ATC Operations in Weather Avoidance Scenarios

ATC Operations in Weather Avoidance Scenarios


This article provides generic advice only on the effective management of air traffic during periods of convective weather avoidance. The guidance provided in this article shall not take precedence over local operating instructions and air traffic management strategies that controllers are required to follow when adverse weather avoidance is in progress.

Cumulonimbus Clouds

Convective clouds present a serious hazard to aviation. Aircraft entering a Cumulonimbus (Cb) cloud may experience severe turbulenceicinglightning, precipitation (especially Hail), and strong winds (both vertical and horizontal). These hazards, individually and collectively can lead to structural damage, injuries to crew and passengers, loss of separation/level bust as a result of an inability to maintain assigned level, and loss of control. Where possible, flight crews will wish to avoid passing within 20 nm of a cumulonimbus cloud.

Particularly intense Cbs, often associated with squall lines, may also present related phenomena such as TornadosGust Fronts, and Microbursts, all of which can have an impact on air traffic management and airport infrastructure.

Aircraft equipped with Weather Radar are able to identify the areas of cloud with the greatest moisture content and navigate through (or if not possible, around) areas of convective activity.

Controllers should note that flight crew workload increases significantly in a weather avoidance scenario not just because of the decision making associated with weather avoidance but also because of Turbulence, management of In-Flight Icing, and increased communications. Particularly dense cells, or groups of cells, can attenuate radar and radio signals thereby causing loss of radar contact and poor quality or lost communications.

Weather Avoidance Characteristics

When air traffic is avoiding Cumulonimbus (Cb) cells, particularly in congested airspace, the workload of the controller increases significantly. In such scenarios the increase in workload is caused by:

  • Non standard traffic flow – the traffic flow is irregular and not easy to anticipate because of:
    • the changing intensity of cells, both vertically and horizontally (for further information see the article "Lifecycle of the Thunderstorm)"
    • the situational awareness of the flight crew and routing decisions they take based on the display on their weather radar (for further information see the article "Weather Radar: Storm Avoidance)"
    • the altitude of aircraft,
    • the onward routing of the aircraft,
    • the training and experience of the flight crews, and
    • operator’s procedures.
  • Reduction in available airspace – controllers will have less airspace volume available for conflict resolution tasks with a consequent impact on sector capacity;
  • New conflict points – new random crossing points are likely to occur as a result of the disrupted and non-standard traffic patterns thus increasing workload associated with conflict detection;
  • Increased frequency occupancy time – radio-communication is likely to be prolonged due to the necessity to clarify the details associated with the avoidance actions as well as revised onward routing clearances. Usage of non standard RTF is likely to increase;
  • Increased verbal (telephone) coordination – telephone coordination with adjacent sectors or ATS units is likely to increase due to the necessity to coordinate the details associated with the avoidance actions (change of routes and flight levels);
  • Rapidly changing situation – isolated Cb cells can quickly evolve into a squall line and make navigation through the line of Cbs increasingly challenging for the pilots;
  • Degradation of RVSM capability – convective weather conditions are associated with moderate to severe turbulence, hence it might be advisable to downgrade the RVSM airspace and introduce 2000 ft vertical separation in areas with reported severe turbulence;
  • Lack of information about traffic in own sector (not on frequency) – situations may arise when traffic deviating from its planned/cleared flight route, due to bad weather, penetrates (or flies close to the boundary of) another sector’s airspace without prior notification of the controller in charge of that sector who is not aware of the crew’s intentions;
  • Limited applicability of radar vectoring - use of radar vectoring to resolve potential traffic conflicts might be limited due to crew inability to maintain the required headings. This is a very significant factor in busy environments where controllers rely heavily on radar vectoring to provide separation;
  • Limited applicability of speed control - pilots may not accept speed increase instructions due to turbulence;
  • Limited applicability of level change - pilots may decline a level change instruction if compliance with it would cause the aircraft enter a Cb. In addition, crews are often likely to request to maintain cruising levels for longer and commence descent much later than usual. This creates two issues:
    • Occupying these levels often means that they are unavailable for other traffic.
    • Other traffic at lower level(s) could be flying on a converging track to the sector exit point. If the descent starts early enough, vertical separation will be established well before horizontal separation becomes unavailable. A delayed descent however could lead to a situation where there is overflying traffic below, with insufficient horizontal spacing. The limited options for vectoring could further aggravate the situation.
  • Airspace constraints - ATC sector overloads can be aggravated by the combination of weather factors (majority of these are Cb-related) and airspace constraints in particular in busy TMAs.

