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Mid-Air Collision

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The original material for this article was contributed by DNV
Article Information
Category: Loss of Separation Loss of Separation
Content source: DNV DNV
Content control: EUROCONTROL EUROCONTROL

One of the most hazardous consequences of a loss of separation between aircraft, including as a result of a level bust, is a mid-air collision

Contents

Definition

A Mid-Air Collision (MAC) is an accident where two aircraft come into contact with each other while both are in flight.

  • Events where aircraft collide on the runway or while one is on the ground and the other in the air close to the ground are covered under Runway Incursion.
  • Events where aircraft collide during taxi or push-back (including collisions with parked aircraft) are covered under Ground Operations.
  • Events where aircraft collide with obstacles (e.g. terrain, buildings, masts, trees etc.) while in flight are covered under CFIT.

Effects

Possible consequences of a MAC are temporary or permanent Loss of Control as a result of damage, avoidance manoeuvre, or mis-handling, potentially resulting in collision with terrain, or an emergency landing as a result of damage to the aircraft and/or injuries to crew and passengers.

It is commonly assumed that any MAC would cause loss of both aircraft and all people on board. In fact, accident and serious incident reports show that there have been a few non-fatal MAC accidents. However, in most cases, total loss is the result.

A crash following MAC may also cause fatalities among people on the ground.

Defences

IVSI.jpg

Example of ACAS II traffic display, indicating a "Climb" RA with a vertical speed of 1500 ft/min.

The main barriers against MAC are:

  • Strategic conflict management, including:
  • Tactical conflict management, which may consist of:
    • ATC conflict management, in which ATCOs provide separation between aircraft.
    • Pilot conflict management, in which pilots are responsible for avoiding other aircraft, sometimes with the assistance of information from ATC.
    • Lateral offset.
  • ATC collision avoidance, including:
    • Short-term conflict alert (STCA)
    • Warning from ATCOs not directly responsible for separation. Although this is not a planned barrier, this type of ad-hoc assistance sometimes helps avoid collisions.
  • Airborne collision avoidance, including:
    • Airborne collision avoidance system (ACAS)
    • Visual airborne collision avoidance (See and Avoid)

Providence (i.e. the chance separation of the two aircraft trajectories in time or space) can also be considered a barrier against MAC. It explains why a loss of separation does not necessarily lead to a collision, even if all the managed collision avoidance barriers are unsuccessful.

Typical Scenarios

Because of the multiple barriers that are in place, most collisions do not have a single cause, but multiple causes, typically one for each unsuccessful barrier. “Unsuccessful” is a general term covering all types of failure causes, including technical reasons, human error (e.g. lack of response or misjudgement), impracticability (e.g. not enough time) or lack of coverage (e.g. equipment not fitted). Barriers may also be by-passed (e.g. if a conflict is created at the tactical stage, strategic conflict management is then inapplicable.

Further details on the causes of unsuccessful tactical conflict management are given in the article on loss of separation.

Examples of the causes of unsuccessful collision avoidance are:

  • Unsuccessful STCA warning:
    • No STCA coverage of area of conflict.
    • STCA failure to give warning in time, e.g. due to transponder failures, surveillance failures, STCA software failures, STCA parameters detuned to minimise false alarms etc.
    • ATCO failure to respond in time, e.g. the ATCO is distracted and misses the warning, or believes the warning is incorrect.
    • ATCO failure to recover separation in time, e.g. due to inadequate communication with the pilot or inadequate response from the pilot.
  • Unsuccessful warning from other ATCOs not directly responsible for separation:
    • No independent ATCO monitoring of area of conflict.
    • Other ATCO failure to detect conflict in time, e.g. for reasons as above.
    • Other ATCO failure to communicate warning to responsible ATCO in time.
    • Responsible ATCO failure to recover separation in time.
  • Unsuccessful ACAS warning:
    • ACAS not installed on the aircraft.
    • ACAS failure to detect the conflicting aircraft or issue a resolution advisory in time.
    • Pilot failure to respond with appropriate timely collision avoidance manoeuvre, e.g. does not respond, or incorrectly prioritises ATC instructions
    • Avoidance action invalidated by incorrect opposing action from the other pilot.
  • Unsuccessful visual warning:
    • Other aircraft in effect concealed, e.g. by IMC, darkness, flight deck surfaces or Empty Field Myopia.
    • Flight crew failure to observe the other aircraft in time to make avoidance action.
    • Pilot failure to respond with appropriate timely collision avoidance manoeuvre.

