UK Airprox Board UK Airprox Board
  • Airprox Barrier Assessment

    Contemporary safety management systems use the concept of ‘safety barriers’ to identify, understand and manage the weaknesses, hazards and risks within their systems. The UKAB has also adopted this concept in recent years and has been evolving a methodology to provide a useful tool for analysing Airprox. Every Airprox is assessed to determine the effectiveness of the available mid-air collision (MAC) safety barriers during that Airprox scenario. These safety barriers are based on those developed by EASA, CAA and MAA, and represent contemporary understanding of the elements that contribute to the prevention of MAC.

    Each Airprox is subjectively assessed for each barrier using relevant word-pictures that describe the barrier’s availability and its function. These assessments are then presented on a chart that displays the weighting of each barrier and how it contributed to the Airprox. The barrier weighting (the length of each barrier) indicates how much a barrier might have contributed to a notional 100% successful collision avoidance if all the barriers were available and functioned perfectly. These weightings were determined for the UKAB by a group of experienced pilots and controllers who provided their expert opinion on the generic importance of each barrier for the 2 scenarios of being within or outside controlled airspace. The weighting (length) of these barrier representations does not change with each Airprox, they only change depending on which airspace type the incident occurred (e.g. in controlled airspace, see-and-avoid is subjectively considered to be 5% of the overall 100% barrier solution, whereas in uncontrolled Class G airspace it is considered to be 20%). It is stressed that these weightings are not mathematically determined; they are based on the subjective view of the group consulted in giving their interpretation of the relative importance of the 9 barriers.

    What does change for each Airprox is the effectiveness colour for each barrier. The UKAB assessment system has developed over the years and so it is important that this is recognised when comparing year-on-year assessments.  Although the differing assessments provide broadly the same outcomes, they are not directly comparable.

    2016 Airprox. The first iteration of the methodology was for the 2016 Airprox reports which were assessed for ‘Availability’ and ‘Functionality’ of each barrier. This used a ‘Traffic Light’ system based on a 3x3 risk matrix to determine their effectiveness (red indicated the barrier was ineffective; yellow indicated it was partially effective; green indicated it was fully effective; and black indicated it was either ‘unassessable’ or ‘not applicable’ (e.g. if ATC were not involved in a particular Class G incident, the ATC-related barriers would have been graded black for ‘not applicable’). 

    2017 and 2018 Airprox. In 2017, the methodology was developed further to recognise that not only might a barrier be not present, it might have been present but not used. Therefore, the 2017 and 2018 assessments replaced the single black ‘unassessable’ grading with a grey ‘not present’ and an open-red ‘not used’ indicator (in the ATC case above, if ATC was not present it was coloured grey, and if it was present but not used then the appropriate ATC barrier would be coloured open-red).

    2019 Airprox. In 2019 the assessment methodology was further refined to clarify the availability and function of each barrier by modifying the word pictures for each barrier and rationalising them under ‘Provision’ and ‘Application’ to determine to what extent the barrier was available and how effectively it was used by the individuals involved. This change also embraced the adoption of multiple contributory factors in Airprox assessment rather than the previous determination of a single cause. Multiple contributory factor assessments provide a much richer understanding of the circumstances behind why an Airprox occurred rather than simply focusing on what happened, as was often the case with single-cause statements.

    It is also important to note that although a barrier may only be partially available, the parts that were present could still have been fully functional.  For example, the circumstances might have been such that only one of the aircraft was fitted with a collision warning system (the ‘Electronic Warning System’ barrier would therefore be only partially available) but the pilot who’s aircraft was equipped might have been able to act fully on the information displayed to him to avoid the other in a timely and effective manner. In this case, the barrier would have been partially available (yellow) but fully effective (green overall).

    The following links provide a breakdown of the associated UKAB scoring scheme for each barrier in the respective years. It is important to refer to the relevant guide for the specific years when viewing Airprox reports and analyses.