Par 4-1 Page 33 CHAPTER 4. FLIGHT PLANNING NOTE: 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.
Aviation meteorologists at the National Weather Service (NWS) Aviation Weather Center (AWC), local NWS Field Offices, major airlines, and private companies prepare icing forecasts and continue to improve upon their accuracy. A review of the current edition of AC 00-45, Aviation Weather Services, is strongly recommended as pilots will need to understand and apply Current Icing Products (CIP), Forecast Icing Products (FIP) and other services to the flight planning and operations discussed in this AC.
4-1. PREFLIGHT PLANNING INFORMATION. Information concerning icing can be obtained through several different sources. Pilot Weather Reports (PIREP) are generally the most useful as they are factual weather reports for specific times and places. Area forecasts, Airmen’s Meteorological Information (AIRMET), and significant meteorological information (SIGMET) provide general information on forecasted inflight icing. Winds aloft forecasts also provide information to determine the approximate freezing level. The graphical CIP and the FIP combined with text based forecasts, provide adequate information for flight planning when icing conditions may exist.
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. Pilots can reasonably expect inflight icing when flying in clouds with temperatures at or below 0 °C (+32 °F). Forecasts, weather reports, and PIREPs of the cloud bases and tops are essential when flight planning for aircraft that have not been certified for flight in icing conditions.
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 NWS 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 a weather advisory issued only to amend the area forecast concerning weather phenomena which are of operational interest to all aircraft and potentially hazardous to aircraft having limited capability because of lack of equipment, instrumentation, or pilot qualifications. A SIGMET is a weather advisory issued concerning weather significant to the safety of all aircraft. Pilots should not rely on these alone for reasons stated before as AIRMETs for icing will not be issued unless the affected area is large, and a severe icing PIREP will not automatically result in an icing SIGMET.
Par 4-1 Page 34
• 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 4-2).
• The ADDS Web site at http://adds.aviationweather.gov/airmets provides a graphical depiction of the areas covered by AIRMETS and SIGMETS.
e. PIREPs. Pilots should consult PIREPs, since AIRMETs and SIGMETs will not necessarily be issued as previously discussed. PIREPs are of high value since they are actual weather reports at specific places and times. However, PIREPs from high-speed aircraft may not represent the actual icing conditions as the ram air temperature rise can mask the true icing conditions. The simplicity and the susceptibility to icing of the small low-speed general aviation airplanes provide the most accurate reports of inflight icing. An example that shows the importance of studying PIREPs is National Transportation Safety Board (NTSB) accident report ERA12FA115.
f. Aviation Routine Weather Reports (METAR). Pilots should be aware that surface observations not augmented by a human observer (“AUTO” in METAR) cannot report freezing rain (FZRA) or freezing drizzle (FZDZ) concurrently with other occurring precipitation. Since supercooled water drops can occur simultaneously with other reported precipitation, pilots on icing-certified airplanes should be vigilant for severe icing when approaching such an airport, particularly if “snow” or “snow and mist” are reported at temperatures slightly below or above freezing. An example that show the limitations of automated surface observations is NTSB accident report DEN05FA051.
g. Air Temperature and Pressure. Icing tends to be found in low-pressure areas and at temperatures at or around freezing. Pilots can reference the surface analysis charts to identify areas of low pressure. Freezing levels can be determined from the winds aloft forecast.
h. 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.
i. 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.
Par 4-1 Page 35
j. Snow. In flight, dry snow is unlikely to pose a hazard with respect to icing; however, wet snow may begin to adhere to aircraft surfaces. If wet 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 and declare an emergency or contact air traffic control (ATC) as necessary. Be aware that freezing drizzle can coexist 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.
k. Freezing Rain and Drizzle. If flying into an airport with no human augmentation of automated weather, be alert for severe ice accretions due to FZRA or FZDZ that would not be automatically detected, particularly if the reported temperature is near freezing and any precipitation (snow, mist, rain, drizzle) is being reported by the automated station.
l. Ice Pellets. Ice pellets by themselves are not a hazard to the airframe with respect to icing, but a ground observation of ice pellets could indicate Supercooled Large Drops (SLD) aloft.
m. 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. It is important to note that aircraft that are approved for instrument flight rules (IFR) operations but not certified for known icing conditions were not tested 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.
4-2. ICING INTENSITY. To report the intensity of icing, such as in a PIREP, the following descriptions are used (see paragraph 1-3 for complete definitions):
a. 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 ¼ inch (6 mm) per hour on the outer wing. The pilot should consider exiting the icing conditions before they become worse.
b. 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 ¼ inch to 1 inch (0.6 to 2.5 cm) per hour on the unprotected part of the outer wing.
The pilot should consider exiting the condition.
c. 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 unprotected part of the outer wing. The pilot should consider exiting the condition as soon as possible.
