The crew of the Beech 1900D (B1900) aircraft had been cleared by the aerodrome controller (ADC) 1 to descend to 4,000 ft and were instructed to join the circuit via a left base leg for runway 12L (northern) circuit at Tamworth. The ADC 1 also instructed the crew to report approaching 4,000 ft. The crew acknowledged the clearance but later advised that they were unable to report approaching 4,000 ft due to frequency congestion. The ADC 1 had assigned 4,000 ft so that a procedural separation standard of 1,000 ft could be maintained with other aircraft until he could see the B1900 and apply visual separation.
The crew of the B1900 subsequently requested confirmation of their assigned level and reported their position when at 2NM north of the aerodrome. The controller then sighted the aircraft and cleared the crew to make a visual approach. The controller also asked the crew if they would need to extend through the centreline of runway 12L. The crew acknowledged the visual approach but did not advise the controller that they would need to extend through the centreline. The visual approach clearance authorised the crew to descend below 4,000 ft.
The crew of the B1900 could not comply with the instruction to join the circuit via a left base for runway 12L because the aircraft was too high and too fast. The co-pilot was the non-flying pilot and was undergoing command upgrade training. He had acknowledged the clearance for the visual approach and had previously acknowledged the instruction to join the northern circuit via a left base leg for runway 12L. At the time that the visual approach clearance was issued, the pilot-in-command was aware that they could not comply with the circuit entry instruction. He believed that the controller would have been aware that they were unable to enter the circuit on a left base leg, given their proximity to the circuit at that time and the height and speed of the aircraft. He also believed that the controller would have taken those circumstances, and the disposition of traffic in the southern circuit, into account when issuing the visual approach instruction.
The B1900 continued to descend and entered the runway 12R (southern) circuit airspace which was under the control of the ADC 2. The ADC 1 notified the ADC 2 that the B1900 had entered the southern circuit. The ADC 2 advised the ADC 1 that there was a Pacific Aerospace Corporation Airtrainer CT4B (CT4) on a right base leg for runway 12R. The ADC 2 provided traffic information about the B1900 to the crew of the CT4. There were four other CT4 aircraft inbound to the circuit at that time but the ADC 2 had not sighted them. The ADC 1 was unable to apply the required separation standard, either 1,000 ft vertically, or a minimum horizontal distance determined using the appropriate `Lateral Separation' standard from the Manual of Air Traffic Services (MATS), between the B1900 and the traffic inbound to the southern circuit. There was an infringement of separation standards.
The ADC 1 reported that he had sighted the CT4 on right base. He then instructed the crew of the B1900 to make a right turn and track via a right base leg for runway 12L. He believed that the B1900 was visually separated from that CT4 and that he could maintain visual separation between the B1900 and that CT4 with that instruction. He did not think that there had been an infringement of separation standards between the B1900 and the aircraft inbound to the southern circuit because all parties had enough time to ensure that visual separation was not infringed.