Recommendation

SUBJECT



Safety implications relating to the provision of a radar advisory
service (RAS) in conjunction with a radar control service.





OCCURRENCE SUMMARY



A Perth air traffic control position was required to respond to
requests from pilots for a radar advisory service while providing a
radar control service. In the occurrence (9503859), immediate
avoidance directions had to be issued to the pilots of two aircraft
under radar control to prevent a potential conflict.



Another occurrence (9603722) revealed that the provision of a
radar advisory service increased the complexity of the controller's
task and resulted in a breakdown of separation.



In both occurrences the controllers' attention was diverted from
the radar control function and radio transmissions from RAS
aircraft increased controller workload, which resulted in a
degradation of the safety nets of the services being
provided.





SAFETY DEFICIENCY



The provision of a radar advisory service diverts controllers'
attention from radar control functions. Radio transmissions from
RAS aircraft increase controller workload and cause distractions
which have safety implications especially when radar separation
instructions are required.





ANALYSIS



Workshops



Prior to the introduction of the RAS by radar control positions,
Airservices Australia conducted a number of workshops to assess the
impact of the provision of a RAS. These workshops were generally
subjective in nature and attempted to develop mitigation techniques
or facilities to minimise the impact of perceived problems with the
provision of a combined radar control and a RAS. Simulation trials
were conducted to gain an appreciation of the workload of a
combined service.



Radio transmission technique



Controllers operating air traffic control positions generally use
a radio transmission management technique which involves, adjusting
the timing and duration of their transmissions to better manage the
available air-ground-air time. The use of this technique assists a
controller to plan the management of their area of responsibility.
Specifically, a controller may make pre-emptive transmissions to a
number of aircraft prior to conducting coordination with other air
traffic control agencies. This enables the controller to minimise
the possibility for interruption from air-ground-air transmissions
while coordination is conducted, and to optimise the use of the
air-ground-air transmission time. The benefits gained by using the
technique vary between controllers and is also subject to aircraft
numbers and traffic complexity.



The radio transmission technique can be of major benefit when an
approach controller is required to vector aircraft for an intercept
of the runway approach path of a navigation aid for a pilot
interpreted approach. The timing of the instructions by a
controller to the pilot for the turn onto final can be critical
when there is a sequence of aircraft. If a controller mistimes the
instruction, the separation between subsequent aircraft in the
sequence may reduce to less than that required. As a consequence, a
controller may be required to resequence an aircraft to regain
separation, resulting in an increase in controller workload and
delays to other aircraft. A departures controller may use the
technique for a departure sequence with similar consequences should
instructions to a pilot be delayed due to other air-ground-air
transmissions.



The use of this technique is predicated by the radiotelephony
discipline of pilots in controlled airspace and the fact that a
majority of the transmissions relate to specific phases of flight.
Conversely, under RAS the radiotelephony of pilots operating
predominantly outside controlled airspace is usually not as
disciplined and consequently there may be a greater number of
air-ground-air transmissions than the ideal. Also, the duration of
the transmission may be longer than necessary.



The combination of radar control services and a RAS at a single
control position require a controller to respond to transmissions
from pilots operating in controlled airspace and outside controlled
airspace. The controller can endeavour to optimise transmissions
for aircraft inside controlled airspace and for known aircraft
outside controlled airspace. However, the controller will not be
able to compensate for those transmissions from the pilot of an
unknown aircraft outside controlled airspace who may request a RAS.
A request for a RAS could occur during a period of control
instruction transmissions. There will be periods when the timing of
a transmission may be critical and consequently safety would be
compromised.



Use of separate frequencies



The two services can be provided using a single control frequency
or a number of separate frequencies. The provision of a separate
frequency (with a mute capability to enable a controller to
prioritize aircraft transmissions) would be of limited benefit as
the controller would still be subject to distraction from RAS
requests. Controller workload and task complexity could be
increased by the need to monitor and differentiate the
transmissions from various aircraft. Additionally, all requests for
a RAS must be acknowledged even if it is only to deny the provision
of the service to the pilot.



Overseas experience



At busy locations in the United Kingdom, controllers are
discouraged from providing RASs to aircraft operating outside
controlled airspace because of the possible detrimental effect on
the air traffic control service.



In the United States, a RAS is offered on a workload-permitting
basis, subject to a number of factors. Controllers are made aware
of the problems that may be encountered when providing a RAS for
aircraft operating outside controlled airspace. Consequently, the
service is often denied, especially in areas adjacent to major
aerodromes where traffic levels are high.



Therefore, the safety implications of providing a combination of
services is appreciated by air traffic service agencies but they
have been unable to manage the issue apart from recommending that
controllers deny the service to the aviation industry. This action
would appear to be of limited benefit to either the industry or the
provider.



Industry expectation



In Australia, controllers are able to deny the request for
provision of a RAS due to workload. Usually, this is the case for
some periods at busy traffic locations. Therefore, it would appear
to be counter-productive to submit the controller to the request
for the service while fully appreciating that it is unlikely to be
performed. This is also the case with the aviation industry's
expectation regarding the service. Anecdotal evidence indicates
that the promotion of the provision of a RAS on a workload basis is
a frustration to pilots. The removal of the provision of a RAS in
conjunction with a radar control service from positions which are
unlikely to be able to perform the service would reduce controller
task complexity and limit pilot frustration.



Workload and complexity



The conduct of two or more tasks concurrently significantly
increases a controller's mental activity, particularly when
different techniques are used for the two tasks. In the situation
under consideration similar conflicts may be required to be
resolved by positive separation in controlled airspace and by the
provision of traffic information outside controlled airspace. An
increase in mental workload results from the limited attention
capacity available to humans when performing different tasks as
decisions are made sequentially. A further issue is that task
complexity is more likely to occur when time is short or in
situations where performance is critical. This situation does occur
for some periods when a controller is operating an ATS
position.



Controller workload is a function of aircraft numbers and the
complexity of the tasks being actioned. A controller providing a
radar control service in conjunction with a RAS is more likely to
experience an increase in mental workload because of the
differences and complexity of the tasks. Consequently, this
increase in workload may lead to a controller's mental processing
becoming overloaded and result in a reduction in performance.



This was highlighted by a recent study undertaken by Airservices
Australia to assess the impact of incorrect and uncorrelated,
secondary surveillance radar codes on the Sydney terminal
manoeuvring area. The study found that "once controllers are forced
outside of individual comfort zones, traffic handling is affected."
Additionally, the report noted that "code errors occurring in high
traffic load situations increased cognitive load to the extent that
on occasion the participants believed they were "losing track of
the full picture." A situation which has serious performance
ramifications." While the report related specifically to code
errors and controller workload, the results can be related to the
cumulative workload issues inherent with the provision of a RAS in
conjunction with a radar control service. The inability to manage
transmissions from pilots of unexpected "pop-up" aircraft
requesting a RAS, the necessary re-prioritizing of tasks and the
manipulation of the radar system, have similar task-complexity
implications.



The current method of assessing controller workload does not
consider human-factors and task-complexity aspects for an air
traffic service position. These issues need to be considered in
conjunction with the expected traffic numbers to better understand
the impact of the workload likely to be experienced by a
controller.