Aviation safety investigations & reports

Boeing Co 737-377, VH-CZA

Investigation number:
Status: Completed
Investigation completed


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.

Factual Information

Sequence of events

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.

Traffic alert and collision-avoidance system (TCAS)

Each aircraft's TCAS functioned in accordance with design parameters. They activated at about the same time, and assisted the crews to respond appropriately.

The first warning the crew of CZX received was an instruction from the controller to turn right immediately. They commented to each other that the controller sounded worried and commenced the turn without delay. As they started the turn they received a TCAS traffic advisory and then a resolution advisory to climb. The pilot in command remembered seeing a "00" indication, which meant the aircraft were at the same height. The crew did not see the other aircraft, due to the turn manoeuvre, but carried out a climb in accordance with company standard operating procedures. They received a "clear of conflict" message after climbing 400 ft. The aircraft remained within a 500 ft excursion of its cleared level.

The crew of CZA had asked for FL370 due to uncomfortable turbulence at FL350. They had been at FL370 for some time and were unaware of the approaching aircraft until the TCAS activated a traffic information advice at the 12 NM extremity of the TCAS display. The pilot in command only had time to consider that advice before the traffic advisory activated and he looked up to see the other B737 straight ahead and in a turn. He heard the controller issue a right turn instruction to CZX and was confident that he could maintain visual separation behind that aircraft. However, when the resolution advisory activated, he commenced a descent in accordance with company standard operating procedures. The aircraft remained within a 500 ft excursion of its cleared level.

Air route and airspace design

Air routes H62 and H29 were subject to heavy air traffic movements and had to cross at some point. Air route design ensured that they crossed in the middle of the Inverell sector while at cruising levels. That design had the effect of producing a common crossing conflict in the centre of the display screen that provided the controller maximum time to recognise and rectify a conflict.

Inverell sector controller

The controller operating the Inverell sector was experienced in the position and reported that the traffic level at the time was moderate. The controller also reported experiencing significant personal factors that would have been likely to cause him considerable stress in the weeks prior to the incident.

The controller had taken several days off duty in the previous two weeks due to illness. He was current in only two of the four positions for which he held a rating, having registered 9.5 hours on the Inverell sector in the past 22 days and 3.5 hours in the last 15 days. The currency requirement was for at least 2 hours every 14 days for each rating.

The Civil Air Traffic Services Operations Administration Manual (CATSOAM) parts 5.2 and 5.3 outline the recency requirements for maintaining a valid air traffic control licence. In practice it was possible for some of the time logged to fulfil the recency requirements for a particular control position to be worked during times of low traffic levels. For example, working a nightshift where several positions were combined may have allowed a controller to credit the hours worked to more than one position.

In the 14 days before the day of the incident, the controller had worked only one shift; a night shift involving combined sectors for which he held ratings. That shift enabled him to meet the recency requirements to exercise the privileges of his air traffic control licence on two of his four ratings, including the control position he occupied at the time of the incident.

During the controller's absence, the Byron Group of sectors was moved to a different aisle within the Brisbane Centre. Although each sector was a stand-alone unit, the sectors each side of Inverell were then different. The controller reported that the repositioning of the sectors made the "feel" of the surroundings seem unusual when he arrived at the console.

When the controller arrived to start work at 1100, he had no immediate console function to perform and decided to read the documentation relevant to his ratings to catch up on the latest changes. While on that break a personal distraction resulted in him having to leave the console. Consequently, he did not take over the sector until approximately 1310.

A short time later (1324:30), the controller gave an instruction to the crew of a Dash 8 that they did not read back correctly. That was not recognised by the controller and was rectified only after an enquiry by the crew.

The controller received the correct coordination in respect of both CZA and CZX. He realised the potential conflict and checked, on at least two occasions, the relative distance of each aircraft from the intersection of the air routes, but did not subsequently take any positive action to ensure separation; even though the aircraft were 10 minutes from that intersection.

The controller was asked by the supervisor to check the radio system and, although initially saying that he had some other tasks to perform and that he would call the supervisor back, he commenced the radio checks soon after receiving the request and during a period of comparatively busy radio activity. The controller reported that he felt under pressure to complete the task without delay. That pressure was largely self-imposed as the supervisor had left the timing of the task to the controller's discretion. However, the controller was aware that the technicians were waiting at a remote location for the check to be performed and he wanted the task completed.