ICAO Procedures

Weather Avoidance Information for Flight Crews

Controllers are expected to provide the most appropriate advice/information to pilots of an aircraft requesting navigational assistance when avoiding areas of adverse weather. ICAO Doc 4444 (PANS-ATM), contains the following provisions on information to be given to flight crews in weather avoidance scenarios:

  • Doc 4444 - 8.6.9 Information regarding adverse weather -
  • Doc 4444 - 8.6.9 Information regarding adverse weather -

Weather Avoidance Communications in Oceanic Airspace

In controlled airspace, a pilot using an aircraft radar and intending to detour around observed weather, must obtain a clearance from the controller before doing so. Even so, controllers should not be surprised if, perhaps because of communications difficulty and the flight safety risks, an aircraft alters course without clearance. If it is necessary to leave controlled airspace the pilot must request permission to re-join. ICAO Doc 4444 (PANS-ATM) provides the following procedures intended for deviations around adverse meteorological conditions:

  • Doc 4444 - Procedures related to emergencies, communication failure and contingencies -
  • Doc 4444 - Procedures related to emergencies, communication failure and contingencies -

Weather Avoidance Procedures in Oceanic Airspace

The crews should notify ATC and request clearance to deviate from track, advising, when possible, the extent of the deviation expected, expressed in new heading and for how long the crew intends to proceed on the deviation heading.

  • Doc 4444 - Actions to be taken when controller-pilot communications are established -
  • Doc 4444 - Actions to be taken when controller-pilot communications are established -


Possible effects of adverse weather avoidance include:

  • Pilots may be unwilling to execute a turn, as instructed by the controller to avoid conflict, due to proximity of adverse weather.
  • Pilots may be unwilling to descend due to proximity of adverse weather area.
  • Pilots may be unwilling to increase speed due to turbulence (which usually exists in or near Cbs). They may also reduce speed without informing the controller. Therefore, the availability speed control as a controller tool may be limited.
  • Pilots setting a heading or altitude not expected by the controller.
  • Pilots changing the assigned heading after clearing weather (CB) without informing ATC. In general pilots request deviation from the planned route due to CBs but sometimes, when clear of weather, they turn back to their planned route without prior notification to ATC.
  • Increased communications with pilots.
  • Increased communications with adjacent ATC units to coordinate avoiding actions.
  • Some flights may not be able to follow missed approach procedure due to thunderstorm areas near the airport (APP/TWR environment)
  • Some flights may initiate a go around on final due to severe turbulence, wind shear, or a flooded runway (APP/TWR environment).
  • More requests to use a different runway for departure or arrival due to sudden changes of wind components in combination with wet runway and radar derived information on adverse weather on the climb out route (APP/TWR environment).
  • Increased controllers’ and pilots’ workload.
  • Reduced sector capacity - The complexity of the traffic situation (traffic demand, non-standard routings, potential conflicts) may necessitate the implementation of flow measures in order to ensure safe ATC service provision during periods of massive adverse weather avoidance.


Operational Supervisor’s (SUP) actions. The ATC shift supervisor should be able to mitigate the impact of severe weather avoidance by air traffic on the controller’s workload by facilitating and engaging in the following actions:

