ATC In-Flight Weather Avoidance Assistance


  • ATC has numerous tools available to pilots which can keep pilots safe from adverse weather

ATC Radar Weather Display:

  • ATC radars are able to display areas of precipitation by sending out a beam of radio energy that is reflected back to the radar antenna when it strikes an object or moisture, which may be in the form of rain drops, hail, or snow
  • The larger the object is, or the more dense its reflective surface, the stronger the return will be presented
  • Radar weather processors indicate the intensity of reflective returns in terms of decibels (dBZ)
  • ATC systems cannot detect the presence or absence of clouds
  • The ATC systems can often determine the intensity of a precipitation area, but the specific character of that area (snow, rain, hail, VIRGA, etc.) cannot be determined
  • For this reason, ATC refers to all weather areas displayed on ATC radar scopes as "precipitation"
  • All ATC facilities using radar weather processors with the ability to determine precipitation intensity, will describe the intensity to pilots as:
    • "LIGHT" (< 30 dBZ)
    • "MODERATE" (30 to 40 dBZ)
    • "HEAVY" (> 40 to 50 dBZ)
    • "EXTREME" (> 50 dBZ)
  • ATC facilities that, due to equipment limitations, cannot display the intensity levels of precipitation, will describe the location of the precipitation area by geographic position, or position relative to the aircraft
  • Since the intensity level is not available, the controller will state "INTENSITY UNKNOWN"
  • ARTCC facilities normally use a Weather and Radar Processor (WARP) to display a mosaic of data obtained from multiple NEXRAD sites
  • There is a time delay between actual conditions and those displayed to the controller
    • The precipitation data on the ARTCC controller's display could be up to 6 minutes old
    • Enroute ATC radar's Weather and Radar Processor (WARP) does not display light precipitation intensity
  • When the WARP is not available, a second system, the narrow-band Air Route Surveillance Radar (ARSR) can display two distinct levels of precipitation intensity that will be described to pilots as "MODERATE" (30 to 40 dBZ) and "HEAVY TO EXTREME" ( > 40 dBZ )
    • The WARP processor is only used in ARTCC facilities
  • ATC radar is not able to detect turbulence
  • ATC radar is not able to detect turbulence. Generally, turbulence can be expected to occur as the rate of rainfall or intensity of precipitation increases. Turbulence associated with greater rates of rainfall/precipitation will normally be more severe than any associated with lesser rates of rainfall/precipitation. Turbulence should be expected to occur near convective activity, even in clear air. Thunderstorms are a form of convective activity that imply severe or greater turbulence. Operation within 20 miles of thunderstorms should be approached with great caution, as the severity of turbulence can be markedly greater than the precipitation intensity might indicate

Weather Avoidance Assistance:

  • To the extent possible, controllers will issue pertinent information on weather or chaff areas and assist pilots in avoiding such areas when requested
  • Pilots should respond to a weather advisory by either acknowledging the advisory or by acknowledging the advisory and requesting an alternative course of action as follows:
    • Request to deviate off course by stating the number of miles and the direction of the requested deviation
      • In this case, when the requested deviation is approved, navigation is at the pilot's prerogative, but must maintain the altitude assigned by ATC and to remain within the specified mileage of the original course
    • An approval for lateral deviation authorizes the pilot to maneuver left or right within the limits specified in the clearance
      • It is often necessary for ATC to restrict the amount of lateral deviation (“twenty degrees right,” “up to fifteen degrees left,” “up to ten degrees left or right of course”)
      • The term “when able, proceed direct,” in an ATC weather deviation clearance, refers to the pilot's ability to remain clear of the weather when returning to course/route
    • Request a new route to avoid the affected area
    • Request a change of altitude
    • Request radar vectors around the affected areas
  • For obvious reasons of safety, an IFR pilot must not deviate from the course or altitude or flight level without a proper ATC clearance
  • When weather conditions encountered are so severe that an immediate deviation is determined to be necessary and time will not permit approval by ATC, the pilot's emergency authority may be exercised
  • When the pilot requests clearance for a route deviation or for an ATC radar vector, the controller must evaluate the air traffic picture in the affected area, and coordinate with other controllers (if ATC jurisdictional boundaries may be crossed) before replying to the request
  • It should be remembered that the controller's primary function is to provide safe separation between aircraft
  • Any additional service, such as weather avoidance assistance, can only be provided to the extent that it does not derogate the primary function
  • It's also worth noting that the separation workload is generally greater than normal when weather disrupts the usual flow of traffic
  • ATC radar limitations and frequency congestion may also be a factor in limiting the controller's capability to provide additional service
  • It is very important, therefore, that the request for deviation or radar vector be forwarded to ATC as far in advance as possible
  • Delay in submitting it may delay or even preclude ATC approval or require that additional restrictions be placed on the clearance
  • Insofar as possible, the following information should be furnished to ATC when requesting clearance to detour around weather activity:
    • Proposed point where detour will commence
    • Proposed route and extent of detour (direction and distance)
    • Point where original route will be resumed
    • Flight conditions (IFR or VFR)
    • Any further deviation that may become necessary as the flight progresses
    • Advise if the aircraft is equipped with functioning airborne radar
  • To a large degree, the assistance that might be rendered by ATC will depend upon the weather information available to controllers
  • Due to the extremely transitory nature of severe weather situations, the controller's weather information may be of only limited value if based on weather observed on radar only
  • Frequent updates by pilots giving specific information as to the area affected, altitudes, intensity, and nature of the severe weather can be of considerable value
  • Such reports are relayed by radio or phone to other pilots and controllers, and also receive widespread teletypewriter dissemination
  • Obtaining IFR clearance or an ATC radar vector to circumnavigate severe weather can often be accommodated more readily in the en-route areas away from terminals, because there is usually less congestion and, therefore, offer greater freedom of action
  • In terminal areas, the problem is more acute because of traffic density, ATC coordination requirements, complex departure and arrival routes, adjacent airports, etc.
  • As a consequence, controllers are less likely to be able to accommodate all requests for weather detours in a terminal area or be in a position to volunteer such routing to the pilot
  • Nevertheless, pilots should not hesitate to advise controllers of any observed severe weather and should specifically advise controllers if they desire circumnavigation of observed weather

