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.
Page 36 Par 5-3
(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:
Par 5-4 Page 37
(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.
Page 40 Par 5-9
CHAPTER 6. IN-FLIGHT OPERATIONS
6-1. AVAILABLE IN-FLIGHT INFORMATION.
a. Flight Watch. There are numerous sources of meteorological information available to pilots while in flight. A principal source of this information is Flight Watch. Flight Watch is an FSS-provided en route flight advisory service designed to provide, upon request, timely weather information pertinent to the type, route, and altitude of flight. Flight Watch is available from 6 AM to 10 PM local time at altitudes generally above 5,000 feet above ground level (AGL). For weather information at other times or at lower altitudes, contact an AFSS via radio on a nearby frequency outlet as depicted on aeronautical charts. The FSSs providing this service are listed in the Airport/Facility Directory and can be contacted on radio frequency 122.0 MHz.
b. Hazardous In-flight Weather Advisory Service. The Hazardous In-flight Weather Advisory Service (HIWAS) provides continuous, recorded hazardous in-flight weather forecasts over selected VHF omnidirectional radio (VOR) outlets within the HIWAS broadcast area. This broadcast area is a geographical zone of responsibility, including one or more HIWAS outlet areas (defined as the area within a 150-nautical mile radius of the HIWAS outlet) assigned to an FSS for hazardous weather advisory broadcasting.
c. Transcribed Weather Broadcast. The Transcribed Weather Broadcast (TWEB) is a continuous recording of meteorological and aeronautical information broadcast on low/medium frequencies and VOR facilities for pilots. The TWEB is based on a route-of-flight concept that, among other information, includes adverse conditions, route forecasts, outlooks, and PIREPs that may contain useful icing-related information.
d. Air Carrier Dispatch. Air carrier flightcrews normally can contact their dispatch facilities on specified company frequencies or through their airplane’s onboard Aircraft Communications Addressing and Reporting System (ACARS). Dispatch can then relay icing information, changes in front movement or speed, or recent icing PIREPs.
e. Weather Radar. Since airborne weather radar detects raindrops, cells painted on radar should be avoided when the temperatures are at or near freezing. Airborne weather radar cannot, however, detect drizzle-size drops or cloud-size drops, and therefore should not be relied upon to detect icing in clouds or freezing drizzle. It also lacks the ability to detect small dry ice crystals that can be in heavy concentrations near convective weather systems.
f. Datalink/Satellite Radio.
(1) Pilots now have the option to subscribe to and receive real-time weather and airspace information via Sirius and XM satellite radio to their panel-mounted or handheld avionics. For a fee, these services provide graphical METARs, TAFs, SIGMETs, and AIRMETs, updated pilot reports, updated winds and temperatures aloft, information on severe thunderstorms, icing levels, and graphical displays of temporary flight restrictions.
(2) Such up-to-date information can assist pilots in avoiding hazardous weather, and identifying potentially hazardous areas of unforecast weather.
Par 6-1 Page 41
6-2. PILOT STRATEGIES. Flight into icing conditions is to be avoided if the aircraft is not certificated for flight into such conditions or otherwise equipped in accordance with § 91.527 (b).
However, even if the aircraft can, by regulation, operate into icing conditions; it should not be regarded by pilots as blanket authority to fly through any weather conducive to aircraft icing.
a. Icing in Stratiform Clouds. Because the icing conditions in stratiform clouds often are confined to a relatively thin layer, either climbing or descending may be effective in exiting the icing conditions within the clouds.
(1) A climb may take the aircraft into a colder section of cloud that consists exclusively of ice particles. These generally constitute little threat of structural icing because it is unlikely that the ice particles will adhere to unheated surfaces.
(2) The climb also may take the aircraft out of the cloud altogether to an altitude where the ice gradually will sublimate or shed from the airframe depending on the conditions. A descent may take the airplane into air with temperatures above freezing, within or below the cloud, where the ice can melt.
b. Icing in Cumuliform Clouds. Hazardous icing conditions can occur in cumulus clouds, which sometimes have very high liquid water content. Therefore, it is not advisable to fly through a series of such clouds or to execute holds within them. However, because these clouds normally do not extend very far horizontally, any icing encountered in such a cloud may be of limited duration; it may be possible to deviate around the cloud.
c. Snow.
(1) Normally snow itself is not a hazard with respect to icing, unless it begins to adhere to aircraft surfaces. If snow does begin to stick, it should then be treated as an icing encounter because ice may begin to form under this accumulation of snow. No aircraft is evaluated in the icing-certification process for this rare situation. If it occurs, the aircraft should exit the conditions as quickly as possible, coordinating with ATC as necessary.
(2) However, as discussed in paragraph 4-17, Visual Cues of SLD Conditions, freezing drizzle can co-exist with snow. If you are flying into or over areas reporting snow, it is important to understand that the presence of snow does not necessarily mean that icing conditions are not present. See following paragraph for further information.
d. Freezing Rain and Drizzle.
