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.
As a result of the investigation, the Australian Transport
Safety Bureau issues the following recommendations:
R20000301
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.
R20000302
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.
- The Inverell controller was experiencing personal stress
factors that may have affected his work performance. - The Inverell controller had minimal recency time on the sector,
which may have affected his performance. - The crew of CZA requested and were approved to climb to
non-standard level FL370. - The Inverell controller did not implement a separation
assurance plan when he became aware of the likelihood of a
conflict. - The Inverell controller allowed secondary considerations to
override separation actions. - The short-term conflict alert activated in sufficient time to
allow remedial action by the controller. - The TCAS on each of the aircraft provided each crew with an
alert indicating a required positive separation action.
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.
Recency
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.
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.