UK Airprox Board UK Airprox Board
  • Airprox Barrier Assessment

    Every Airprox is assessed to determine the effectiveness of the available mid-air collision (MAC) safety barriers during that Airprox scenario.  These safety barriers were developed by EASA, CAA and MAA and represent contemporary understanding of the elements that contribute to the prevention of MAC. 

    Each Airprox assessment is presented on a chart that displays the risk weighting of each barrier and how the barrier contributed to the Airprox. The risk weighting (the length of each barrier) shows the importance that the barrier would have contributed to a notional 100% successful collision avoidance where all barriers were available.  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 length of these barrier representations does not therefore change with each Airprox, they only change depending on which airspace type the incident occurred (e.g. in controlled airspace, see-and-avoid is generically considered to be 5% of the overall 100% barrier solution, whereas in uncontrolled Class G airspace it is considered to be 20%).

    What does change for each Airprox is the effectiveness colour for each barrier.  In this respect, the Airprox assessment is broken down to determine the ‘Availability’ and ‘Functionality’ of each barrier using a ‘Traffic Light’ system based on the 3x3 risk matrix to determine its effectiveness (red indicates the barrier was ineffective; yellow indicates it was partially effective; green indicates it was fully effective; and black indicates it was either unassessable or the barrier was not applicable (e.g. ATC may not have been involved in a particular Class G incident)).  It is important to note that a barrier may only be partially available but the parts that were present could still have been fully functional in ensuring the flight continued safely; the barrier would therefore be considered fully effective in such a case, even though it was only partially available.

    The following links provide samples of the Barrier Effectiveness Tables (with the barrier weighting for aircraft operating inside and outside controlled airspace) and a breakdown of the associated UKAB barrier 'availability' and 'functionality'  scoring scheme for each barrier.