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.