Interim Recommendation IR19990079

Interim Recommendation issued to: AirServices Australia

Recommendation details
Output No: IR19990079
Date issued: 08 June 1999
Safety action status:



The current published procedures that place reliance on only two radio broadcasts to facilitate alerted see-and-avoid between aircraft at non-controlled aerodromes are inadequate for collision avoidance.


Related occurrences

Occurrence 9900970

The pilot of a Beechcraft Sundowner (Beechcraft) was taxiing at Hoxton Park to begin his first solo navigation training exercise. The pilot of a Pitts Special (Pitts) had earlier flown from Bankstown to the training area and then to Hoxton Park, in order to practice circuits and landings at Hoxton Park before returning to Bankstown. The pilot of the Pitts reported that he transmitted a broadcast on 127.0 MHz as he entered the circuit at Hoxton Park, before reducing speed in order to maintain separation with a preceding aircraft in the circuit.

After that aircraft had landed, the pilot of the Beechcraft taxied onto the threshold of runway 34 and commenced to take off. In the meantime, the Pitts was approaching the same runway. The pilot of the Pitts reported that he had momentarily lowered the nose of the aircraft on a number of occasions during the approach, in order to check that the runway was clear before committing himself to a landing. The pilot reported that he did not see any aircraft on the runway or on the taxiway adjacent to the runway.

As the Beechcraft was accelerating during its take-off roll, the Pitts landed on top of it. Both aircraft became entangled and travelled approximately 100 m down the runway, before turning sharply left and stopping. The pilot of the Beechcraft received fatal injuries and both aircraft sustained substantial damage.

An onsite investigation found that the radios of both aircraft were tuned to the Hoxton Park Common Traffic Advisory Frequency (CTAF) frequency (127.0 MHz) at the time of the accident. Each radio was checked and found to be capable of normal operation. The investigation team subsequently conducted a reconstruction of the sequence of events leading up to the accident. That reconstruction revealed that the Beechcraft would have been very difficult to discern if it had been stopped on the threshold "piano keys" at the time the pilot of the Pitts was scanning the runway.

Occurrence 9802022

A Piper Archer (Archer) collided with a Piper Tomahawk (Tomahawk) at an altitude of about 1,200 ft as the Archer was tracking to enter the crosswind leg for a landing on runway 34 at Hoxton Park aerodrome. The collision occurred in fine and clear conditions, about 1 NM to the east of the upwind end of the runway. Both aircraft were being flown under the visual flight rules. The pilot of the Archer was able to maintain control of his aircraft and make a successful approach and landing on runway 34, although the nose landing gear had been substantially damaged in the collision. The aircraft stopped on the runway, resting on the collapsed nose landing gear.

The Tomahawk was observed by witnesses to immediately spiral down and crash into an unoccupied house in a suburban housing area. Both occupants received fatal injuries. There were no injuries to personnel on the ground.

The pilot of the Archer reported that he had tuned to the Hoxton Park CTAF frequency at Bonnyrigg, 2 NM north-east of Hoxton Park, and made an inbound broadcast. He reported that he made a further broadcast as he was joining crosswind for runway 34. Just prior to the collision, he had observed an aircraft turning onto crosswind after departure from runway 34 and another aircraft on downwind. The investigation was unable to determine the flightpath of the Tomahawk nor was it able to determine if the pilot of the Tomahawk had made the required CTAF broadcasts, as there was no automatic recording of that frequency.

Common Traffic Advisory Frequency (CTAF) procedures

CTAFs are normally located outside controlled airspace and within 5 NM radius of a designated aerodrome up to and including 3,000 ft above ground level (AGL), unless otherwise specified. Because of its proximity to Bankstown aerodrome, and the lower limit of controlled airspace above the aerodrome (2,000 ft), the Hoxton Park CTAF is limited to an area of 2 NM radius up to and including 1,700 ft AGL. Pilots of aircraft operating within that airspace are required to comply with CTAF radio communications procedures published in Aeronautical Information Publication (AIP) ENR 3.1.

The carriage and use of radio at CTAF aerodromes is not mandatory, but pilots of radio equipped aircraft are required to make broadcasts on the CTAF. Pilots are only required to make one broadcast when operating into a CTAF aerodrome (inbound), and similarly only one broadcast when departing from a CTAF aerodrome (taxiing). Due to the airspace constraints mentioned above, pilots adhering to the minimum requirements of the published CTAF procedures at Hoxton Park, are entering the circuit at the same time that they make their inbound broadcast.

Adherence to CTAF procedures varies considerably from one aerodrome to another, with some pilots limiting their radio broadcasts to the minimum required whilst others appear to make a number of broadcasts, announcing their location at a variety of key locations within the CTAF. In addition, the degree to which pilots use the radio to arrange self-separation in order to minimise the risk of collision also varies between aerodromes and pilots. There have been instances where pilots have been unable to establish the position of other aircraft in CTAFs due to a significant amount of radio traffic between other pilots attempting to arrange self-separation.