During that time the controller also changed the transmitter and receiver functions on the voice switching and communication system, thinking he was carrying out the changes requested by the supervisor. However, equipment-fail "bleeps" were evident as he was asking a pilot to comment on the quality of the radio transmission. Those bleeps indicated that the selections made were not correct and the controller's discussion with the supervisor confirmed that he had made an incorrect selection.

The controller stated that although he had acknowledged that CZA was at FL370, he was not fully aware that the aircraft was at a non-standard level. He was concentrating on the voice switching and communication system and the potential problems for sequencing aircraft that were soon to enter his airspace.

The controller stated that had allowed his "scan" to be diverted and, when the short-term conflict alert activated, he knew immediately what the problem was and acted to rectify the infringement of separation standards. He was unable to explain either his poor task prioritisation or his memory lapse.

Short-term conflict alert

The short-term conflict alert was designed to activate when the system checks determined that the aircraft were within 1 minute of coming within 4.1 NM and 600 ft. Analysis of the recording medium indicated that the alarm provided approximately 16 NM warning of the point of closest proximity. As the closing speed was approximately 15 NM/min, indications were that the system activated within reasonable tolerances.

Operational shift supervisor

The supervisor was dealing with radio frequency outlet problems on two sectors. He tasked the other sector first, as Inverell was the busier. Radio technicians at Point Lookout found equipment that needed replacement, but the task required the primary transmitter to be unavailable for about 24 hours. Before authorising its removal, the supervisor needed to check that the standby arrangements for the Inverell sector were satisfactory, and contacted the controller to have them checked. When the controller replied that he was busy, the supervisor waited for his return call. Two minutes later, the controller returned the call. The supervisor said that he thought that it was acceptable to the controller to commence conversation, otherwise he would not have returned his call. During that conversation the supervisor heard an alarm and immediately terminated the call. Although he did not know the nature of the alarm at the time, it was the short-term conflict alert that the supervisor heard.


Both aircraft crews acted in accordance with company procedures and followed the TCAS resolution advisories.

The incident occurred at a well-known point of conflict within an en-route sector. The experienced controller was operating a control position with which he was familiar and with traffic volume and complexity that should have been well within his ability. However, he did not take action to ensure separation between two aircraft that he had earlier recognised as being in potential conflict. That was most likely the result of a number of predisposing factors, including the effects of stress, limited recency, distraction, and not using a memory aid.

At the time of the occurrence the controller was testing the serviceability of the secondary radio transmitter. That was a routine and relatively unimportant task. However, the controller felt under pressure, largely self-imposed, to complete it without undue delay. Initially the controller was mistaken as to the exact test required and that led to some confusion on his part. Consequently, he was distracted for longer than anticipated and his awareness of the developing traffic situation was compromised.

Correct prioritisation is fundamental to any complex operating task. The controller was faced with several competing demands for his attention. In addition to monitoring the conflicting aircraft he carried out routine tasks such as issuing instructions to aircraft and transferring aircraft to or from other control sectors, and attempted to assist the supervisor with the radio checks.

Inadequate prioritisation committed the controller to remembering the unresolved aircraft conflict in order to deal with it in a timely manner. However, distraction and a subsequent memory lapse left the conflict unresolved until the activation of the short-term conflict alert.

There were other aspects of the controller's performance that, while not significant in isolation, were possibly indicative of a lower general level of performance at the time of the occurrence. When the southbound aircraft entered the controller's airspace, he did not appreciate that the aircraft was at a non-standard flight level, even though he was well familiar with the airways route. Later, approximately 10 minutes before the incident, the controller did not correct an oversight by a Dash 8 crew. Further, when asked by the supervisor to carry out a frequency check, the controller had some difficulty with what was a relatively straightforward task.


