Incident Boeing 737-33A VH-CZX,
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ASN Wikibase Occurrence # 190872
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Date:Tuesday 23 May 2000
Type:Silhouette image of generic B733 model; specific model in this crash may look slightly different    
Boeing 737-33A
Owner/operator:Ansett Australia
Registration: VH-CZX
MSN: 24029/1601
Fatalities:Fatalities: 0 / Occupants:
Aircraft damage: None
Location:19 km S Gibraltar, (NDB) -   Australia
Phase: En route
Nature:Passenger - Scheduled
Departure airport:Maroochydore, QLD
Destination airport:Sydney, NSW
Investigating agency: ATSB
Confidence Rating: Information verified through data from accident investigation authorities
A Boeing 737, VH-CZA, departed Maroochydore for Sydney and was tracking southbound on air route H62. The crew planned to cruise at flight level (FL) 350 but experienced turbulence at that level. Several other aircraft had encountered turbulence at various levels up to FL350 but those at FL370 were in smooth flying conditions. Consequently, the crew had obtained approval from air traffic control to climb to non-standard level FL370 well before the aircraft was handed off to the Inverell sector controller.
A Boeing 737, VH-CZX, departed Melbourne for Brisbane, tracking via air route H29 and on climb to the flight planned level, FL370.
The air routes crossed near the Gibraltar non-directional navigation beacon.
The crew of CZA reported maintaining FL370 to the Inverell sector controller at 1322 Australian Eastern Standard Time.
At 1323, CZX was identified to the Inverell sector controller by the previous sector controller. The crew of CZX made radio contact at 1327, and reported that they were maintaining FL370. Between 1327 and 1329, the controller issued arrival and sequencing information to the crew. This information did not change the aircraft's current route or flight level.
The operational shift supervisor commenced a conversation with the Inverell sector controller at 1330 regarding the selection and testing of radio frequency outlets for that sector. The controller was performing other tasks and said that he would call the supervisor back. Between 1330 and 1332, the controller spoke with the crews of seven aircraft, including asking one of them for a radio check of readability after changing the radio outlet settings.
At 1332:30, the controller contacted the supervisor to discuss the selection of primary and secondary transmitters and receivers. During this exchange, the short-term conflict alert activated at 1333:22 due to confliction between CZA and CZX. The supervisor deselected the intercom line without further conversation.
The controller immediately issued a radar vector to the crew of CZX to effect avoiding action. The crew commenced the turn as instructed and received a traffic alert and collision-avoidance system (TCAS) resolution advisory to climb. They carried out the TCAS climb during the turn.
In the next 15 seconds, the controller passed traffic information on CZX to the crew of CZA. The crew of CZX then informed the controller that they were initiating a TCAS climb, and the crew of CZA broadcast that they were descending in accordance with a TCAS resolution advisory.
Analysis of the radar display data indicated that the radar vector took CZX across the flight path of CZA at 1333:46 while maintaining FL370. At that time, CZA was 10 NM north of CZX and also at FL370.
The point of closest proximity was 3.9 NM at 1334:11, when CZA was descending through FL369 and CZX was maintaining FL370. The required separation for that situation was either 5 NM horizontally or 2,000 ft vertically.


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