Loss of Separation - Pilot-induced Situations

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

Description

Loss of separation between aircraft sometimes occurs as a result of an aircraft deviating from the cleared track without clearance.

Loss of separation may be either in a vertical or a horizontal plane, or both.

Effects

• Loss of separation from other aircraft may result in collision. This is especially likely when Reduced Vertical Separation Minima (RVSM) procedures are in force.

• Injury, especially to cabin crew or passengers, may result from violent manoeuvres to avoid collision with other aircraft or the ground.

• Injury may also result from wake vortex turbulence encounter.

• High levels of stress for the pilots and controllers involved, which may lead to reduced performance.

Defences

• Standard Operating Procedures, on the flight-deck, which detail procedures to be followed to reduce the chance of loss of separation.

• Onboard aircraft equipment designed to warn of potential collision with other aircraft (TCAS).

• Ground-based equipment designed to warn of potential collision with other aircraft: (Short Term Conflict Alert (STCA)).

Typical Scenarios

• Flight deviates from cleared track or level without clearance:
  • Due to pilot inattention, equipment malfunction or the mis-setting of aircraft equipment;
  • To avoid a visually-perceived loss of separation from another aircraft;
  • To avoid severe weather if Instrument Flight Rules (IFR) or to remain in Visual Meteorological Conditions (VMC) if Visual Flight Rules (VFR).

• Pilot fails to follow ATC clearance or delays their actioning of an accepted clearance.

• Instruction not received or not understood by pilot due to ineffective air-ground communications.

• Pilot receives a TCAS RA but fails to follow it correctly.

• Pilot enters notified airspace without clearance.

Contributory Factors

• Weather (e.g. thunderstorm activity).

• Aircraft performance (e.g. high or low rate of climb or descent).

• Pilot workload.

Solutions

• Improve standard of pilot training, especially in:
  • Air-ground communications safety;
  • Loss of situation awareness;
  • Use of TCAS.

• Provide more and improved safety nets, e.g. Short Term Conflict Alert (STCA).

Related Articles

• Loss of Separation
• Loss of Separation - ATCO-induced Situations
• Accident and Serious Incident Reports: LOS contains examples of pilot-induced loss of separation.
• Visual Scanning Technique
• Loss of Separation During Weather Avoidance

Further Reading

• Safety Management System Manual v10.1, by the British Gliding Association (BGA), 26 February 2016

HindSight Articles:

• HindSight1 Analysis of an AIRPROX between a B737 and an A340;
• HindSight2 Loss of Separation - an Incident at Paris/Orly;