Procedures for Weather Deviations and Other Contingencies in Oceanic Controlled Airspace:

  • When the pilot initiates communications with ATC, rapid response may be obtained by stating "WEATHER DEVIATION REQUIRED" to indicate priority is desired on the frequency and for ATC response
  • The pilot still retains the option of initiating the communications using the urgency call "PAN-PAN" 3 times to alert all listening parties of a special handling condition which will receive ATC priority for issuance of a clearance or assistance
  • ATC will:
    • Approve the deviation
    • Provide vertical separation and then approve the deviation; or
    • If ATC is unable to establish vertical separation, ATC shall advise the pilot that standard separation cannot be applied; provide essential traffic information for all affected aircraft, to the extent practicable; and if possible, suggest a course of action. ATC may suggest that the pilot climb or descend to a contingency altitude (1,000' above or below that assigned if operating above FL 290; 500' above or below that assigned if operating at or below FL 290)


  • "Standard separation not available, deviate at pilot's discretion; suggest [Climb/Descent] to [Altitude]; Traffic [Location]; Report deviation complete"

  • The pilot will follow the ATC advisory altitude when approximately 10 NM from track, as well as execute the procedures detailed in paragraph 7-1-14c5
  • If contact cannot be established or revised ATC clearance or advisory is not available and deviation from track is required, the pilot shall take the following actions:
    • If possible, deviate away from an organized track or route system
    • Broadcast aircraft position and intentions on the frequency in use, as well as on frequency 121.5 MHz at suitable intervals stating: flight identification (operator call sign), flight level, track code or ATS route designator, and extent of deviation expected
    • Watch for conflicting traffic both visually and by reference to TCAS (if equipped)
    • Turn on aircraft exterior lights
    • Deviations of less than 10 NM or operations within COMPOSITE (NOPAC and CEPAC) Airspace, should REMAIN at ASSIGNED altitude. Otherwise, when the aircraft is approximately 10 NM from track, initiate an altitude change based on the following criteria:

    Route Centerline/Track Deviations > 10 NM Altitude Change
    East 000-179°M Left/Right Descend/Climb 300'
    West 180-359°M Left/Right Descend/Climb 300'

    Pilot Memory Slogan: "Easy right up, West right now"

  • When returning to track, be at assigned flight level when the aircraft is within approximately 10 NM of centerline
  • If contact was not established prior to deviating, continue to attempt to contact ATC to obtain a clearance. If contact was established, continue to keep ATC advised of intentions and obtain essential traffic information

Flight Information Service (FIS):