Bankstown General Aviation Airspace Procedures

Bankstown aerodrome is located approximately 7 NM to the east of Hoxton Park. The procedures to be followed at Bankstown aerodrome are described in Enroute Supplement Australia. Pilots of aircraft departing Bankstown who intend to remain outside controlled airspace, are to change frequency when 5 NM from Bankstown. Many pilots track to Hoxton Park from Bankstown for circuit and landing practice. If the procedures described above were strictly followed, pilots would be on the boundary of the Hoxton Park circuit area/CTAF before they changed to the CTAF.

Some aircraft are equipped with dual radio systems which allow pilots to maintain a listening watch on the Hoxton Park CTAF while tuned to and operating on the Bankstown frequency; however, many light general aviation aircraft are not so equipped. This means that many pilots operating into the Hoxton Park CTAF may not have any prior knowledge of traffic within the CTAF until they change to that frequency to make an inbound broadcast. This also occurs to a lesser degree with pilots tracking to Hoxton Park from other locations, who have been operating on the area frequency. However, in that circumstance there is greater latitude for pilots to change to the Hoxton Park CTAF prior to reaching its boundary.

Limitations of unalerted see-and-avoid procedures

A previous BASI report on "The Limitations of See and Avoid" (1991) stated the following:

"A traffic search in the absence of traffic information is less likely to be successful than a search where traffic information has been provided because knowing where to look greatly increases the chance of sighting the traffic (Edwards and Harris 1972).

Field trials conducted by John Andrews found that in the absence of a traffic alert, the probability of sighting a threat aircraft is generally low until a short time before impact. Traffic alerts were found to increase search effectiveness by a factor of eight. A traffic alert from ATS or from a radio listening watch is likely to be similarly effective (Andrews 1977, Andrews 1984, Andrews 1987)".

One of the recommendations of that report was that "pilots should recognise that they cannot rely entirely on vision to avoid collisions. Consequently, they should attempt to obtain all available traffic information, whether from Air Traffic Services or a listening watch, to enable them to conduct a directed traffic search".

The Civil Aviation Safety Authority (CASA) proposed changes to mandatory radio requirements for Class G terminal airspace

A summary of responses to a previous Notice of Proposed Rule Making (NPRM) 9702RP was issued in January 1999. Included in that document were a series of proposed changes to current radio requirements for aircraft operating into aerodromes in Class G airspace. This included both CTAF and Mandatory Broadcast Zone (MBZ) aerodromes. In that document, CASA advised that apart from the mandatory carriage and use of radio in MBZs, "procedures used in MBZs and CTAFs are otherwise identical. Compliance with procedures is generally believed to be higher in MBZs than in CTAFs, although there is evidence to suggest that compliance rates are related to traffic densities rather than the designation of the airfield". The proposed amendments to the radio requirements will apply to any licensed aerodrome and include a "more comprehensive list of recommended calls which should be made when safety will be enhanced". For a circuit entry approach, for example, up to seven broadcasts at various locations have been recommended. The NPRM recognises that the current list of broadcast requirements listed in the AIP is not all-inclusive and that the use of a more comprehensive list of recommended broadcasts would reduce unnecessary dialogue between individual aircraft.


Related occurrences

In occurrence 990970, it appears likely that the pilot of the Beechcraft was preoccupied with his preparations for departure for his first solo navigation exercise. He may have broadcast his taxi report before the Pitts pilot first tuned to the Hoxton Park CTAF. Subsequently, the Beechcraft pilot may not have heard the "joining crosswind" broadcast made by the Pitts pilot, or he may have heard the broadcast but did not respond. It is also possible that he believed that an aircraft that had landed (before the Pitts) was the same aircraft that had reported on crosswind, and as such, considered that the circuit was clear for him to take off.

Alternatively, the Beechcraft pilot may not have been tuned to the Hoxton Park CTAF until after the Pitts pilot had made his crosswind broadcast. In either case, by missing one critical broadcast, both pilots were totally reliant on visual scanning in order to determine each other's location. Without some prior indication of the presence and location of aircraft in the circuit, the chances of sighting each other were reduced. Had the pilot of the Beechcraft broadcast that he was entering the runway for departure, it is likely that the Pitts pilot would have been alerted to the conflict and taken appropriate action.

Whilst the investigation was not able to determine what, if any, CTAF broadcasts were made by the pilot of the Tomahawk in occurrence 9802022, it is again possible that one of the pilots was not tuned to the CTAF when the other pilot made a vital broadcast. Even though the Archer pilot made an inbound broadcast at 2 NM from the airfield, the Tomahawk pilot may have still been tuned to the Bankstown frequency or the area frequency and not have been aware of the position of the Archer. It is also possible that the pilot of the Tomahawk had broadcast an inbound call before the Archer pilot changed to the CTAF.

