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.