Loss of Separation

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. Standard lateral and vertical separations in radar-controlled airspace are 5 nm and 1000 feet respectively although with radar control in terminal airspace, regulations may permit a 3 nm lateral separation minimum where radar cover is adequate.
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 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 restricted airspace is dealt with under Airspace Infringement;
• Loss of separation from the ground is dealt with under CFIT.

(Loss of sepatation 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 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 ocupants may also result from a wake vortex turbulence encounter.

Defences

• Pilot situational awareness of the location and intent of other aircraft gained from listening to radio traffic, visual identification and monitoring and ACAS, especially when not in receipt of a ATS radar or procedural control or when operating outsided controlled airspace;

• Standard Operating Procedures, both on the flight deck and in the 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 (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 EU-OPS 1.668);

• Ground-based equipment designed to warn of potential collision with other aircraft (STCA).
• Ground-based equipment designed to warn of potential conflict between aircraft in flight (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

• Pilot-induced situations: Flight in controlled airspace (CAS) deviates from cleared track or level without clearance due to flight crew:
  • Inattention to equipment malfunction;
  • Mis-setting of aircraft equipment;
  • Mis management of FMS inputs;
  • Failure to follow ATC clearance;
  • 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;
  • Failure to properly follow a TCAS 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;
  • Ineffective visual "look out" when operating VFR;
  • Not flying instructed or expected speeds or rates of climb and descent which have been the basis of a controllers flight sequence management;
  • Allows their aircraft to enter CAS without ATC clearance

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.
• 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

Solutions

• Encourage or mandate the use of high quality communication systems, Transponders (Mode C and Mode S) and 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. STCA.
  • Safety Nets must be set up so as to generate low levels of Nuisance alerts whilst providing useful warning time.

Content source:

SKYbrary