Par 4-2 Page 36
d. 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 any other areas identified by the manufacturer.
A representative accretion rate for reference purposes is more than 3 inches (7.5 cm) per hour on the unprotected part of the outer wing. By regulation, immediate exit is required.
4-3. PIREP CAUTIONS. Although PIREPs are excellent sources of information about in-flight icing, there are situations when these reports can be misleading.
a. No Recent PIREP. 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. No Icing Reported. An aircraft encountered icing, but the pilot did not report it. Pilot workloads might prevent a pilot from making a report, particularly when making an approach.
An aircraft might also encounter icing conditions that are more serious than those are reported in any recent PIREPs in the given area. There are several possible reasons for this:
(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 slower 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?
Par 4-3 Page 37 (and 38)
(4) When icing conditions exist, reporting may alert other crews to maintain vigilance, and pilots are reminded that such reporting of meteorological hazards is a regulatory requirement under 14 CFR part 91, § 91.183; part 121, § 121.561; and part 135, § 135.67. 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. Pilots are encouraged to provide in-flight icing observations as part of the weather forecasting process as often as practical. The importance of PIREPs to provide additional input into NWS forecasting products cannot be stressed enough. A report of no icing in a particular PIREP can be just as useful as those reporting ice subsequently improving the quality of CIP/FIP.
Par 5-1 Page 39 CHAPTER 5. ICING OPERATIONS
5-1. GENERAL. This chapter focuses primarily on how to safely fly an aircraft certificated for flight in icing conditions, what is expected regarding communications of 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 Airplane Flight Manual (AFM) or pilot’s operating handbook (POH) for approved checklists and operations in their particular aircraft.
5-2. REGULATIONS FOR ICING OPERATIONS. Title 14 CFR parts 91, 121, 125, and 135 specify the responsibilities of flightcrews concerning flight in icing conditions. Pilots are advised to check the current regulations for revisions. An important distinction in each of these regulations is the restriction on flight into known or forecast conditions. Because of the limitations of icing forecasts, it is admittedly difficult for pilots to be certain whether the conditions in which they are flying actually will result in an icing encounter, and it is even more difficult to determine the severity of the possible encounter. Pilots can be caught inadvertently in icing conditions that exceed these legal limits. General operating and flight rules for General Aviation (GA) aircraft are found in part 91, but not all rules within part 91 are applicable to all GA aircraft. Part 91, § 91.501 states that the rules in subpart F apply only to large and turbojet-powered multiengine airplanes and fractional ownership program aircraft that are not covered by parts 121, 125, 129, 135, and 137. Section 91.527, Operating in icing conditions, falls within subpart F and thus is not applicable to all GA aircraft. For aircraft not covered by subpart F, there are no specific icing regulations; however, § 91.9 prohibits you from flying without complying with the operating limitations in the POH or placards. For part 121 aircraft, refer to part 121, §§ 121.341 and 121.629; for part 125 aircraft, refer to part 125, § 125.221; and for part 135 aircraft, refer to part 135, § 135.227. The operating rules have the following language on severe icing:
a. Severe Icing. No pilot may fly a nontransport category airplane type-certificated (TC) after December 31, 1964 into known or forecast severe icing conditions unless one or more of the following apply:
(1) The aircraft has ice protection provisions that meet part 135 appendix A.
(2) The aircraft has ice protection provisions that meet the requirements for transport category airplane type certification.
CAUTION: Even airplanes approved for flight into known icing conditions should not fly into severe icing. Many AFM Limitations Sections require an immediate exit when these types of conditions are encountered.
CAUTION: Airplane certification for flight into known icing conditions does not include freezing drizzle and freezing rain. In fact, some airplanes are prohibited from flying into freezing drizzle or freezing rain, regardless of its intensity. These conditions are very dangerous and can cause ice to form behind the protected areas.
Par 5-2 Page 40
b. Communications with Air Traffic Control (ATC). When encountering icing, controllers will not know if an aircraft is certificated or equipped for icing, the severity of the conditions, or what anti-icing or deicing equipment is installed on the aircraft. The pilot should communicate to ATC the severity of the icing conditions, its effect on their aircraft and continued operations, whether an alteration of the current course and altitude is required, and, if necessary, if an alternate destination is needed. If an aircraft that is not certificated for flight in icing conditions inadvertently encounters ice, exit icing conditions as expeditiously as possible and declare an emergency to ATC. Inform the controller of what actions are being taken by the pilot to cope with the emergency and coordinate with ATC for additional instructions, altitudes, or headings needed to resume safe flight operations.
(1) 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 safety is compromised. In this case, it is recommended that the pilot declare an emergency and exit the conditions as soon as practical.
(2) If an aircraft certificated for flight in icing conditions encounters freezing rain or freezing drizzle, advise ATC and do not attempt sustained flight in these conditions. Final authority and responsibility for the safety of a flight rests with the pilot in command (PIC).
