Piper PA-31 Navajo, VH-ITF, Embraer EMB110-P2 Bandeirante, VH-MWX, 50 km south of Ballidu, Western Australia, on 4 February 1991

Circumstances:

The two aircraft were on identical tracks, with the slower aircraft (VH-ITF) in front and maintaining 8000 feet. VH MWX was faster and maintaining 10000 feet. Due to military flying activity, they were unable to fly from Ballidu direct to Perth but obtained a clearance to track via Clackline. VH-ITF obtained a clearance to enter controlled airspace (65 nautical miles from Perth) at 6000 feet, while VH-MWX was cleared to enter at 5000 feet. Although both aircraft had reported at Ballidu at the same time, the pilots were unable to see the other aircraft. VH-ITF descended to 6000 feet and the pilot of VH-MWX elected to descend to 7000 feet until he could maintain visual contact with the other aircraft. If he had not seen the other aircraft before entering controlled airspace (CTA) he intended orbiting outside CTA until separation was obtained. The pilot of VH-MWX saw the other aircraft about 13 kilometres before the airspace boundary and commenced descent to 5000 feet. He was overtaking to the left of the other aircraft and at the same altitude when VH-ITF was instructed to make a left orbit by the controller. This action was halted when the pilot of VH-MWX advised the other pilot and the controller against the turn because his own aircraft was in the way. VH-MWX subsequently descended to 5000 feet and VH-ITF was instructed to orbit to gain lateral spacing with the other aircraft. Identification of VH-MWX on radar was completed at 57 nautical miles (103 kilometres) from Perth, after the pilot had reported at 5000 feet.

Significant Factors:

The following factor was considered relevant to the development of the incident:

1. The controller used a poor technique in allocating clearances. This incident was not the subject of an on-scene investigation.