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Interim Recommendation issued to: Airservices Australia

Recommendation details
Output No: IR19960009
Date issued: 05 August 1997
Safety action status: Closed


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


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.


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.



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.

Output text

The Bureau of Air Safety Investigation recommends that Airservices Australia re-assess the safety implications of providing a radar advisory service in conjunction with a radar control service.

Initial response
Date issued: 05 December 1997
Response from: AirServices Australia
Action status: Monitor
Response text:

Subject: Occurrence 9503859 generating Interim recommendation IR960009

Action by Airservices to re-assess the provision of a radar advisory service in conjunction with a radar control service was held in abeyance pending the outcome of AIRSPACE 2000 initiatives.

Now that Airservices has a clearer understanding of the airspace architecture in the medium term, it is proposed to conduct a re-assessment of the provision of joint radar advisory/separation services. This re-assessment will focus on workload and human factors issues.

[name supplied], Gen Mgr ATS Division 3/12/97

Further correspondence
Date issued: 20 January 1999
Response from: AirServices Australia
Response status: Monitor
Response text:

Thank you for your letter of 19 January 1999 and the copy of the BASI recommendation IR960009.

Not being completely familiar with all the terms used by BASI when dealing with responses to BASI recommendations, I would like to get a clearer idea of what the BASI classification "Closed-Accepted" means with respect to our continued provision of RAS for the next little while.

Having made a formal recommendation to Airservices about aspects of RAS, does the BASI classification of our response indicate that BASI accepts, as we do, that the risk associated with continuing to provide RAS in the short term is low enough to justify its continuation?

Notwithstanding any possible answer you may give to my question, Airservices still intends to fulfil its undertaking to BASI that RAS will be reassessed as part of the broader issues associated with Airspace 2000 and the commissioning of TAAATS.

ATSB response:

The following letter was sent to Airservices Australia on 6 December 1999:

Subject: Outstanding airspace related safety actions

Responses to some previous safety recommendation to Airservices Australia and the Civil Aviation Safety Authority (CASA) proposed actions that were subject to the latter's Airspace 2000 initiative or the resolution of related issues.

Since the Minister of Transport and Regional Services' statement clarifying the roles and responsibilities for airspace management it has become apparent that, with the change in roles, CASA may not be in a position to implement all of their proposed airspace related safety actions. Consequently, there is the likelihood that there will be less safety enhancement of airspace than there previously might have been.

However, should Airservices Australia review the safety recommendations previously forwarded to CASA and the subsequent responses, during the development of the new airspace project, the knowledge gained from past air safety investigations has the potential to be retained. Consequently, the ATSB would appreciate Airservices consideration and advice of any subsequent proposed action, of interim recommendation (IR) 19970155 issued on 30 January 1998 and the CASA response of 25 August 1999 (both attached).

Similarly, advice of any proposed action in regard to IR19960009, IR19980005 and IR19980059 issued to Airservices, where further action was also subject to CASA's Airspace 2000, would assist the ATSB in understanding the integration of safety lessons with future airspace developments.

Further correspondence
Date issued: 17 May 2000
Response from: AirServices Australia
Response status: Closed - Accepted
Response text:

Outstanding Airspace Related Safety Actions

I apologise for the late response to your letter dated the 6th of December 1999, concerning the resolution of matters relating to the Airspace 2000 initiative. We note the recent statement by the Minister for Transport and Regional Services, and its effect in clarifying the roles and responsibilities for airspace management.

In the current airspace management arrangements, Airservices holds the legislative responsibility for the declaration of airspace in accordance with the ICAO "alphabet" menu of airspace. Responsibility for procedures applicable within airspace classes, including Class G airspace, together with ongoing training and pilot education remains with the Civil Aviation Safety Authority.

In that context, we believe that Recommendation IR19970155, relating to separation assurance in Class G airspace is more properly directed to CASA.

In relation to Recommendations IR19960009, IR19980005 and IR19980059, I offer the following advice.

IR19960009: Whilst Airservices disagrees in part with the investigation report, the recommendation is accepted.

Airservices has conducted extensive re-assessment of safety implications through the conduct of Safety Cases associated with the transitioning into TAAATS.

In relation to your statement regarding the integration of safety lessons with future airspace developments, Airservices is acutely aware of the primacy of safety in all aspects of our operations. In work being undertaken with industry to achieve reforms in the provision of services in low level airspace, detailed examination of hazards, potential mitigations, safety analysis and risk modelling are a fundamental part of our considerations. We would welcome the opportunity to provide you and your staff with a briefing on that reform program at your convenience.

I trust that this addresses your concerns. Please do not hesitate to contact me if you require further clarification.

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Last update 01 April 2011