The pilot of the C182 had been cleared to operate over the metropolitan area at 5,000 ft, provided he remained 2 NM west of the extended centreline of Perth runways 03/21. This clearance had been issued several hours before, by a different controller to the one involved in this incident.
A C172 had also been operating over the metropolitan area at 1,500 ft and had been cleared to return to Jandakot.
The B767 was departing Perth from runway 21 and the crew intended to turn right and depart towards the north-west.
During the day, the pilot of the C182 had been following a regular, orbiting flight path which was displaced from the B767's planned departure track. After the B767 took off the C182 made two irregular orbits which took it further south than previously, and closer to the B767's path.
The approach-west controller was aware of a possible conflict between the B767 and both the C172 and C182, prior to the B767's departure. He had formulated a plan to ensure that separation was maintained. The plan required the B767 to maintain runway heading until it was clear of both aircraft. The initial clearance to the crew of the B767 was to maintain runway heading after take-off and to climb to, and maintain 6,000 ft. The altitude requirement was to keep the B767 clear of military airspace.
When the crew of the B767 gave their departure report the approach-west controller was engaged in a lengthy coordination discussion with another controller and he was not maintaining a continuous radar watch on the airborne traffic. He glanced at the radar and noted that the B767 was clear of the C172 before he directed the crew of the B767 to turn right onto 280 degrees. This turn was to allow the aircraft to use a preferred noise abatement track, one of a number that have been established in the metropolitan area. The controller had forgotten about the C182 and did not note its location when he glanced at the radar.
As the B767 established a heading of 280 degrees the C182, which was at the end of its southerly track, turned onto an easterly heading. This resulted in a reduction in vertical separation to 400 ft as the aircraft passed 2.8 NM apart. This was below the requirement to maintain at least 1,000 ft if the aircraft are less than 3 NM apart. The crew of the B767 received an electronic warning of the C182's proximity and also acquired it visually. No evasive action was necessary.
The evidence indicates that C182's long-standing clearance and its regular pattern may have caused the controller to establish a mind-set that the B767/C182 separation problem was less important than it actually was. The change in pattern was unexpected and went unnoticed. The controller's attention was distracted from the radar console by his discussion with another controller.
The combination of the unexpected and unobserved change in the C182's flight path and the distraction resulted in the controller only ensuring the B767 was clear of the C172 before he cleared the crew to turn onto a new heading.
The controller also indicated that he was mindful of the need to comply with noise abatement procedures and the priority he assigned to this may also have been a factor in his missing the impending confliction.