLD icing on particular aircraft types. The pilot should consult the aircraft AFM or POH for descriptions of any such cues.
Par 5-1 Page 35 CHAPTER 5. FLIGHT PLANNING This chapter describes sources of flight planning information as well as elements pilots can identify from the material made available to them to adequately manage their in-flight icing risk.
5-1. GENERAL AVIATION RESOURCES.
a. The primary means available to general aviation pilots for obtaining preflight planning information is the FSS. An FSS briefer is available via telephone by dialing 1-800-WX-BRIEF (1-800-992-7433). The briefer will provide weather information that may include the location of frontal systems, available PIREPs, cloud coverage, temperatures, and wind direction and speed.
b. Another source of information is the Direct User Access Terminal (DUAT), which is an information system that enables pilots to conduct their own weather briefings. The computer-based system acquires and stores a number of NWS and FAA products commonly used in pilot weather briefings and can be accessed through the World Wide Web. Pilots also can file and amend flight plans while logged into the system. Further information about DUAT can be obtained from any FSS, Flight Standards District Office (FSDO), or on the Web.
5-2. AIR CARRIER DISPATCH SERVICES.
a. U.S. air carriers that are authorized to conduct domestic or flag operations must have professional dispatch services that provide their pilots with relevant weather data for each flight.
b. Sometimes, these dispatchers rely on forecasts developed by the NWS, but some air carriers have an expanded capability, with their own meteorologists on staff who analyze raw data in addition to available forecasts to build a more detailed picture of existing and predicted weather conditions, including icing conditions.
NOTE: Title 14 CFR part 121 supplemental and part 135 commuter/on-demand operations do not require dispatchers.
5-3. PIREPs.
a. PIREPs are the only direct evidence of aircraft icing in a given area. They constitute an extremely valuable source of information for flight planning.
b. The AIM lists the following essential elements for all PIREPs: message type, location, time, flight level, type of aircraft, and at least one weather element encountered. When not required, elements without reported data are omitted. All altitudes are referenced to mean sea level unless otherwise noted. Distances are in nautical miles and time is in Coordinated Universal Time. The phenomenon is coded in contractions and symbols. A PIREP usually is transmitted as an individual report but can be appended to a surface aviation weather report or placed into groups of PIREPs.
c. To report the intensity of icing in a PIREP, the following descriptions should be used (refer to Chapter 2 for complete definitions.):
(1) Trace Icing. Ice becomes noticeable. The rate of accumulation is slightly greater than the rate of sublimation. A representative accretion rate for reference purposes is less than 1/4 inch (6 mm) per hour on the outer wing. The pilot should consider exiting the icing conditions before they become worse.
(2) Light Icing. The rate of ice accumulation requires occasional cycling of manual deicing systems to minimize ice accretions on the airframe. A representative accretion rate for reference purposes is 1⁄4 inch to 1 inch (0.6 to 2.5 cm) per hour on the outer wing. The pilot should consider exiting the condition.
(3) Moderate Icing. The rate of ice accumulation requires frequent cycling of manual deicing systems to minimize ice accretions on the airframe. A representative accretion rate for reference purposes is 1 to 3 inches (2.5 to 7.5 cm) per hour on the outer wing. The pilot should consider exiting the condition as soon as possible.
(4) Heavy Icing. The rate of ice accumulation requires maximum use of the ice protection systems to minimize ice accretions on the airframe. A representative accretion rate for reference purposes is more than 3 inches (7.5 cm) per hour on the outer wing. Immediate exit from the conditions should be considered.
(5) Severe Icing. The rate of ice accumulation is such that ice protection systems fail to remove the accumulation of ice and ice accumulates in locations not normally prone to icing, such as areas aft of protected surfaces and areas identified by the manufacturer. Immediate exit from the condition is necessary.
5-4. PIREP CAUTIONS. Although PIREPs are excellent sources of information on in-flight icing, there are situations when these reports can be misleading. Two possibilities are discussed below.
a. An aircraft encounters icing conditions in an area where there were no recent icing PIREPs. There are several possible reasons for this:
(1) No aircraft recently flew in the area.
(2) Some aircraft recently flew in the area but did not encounter the icing conditions.
This is a common occurrence, especially if the area has limited air traffic. Icing conditions are extremely variable in both space and time. A slight change in altitude or flightpath or the passage of just a few minutes can mean the difference between encountering and not encountering icing.
