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Difference between revisions of "Loss of Separation During Weather Avoidance"

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==Introduction==
 
==Introduction==
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 ([[Cumulonimbus|CB]]s, [[Cumulus|TCU]]s, [[Turbulence|severe turbulence]], [[Hail|hail]], etc).
+
This article provides a general overview for controllers of some typical scenarios involving loss of separation (LOS) during weather avoidance. It offers some suggestions to help controllers provide as much support as possible to the aircraft concerned and considerations for maintaining 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 weather which may be considered by the pilot in command to be prejudicial to either the safety of their aircraft or to the comfort of its passengers such as a [[Thunderstorm]], [[Cumulonimbus]] (CB) or [[Cumulus|Towering Cumulus]] (TCU) cloud and the [[Turbulence|severe turbulence]] and [[Hail|hail]] which may be associated with them.
  
''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.''
+
''There is no set of ready, out-of-the-box rules which can be universally applied. Controllers should aim to exercise their best judgment when dealing with LOS risks. The advice and illustrations provided in this article are not intended to be exhaustive and should not prejudice the application of local ATC procedures.''
  
 
==Description==
 
==Description==
[[Loss of Separation|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.
+
[[Loss of Separation|Loss of separation]] during weather avoidance is usually caused by an unexpected and significant change to the trajectory of one or more aircraft. The options for manoeuvring and restoring any lost separation will often be very limited.
  
==Useful to know==
+
==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.   
+
In adverse weather scenarios, reduction of available airspace is to be expected in both lateral and vertical dimensions. In some countries, attempts to prevent hail by active intervention and the corresponding activation of danger areas may further reduce the volume of available airspace. Depending on the scale and position of the adverse weather, some sector entry and/or exit points may not be usable and normally discrete entry and exit traffic flows may be merged over one single sector entry/exit point. It may not be possible to apply normal procedures which are included in Letters of Agreement (LoA) between adjacent ATC units, such as flight level allocation schemes for transfer of control and this will impose an increased burden in respect of 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).
+
Speed control as a means to achieve separation can be ineffective during weather avoidance because of uncertainty on likely aircraft future track. This means of separation is also likely to be more difficult to apply because most aircraft types are subject to [[Aircraft Flight Manual (AFM)]] limitations which impose a speed limit (Va) for the maximum deflection of any primary flight control which cause pilots to consider proactively reducing speed if they consider that maintaining aircraft control might require such inputs. It is also usual for aircraft to have a recommended maximum "rough air" speed (Vra) for all flight in significant turbulence in order to reduce the effect of turbulence 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.  
+
The opportunities for radar vectoring may be constrained in the presence of significant areas of convective instability. A pilot would be likely to reject any instruction that would take their aircraft into close proximity with any CB cloud. Also, while ground weather radar systems have a greater range than most on-board equipment, even if weather radar display is available to controllers, it often presents a rather old “picture” compared to on-board weather radar data. All of this means that pilots are usually in the best position to determine 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.
+
Unfortunately, pilots will sometimes commence avoiding action without obtaining ATC clearance or even advising of their action at the point of commencement. Such action represents a potentially significant safety threat, especially in congested airspace and is contrary to most aircraft operator [[SOP]]s and may sometimes occur when a decision to deviate around adverse weather has been delayed beyond the point where the majority of pilots would have acted.
  
 
==Typical Scenarios==
 
==Typical Scenarios==
*Sudden change of heading an aircraft unexpectedly turns towards another one;  
+
*Sudden change of heading into conflict– 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;
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*Sudden change of heading/level – in the event of a sudden encounter with severe icing/turbulence an aircraft may promptly leave the hazardous area without prior communication with ATC;
*[[VFR]] flight avoiding clouds – in class B or C airspace this might lead to a LOS event with an [[IFR]] flight;
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*[[VFR]] flight avoiding cloud – in class B or C airspace this might lead to a LOS event with an [[IFR]] flight;
*Degradation of [[RVSM]] capability – in case of severe turbulence aircraft may not be able to maintain assigned flight level;
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*Degradation of [[RVSM]] capability – in the case of severe turbulence, aircraft may not be able to accurately maintain assigned flight level;
  
 
==Contributory Factors==
 
==Contributory Factors==
*Surprise – the first avoiding action by an aircraft may come unexpected for the controller;  
+
*Surprise – the first adverse weather avoidance action of a subsequent sequence may be unexpected by the controller;  
*Traffic complexity - and respectively workload - is increased due to the non-standard routes used and the dynamic conflict points;
+
*Traffic complexity - and the corresponding controller workload - is increased due to the non-standard routes being 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;
 
*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;
 
*Frequency congestion  - may be caused by the significant increase in air-ground communication exchange;
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==Defences==
 
==Defences==
 
As a controller:
 
As a controller:
*Inform pilots as soon as practicable about reported adverse weather along the route, especially about turbulence and in-flight icing.
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*Inform pilots as soon as practicable about any reports of adverse weather ahead, especially about those relating to 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.
+
*In the case of turbulence avoid, insofar as is possible, the use of opposite flight levels to solve crossing conflicts. Vectoring or 2000ft separation are usually more appropriate 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.
+
*Be prepared to ask pilots about the likelihood of their taking weather avoiding action or making a weather-predicated change of track within the next few, say 3-5, minutes in order to help build an adequate short-term plan.
 
