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The reason why the crew of the Dash 8 did not hear the Wee Waa departure report by the pilot of the Chieftain, or if they did, why they did not appreciate that there was the potential for conflict, was not determined.

The crew of the Dash 8 had previously reported their position and estimate for Narrabri on first contact with flight service. The reason the crew subsequently did not comply with AIP and company operating procedures, regarding a position report prior to descent, was also not determined.

The monitoring of the Tamworth ATIS transmission by the pilot of the Chieftain may have prevented him from hearing the Dash 8 crew report on descent. Consequently, he was unalerted to the Dash 8. If he had heard the transmission he may have initiated communication to ascertain if there was the potential for conflict. Under the current VFR procedures there was no third party, such as flight service, to alert the pilot to the position of conflicting traffic.

Environmental aspects

The Chieftain pilot reported that he was using the visor in the cabin but that he did not believe that his view from the aircraft was impaired. However, the fact that he was using the visor would indicate that the rising sun was of some concern and this aspect, in conjunction with the hazy conditions, probably limited his ability to sight potentially conflicting aircraft in his forward field of vision.

Unalerted "see-and-avoid" principle

Despite the Chieftain pilot's departure report, which was additional to that required for VFR flights, the crew of the Dash 8 remained unalerted to the pending conflict. This may have been due to the difficulty of assessing the potential conflict of other traffic from the limited details included in departure reports. In the dynamic aircraft operational environment, even with multiple flightcrew, it is not an easy task to develop a situational awareness of all potential conflicting aircraft.

The provision of a position report by the crew of the Dash 8 prior to commencing descent would have been the main opportunity for the pilot of the Chieftain to be alerted to the potential conflict. However, the report did not contain a reference to the position of the Dash 8 and the Chieftain pilot was unaware of the location of the Dash 8. Additionally, it was possible that the pilot of the Chieftain may not have heard or appreciated the significance of the Dash 8 pre-descent position report even if it had been provided, as he was probably monitoring the Tamworth ATIS at the time. Consequently, in either situation, separation was solely reliant on the pilots' ability to sight other aircraft and to manoeuvre their aircraft in sufficient time to avoid a conflict.

The limited radio reports required from pilots operating VFR flights place an over-reliance on the unalerted "see-and-avoid" principle to maintain separation from other aircraft. This aspect was one of the conclusions in the Bureau's research report titled "Limitations of the See-and-Avoid Principle" (1991) which stated, "Unalerted see-and-avoid has a limited place as a last resort means of traffic separation at low closing speeds but is not sufficiently reliable to warrant a greater role in the air traffic system. BASI considers that see-and-avoid is completely unsuitable as a primary traffic separation method for scheduled services". In that research report BASI recommended that "the CAA should take into account the limitations of see-and-aviod when planning and managing airspace and should ensure that unalerted see-and-avoid is never the sole means of separation for aircraft providing scheduled services".

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