  • Establish a co-ordination pattern with the MET office for the provision of periodic weather updates and forecasts for the affected area. Following an established protocol, which outlines the roles and responsibilities of involved persons, will be of advantage.
  • Use all available information: MET updates/forecasts, traffic load forecasts and availability of ATC personnel to assess the situation and establish with the help of local flow management position (FMP) possible tactical measures.
  • Inform affected adjacent units of the (flow) measures taken.
  • Assess whether it is safe and possible to follow the procedures described in local letters of agreement (LoAs) with adjacent ATC units. As necessary, agree with the SUPs of the neighbouring unit(s) special coordination procedures to substitute the normal operating procedures (i.e. flight level allocations, points of transfer etc.). It is important to point out to the controllers the need for dedicated coordination in these exceptional cases.
  • Provide additional (third) controller as necessary at the sector to help the sector team with coordination / monitoring / planning tasks, as applicable.
  • Apply dynamic sectorisation management - the OPS Supervisor should monitor the situation and activate the most appropriate sector configuration depending on the traffic volume and complexity, and the scale of the weather avoidance. For example, in case of large deviations from planned routes vertically split sectors may be more appropriate than laterally split ones.
  • Open additional sectors in order to deal with possible capacity problems and avoid sector overloads.
  • Consider the application of reduced departure rate or of a specific departure separation (e.g. individual approval request; departure interval of 5 minutes etc.) for aerodromes within the affected airspace and coordinate this course of action with the relevant units.
  • If relevant, coordinate with respective parties release of temporary restricted airspace for use by general air traffic or its use under special crossing co-ordination procedure (e.g. use of dedicated SSR code).
  • Consider the rejection/adjournment of planned maintenance work on any technical equipment used for ATS provision.
  • Consider application of the so-called “one-airway” procedure, i.e. closely situated airways are considered as one airway for traffic separation purposes.

Flow Management Position’s (FMP) actions. The FMP should provide the necessary assistance to the OPS Supervisor and facilitate the management of the severe weather by timely activation of coordinated flow control measures in order to prevent sector overloads. During the normalization period, special consideration must be given to possible bunching of traffic at the end of the regulated period. It is considered that return to normal capacity following flow control measures is usually more efficient if implemented on a gradual (step-by-step) basis.

The FMP should consider passing timely information to regional flow management unit (NMOC for the European region) about the forecast and actual convective weather and its impact on ATC operations.

Controllers’ actions. Controllers should exercise their best judgement and expertise when dealing with adverse weather avoidance scenarios; in particular they should be prepared to:

  • Maintain awareness of the adverse weather location, its evolution (laterally and vertically) and of the possible deviation routes. A controller may be alerted to the presence of adverse weather by a variety of sources including: radar observations, adjacent ATS units, MET office reports, unit briefings and reports from pilots. Being constantly aware of any ongoing deviations and flight crews’ intentions should provide precious time for separation of affected nearby traffic.
  • Develop strategies – the executive (radar) and planner controller should develop strategies and practice mutual crosschecks of the current, planned and intended weather avoidance actions.
  • Provide timely information to and coordinate with the adjacent sectors of any deviations which will affect them.
  • Pro-actively seek information regarding traffic which is likely to enter own sector.
  • Request any necessary details from the flight crews on the planned avoiding actions i.e. heading(s) on which the aircraft will be flying, as well as the estimated duration and/or the distance the aircraft will proceed on the heading(s).
  • Provide extra room for manoeuvring, if in doubt that the traffic could request further deviation provide extra space for separation, issue instructions for flight level change as necessary, provide traffic information, as necessary.
  • Inform pilot if weather avoidance will take pilot outside controlled airspace and offer an appropriate service.

Organisational measures.

  • Provision of sufficient number of controllers during periods with forecasted severe convective weather.
  • Use of weather radars/ weather displays to enhance information provided to controllers.
  • Use SIGMETS and associated weather forecasts to improve prediction of sector loading.
  • Train controllers to deal with weather during live training; use simulator training to build in more resilience in controllers’ skills.
  • Provide a periodic refresher course to OPS supervisors.
  • Consider adoption of a generic checklist for OPS supervisors.

Weather Avoidance Decision Support Systems

It is generally agreed that decision support tools may be instrumental in the management of convective weather avoidance scenarios in congested airspace. Such tools will use the weather forecasts to estimate the impact on ATC provision and consequently suggest air traffic management strategies.

In research funded by NASA Ames Research Centre, the MIT Lincoln Laboratory has developed an en-route Convective Weather Avoidance Model that outputs three-dimensional weather avoidance fields. “The probabilistic Weather Avoidance Fields identify regions of airspace that pilots are likely to avoid due to the presence of convective weather” (for details see Further Reading).

The concept for the future trajectory-based operations is that it will be necessary to automatically generate flight trajectories through or around convective weather that pilots will find acceptable.

Related Articles

NATS Flight Deck Procedures Video

There will be times when controllers will have to cope with unusual situations such as weather avoidance or aircraft emergencies. It is important for controllers to have knowledge of the flight deck procedures that will be used by aircrew in such situations. The following video describes the generic procedures followed by aircrew in certain unusual situations:

Further Reading


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