In either case, it appears that neither pilot had knowledge of the other's presence until it was too late to avoid a collision. Had both pilots been tuned to and/or maintaining a listening watch on the Hoxton Park CTAF earlier in the sequence of events, the collision may have been avoided. In addition, it may have been prudent for both inbound aircraft to have remained at 1,700 ft until overhead the aerodrome in order to gain an appreciation of the circuit traffic before descending to circuit height.

CTAF procedures

Despite the correct use of prescribed procedures and the application of sound airmanship by pilots, reliance on a single radio broadcast for arrival and departure of aircraft at non-controlled aerodromes appears to be inadequate for collision avoidance. If one of these critical broadcasts is not heard, or the significance of that broadcast is not appreciated, unalerted visual scanning is the only defence available to a pilot to avoid a potential collision. Effectively, a critical safety defence has been removed, with a subsequent reduction in the level of safety.

Whilst the proposed changes to mandatory radio requirements are commendable and represent a potentially significant improvement in the level of safety at non-controlled aerodromes, the listed broadcasts have been referred to as "recommended", rather than mandatory. Such advice will allow for significant differences, which already exist in the level and type of radio usage at those aerodromes. The potential exists for no change at all in the current level and type of radio usage, and thus no change in the level of safety.

The Bureau agrees that allowing pilots to use the radio for self-separation as the primary means of alerted see and avoid can lead to significant congestion of the radio frequency with the potential for critical broadcasts to be missed. Similarly, at busy aerodromes in particular, requiring pilots to make up to seven radio broadcasts may create the same congestion. Some rationalisation of this proposal, with the emphasis on mandatory radio broadcasts for identified critical locations for departures and arrivals, would be more appropriate. Other broadcasts could be discretionary and only used by pilots if they considered them necessary in the circumstances.

The Bureau also believes that the proposed mandatory radio requirements should encompass both licensed aerodromes and any unlicensed aerodrome into which fare-paying passenger services operate. Such action would achieve a greater consistency in the use of radio and would provide further opportunities for pilots to be alerted to potential conflict situations.

Hoxton Park CTAF

The Bureau does not believe that recommending Hoxton Park be upgraded to an MBZ would provide any significant safety benefit. Evidence suggests that the carriage and use of radio by aircraft operating at Hoxton Park is not dissimilar to aircraft operating at MBZs. In addition, the airspace constraints of Hoxton Park would not allow for any significant expansion of the dimensions of the CTAF.

One area of concern at Hoxton Park is the limited opportunity for pilots to change to the Hoxton Park CTAF after leaving adjacent airspace, in particular, Bankstown. If pilots made an inbound broadcast to Hoxton Park at the same time they passed the Bankstown control zone boundary they would have, at best, 3 NM before they reached the boundary of the Hoxton Park CTAF. As the Bankstown procedures advise pilots to change frequency 3 NM from the control zone boundary, it is likely that some pilots departing Bankstown for Hoxton Park would not change to the Hoxton Park frequency and make an inbound broadcast, until they were at or near the boundary of the CTAF. Even if a listening watch were maintained from the time the pilot departed the Bankstown zone, a limited opportunity would be afforded to the pilot to gain a proper appreciation of the circuit traffic at Hoxton Park.

Output text

The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority amend current procedures/airspace for aircraft operating into and departing from Hoxton Park in order to reduce the potential for further aircraft collisions.

As a result of the investigation of this safety deficiency, the Bureau simultaneously issues the following recommendations to the Civil Aviation Safety Authority:


The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority increase the number of mandatory broadcasts at non-controlled aerodromes to include a set of critical location broadcasts for those locations where the risk of collision is increased.


The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority extend the proposed radio requirements as outlined in Notice of Proposed Rule Making (NPRM 9702RP) to include all licensed aerodromes and any unlicenced aerodrome into which fare-paying passenger services operate.

Initial response
Date issued: 14 July 1999
Response from: AirServices Australia
Response text:

Thank you for the advice on the above Interim recommendations. Whilst these recommendations are not specifically addressed to Airservices I would like to provide the following comment:


Airservices agrees that the preferred situation would see aircraft operating in the vicinity of Hoxton Park equipped with two radios and with the suggestion that more appropriate position reports in the circuit area would assist in situational awareness.

We are also aware of anecdotal evidence that pilots interpret the requirements for radio calls in the MBZ differently which itself can cause confusion and loss of situational awareness.

ATSB Note: As this recommendation was not issued to Airservices Australia a response status has not been classified.

Last update 01 April 2011