(1) Freezing rain forms when rain becomes supercooled by falling through a subfreezing layer of air. Thus, it may be possible, to exit the freezing rain by climbing into the warm layer.
(2) Because freezing drizzle often forms by the collision-coalescence process, the pilot should not assume that a warm layer of air exists above the aircraft. A pilot encountering freezing drizzle should exit the conditions as quickly as possible either vertically or horizontally.
The three possible actions are to ascend to an altitude where the freezing drizzle event is less Page 42 Par 6-2
Par 6-2 Page 43 (and 44) intense, to descend to an area of warmer air, or to make a level turn to emerge from the area of freezing drizzle.
e. Ice Pellets. Ice pellets by themselves are not a hazard to the airframe with respect to icing. However, a ground observation of ice pellets could indicate SLD aloft, so avoid such areas.
f. Ice Crystals. Ice crystals can exist in high concentrations at high altitudes near convective weather systems. Turbine engines can ingest and accrete some ice crystals that can ultimately cause power loss. Therefore, flying through clouds near convective weather should be avoided and engine nacelle anti-ice systems should be activated. Refer to your airplane’s AFM for specific information.
g. Communications with ATC. If an aircraft that is not approved for flight in icing conditions inadvertently encounters ice, controllers will not know if the aircraft is certificated or equipped for icing, the severity of the conditions, or what anti-icing or deicing equipment is installed on the aircraft. Therefore, it is imperative that ATC be thoroughly informed and a change in altitude or heading requested as soon as possible in order to exit the icing conditions.
(1) Communications from the pilot should focus on the icing conditions, whether an alteration of the current course and altitude is required, and, if necessary, an alternate landing site selected. In the congested airspace that exists in some parts of the country, along with the intensity of radio communications in such areas, it is possible that a pilot who encounters icing will not receive a clearance in time to exit the conditions before the safety of the flight is compromised.
(2) In such cases, pilots should declare an emergency to ATC and inform the controller of what actions are being taken by the pilot to cope with the emergency. If an aircraft certificated for flight in icing conditions encounters freezing rain or freezing drizzle, flightcrews should advise ATC and should not attempt sustained flight in these conditions. Final authority and responsibility for the safety of a flight rests with the pilot in command.
(3) Therefore, the pilot should not hesitate to reject a controller’s instructions if, in the judgment of a pilot, those instructions would result in an unsafe condition. To make correct decisions, pilots should have at their disposal as much information as possible, including the capability of the aircraft and its systems, and an awareness of the current and evolving weather situation.
CHAPTER 7. ICING CONSIDERATIONS FOR THE PHASES OF FLIGHT The following is a description of icing considerations as they apply to the phases of flight. This chapter focuses primarily on how to safely fly an aircraft certificated for flight in icing conditions, and when it is advisable to exit those conditions. The following is only a sampling of icing-related items to consider when planning a flight. Pilots should consult the aircraft’s AFM or POH for approved checklists.
7-1. PREFLIGHT. The first step in preparing for any flight is to obtain a thorough weather briefing. This becomes especially crucial when icing conditions are likely. When obtaining a weather briefing, make special note of the following items:
a. Freezing Level. Pilots should locate freezing levels on forecast maps. This will assist in developing a contingency plan in the event icing is encountered.
b. AIRMET and SIGMET. These advisories will alert a pilot to areas of moderate and severe icing. AIRMET “Zulu” is the classification of AIRMET issued for icing. SIGMETs are issued to indicate severe icing not associated with a thunderstorm. Anytime a Convective SIGMET is issued, expect severe icing.
c. PIREPs. PIREPs are an excellent source of information on icing location and type.
d. Precipitation. Knowing the location and type of precipitation forecast will assist in avoiding areas conducive to severe icing.
e. Graphical Data. Supplement your briefing by checking theADDS at:
http://adds.aviationweather.gov. (This site depicts all of the above data in easy to read color graphics, and has the CIP and FIP. The CIP and FIP are updated every hour.)
f. Icing conditions require some additional considerations.
(1) When determining routes of flight, make note of airports along the way and highlight them on the chart for easy identification in case an alternate is needed.
(2) When determining routes, consider the climb performance of the airplane and the route’s minimum altitude, particularly in mountainous terrain. Because the airplane’s climb performance will be degraded in icing conditions, consult the AFM to see if there is degradation data.
(3) Determine icing exit strategies during preflight. Determine if climbing or descending will be viable options.
(4) When choosing alternate airports, remember that if structural icing occurs, higher approach speeds and, consequently, longer runways may become necessary.
(5) Consider carrying a high intensity flashlight for use in locating ice accumulation on the aircraft at night or in low visibility conditions.
Par 7-1 Page 45
Page 46 Par 7-1
(6) Consider using a transceiver as a backup radio in case of a communications loss caused by an antenna icing up and/or breaking off.
(7) If the aircraft is loaded near maximum gross weight, climb performance will be degraded, possibly increasing time spent in icing conditions.