Using low traffic movement periods to combine sectors in order to maintain adequate work levels may result in an individual meeting the formal recency requirements, but not actually having sufficient exposure to a particular sector to warrant meaningful traffic practice. In this occurrence, the amount and type of recent control work that the controller had completed may not have been sufficient to ensure performance to the standard that the recency requirements were intended to maintain; taking into consideration the variation in the level/complexity of traffic during different periods. It is possible that the controller's performance was affected by the relatively short time he had worked in the control position during the previous fortnight.

Stress and performance

At the time of the incident the performance of the Inverell controller may have been adversely affected by stress. Recent significant personal factors may have been likely to cause him considerable chronic stress. The extent to which stress related to non-work factors can affect work performance is often underestimated. Major life events can markedly affect stress levels. In addition, the acute stress of leaving work to attend to an urgent personal matter may also have influenced the controller's performance.

Individuals are often unaware of the extent to which their performance is affected by stress. They may try to "work on" despite problems or difficulties. Individuals may be reluctant to admit, even to themselves, that they are suffering from stress because of a perception that this will be seen as a sign of weakness or failure.

Research has shown that stress can produce errors such as inappropriately delaying necessary actions and forgetting to carry out required actions at a time of high workload or distraction. Stress can result in perceptual and cognitive narrowing, where attention and decision making are focussed on a restricted range of information and tasks. For example, a controller's scan pattern may be disrupted. Stress can also lead to task shedding. This can result in the neglect of crucial matters while time may be spent on tasks of lesser importance. Memory can be significantly inhibited by stress.

In this incident, stress may have reduced the controller's capacity to handle what would normally have been a moderate workload. The controller allowed himself to be distracted by testing the radio equipment, to the detriment of his primary task; that of managing air traffic.

Memory aids

Because the potential conflict remained unresolved, the controller had to keep the task in short-term memory. Omitting to carry out planned actions - a failure of prospective memory - is one of the most common forms of memory lapse. A necessary condition for a memory lapse to occur is that attention is captured by either an external distraction or an internal preoccupation. The use of an appropriate memory aid would have guarded against the separation task being forgotten and a number of such methods were available to controllers.

Significant Factors

  1. The Inverell controller was experiencing personal stress factors that may have affected his work performance.
  2. The Inverell controller had minimal recency time on the sector, which may have affected his performance.
  3. The crew of CZA requested and were approved to climb to non-standard level FL370.
  4. The Inverell controller did not implement a separation assurance plan when he became aware of the likelihood of a conflict.
  5. The Inverell controller allowed secondary considerations to override separation actions.
  6. The short-term conflict alert activated in sufficient time to allow remedial action by the controller.
  7. The TCAS on each of the aircraft provided each crew with an alert indicating a required positive separation action.

Safety Action

As a result of the investigation, the Australian Transport Safety Bureau issues the following recommendations:


That Airservices Australia review the documentation in relation to air traffic controller recency requirements, in particular, the methodology of how individuals can meet the requirements.


That the Civil Aviation Safety Authority review the requirements for air traffic controller recency, such as the number of hours required, shifts that do or do not qualify for such recency, and documentation for recording such recency.

Related safety action

Related issues have been found in several Australian Transport Safety Bureau investigation reports and the Bureau is developing recommendations in respect of the affect of personal stress on controllers.

General details
Date: 23 May 2000   Investigation status: Completed  
Time: 1333 hours EST    
Location   (show map): 19 km S Gibraltar, (NDB)    
State: New South Wales   Occurrence type: Loss of separation  
Release date: 02 November 2001   Occurrence category: Incident  
Report status: Final   Highest injury level: None  

Aircraft 1 details

Aircraft 1 details
Aircraft manufacturer The Boeing Company  
Aircraft model 737  
Aircraft registration VH-CZA  
Serial number 23653  
Type of operation Air Transport High Capacity  
Damage to aircraft Nil  
Departure point Maroochydore, QLD  
Departure time 1300 hours EST  
Destination Sydney, NSW  

Aircraft 2 details

Aircraft 2 details
Aircraft manufacturer The Boeing Company  
Aircraft model 737  
Aircraft registration VH-CZX  
Serial number 24029  
Type of operation Air Transport High Capacity  
Damage to aircraft Nil  
Departure point Melbourne, VIC  
Departure time 1212 hours EST  
Destination Brisbane, QLD  
Last update 13 May 2014