Airprox Report 2013085

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Figure 3
A high level of thunderstorm activity caused both aircraft to deviate off track to avoid weather and
into closer proximity with each other. Also, due to the thunderstorm activity, the DH8D was
descending at a significantly slower rate than the A320. This was not noticed by the Welin
controller.
Summary
An Airprox was reported following TCAS RAs being received by an A320 and a DH8D.The TC Welin
controller did not notice that the descent rate of the DH8D was significantly less than that of the A320
and the two aircraft came into closer proximity than anticipated by the Welin controller. Both the
Cowly and Welin controllers issued avoiding action following low level STCA.
PART B: SUMMARY OF THE BOARD’S DISCUSSIONS
Information available included reports from the pilots and air traffic controllers involved, radar
recordings, transcripts of the relevant RT frequencies and reports from the appropriate ATC and
operating authorities.
The Board first considered the actions of the Welin and Cowly sector controllers. Although the
Airprox occurred within the Cowly sector, the Welin controller was in control of the DH8D, having co-
ordinated it into the Cowly sector. The Board noted that both controllers had reported conducting
significant weather avoidance within their sectors, which increased their respective workloads
considerably. However, civil ATC members commented that, in conditions when aircraft may ask for
heading changes for weather avoidance, it was prudent to ensure vertical separation as the
fundamental method. The Board opined that in deciding to clear the DH8D to descend through the
level of the A320, the Welin controller should have been more pro-active in monitoring the DH8D’s
descent profile. In this respect, the clearance issued by the Welin controller was not ‘fail-safe’ but
relied on his ability to monitor the aircraft in a busy traffic environment. One ATC member thought
that the Cowly controller could have stopped the A320’s descent at FL210, when the aircraft were
about 19nm apart and the DH8D was passing FL201. However, the Board considered that it was the
Welin controller’s responsibility to ensure separation between the two flights.
Turning to the actions of the pilots, the Board noted that both aircraft were being operated in
accordance with their clearances, albeit the DH8D was making a ‘slow’ descent at a rate of 500fpm to