Interim Recommendation IR19970175

Interim Recommendation issued to: Airservices Australia

Recommendation details
Output No: IR19970175
Date issued: 30 January 1998
Safety action status: Closed
Background: Why this Interim Recommendation was developed


Maximisation of the use of "separation assurance" techniques in conjunction with "alerted see-and-avoid" procedures by flight crews of aircraft operating in Class G airspace.


"Alerted see-and-avoid" is a procedure where flight crew, having been alerted (usually by radio communications) to the existence and approximate location of other aircraft in their immediate vicinity, seek to sight and avoid colliding with those known aircraft.

"Unalerted see-and-avoid" is a procedure where flight crew, who have no specific knowledge of other aircraft in their immediate vicinity, rely solely on their ability to physically sight and avoid colliding with aircraft that may be in their immediate vicinity.

"Separation assurance" techniques involve positive separation by mutual arrangement of a vertical or lateral distance that will ensure separation will be maintained, even in the event of a subsequent radio failure or when the crews of either aircraft do not see the conflicting traffic.


The crew of an Instrument Flight Rules (IFR) regular public transport (RPT) De Havilland Dash 8 (Dash 8) aircraft chose to proceed from controlled airspace, on descent to non-controlled airspace, using "alerted see-and-avoid" procedures. The crew had been provided with traffic information on an IFR RPT Embraer Brasilia aircraft which was proceeding in the opposite direction. Because of frequency congestion, there was no communication between the crews until after the Dash 8 passed underneath the Brasilia.

In another occurrence, the pilot of an IFR Cessna 310 (C310) aircraft descended in non-controlled airspace using "unalerted see-and-avoid" procedures and came in close proximity to an IFR Cessna 402 (C402). The pilot of the C402 estimated that the two aircraft had passed with approximately 200 ft lateral separation and at the same level. There had been no communication between the pilots of the two aircraft as they were operating on different frequencies (one on air traffic control and the other on the Common Traffic Advisory Frequency).

The crew of an IFR RPT Dash 8 aircraft, on descent to Taree in non-controlled airspace using "alerted see-and-avoid" procedures, was delayed in obtaining traffic information from flight service due to the number of aircraft using the area frequency. After receiving and analysing the information, the crew discontinued the descent at 10,000 ft due to a slower aircraft ahead of them which was maintaining 9,000 ft. Had the crew of the Dash 8 experienced a further delay in obtaining the traffic information, their continued descent would have possibly resulted in a proximity occurrence.


Flight crews of aircraft are not establishing communications prior to entering Class G airspace in sufficient time to arrange positive separation. They are placing an over reliance on their ability to see conflicting traffic in adequate time to manoeuvre and avoid other aircraft.


Current situation

As a result of the investigations of these incidents, two regional airlines have amended their flight operations manuals to emphasise the use of "separation assurance" in conjunction with "alerted see-and-avoid" procedures. Crews are required to establish vertical or lateral separation with conflicting traffic. A third regional airline is also considering similarly amending their flight operations manual.

A survey of chief pilots from five other low-capacity airlines revealed that only one company had published "separation assurance" techniques for self-separation of aircraft in Class G airspace. Three of these airlines had unwritten procedures that were common practice within their companies while the last airline only used the "alerted see-and-avoid" procedure.

The Aeronautical Information Publication (AIP) details procedures to enable flight crews to be alerted to possible conflicting traffic either by ATS operators or by communications on the appropriate area or Mandatory Broadcast Zone (MBZ)/Common Traffic Advisory Frequency (CTAF) frequency. However, the AIP does not require flight crews to use "separation assurance". Consequently, they would be totally reliant on their ability to sight the other aircraft in adequate time to safely implement avoiding action.

Operational technique

Generally, crews of IFR flights which are required to leave or descend from controlled airspace into non-controlled airspace expect that they will be able to establish timely communications with crews of conflicting aircraft. Having established communications, the crews can then arrange separation if necessary, prior to the flight leaving the protection provided by controlled airspace. However, this process does not account for those periods when the Class G airspace area frequency is so busy that communications cannot be established with any potential conflicting traffic.

