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.
• 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.
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.
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?
(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.