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Category:	Loss of Separation
Content source:	SKYbrary
Content control:	SKYbrary

Description

A defined loss of separation between airborne aircraft occurs whenever specified separation minima in controlled airspace are breached. Minimum separation standards for airspace are specified by ATS authorities, based on ICAO standards.

A loss of separation between aircraft which are responsible for their own separation by visual lookout is not subject to definition.

Usually, the occurrence of a 'near miss', termed an AIRPROX by International Civil Aviation Organisation (ICAO), is defined only by the opinion of one or more of the parties involved, whereas Near Midair Collision (NMAC) is an AIRPROX that meets specified criteria.

Types of Loss of Separation

Loss of separation may be either in a vertical or a horizontal plane, or both;
Loss of separation between aircraft may be a consequence of a Level Bust;
Loss of separation between aircraft may result in encounters with Wake Vortex Turbulence;
Loss of separation from notified airspace is dealt with under Airspace Infringement;
Loss of separation from the ground is dealt with under Controlled Flight Into Terrain (CFIT).
Loss of separation between aircraft on the ground is dealt with under Ground Operations

RVSM

If the required equipment is carried as prescribed, then the risk of loss of separation in Reduced Vertical Separation Minima (RVSM) airspace is no greater (and no less) than in non-RVSM airspace.

Effects

Loss of separation from other aircraft may result in collision;
Injury, especially to unsecured cabin crew or passengers, may result from violent manoeuvres to avoid collision with other aircraft;
Injury to aircraft occupants may also result from a wake vortex turbulence encounter.
High levels of stress for the pilots and controllers involved, which may lead to reduced performance.

Defences

Pilot situational awareness of the location and intent of other aircraft gained from listening to radio traffic, visual identification and monitoring and Airborne Collision Avoidance System (ACAS), especially when not in receipt of an ATS radar or procedural control or when operating outside controlled airspace;
Standard Operating Procedures, both on the flight deck and in the Air Traffic Services Unit (ATSU), which detail procedures to be followed to reduce the risk of loss of separation;
Aircraft onboard equipment which warns of potential collision with other aircraft (Airborne Collision Avoidance System (ACAS)) and allows an appropriate procedural response to risk. However, note that not all aircraft are required to be fitted with ACAS - only civil turbine-powered aircraft having a maximum certified takeoff mass in excess of 5,700 kg, or a maximum approved passenger seating configuration of more than 19.(ICAO Annex 6 Part I Chapter 6 Para 6.18 and EASA IR-OPS CAT.IDE.A.155, EU-OPS 1.668);
Ground-based equipment designed to warn of potential collision with other aircraft (Short Term Conflict Alert (STCA)).
Ground-based equipment designed to verify that the current clearances provide adequate separation in the short term (Tactical Controller Tool (TCT)).
Ground-based equipment designed to warn of potential conflict between aircraft in flight (Medium Term Conflict Detection (MTCD)).

Typical Scenarios

ATCO-induced situations

Flight clearance does not provide adequate separation from other traffic:
- Controller is aware but makes a misjudgement.
- Controller is unaware.
- A trainee Controller is being mentored and the mentor fails to intervene appropriately when the trainee allows a potentially hazardous situation to develop
- Failure in sector or unit co-ordination
- The controller fails to see the conflict due to the blind spot effect.

Pilot-induced situations

Flight in controlled airspace (CAS) deviates from cleared track or level without clearance due to flight crew:
- Mis-setting of aircraft equipment;
- Mismanagement of Flight Management System inputs;
- Failure to follow ATC clearance;
- Not flying instructed or expected speeds or rates of climb and descent which have been the basis of a controllers flight sequence management;
- Inattention to equipment malfunction;
- Avoiding a perceived (visual) loss of separation with another aircraft;
- Avoiding Weather perceived as potentially hazardous when unable to make timely contact with ATC on a busy frequency;
Flight outside CAS:
- Ineffective visual "look out" when operating Visual Flight Rules (VFR);
- Allowing the aircraft to enter CAS without ATC clearance;
- Failure to follow an ATC Clearance;
Response to TCAS RA:
- Failure to properly follow a Airborne Collision Avoidance System (ACAS) RA, including failure to terminate the deviation in a prompt manner when the RA ceases which may lead to a 'chain reaction' causing secondary TCAS RAs for aircraft in the same vicinity at adjacent levels.

Any of the above scenarios is exacerbated by high traffic density or a rapidly changing traffic situation and many can also lead to inadequate separation outside controlled airspace associated, for example, with military fast jet low flying. Entry to an ATZ outside CAS without clearance has also caused many losses of adequate separation.

Contributory Factors

Volume of traffic/Traffic density.
Lack of appreciation by IFR pilots of the absence of Separation Standards against VFR traffic within some classes of airspace
Weather (e.g. convective activity or clear air turbulence (CAT);
Aircraft performance (e.g. high or low rate of climb or descent)
Proximity of military operational or training areas.
Flight outside Controlled Airspace
Obscured track labels (e.g. due overlapping, filters, colour representation, etc.).

Solutions

Encourage or mandate the use of high quality communication systems, Transponder (Mode C and Mode S) and Airborne Collision Avoidance System (ACAS);
Improve standard of pilot and ATCO training, especially in:
- Air-ground communications safety;
- Loss of separation awareness;
- Crew resource management;
Provide more and improved ATC safety nets, e.g. Short Term Conflict Alert (STCA).
- Safety Nets must be set up so as to generate low levels of Nuisance alerts whilst providing useful warning time.
Use of structured scan.

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