The options for the crew of the aircraft entering Class G airspace are either to request an amended clearance to remain in controlled airspace until communications are established, or to continue into Class G airspace under "see-and-avoid" procedures. In the latter case, potentially conflicting traffic could remain "unalerted" with no assurance of separation.

Use of "alerted see-and-avoid" procedures

Evidence indicates that crews are relying on the "alerted see-and-avoid" procedure as the prime means for separation from conflicting traffic in non-controlled airspace. However, a crew's ability to visually acquire other traffic is limited by a number of factors.

The problems with human visual perception are well documented and have been the subject of considerable study. The BASI research report "Limitations of the See-and-Avoid Principle", published in 1991, related specifically to "unalerted see-and-avoid" procedures and concluded that "the most effective response to the many flaws of see-and-avoid is to minimise the reliance on see-and-avoid in Australian airspace". An article in the Airline Pilot magazine of October 1988 stated "Recent research shows that the concept of see-and-avoid is more seriously flawed than anyone, except perhaps pilots, suspected". Similarly, H E Mathinson of the US Airline Pilot's Association wrote in an article that "the see-and-avoid concept is an outmoded, antiquated system of traffic separation".

A recent Flight Safety Foundation article described research which analysed visual acquisition and the probability of detection of aircraft. This research found that "the probability of target detection was quite low in most cases".

Procedure guidance

The Aeronautical Information Publication (AIP) NCTL 51.1 describes the actions to be taken by the crew of an aircraft descending into Class G airspace. However, this does not provide particular guidance in relation to the actions required when the area frequency is congested.

Conversely, the AIP CTL 21.1.6 advises pilots to consider aircraft performance and the possibility of frequency congestion when determining the time to make a request for a clearance to enter controlled airspace. Also, AIP CTL 21.1.7 advises crews of dual VHF-fitted aircraft to monitor the area frequency during the clearance request.

Similar guidance in the AIP NCTL section should cover the actions that may be required prior to entering Class G airspace. The establishment of communications, prior to leaving controlled airspace, with the flight crew of aircraft operating in Class G airspace, is an essential precursor to establishing "separation assurance".

Having established communications with potential conflicting traffic, the flight crews should then arrange to have appropriate vertical or lateral separation applied between their aircraft.


The greater use of larger and faster aircraft for RPT flights in non-controlled airspace increases the need for the adoption and use of "separation assurance" techniques in conjunction with "alerted see-and-avoid" procedures by all flight crews.

Although, the inclusion of "separation assurance" techniques in airline flight operations manuals would address the deficiency to some extent, procedures for other IFR and the majority of Visual Flight Rules (VFR) flights are also essential. Many pilots may require guidance and training on "separation assurance" techniques.

"Alerted see-and-avoid" procedures used in conjunction with "separation assurance" techniques provide a fail-safe method of self-separation which enhances safety.

Output text

The Bureau of Air Safety Investigation recommends that Airservices Australia in conjunction with the Civil Aviation Safety Authority:

Review aviation regulations and instructions, with the aim of maximising the use of "separation assurance" procedures in conjunction with "alerted see-and-avoid" procedures by pilots of flights in Class G airspace.

The recommendation was sent to the Civil Aviation Safety Authority as IR970155. The Bureau of Air Safety Investigation further recommended that the Civil Aviation Safety Authority:

Implement an education program for pilots to promote the use of "separation assurance" procedures in Class G airspace.

Initial response
Date issued: 20 March 1998
Response from: AirServices Australia
Action status: Closed - Accepted
Response text:

Reference is made to your letter of 30 January 1998 regarding BASI Interim recommendation IR970175.

This Interim Recommendation more properly falls within the province of CASA to provide further guidance within the AIP about broadcast arrangements for aircraft leaving controlled airspace into uncontrolled airspace and should be addressed by CASA.

However, inclusion of text similar to that at OPS CTL - 1 paragraph 14.2 "When determinining when to make broadcasts on the Area VHF, the pilot should consider the performance of the aircraft and the possibility of frequency congestion, if the airspace is known to be busy "would appear appropriate in the NCTL section to reinforce the need for timely broadcasts to be made in these circumstances. Airservices would, therefore support the intent of the BASI recommendation.

Last update 18 March 2016