*Assign safe (non-conflicting) flight levels to climbing and descending aircraft.  
 
*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).  
+
*Issue instructions for traffic to cross other traffic at conflicting levels only after ensuring that the crossing will be safe using rates, locked headings or equivalent conditions.  
*If unsure, ask pilots if an instruction is executable before issuing it.
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*If unsure whether an intended clearance in relation to adverse weather will be acceptable, ask pilots that question before issuing it.
*Assign rates of climb/descent to make sure climbing/descending aircraft remain separated even in case of a sharp unexpected turn.
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*Assign required minimum or maximum 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.
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*Remember that assigning different flight levels to aircraft on converging flight paths is generally preferable to speed control or vectoring. Be aware that the number of aircraft on converging flight paths may 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.
+
*Advise adjacent sectors or units for aircraft that are unable to maintain RVSM due to turbulence.
 +
*Think ahead - work out the essentials of alternative plans for separating flights in adverse weather conditions; take account of things like an aircraft being be too heavy to climb higher and the natural reluctance of pilots to accept significantly lower cruise levels due to concerns about increased fuel burn.
  
==Organizational Defences==
+
==Organisational 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.   
+
*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 should improve ATCO performance when encountering actual or potential loss of separation during weather avoidance scenarios.   
 
*Workload management. Possible measures include:
 
*Workload management. Possible measures include:
**Sector configuration management (e.g. opening more sectors);
+
**Sector configuration (e.g. opening more sectors than would normally be used at given traffic levels);
**Additional controller at the sector;
+
**Adding an additional controller for the affected sector(s);
**Flow control measures.
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**Implementing appropriate tactical flow control measures.
  
==Further Reading==
+
==Related Articles==
 
*[[Loss of Separation]]
 
*[[Loss of Separation]]
 
*[[Loss of Separation - Pilot-Induced Situations]]
 
*[[Loss of Separation - Pilot-Induced Situations]]
 
*[[ATCO Actions in Case of Loss of Separation]]
 
*[[ATCO Actions in Case of Loss of Separation]]
 
*[[ATC Operations in Weather Avoidance Scenarios]]
 
*[[ATC Operations in Weather Avoidance Scenarios]]
 +
*[[Aligned Weather Impact Management]]
 
*[[Loss of Separation at Sector Boundaries]]
 
*[[Loss of Separation at Sector Boundaries]]
 +
*[[Conflict Detection with Adjacent Sectors]]
 
*[[ACAS: Guidance for Controllers]]
 
*[[ACAS: Guidance for Controllers]]
 +
 +
==Further Reading==
 +
*[http://www.skybrary.aero/bookshelf/books/2438.pdf Green Paper on the gains for the European ATM Network of aligned weather impact management], EUROCONTROL, September 2013
 +
*[http://www.skybrary.aero/bookshelf/books/2443.pdf Severe weather risk management survey], EUROCONTROL, April 2013
 +
*[http://www.skybrary.aero/bookshelf/books/1544.pdf UK AIC: P 056/2010, "The Effect of Thunderstorms and Associated Turbulence on Aircraft Operations", 12 Aug 2010.]
 +
*[http://www.skybrary.aero/bookshelf/books/1546.pdf CAP 493 - Manual of Air Traffic Services Part 1]: UK CAA. Paragraph 17: "Weather Avoidance".
 +
 +
[[Category:Loss of Separation]]

Latest revision as of 11:27, 29 March 2016

Article Information
Category: Loss of Separation Loss of Separation
Content source: SKYbrary About SKYbrary
Content control: EUROCONTROL EUROCONTROL


Introduction

This article provides a general overview for controllers of some typical scenarios involving loss of separation (LOS) during weather avoidance. It offers some suggestions to help controllers provide as much support as possible to the aircraft concerned and considerations for maintaining 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 weather which may be considered by the pilot in command to be prejudicial to either the safety of their aircraft or to the comfort of its passengers such as a Thunderstorm, Cumulonimbus (Cb) (CB) or Towering Cumulus (TCU) cloud and the severe turbulence and hail which may be associated with them.

