Traffic alert and collision avoidance system, Resolution Advisory, Western Australia

On 19 April 2006, a Fairchild Industries Inc SA227-DC (Metro) aircraft was being operated from Newman, WA to Perth, WA at flight level (FL) 200. A Boeing Company 717-200 (717) was being operated from Karratha, WA to Perth at FL320. Both aircraft were in controlled airspace and were on tracks that converged.

The crew of the 717 had been provided with a clearance to descend to FL210 by the air traffic controller managing the airspace the aircraft were operating in. This level provided the required minimum vertical separation standard of 1,000 ft with the Metro.

As the crew of the 717 were approaching FL225 the traffic alert and collision avoidance system (TCAS) provided a traffic advisory relating to the Metro. A few seconds later the crew received a TCAS 'reduce descent rate' resolution advisory (RA). The crew advised the controller 'we've got a TCAS resolution advisory we're leaving F210 on descent'. The controller responded by instructing the crew of the 717 to maintain FL210.

Later analysis of Airservices Australia recorded radar data and audio recordings demonstrated that the crew of the 717 had descended to, and maintained, FL210. There was no infringement of separation standards.

The crew of the 717 maintained a high rate of descent approaching their assigned level, which resulted in the TCAS 'reduce descent rate' resolution advisory. The controller did not recognise the rate of descent of the 717, together with the relative positions of the two aircraft, may have generated a RA.