A Boeing 737-8FE (737) operating under the instrument flight rules (IFR) was en route from Coolangatta and descending for a landing at Melbourne. A Cessna Aircraft Company 421B (C421) operating under the visual flight rules (VFR) was en route from Moorook, South Australia, to Essendon, Victoria, at flight level (FL) 175. As the 737 was descending through FL206, the controller instructed the crew of the 737 to maintain FL180. A short time later the crew of the 737 received a resolution advisory from their traffic alert and collision avoidance system (TCAS) about the C421. Because the crew of the 737 had the C421 in sight, they chose to maintain visual separation rather than follow the TCAS RA. That action was in accordance with company policy. Based on analysis of recorded radar data, the vertical spacing between the 737 and the C421 was 400 ft when there was approximately 1.5 NM laterally between the two aircraft. The minimum vertical spacing reached was 300 ft when there was 2.74 NM between the two aircraft.
The C421 was in Class E airspace and the 737 entered Class E airspace on reaching FL180. There was no prescribed separation standard applicable in these circumstances, therefore there was no infringement of separation standards. While the ATSB initially assessed that this was not an airprox, a subsequent review of evidence against the definition of airprox has determined that although not considered serious, this was an airprox occurrence. In Class E airspace, pilots operating aircraft under IFR and VFR are required to maintain vigilance so as to see, and avoid, other aircraft (Civil Aviation Regulation. 163A).
A third aircraft, a Raytheon Beechcraft King Air 200, was part of the air traffic control sequence, but was not a factor in the reported occurrence.
FACTUAL INFORMATION
At 22:57:33 Co-ordinated universal time (09:57:33 ESuT), the pilot of the C421 contacted Melbourne air traffic control and requested an airways clearance. Although the pilot did not require an airways clearance in Class E airspace while operating under the VFR, the pilot intended to enter Class C airspace en route to Essendon. All aircraft require an airways clearance from ATC to operate in Class C airspace. About 10 seconds after the pilot of the C421 contacted ATC, the 737 crew made their first contact with the same controller. The 737 was descending from FL400. The controller acknowledged both calls and authorised the 737 crew to descend their aircraft to 8,000 ft.
At 22:58:17 the controller obtained flight details from the pilot of the C421 and identified the aircraft on his air situation display (ASD). The C421 was transmitting a transponder mode A, code 1200. The controller was not receiving any altitude information from the aircraft's transponder. Under NAS phase 2b introduced on 27 November 2003, the carriage and activation of a serviceable transponder was mandatory in Class E airspace and pilots were required to activate the altitude function (mode C) unless in receipt of a general exemption. The pilot of the C421 was unaware that his aircraft's transponder was not transmitting mode C altitude information and at 22:59:10 the controller informed the pilot. The pilot of the C421 subsequently reselected mode C and the altitude was then displayed to the controller on the ASD. Analysis of the recorded radar data showed that, at that time, the 737 was approximately 51 NM laterally and 20,870 ft vertically from the C421.
The 737 was equipped with a serviceable TCAS. That system was capable of determining the range, bearing and relative altitude of another aircraft transmitting mode 'A' and altitude information from the transponder of the other aircraft once that other aircraft came within encoding range of the TCAS. At 22:59:10, when the C421 began transmitting altitude information, it was not within range of the TCAS on board the 737. The operator advised that the TCAS fitted to this aircraft detects targets but does not display them until within 2,700 feet vertically of their aircraft unless the system calculates that the aircraft are on a collision course. The crew of the 737 were aware of the C421 because they heard the controller provide information about the location of the C421 to the pilot of another aircraft. As a result of that information, the crew of the 737 reduced their rate of descent from 3,000 ft/min to 1,000 ft/min until they could acquire the C421 either visually or on the TCAS display in their cockpit.
At 23:03:41, when there was approximately 18 NM and 7,800 ft between the 737 and the C421, the controller provided traffic information about the location of the C421 to the crew of the 737, relative to a tracking point that was common to the routes being flown by both aircraft. The crew of the 737 acknowledged that traffic information. They later reported that they did not have the C421 in sight at that time. At 23:04:14, when the two aircraft were approximately 14 NM and 6,100 ft apart, the controller passed traffic information on the location of the 737 to the pilot of the C421.
The controller became concerned that a collision risk could exist between the 737 and the C421 if the 737 continued descent through FL175 to 8,000 ft. At 23:05:23 the controller instructed the crew of the 737 to maintain FL180, even though there was no requirement to intervene. At that time there was approximately 7 NM and 3,100 ft between the two aircraft. When there was approximately 5 NM between the aircraft, the crew of the 737 identified the C421 on their TCAS and subsequently saw the aircraft.
Controller discretion to intervene was authorised under section 4.1.1.3 of the Manual of Air Traffic Services (MATS) which stated that 'Nothing in this chapter precludes a controller from using discretion and initiative in any particular circumstance where these procedures appear to be in conflict with the requirement to promote the safe conduct of flight'. That discretion was also authorised in the MATS prior to NAS phase 2b implementation.
The pilot of the C421 had intended to continue operating under the VFR to Essendon. However, due to weather in the Melbourne terminal area, the controller advised the pilot that an airways clearance to Essendon would not be available unless he could operate under the IFR. The pilot then advised the controller that he was upgrading to IFR.
At 23:06:05 the controller instructed the pilot of the C421 to turn right onto a heading of 270 degrees to facilitate the provision of an IFR clearance. At 23:06:36, after the C421 had commenced the right turn, the crew of the 737 reported to ATC that they had the C421 in sight. The pilot of the C421 later reported that he never saw the 737. At 23:07:06, the crew of the 737 reported to ATC that they had received a resolution advisory on their TCAS on the C421 as it passed in front of, and beneath their aircraft, that they were clear of the C421, and were requesting further descent. The controller subsequently issued a clearance to the crew of the 737 to descend to 8,000 ft. At 23:11:47, the controller confirmed with the pilot of the C421 that he was now operating under the IFR. The controller then issued the pilot of the C421 with an airways clearance and subsequently became responsible for the provision of separation between the 737 and the C421.
Prior to NAS phase 2b, the airspace in which the 737 and the C421 were operating at the time of the occurrence was classified as Class C airspace. In Class C airspace, both aircraft would have been subject to an ATC airways clearance and would have been separated in accordance with prescribed standards.
Under NAS, IFR aircraft are provided with an ATC service for separation with other aircraft operating under the IFR. ATC also provides information on the location of other aircraft operating under the VFR unless it is impractical to do so. A change from operations under the VFR to operations under the IFR in Class E airspace results in ATC ultimately assuming responsibility to provide separation between aircraft operating under the IFR. However, the pilot of an aircraft changing to IFR is responsible for separation with all advised traffic, until an alternative ATC separation standard exists.