There are many documented cases of aircraft flying through approximately the same area at similar altitudes at approximately the same time with one aircraft experiencing substantial icing and the other experiencing none.
b. An aircraft encountered icing, but the pilot did not report it. An aircraft encounters icing conditions that are more serious than those are reported in any recent PIREPs in the given area.
c. Again, there are several possible reasons for this.
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(1) Icing conditions are extremely variable in space and time, as previously noted.
PIREPs depend on the type and ice protection of the reporting aircraft. If the pilot’s aircraft is smaller, has less power, or has less ice protection than the reporting aircraft, it may experience more serious icing than the reporting aircraft in the same exact meteorological conditions. For example, a Boeing 747 may report light icing when flying through conditions that would cause a Mooney to experience severe icing.
(2) PIREPs are subjective, depending on the pilot’s observations, how the pilot operates the ice protection on the aircraft, and the pilot’s experience level with in-flight icing.
For example, there are documented cases of pilots reporting “light” icing conditions when ice was accreting on an ice evidence probe at a rate of approximately 1 inch per minute. In addition, observation and assessment of icing is more difficult at night.
(3) Although PIREPs from similar aircraft are most relevant to the pilot’s aircraft, direct translation to the pilot’s aircraft may still present difficulties. In addition to pilot subjectivity, other relevant questions are: Was the reporting aircraft flying slowly, or climbing, at a high AOA (which is conducive to accumulation over a larger area of the aircraft)? What kind of ice protection does the reporting aircraft have, and is it functioning properly?
(4) When icing conditions exist, reporting may alert other crews to maintain vigilance. Flightcrews should ensure that when submitting a PIREP of observed icing conditions, they accurately state the conditions and effects of the icing observed and report them in a timely fashion to make the PIREP as useful as possible.
5-5. ICING FORECASTS.
a. Aviation meteorologists at the NWS Aviation Weather Center (AWC), local NWS Field Offices, major airlines, and private companies prepare icing forecasts. In addition, guidance products are available on the Web. Icing AIRMETs, prepared at AWC, cover a 6-hour forecast window and are updated four times daily. Because they cover an extended time and may cover an area of several states, they are necessarily somewhat broad. Consulting a local FSS or NWS for a more specific preflight briefing will provide more detail and allow interaction with the briefer for more specific questions.
b. Technologies for forecasting icing are undergoing rapid development. PIREPs remain the major source of information for icing location and severity. The forecaster matches the PIREP to local weather features, assesses where those features will be moving during the forecast period, and estimates areas of expected icing, assigning their type and severity in this manner. Automated methods for assessing these icing attributes have been developed and are now available. (See the discussion on Current and Forecast Icing Product (CIP and FIP) later in this section.)
5-6. TERMINOLOGY USED IN ICING FORECASTS.
a. The AWC uses the same terms for icing intensity in its forecasts as those given in the AIM for use in PIREPs. These forecasts are based on:
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(1) Weather analysis, using both traditional tools and those developed in recent years by the research community.
(2) PIREPs.
b. Since icing intensity is aircraft-dependent, a standard needs to be adopted by the NWS for all icing forecasts. Currently, there is one standard for AIRMET in which light or moderate icing is related to anticipated operational effects on a reciprocating engine, straight wing aircraft of intermediate size. Nevertheless, pilots of other aircraft are expected to familiarize themselves with AIRMETs also. However, there is no similar standard for severe icing in SIGMETs. Therefore, any SIGMET for severe icing must be interpreted as forecasting severe icing for all classes of aircraft.
c. Icing PIREPs are valuable to forecasters for the same reasons they are valuable to pilots. The PIREPs constitute the only direct observation of the phenomenon that is forecast.
Thus the weather forecasters carefully evaluate the results of their weather analysis against the PIREPs for the forecast area and modify their forecasts if necessary. SIGMETs for severe icing are rarely issued unless there has been a PIREP of severe icing in the forecast area. PIREPs are also used in the CIP algorithm, described in the next chapter.
d. Icing forecasts also use the previously defined terms clear, rime, and mixed to describe types of ice. A convention that has evolved in forecasting practice is to associate ice type mainly with cloud drop size, essentially ignoring other factors such as temperature, liquid water content, drop size, and aircraft velocity. Thus a forecast of rime icing (RIME ICG) indicates smaller drop sizes, whereas a forecast of mixed (MXD ICG) or clear (CLR ICG) indicates larger drop sizes, with the line of demarcation not firmly established.
