Interim Recommendation IR19980059

Interim Recommendation issued to: AirServices Australia

Recommendation details
Output No: IR19980059
Date issued: 23 April 1998
Safety action status:


Between October 1995 and July 1997, the Bureau of Air Safety Investigation undertook a study of the safety of Australian regional airlines. The objectives of this BASI safety study were to:

(a) identify safety deficiencies affecting regional airline operations in Australia; and

(b) identify means of reducing the safety impact of these deficiencies.

The Regional Airlines Safety Study involved analysing data obtained from:

(a) responses to a survey of Australian regional airline employees;

(b) discussions with Australian regional airline employees and managers; and

(c) air safety occurrence reports in the BASI database involving regional airlines over a 10-year period (1986 - 1995).

This interim recommendation addresses one of the safety deficiencies identified as a result of this study.

Safety deficiency

The combining of air traffic service operator positions during busy traffic periods can limit the ability of flight crew to exchange essential position, emergency or traffic information.

Related Occurrences


A de Havilland Dash 8 and a Lockheed C130, travelling in opposite directions on the same route, passed within 400 m of each other while at the same level. Pilots of other aircraft operating on the area frequency prevented the flight crews of the Dash 8 and the C130 from communicating with each other in adequate time to arrange separation. Two flight service positions were combined at one console and there were reported to be 21 aircraft using the two area frequencies, which were linked via a retransmission facility.


The pilot in command (PIC) of a Beech B200 taxied at Smithton for an IFR flight to Launceston. On departure he was unsuccessful in establishing communications on 122.6 MHz to obtain traffic information due to the volume of traffic on the frequency. The PIC reported that the communications problem was due to the increased traffic calls by aircraft taxiing at King and Flinders Islands, visual flight rules (VFR) and instrument flight rules (IFR) aircraft position reports and traffic requests for descent. The problem was further exacerbated by the retransmit facility which rebroadcast all transmissions on eight separate frequencies. The pilot advised that he made six attempts before contacting Melbourne to advise his departure, obtain traffic and to obtain a clearance to enter controlled airspace.

Discussions with other pilots who operate in the same area and air traffic service personnel in Melbourne, revealed that the frequency congestion problems were most critical at weekends when frequencies are combined.


At approximately 100 ft AGL after takeoff at Williamtown the left engine on a Beech 1900D failed. The crew conducted emergency checks and returned to the aerodrome and landed without further incident. An emergency call to Sydney Flight Service was not possible due to the large number of other pilots operating on the area frequency.

Regional airline safety study

The results of the Regional Airlines Safety Study revealed that 72% of respondents had experienced delays of more than two minutes due to frequency congestion, while using flight service area frequencies.

The study noted that frequency congestion appeared to be a problem at a large number of locations across Australia, suggesting that the problem was not isolated to one particular region. Pilots'comments from the study indicated that congestion was worse when frequencies were combined and particularly at weekends (when a number a frequencies were usually combined and allocated to a single operator position).

Reports since 1996

Reports since the study indicate that the problem of frequency congestion is still occurring. However, it is now being experienced on air traffic control frequencies since the transfer of some flight service functions to air traffic control.

Equipment limitation

Aeronautical Information Circular H34/97 - Flight Information Service Retransmission Facility, (December 1997) from Airservices Australia, detailed an equipment limitation which can inhibit the provision of services by air traffic services. The limitation was a function of the retransmit facility used to network a number of radio frequencies at an operator position. Airservices Australia acknowledged the limitation and provided advice to assist pilots operating on frequencies that may be retransmitted.


Although Airservices Australia has endeavoured to address the limitation inherent in the current equipment by issuing Aeronautical Information Circular H34/97, more consideration of the possible impact of the combining of air traffic service positions is required.

Ready access to and use of the area frequency is essential for crews of IFR flights to obtain traffic information from the air traffic service operator. Ready access to the area frequency is also essential for the exchange of position information between the crews of IFR and VFR flights, in order for an assessment of potential conflict to be made. The combining of two or more area frequencies using retransmission may create a hazard for aircraft operations.

Airservices Australia should develop procedures to assist operators in evaluating the advantages of combining air traffic service positions against the consequent potential for frequency congestion and the possibility of a degradation of safety in the aviation system.

Output text

The Bureau of Air Safety Investigation recommends that Airservices Australia review air traffic service procedures relating to the combining of a number of operator positions and/or frequencies with a view to reducing the impact of frequency congestion.

Initial response
Date issued: 11 November 1999
Response from: AirServices Australia
Action status: Open
Response text:

Reference is made to BASI's Air Safety Interim Recommendation No: IR 980059 regarding the use of ATS frequencies.

In addition to AIC H34/97, Airservices provides direction to ATS staff regarding the use of retransmit facilities in MATS 12-4-2 which shows "To reduce frequency congestion and interference on pilot broadcasts or other pilot-to-pilot communications being used for self separation, the retransmit facility should be operated in the "OFF" mode whenever practicable".

A number of other factors, in the context of the Airspace 2000 initiatives, relating to the provision of services are currently being discussed with CASA. Implementation of aspects of these initiatives would affect not only the way services are provided but the way in which ATS frequencies are used.

Airservices will continue to monitor the effects of retransmit facilities pending resolution of a number of issues associated with the implementation of CASA's Airspace 2000 initiatives.

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

Further correspondence
Date issued: 11 July 2000
Response from: AirServices Australia
Response status: Closed - Accepted
Response text:

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:

IR19980059: Whilst Airservices disagrees in part with the investigation report, the recommendation is accepted.

During the transition to TAAATS, Safety Cases have been conducted. These Safety Cases include consideration of a variety of hazards, including frequency congestion. This is particularly the case in relation to the current initiatives to absorb the provision of Directed Traffic Information services 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

Last update 01 April 2011