The causes of barriers being unsuccessful are not necessarily independent. In fact, the most important causes include ones that make several barriers unsuccessful (known as common-cause failures). These are considered further under contributory factors below.

Contributory Factors

In addition to the specific causes of barrier failure, there are many other factors that can contribute to MAC or influence its likelihood.

The influences include:

  • Traffic conditions. This includes the traffic density, complexity, mixture of aircraft types and capabilities etc.
  • ATCO performance. This includes fundamental issues such as workload, competence, teamwork, procedures, commitment etc, as well as the influence of ANSP safety management on these.
  • Flight crew training and corporate culture. This includes the same fundamental issues as for ATCOs, and the influence of aircraft operator safety management.
  • ATC systems. This includes systems such as flight data processing, communication, STCA etc, as well as the interaction with the human operator and the aircraft systems, and the procurement policy of the ANSP.
  • Aircraft equipment. This includes aircraft systems such as autopilots, transponders and ACAS, but also aircraft performance (e.g. rate of climb) and their physical size.
  • Navigation infrastructure. This includes the coverage and quality of navigation infrastructure.
  • Surveillance. This includes the coverage and quality of surveillance systems.
  • Flight plan processing. This includes the efficiency and reliability of flight plan submission, approval and distribution.
  • Airspace. This includes the quality and complexity of airspace design, route layout, extent of controlled or uncontrolled airspace, proximity of military operational or training areas etc.
  • Weather. This includes the occurrence of IMC conditions, storm activity and other turbulence that may influence conflict management and collision avoidance.

Key influences (common-causes) that may affect several barriers at once include:

  • ATCO performance. This is critical for tactical conflict management and ATC collision avoidance, but may also influence flight crew performance, and hence airborne collision avoidance. An example occurred in the Überlingen accident, where the pilot incorrectly prioritised late ATCO instructions over an ACAS RA.
  • Flight crew inappropriate response to an ACAS RA, or mishandling of a response to an ACAS RA.
  • Common information sources. Any information downlinked from the aircraft to the ATC is a potential source of common cause failures. For example, if the aircraft location is supplied by Mode C to both ACAS and ATC surveillance, any failures in the transponder or inaccurate height information will affect tactical conflict management, STCA and ACAS warning. This may also occur for aircraft without transponders or where a military aircraft is part of a formation and not transponding Mode C. This may leave see & avoid as the only available barrier.

Solutions

Reductions in collision risk can be achieved by reducing the most important reasons why the individual barriers are unsuccessful, especially common-causes; improving beneficial influences that may make existing barriers more successful; and introducing new barriers, if this can be done without degrading the ones that are already there. As well as reducing collision risk, it is also desirable to maintain awareness of reasons why collision risks have been made as low as they are, so as to prevent deterioration in the future.

Key areas with potential for improvement include:

  • Improved positive Safety Culture. This includes improving crew/team resource management, air ground communications, compliance with ACAS warnings etc.
  • More extensive fitment of safety nets (STCA and ACAS). This includes developing STCA suitable for terminal areas.
  • Improved reliability and consistency of safety nets. These need to provide early and dependable warning, and to reduce nuisance alerts. This includes using information downlinked from the aircraft, providing this is sufficiently reliable to offset the extra hazard potential of common-cause failures.
  • Improved aircraft systems to alert pilots to any non-availability of transponders and ACAS.
  • Improved ATC systems and procedures to enhance conflict management during any degradation of surveillance or STCA.
  • Improved communications systems and procedures, such as controller-pilot datalink. This has the potential to reduce VHF congestion and communication errors, providing it is sufficiently reliable to offset the lost benefits of broadcast voice communication.
  • Improved predictability of aircraft trajectories, so that conflicts can be predicted and resolved at an earlier stage, using MTCD and similar systems, and ATCOs need to make fewer interventions to maintain separation.

Related Articles

In the "Loss of Separation" Category:

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