5-7. CURRENT AND FORECAST ICING PRODUCTS (CIP AND FIP).
a. There is a new Web-based preflight planning tool available to pilots that gives a graphical presentation of the current and forecast potential for icing. The URL for this product is http://adds.aviationweather.gov/icing/.
b. The CIP represents the latest analysis of possible icing regions. CIP combines weather sensor and numerical model data, as well as PIREPS, to provide a three-dimensional diagnosis of the icing environment.
(1) The new version of CIP output consists of a probability of icing and a severity of icing, as well as an indication of the possible presence of SLD.
(2) Water drops larger than 50 micrometers (diameter), which includes freezing drizzle and freezing rain aloft characterize SLD icing conditions. It has been demonstrated that these conditions, which are outside the icing certification envelopes (part 25, appendix C) can be particularly hazardous to some aircraft. Thus the information can be valuable for flight planning.
c. The FIP is an automatically generated forecast of the potential for icing 2, 3, 6, 9 and 12 hours in the future. This icing potential demonstrates the confidence that an atmospheric location, represented by a three-dimensional model grid box, will contain supercooled liquid Page 38 Par 5-6
Par 5-7 Page 39 water that is likely to form ice on an aircraft. The algorithm analyzes the model output from a vertical column, determines the cloud top and base heights, checks for embedded cloud layers, and identifies a precipitation type. Once the likely locations of clouds and precipitation are found, the physical icing situation is determined, and a physically-based, fuzzy logic method is used to determine the icing potential. The fuzzy logic interest maps are based on clues from the model output reflecting relevance to the presence of icing. After the information is extracted, the interest maps are combined in a manner that reflects their significance for icing, given the physical situation present. The entire model domain is examined and the result is a three-dimensional depiction of the potential for icing and SLD at the model valid time. Note that plans are for FIP to depict icing probability and severity, similar to CIP, in mid-2008.
5-8. PREFLIGHT PLANNING INFORMATION. All pilots, whether they are general aviation (GA) or air carrier pilots, are responsible for obtaining as much information as possible about all meteorological conditions, including icing conditions, before departure. Some important meteorological facts that pilots should be aware of with respect to icing are described below.
a. Location of Fronts. Fronts play an important part in the formation of icing conditions. Pilots should be aware of a front’s location, type, speed, and direction of movement.
Pilots should try to keep a mental picture of where the front is moving and look for indications of frontal activity or frontal passage, such as a wind shift or temperature change.
b. Cloud Layers. While information on the types of clouds present may not be available without direct observation, the bases and tops of clouds might be known. This information is valuable because pilots need to know whether they will be able to climb above the icing layers of the clouds or descend beneath those layers into warmer air if icing is encountered.
c. Freezing Levels. It is critically important for pilots to obtain the freezing levels for the areas in which they will be flying to be able to make educated decisions on how to exit icing conditions if they are encountered. It is also important for pilots to know if there are any temperature inversions aloft that might alter the normal relationship between altitude and air temperature. Pilots should be aware of multiple freezing levels and their locations. The National Weather Service Aviation Digital Data Service (ADDS) Web site at http://adds.aviationweather.gov/icing/ provides a graphical depiction of the freezing level.
d. AIRMET and SIGMET. An AIRMET is information of significant weather phenomena, but describes conditions at intensities lower than those described that trigger a SIGMET. An AIRMET is intended for dissemination to all pilots in the preflight and en route phase of flight.
• SIGMETs and Convective SIGMETs advise of weather that is potentially hazardous for all aircraft, such as severe icing. A SIGMET for severe icing applies to all aircraft, from small GA aircraft to transport jets (see also the discussion in paragraph 5-6, Terminology Used in Icing Forecasts).
• The ADDS Web site at http://adds.aviationweather.gov/airmets provides a graphical depiction of the areas covered by AIRMETS and SIGMETS.
e. Air Temperature and Pressure. Icing tends to be found in low-pressure areas and at temperatures at or around freezing. For areas of low pressure, pilots can scan the surface analysis charts, and for freezing levels, the winds aloft chart can be used because the wind direction and speed grouping also provide air temperature at that altitude.
f. Alternatives. When contemplating flight into possible icing conditions in an aircraft approved for flight in icing conditions, a major consideration of preflight planning is to have alternative courses of action if conditions are worse than expected. These alternatives could be a change in altitude, heading, airspeed, or an alternate airport with adequate runway length.
g. It is important to note that aircraft that are approved for instrument flight rules (IFR) operations but not certified for known icing conditions are not testing during the certification process for inadvertent icing encounters. Therefore, pilots in such aircraft should emphasize ice avoidance during preflight planning and pay special attention to planning an alternate course of action in case actual icing is encountered.
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