(8) Extra fuel may be necessary because excess drag caused by ice formation may require extra power, increasing fuel consumption.
(9) When performing an aircraft preflight inspection, remove all frost, snow, and ice from the aircraft surfaces because even very small amounts may adversely affect the aerodynamic properties of a wing. Placing an aircraft in a heated hangar is a good method of removing frost, snow, and ice; however, a pilot should ensure the aircraft is dry before removing it from the hangar to prevent the moisture from refreezing on the surface.
g. Frost may appear on an aircraft even if the air temperature is well above freezing.
This can occur if the airplane is flown in temperatures below freezing, causing the fuel to become cold-soaked. In flight or after landing, ice and frost can form on the wing in the same fashion that condensation forms on a cold soft drink can. Certain airplanes may be more vulnerable to ice formation from cold-soaked fuel than others may. An airplane sitting outside on a clear night may cause the airplane’s skin temperature to fall below freezing due to radiation cooling. If there is moisture present, frost can form on the airplane skin.
h. Frost, snow, or ice also can be removed with freezing-point depressant fluids. See AC 120-58, Pilot Guide Large Aircraft Ground Deicing, or AC 135-17, Pilot Guide Small Aircraft Ground Deicing, for discussion of the proper use of the fluids and protection provided under various environmental conditions as summarized in holdover timetables. Anti-icing fluids are designed to be shed by the time the aircraft reaches rotation speed; consequently, they provide no protection once the aircraft is airborne. Even though holdover times for freezing drizzle and light freezing rain exist, aircraft certificated for flight in icing conditions are not evaluated for flight in freezing drizzle or freezing rain.
i. Caution should be exercised when choosing an anti-icing fluid because Type II and IV fluids are not appropriate for use on aircraft with a rotation speed of less than 110 knots due to incomplete flowoff and resulting lift losses and control problems. Check the AFM to see if the use of Type II, III, or IV fluids is approved. In some cases, there may be limitations on takeoff procedure or minimum outside air temperature. If fluids are not mentioned in the AFM, consult with airplane manufacturer.
j. Ensure there is no ice that may interfere with control surface movement, braking, or steering. Check the pitot heat, pitot tube opening, and stall warning system. Check for proper functioning of any anti-icing or deicing systems. For fluid systems, make sure you see fluid along the entire leading panels. It may take several minutes to “prime” the fluid system, particularly if it has not been used in a while. Do not assume that any contamination, even snow, will blow off during takeoff. Wet snow may not blow off, and there may be a layer of ice under the snow.
7-2. TAXI. Always perform a pretakeoff check of the anti-ice/deice systems in accordance with the AFM or POH prior to takeoff.
a. While taxiing in snow or ice, leave extra space around your aircraft and taxi at a slower rate. Be careful when braking to prevent the wheels from skidding.
b. When stopping, begin to brake earlier than normal because the aircraft may require more distance to stop. Leave additional space in front of the aircraft during an engine run-up; the aircraft may begin to slide on ice. Carefully check the braking action of the aircraft to ensure that snow or ice is not building up on any of the components of the brake system.
c. If the aircraft is equipped with wheel fairings, be aware that snow may accumulate in the wheel fairings and freeze during flight. Make sure that all controls have full range of motion, and, if applicable, check that the carburetor heat is working.
d. If the aircraft is not equipped with windshield anti-icing or deicing system, turn the defroster on high and leave it on. This may help to prevent ice from forming on the windshield during flight.
7-3. TAKEOFF AND CLIMBOUT. Depending on the recommendations of the manufacturer, the POH, or the AFM, on small aircraft and on certain light aircraft, it may be advisable during climbout to apply the brakes and cycle the landing gear to break loose any snow, slush, or ice that may have accumulated during taxi and takeoff.
a. Verify that the airspeed indicator is working properly and that the pitot heat is on.
Because of ATC restrictions and other traffic, climbout may not always be expeditious.
b. Airplanes are vulnerable to ice accumulation during the initial climbout in icing conditions because lower speeds often translate into a higher AOA. This exposes the underside of the airplane and its wings to the icing conditions and allows ice to accumulate further aft than it would in cruise flight.
c. Consequently, any ice that forms may be out of the pilot’s view and go undetected.
Extreme vigilance should be exercised while climbing with the autopilot engaged. Climbing in Vertical Speed (VS) mode in icing conditions is highly discouraged.
d. As ice accretes on aircraft without autothrottles, the autopilot will attempt to hold a vertical speed without regard for airspeed, leading to a potential stall situation. It is critical that the pilot monitor airspeed to assure that at least the minimum flight speed for the configuration and environmental conditions is maintained.
7-4. CRUISE. An aircraft that is certificated for flight in icing conditions will be able to cope with most icing encounters provided that its ice protection systems are operating properly and that the exposure is not extended beyond their capabilities. However, if it is possible to exit the icing conditions by a change in altitude or a minor change in flight path, this is certainly advisable. During any icing encounter, the pilot should carefully monitor the behavior of the aircraft.
Par 7-2 Page 47