Airprox Report 2013085

2
When N of DTY, inbound to HON, the pilot requested a turn onto heading 335° to avoid weather.
Having approved this request, he noticed that the A320 was level at FL200 and the DH8D was
passing FL197 and seemed to have stopped descending. He issued an avoiding action turn onto
heading 280° and the Cowly controller also gave the A320 avoiding action. Both aircraft received
TCAS RAs. He reported there was no loss of separation.
THE LTC COWLY CONTROLLER reports weather avoidance was taking place. He was monitoring a
trainee on TC Cowly when the A320, descending to FL200, requested a weather avoidance heading
of 135°. This was approved and notified to the Welin controller who had the DH8D heading 335°,
descending to FL170. A short time later the STCA flashed red when the two aircraft were about 10nm
apart with the A320 level at FL200 and the DH8D observed at FL197. He took over the RTF and gave
avoiding action to the A320 (left turn onto 090° degrees (he thought) and traffic information (TI) was
issued). The pilot reported a TCAS RA and was seen to be climbing. The DH8D was also given
avoiding action (by TC Welin) and reported a TCAS RA. There was no loss of separation.
Factual Background
Required minimum separation between the subject aircraft was 3nm horizontal and/or 1000ft vertical.
Analysis and Investigation
CAA ATSI
An Airprox was reported in Class C airspace by an Airbus A320 (A320), descending to FL200 in
the vicinity of Daventry when it received a TCAS RA against a Bombardier DHC-8-402 (DH8D),
which was descending into Class A airspace to FL90. ATSI had access to both pilot reports,
reports from the Cowly and Welin controllers, recorded area surveillance and transcription of
frequencies 121.025MHz and 130.925MHz, together with the unit report.
The A320 was operating IFR on a flight inbound to LHR, displaying SSR code 7664 and was in
receipt of a Radar Control Service from the London Terminal Control (LTC) Cowly sector on
frequency 121.025MHz.
The DH8D was operating an IFR flight inbound to BHX, displaying SSR code 4406 and was in
receipt of a Radar Control Service from the LTC Welin sector on frequency 130.925MHz.
Training was in progress on the Cowly sector. There was a high level of Thunderstorm activity in
the area which brought a high level of workload complexity for the Welin controller.
At 1900:15 the DH8D pilot contacted the Welin controller descending to FL220 heading 310° and
was given descent to FL170. At 1902:15 the DH8D was given further descent to FL90.
At 1903:32 the A320 pilot contacted the Cowly controller descending to FL200 heading 135° due
to weather. The Cowly controller advised Welin that the A320 was avoiding weather and the A320
pilot subsequently asked for a 5° right-turn heading 140°, which was approved by Cowly.
At 1905:13 the DH8D pilot requested a 20° right turn onto heading 335° to avoid weather which
was approved by the Welin controller (Figure 1). The two aircraft were 19.2nm apart and the
DH8D was 2100ft below the A320, however, the DH8D was only descending at a rate of 500fpm
while the A320, descending to FL200, was descending at a rate of 2000fpm.