de Havilland Canada DHC-8-201, VH-TQG

The De Havilland Dash 8 was tracking inbound to Tamworth via the
040 radial of the Tamworth VOR (a radio navigation aid) at 3,500 ft
in accordance with air traffic control instructions. Meanwhile, a
TB10 Tobago was about to depart Tamworth via the 018 VOR radial, a
track that would require the Tobago to turn across the inbound
track of the Dash 8. Air traffic control had decided to limit the
initial climb of the Tobago to 2,500 ft in order to ensure vertical
separation with the Dash 8.

The aerodrome control task was being performed by a trainee
controller under the supervision of a rated controller. The pilot
of the Tobago was instructed to "line-up" and given an altitude
restriction of 2,500 ft, which was read back by the pilot. However,
the readback was not clear and the aircraft subsequently took off
with both the trainee controller and the rated controller uncertain
as to the actual altitude read back by the pilot, who was from a
non-English speaking background and had an accent that was, on
occasions, difficult to understand. It was subsequently determined
that the pilot of the Tobago had understood the controller to say
3,500 ft, and that was the altitude he had read back to air traffic
control. The crew of the Dash 8 were given traffic information on
the position of the Tobago by air traffic control and commenced a
look-out for that aircraft.

Because both controllers were unsure that the 2,500 ft
restriction had been correctly acknowledged, it was agreed that the
trainee controller would request the pilot of the Tobago to confirm
that he was maintaining that altitude. Before that could take
place, the pilot of the Tobago asked the controller to confirm his
assigned altitude. Again the transmission was not easy to
understand. The trainee controller confirmed 2,500 ft and passed
traffic information on the Dash 8 at the same time. The only reply
from the pilot of the Tobago was the word "affirm" and his
callsign.

In fact, the Tobago had been maintaining 3,500 ft, and it was
that later transmission from air traffic control that made the
pilot realise he should have been maintaining 2,500 ft. He
commenced an immediate descent from 3,500 ft, but did not make any
radio transmission to that effect.

The crew of the Dash 8 heard the exchange between air traffic
control and the pilot of the Tobago. At almost the same time, they
saw the Tobago about 400 m ahead, at the same level. They commenced
an immediate descent then realised the Tobago was moving to their
right. The aircraft passed with a horizontal separation of
approximately 200 m, and no discernible vertical separation. The
required standard was 1,000 ft vertical separation until the
aircraft had passed.