(3) At any time, a 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. Pilots should not accept an airspeed assigned by ATC that is inconsistent with their manuals or the airplane manufacturer recommended airspeed, or if there is no specific icing airspeed information provided in the manuals, inconsistent with 50 percent to 60 percent margin above stall speed (Vs) as recommended.
5-3. AVAILABLE IN-FLIGHT INFORMATION. Available services and products are best described in the current edition of AC 00-45, Aviation Weather Services. Brief descriptions of some of the basic services are provided below:
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 a Flight Service Station (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 automated flight service station (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.
b. Hazardous In-Flight Weather Advisory Service (HIWAS). The HIWAS provides continuous, recorded hazardous in-flight weather forecasts over selected Very High Frequency (VHF) Omnidirectional Range (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.
Par 5-3 Page 41
c. Transcribed Weather Broadcast (TWEB). The 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 includes, among other information, 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, pilots should avoid cells painted on radar 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, ice crystals that have little to no liquid water present that can be in heavy concentrations near convective weather systems.
f. Data Link/Satellite Radio. Pilots now have the option to subscribe to and receive near-real-time weather and airspace information via satellite to their panel-mounted or handheld avionics which may include free products via Flight Information Service-Broadcast (FIS-B), a component of Automatic Dependent Surveillance-Broadcast (ADS-B), transmitted to the cockpit avionics (refer to the current edition of AC 00-63, Use of Cockpit Displays of Digital Weather and Aeronautical Information, Appendix 1). These services can provide textual or graphical METARs, Terminal Area Forecasts (TAF), SIGMETs, and AIRMETs, updated pilot reports, updated winds and temperatures aloft, information about severe thunderstorms, icing levels, and graphical displays of temporary flight restrictions. Such up-to-date information can assist pilots in avoiding hazardous weather and identifying potentially hazardous areas of unforecast weather.
Pilots must understand that some of the products available to them may not include pertinent icing information. Pilots are advised to verify the products they are receiving include the necessary information consistent with their aircraft capabilities and route of flight.
5-4. PREFLIGHT. The first step in preparing for any flight is to obtain a thorough weather briefing as previously discussed in Chapter 4 and 5 using products presented in AC 00-45.
a. Considerations. Operations in or around 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 for any degradation data.
(3) Determine icing exit strategies during preflight. Determine if climbing or descending will be viable options based on the planned route of flight which includes required
Par 5-4 Page 42 altitudes to maintain clearance from terrain, airspace, published routes, departure procedures, arrival procedures and approaches.
(4) When choosing alternate airports, remember that if structural icing occurs, higher approach speeds and, consequently, additional runway length may be required for landing.
(5) Consider carrying a high intensity flashlight for use in locating ice accumulation on the aircraft at night or in low visibility conditions.
(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 of additional fuel needed to operate icing systems or from excess drag or weight caused by ice formation that may require extra power to maintain altitude or airspeed, 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.
b. Frost. Frost can form on an airplane sitting outside on a clear night when there is moisture present in the air as the airplane’s skin temperature falls below freezing due to radiation cooling.
(1) Certain airplanes may be more vulnerable to ice formation from cold-soaked fuel than others depending on how the fuel tanks are arranged and how much fuel they contain (more information regarding icing certification and testing can be found in the current edition of AC 20-147, Turbojet, Turboprop, Turboshaft, and Turbofan Engine Induction System Icing and Ice Ingestion). Most aircraft specify allowances for the amount of frost that may be present underneath the wing. On the Ground, clear ice can form on the upper surfaces of the wing when cold-soaked fuel (due to aircraft prolonged operation at high altitude) remains in contact with the fuel tanks’ upper surfaces after landing, and during time on the ground when the airplane is exposed to conditions of atmospheric moisture (for example, fog, precipitation, and condensation of humid air) at ambient temperatures above freezing. Atmospheric moisture, when in contact with cold wing surfaces, may freeze. This can even occur if conditions remain above freezing and are not expected to be, or recognized as, icing conditions.
(2) Frost, snow, or ice also can be removed with freezing-point depressant fluids. Refer to the current edition of 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 so that most of the fluid will flow off the aircraft by the time the aircraft reaches rotation speed; consequently, they provide no icing protection once the
Par 5-4 Page 43 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.
(3) 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.
(4) 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.
CAUTION: If undetected and still present during takeoff, ice is most likely to shed when the wings flex at takeoff rotation. Simultaneous ice shedding from both wings of an airplane with aft mounted engines has been known to result in ice ingestion damage and power loss in both engines during takeoff.
5-5. TAXI. Always perform a pretakeoff check of the anti-ice/deice systems in accordance with the AFM or POH prior to takeoff.
a. Speed. 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. Braking. 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 runup; 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. Wheel Fairings. 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. Defroster. 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.
5-6. 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. Preparation. 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.