Newcastle's Williamtown airport in Australia has made changes to its air traffic control procedures after a breakdown of separation occurred on 1 February 2011 between a Boeing 737-700 and an Israel Aircraft Industries Westwind 1124 (WW24).

An investigation by the Australian Transport Safety Bureau (ATSB) found that a series of errors had been made by the airport controllers involving separation assurance, coordination and communication and compromised safety recovery.

The 737, registration VH-VBK, was operated by Virgin Australia while the WW24, registration VH-AJG, was on a charter flight. The incident occurred when the 737 departed on a scheduled service while the WW24 was descending.

The airport, which also serves as a military airbase, employs Department of Defence (DoD) air traffic controllers. It is also divided into two jurisdictions: approach low (APP (L)) and approach high (APP (H)). Because of reduced military flying over the Christmas stand-down period, the controllers had become accustomed to combining the two sectors into one control position.

At the time of the incident, however, APP (L) and APP (H) were being operated as separate control positions.

At 13:00:33, the Williamtown Aerodrom Controller (ADC) coordinated with APP (L) for the departing 737 to climb to 5,000 ft and turn left to intercept its outbound track.

At 13:00:53, APP (H) told the inbound WW24 to descend to 5,000 ft visual, with the type of approach to be advised later. This contravened local air traffic control procedures, which mandated that the standard altitude APP (H) could assign was 6,000 ft. When the APP (H) and APP (L) were operated as one control position, however, the routine was to assign descending aircraft the 5,000 ft level.

"Assigning the arriving WW24 with the same level as the departing 737 (5,000 ft), without any other separation standard, resulted in a loss of separation assurance between the two aircraft," says the ATSB.

"At the point of closest radar separation, there was 400 ft vertical separation between the two aircraft," it said, adding that this was only achieved because the 737 crew had responded to a warning from its traffic alert and collision avoidance system (TCAS).

The required radar separation and vertical separation standards were 5.6km and 1,000 ft respectively, both of which were compromised.

Another significant safety issue spotted by the ATSB in its investigation, was that the DoD's air traffic controllers did not receive training in "compromised separation recovery" techniques and thus did not manage to recover the situation effectively.

The APP (H) traffic controller also had "limited opportunity" to operate the Williamtown approach control sectors when they were operated as independent control positions.

The APP (L) controller, meanwhile, did not verbally coordinate the departure of the 737 when the radar track remained uncoupled, or coordinate the vertical and tracking restrictions for the departing aircraft. He also failed to communicate the two aircraft's separation plans to its peer, said the ATSB.

The DoD has since increased its focus on safety alerts in its training school and job training materials. Safety alert phraseology has also been emphasized in all core theory exams and additional theory time on safety alerts has been incorporated into its school's training documents and simulator exercises.

After reviewing its procedures, DoD made changes to help controllers better differentiate between intended tracking and actual tracking as well as other procedural changes.

The operator of the WW24 has decided to install TCAS systems on all of its aircraft by this year.

Between 1 January 2010 and 30 June 2011, nine breakdowns of separations involving the Williamtown air transport controllers were reported to the ATSB.

Source: Air Transport Intelligence news