There is no set of ready, out-of-the-box rules which can be universally applied. Controllers should aim to exercise their best judgment when dealing with LOS risks. The advice and illustrations provided in this article are not intended to be exhaustive and should not prejudice the application of local ATC procedures.

Description

Loss of separation during weather avoidance is usually caused by an unexpected and significant change to the trajectory of one or more aircraft. The options for manoeuvring and restoring any lost separation will often 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, attempts to prevent hail by active intervention and the corresponding activation of danger areas may further reduce the volume of available airspace. Depending on the scale and position of the adverse weather, some sector entry and/or exit points may not be usable and normally discrete entry and exit traffic flows may be merged over one single sector entry/exit point. It may not be possible to apply normal procedures which are included in Letters of Agreement (LoA) between adjacent ATC units, such as flight level allocation schemes for transfer of control and this will impose an increased burden in respect of verbal coordination between the ATC units concerned.

Speed control as a means to achieve separation can be ineffective during weather avoidance because of uncertainty on likely aircraft future track. This means of separation is also likely to be more difficult to apply because most aircraft types are subject to Aircraft Flight Manual (AFM) limitations which impose a speed limit (Va) for the maximum deflection of any primary flight control which cause pilots to consider proactively reducing speed if they consider that maintaining aircraft control might require such inputs. It is also usual for aircraft to have a recommended maximum "rough air" speed (Vra) for all flight in significant turbulence in order to reduce the effect of turbulence on passenger comfort).

The opportunities for radar vectoring may be constrained in the presence of significant areas of convective instability. A pilot would be likely to reject any instruction that would take their aircraft into close proximity with any CB cloud. Also, while ground weather radar systems have a greater range than most on-board equipment, even if weather radar display is available to controllers, it often presents a rather old “picture” compared to on-board weather radar data. All of this means that pilots are usually in the best position to determine the most suitable avoiding action.

Unfortunately, pilots will sometimes commence avoiding action without obtaining ATC clearance or even advising of their action at the point of commencement. Such action represents a potentially significant safety threat, especially in congested airspace and is contrary to most aircraft operator SOPs and may sometimes occur when a decision to deviate around adverse weather has been delayed beyond the point where the majority of pilots would have acted.

Typical Scenarios

  • Sudden change of heading into conflict– an aircraft unexpectedly turns towards another one;
  • Sudden change of heading/level – in the event of a sudden encounter with severe icing/turbulence an aircraft may promptly leave the hazardous area without prior communication with ATC;
  • Visual Flight Rules (VFR) flight avoiding cloud – 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 the case of severe turbulence, aircraft may not be able to accurately maintain assigned flight level;

Contributory Factors

  • Surprise – the first adverse weather avoidance action of a subsequent sequence may be unexpected by the controller;
  • Traffic complexity - and the corresponding controller workload - is increased due to the non-standard routes being 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.

Defences

As a controller:

  • Inform pilots as soon as practicable about any reports of adverse weather ahead, especially about those relating to turbulence and in-flight icing.
  • In the case of turbulence avoid, insofar as is possible, the use of opposite flight levels to solve crossing conflicts. Vectoring or 2000ft separation are usually more appropriate better options.
  • Be prepared to ask pilots about the likelihood of their taking weather avoiding action or making a weather-predicated change of track within the next few, say 3-5, minutes in order to help build an adequate short-term plan.
  • Assign safe (non-conflicting) flight levels to climbing and descending aircraft.
  • Issue instructions for traffic to cross other traffic at conflicting levels only after ensuring that the crossing will be safe using rates, locked headings or equivalent conditions.
  • If unsure whether an intended clearance in relation to adverse weather will be acceptable, ask pilots that question before issuing it.
  • Assign required minimum or maximum rates of climb/descent to make sure climbing/descending aircraft remain separated, even in case of a sharp unexpected turn.
  • Remember that assigning different flight levels to aircraft on converging flight paths is generally preferable to speed control or vectoring. Be aware that the number of aircraft on converging flight paths may increase if exit points become unavailable.
  • Advise adjacent sectors or units for aircraft that are unable to maintain RVSM due to turbulence.
  • Think ahead - work out the essentials of alternative plans for separating flights in adverse weather conditions; take account of things like an aircraft being be too heavy to climb higher and the natural reluctance of pilots to accept significantly lower cruise levels due to concerns about increased fuel burn.

Organisational 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 should improve ATCO performance when encountering actual or potential loss of separation during weather avoidance scenarios.
  • Workload management. Possible measures include:
    • Sector configuration (e.g. opening more sectors than would normally be used at given traffic levels);
    • Adding an additional controller for the affected sector(s);
    • Implementing appropriate tactical flow control measures.

Related Articles

Further Reading