  • FIS is a method of disseminating meteorological (MET) and aeronautical information (AI) to displays in the cockpit in order to enhance pilot situational awareness, provide decision support tools, and improve safety
  • FIS augments, but does not replace, traditional pilot voice communication with Flight Service Stations (FSSs), ATC facilities, or Airline Operations Control Centers (AOCCs)
  • FIS, however, can provide textual and graphical information that can help abbreviate and improve the usefulness of such communications
    • Data link Service Providers (DLSP): DLSP deploy and maintain airborne, ground­based, and, in some cases, space­based infrastructure that supports the transmission of AI/MET information over one or more physical links
    • DLSP may provide a free of charge or for­fee service that permits end users to uplink and downlink AI/MET and other information
      • FAA FIS­B:
        • A ground­based broadcast service provided through the ADS­B Universal Access Transceiver (UAT) network
        • The service provides users with a 978 MHz data link capability when operating within range and line­of­sight of a transmitting ground station
        • FIS­B enables users of properly equipped aircraft to receive and display a suite of broadcast weather and aeronautical information products
      • Non­FAA FIS Systems:
        • Several commercial vendors provide customers with FIS data over both the aeronautical spectrum and on other frequencies using a variety of data link protocols
        • Services available from these providers vary greatly and may include tier based subscriptions
        • Advancements in bandwidth technology permits preflight as well as inflight access to the same MET and AI information available on the ground
        • Pilots and operators using non­FAA FIS for MET and AI information should be knowledgeable regarding the weather services being provided as some commercial vendors may be repackaging NWS sourced weather, while other commercial vendors may alter the weather information to produce vendor-tailored or vendor-specific weather reports and forecasts
    • Three Data Link Modes: There are three data link modes that may be used for transmitting AI and MET information to aircraft
    • The intended use of the AI and/or MET information will determine the most appropriate data link service
      • Broadcast Mode: A one­way interaction in which AI and/or MET updates or changes applicable to a designated geographic area are continuously transmitted (or transmitted at repeated periodic intervals) to all aircraft capable of receiving the broadcast within the service volume defined by the system network architecture
      • Contract/Demand Mode: A two­way interaction in which AI and/or MET information is transmitted to an aircraft in response to a specific request
      • Contract/Update Mode: A two­way interaction that is an extension of the Demand Mode. Initial AI and/or MET report(s) are sent to an aircraft and subsequent updates or changes to the AI and/or MET information that meet the contract criteria are automatically or manually sent to an aircraft
    • To ensure airman compliance with Federal Aviation Regulations, manufacturer's operating manuals should remind airmen to contact ATC controllers, FSS specialists, operator dispatchers, or airline operations control centers for general and mission critical aviation weather information and/or NAS status conditions (such as NOTAMs, Special Use Airspace status, and other government flight information). If FIS products are systemically modified (for example, are displayed as abbreviated plain text and/or graphical depictions), the modification process and limitations of the resultant product should be clearly described in the vendor's user guidance
    • Operational Use of FIS. Regardless of the type of FIS system being used, several factors must be considered when using FIS:
      • Before using FIS for inflight operations, pilots and other flight crewmembers should become familiar with the operation of the FIS system to be used, the airborne equipment to be used, including its system architecture, airborne system components, coverage service volume and other limitations of the particular system, modes of operation and indications of various system failures. Users should also be familiar with the specific content and format of the services available from the FIS provider(s). Sources of information that may provide this specific guidance include manufacturer's manuals, training programs, and reference guides
      • FIS should not serve as the sole source of aviation weather and other operational information. ATC, FSSs, and, if applicable, AOCC VHF/HF voice remain as a redundant method of communicating aviation weather, NOTAMs, and other operational information to aircraft in flight. FIS augments these traditional air-traffic-control/FSS/AOCC services and, for some products, offers the advantage of being displayed as graphical information. By using FIS for orientation, the usefulness of information received from conventional means may be enhanced. For example, FIS may alert the pilot to specific areas of concern that will more accurately focus requests made to FSS or AOCC for inflight updates or similar queries made to ATC
      • The airspace and aeronautical environment is constantly changing. These changes occur quickly and without warning. Critical operational decisions should be based on use of the most current and appropriate data available. When differences exist between FIS and information obtained by voice communication with ATC, FSS, and/or AOCC (if applicable), pilots are cautioned to use the most recent data from the most authoritative source
      • FIS aviation weather products (for example, graphical ground-based radar precipitation depictions) are not appropriate for tactical (typical time-frame of less than 3 minutes) avoidance of severe weather such as negotiating a path through a weather hazard area. FIS supports strategic (typical time-frame of 20 minutes or more) weather decision-making such as route selection to avoid a weather hazard area in its entirety. The misuse of information beyond its applicability may place the pilot and aircraft in jeopardy. In addition, FIS should never be used in lieu of an individual preflight weather and flight planning briefing
