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Loss of Separation During Weather Avoidance

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Category: Loss of Separation Loss of Separation
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This article provides a general overview to controllers of some typical scenarios of loss of separation (LOS) during weather avoidance. It offers some consideration to help controllers provide as much support as possible to the aircraft concerned and maintain the safety of other aircraft in the vicinity of the potential conflict. In the context of this article the term ‘weather avoidance’ is used to describe avoiding actions taken by a pilot to circumnavigate adverse weather (CBs, TCUs, severe turbulence, hail, etc).

There is no set of ready out-of-the-box rules to be followed universally. The controllers should exercise their best judgment when dealing with LOS. The examples and advice provided in this article are not intended to be exhaustive and shall not have prejudice to local ATC procedures.


Loss of separation during weather avoidance is usually caused by an unexpected and considerable change to the flight trajectory. The options for manoeuvring and restoring the separation are likely to be very limited.

Useful to know

In adverse weather scenarios reduction of available airspace is to be expected in both lateral and vertical dimensions. In some countries, hail prevention shooting associated with danger area activation reduces further available airspace. Depending on the scale and position of the adverse weather some sector entry and/or exit points may not be usable and otherwise separated entry and exit traffic flows may be merged over one single sector entry/exit point. Some procedures fixed in the Letters of Agreement (LoA) between adjacent ATC units, such as flight level allocation schemes for transfer of control, may not be possible to apply, which will cause increased verbal coordination between the ATC units concerned.

Speed control is less effective during CB avoidance given the uncertainty of the future flight path. Also, this separation method may not be effective as aircraft have reduced speed range in case of turbulence (pilots may reduce speed to weaken the effect on passenger comfort).

Vectoring may not always be useable during CB avoidance. A pilot would most likely neglect an instruction that would take the aircraft into a CB area. Also, while ground systems have greater range than onboard equipment, the weather radar products made available to controller present a rather old “picture” compared to on-board weather radar data. Hence pilots are better placed to decide on the most suitable avoiding action.

Sometimes pilots start the avoiding action before obtaining ATC clearance or before advising the controller of the intended manoeuvre. Such practices are considered significant safety threat as controllers are not aware of the avoidance manoeuvres and may lead to loss of separation, in particular in congested airspaces.

Typical Scenarios

  • Sudden change of heading – an aircraft unexpectedly turns towards another one;
  • Sudden change of heading/level – in case of severe icing/turbulence an aircraft may promptly leave the hazardous area without prior notification to ATC;
  • Visual Flight Rules (VFR) flight avoiding clouds – in class B or C airspace this might lead to a LOS event with an Instrument Flight Rules (IFR) flight;
  • Degradation of Reduced Vertical Separation Minima (RVSM) capability – in case of severe turbulence aircraft may not be able to maintain assigned flight level;

Contributory Factors

  • Surprise – the first avoiding action by an aircraft may come unexpected for the controller;
  • Traffic complexity - and respectively workload - is increased due to the non-standard routes used and the dynamic conflict points;
  • Communication – safe operations during weather avoidance require increased air-ground (with pilots) and ground-ground (with adjacent sectors) communication;
  • Frequency congestion - may be caused by the significant increase in air-ground communication exchange;
  • Unpredictable avoidance manoeuvres – different airlines have different SOPs regarding weather avoidance.


As a controller:

  • Inform pilots as soon as practicable about reported adverse weather along the route, especially about turbulence and in-flight icing.
  • In case of turbulence avoid, as far as possible, the use of opposite flight levels to solve crossing conflicts. Vectoring or 2000ft separation are usually better options.
  • Ask pilots about the likelihood of an avoiding action or course change within the next few (e.g. 3-5) minutes in order to build an adequate short-term plan.
  • Assign safe (non-conflicting) flight levels to climbing and descending aircraft.
  • Issue instructions to cross conflicting levels after making sure the crossing will be safe (e.g. using rates, locked headings).
  • If unsure, ask pilots if an instruction is executable before issuing it.
  • Assign rates of climb/descent to make sure climbing/descending aircraft remain separated even in case of a sharp unexpected turn.
  • Be aware that assigning different flight levels to aircraft on converging flight paths is generally preferable than speed control or vectoring. Be aware that number of aircraft on converging flight paths will increase if exit points become unavailable.
  • Think ahead of the events- consider alternative plans for separating flights in adverse weather conditions - an aircraft might be too heavy to climb higher and descent might not be an option.

Organizational Defences

  • Training and awareness. The rating training phase usually includes some exercises focused on operations during adverse weather. In addition the ANSPs should consider introducing seasonal refresher training (e.g. before the start of the summer season). This would improve ATCO performance when facing such scenarios.
  • Workload management. Possible measures include:
    • Sector configuration management (e.g. opening more sectors);
    • Additional controller at the sector;
    • Flow control measures.

Further Reading