      • DLSP offer numerous MET and AI products with information that can be layered on top of each other. Pilots need to be aware that too much information can have a negative effect on their cognitive work load. Pilots need to manage the amount of information to a level that offers the most pertinent information to that specific flight without creating a cockpit distraction. Pilots may need to adjust the amount of information based on numerous factors including, but not limited to, the phase of flight, single pilot operation, autopilot availability, class of airspace, and the weather conditions encountered
      • FIS NOTAM products, including Temporary Flight Restriction (TFR) information, are advisory-use information and are intended for situational awareness purposes only. Cockpit displays of this information are not appropriate for tactical navigation - pilots should stay clear of any geographic area displayed as a TFR NOTAM. Pilots should contact FSSs and/or ATC while en route to obtain updated information and to verify the cockpit display of NOTAM information
      • FIS supports better pilot decision-making by increasing situational awareness. Better decision- making is based on using information from a variety of sources. In addition to FIS, pilots should take advantage of other weather-and-atmosphere/NAS status sources, including, briefings from Flight Service Stations, FAA's en route “Flight Watch” service, data from other air traffic control facilities, airline operation control centers, pilot reports, as well as their own observations
      • FIS-B Over UAT Product Update and Transmission Intervals
        Figure 1: FIS-B Over UAT Product Update and Transmission Intervals
      • FAA's Flight Information Service-Broadcast (FIS-B):
        • FIS-B is a ground-based broadcast service provided through the FAA's Automatic Dependent Surveillance–Broadcast (ADS-B) Services Universal Access Transceiver (UAT) network. The service provides users with a 978 MHz data link capability when operating within range and line-of-sight of a transmitting ground station. FIS-B enables users of properly-equipped aircraft to receive and display a suite of broadcast weather and aeronautical information products
        • The following list represents the initial suite of text and graphical products available through FIS-B and provided free-of-charge. Detailed information concerning FIS-B meteorological products can be found in Advisory Circular 00-45, Aviation Weather Services, and AC 00­63, Use of Cockpit Displays of Digital Weather and Aeronautical Information. Information on Special Use Airspace (SUA), Temporary Flight Restriction (TFR), and Notice to Airmen (NOTAM) products can be found in Chapters 3, 4 and 5 of this manual
          • Text: Aviation Routine Weather Report (METAR) and Special Aviation Report (SPECI);
          • Text: Pilot Weather Report (PIREP);
          • Text: Winds and Temperatures Aloft;
          • Text: Terminal Aerodrome Forecast (TAF) and amendments;
          • Text: Notice to Airmen (NOTAM) Distant and Flight Data Center;
          • Text/Graphic: Airmen's Meteorological Conditions (AIRMET);
          • Text/Graphic: Significant Meteorological Conditions (SIGMET);
          • Text/Graphic: Convective SIGMET;
          • Text/Graphic: Special Use Airspace (SUA);
          • Text/Graphic: Temporary Flight Restriction (TFR) NOTAM; and
          • Graphic: NEXRAD Composite Reflectivity Products (Regional and National)
        • Users of FIS-B should familiarize themselves with the operational characteristics and limitations of the system, including: system architecture; service environment; product life-cycles; modes of operation; and indications of system failure
        • FIS-B products are updated and transmitted at specific intervals based primarily on product issuance criteria. Update intervals are defined as the rate at which the product data is available from the source for transmission. Transmission intervals are defined as the amount of time within which a new or updated product transmission must be completed and/or the rate or repetition interval at which the product is rebroadcast. Update and transmission intervals for each product are provided in TBL 7-1-1
        • Where applicable, FIS-B products include a look-ahead range expressed in nautical miles (NM) for three service domains: Airport Surface; Terminal Airspace; and Enroute/Gulf-of-Mexico (GOMEX). TBL 7-1-2 provides service domain availability and look-ahead ranging for each FIS-B product
        • Prior to using this capability, users should familiarize themselves with the operation of FIS-B avionics by referencing the applicable User's Guides. Guidance concerning the interpretation of information displayed should be obtained from the appropriate avionics manufacturer
        • FIS-B malfunctions not attributed to aircraft system failures or covered by active NOTAM should be reported by radio or telephone to the nearest FSS facility
  • Non-FAA FIS Systems: Several commercial vendors also provide customers with FIS data over both the aeronautical spectrum and on other frequencies using a variety of data link protocols. In some cases, the vendors provide only the communications system that carries customer messages, such as the Aircraft Communications Addressing and Reporting System (ACARS) used by many air carrier and other operators
    • Operators using non-FAA FIS data for inflight weather and other operational information should ensure that the products used conform to FAA/NWS standards. Specifically, aviation weather and NAS status information should meet the following criteria:
      • The products should be either FAA/NWS “accepted” aviation weather reports or products, or based on FAA/NWS accepted aviation weather reports or products. If products are used which do not meet this criteria, they should be so identified. The operator must determine the applicability of such products to their particular flight operations
      • In the case of a weather product which is the result of the application of a process which alters the form, function or content of the base FAA/NWS accepted weather product(s), that process, and any limitations to the application of the resultant product, should be described in the vendor's user guidance material
    • An example would be a NEXRAD radar composite/mosaic map, which has been modified by changing the scaling resolution. The methodology of assigning reflectivity values to the resultant image components should be described in the vendor's guidance material to ensure that the user can accurately interpret the displayed data

Product Parameters for Low/Medium/High Altitude Tier Radios
Figure 2: Product Parameters for Low/Medium/High Altitude Tier Radios