Piper PA-31-350, VH-LTW

On 31 August 2004, at about 1000 eastern standard time, the pilot of a Piper Aircraft Corporation PA-31 (Chieftain), registered VH-LTW, was conducting an instrument flight rules (IFR) flight from King Island to Devonport. The aircraft was maintaining 7,000 feet in visual meteorological conditions. The pilot saw an aircraft flying in the opposite direction passing between 100 ft and 200 ft, down the left of the Chieftain. He took immediate avoiding action. The pilot estimated that there was two to three seconds between initially seeing and then passing the other aircraft.

The other aircraft was later identified as an Aero Commander 500-S (Shrike), registered VH-LET, on a visual flight rules (VFR) flight at 6,500 feet from Launceston to King Island via overhead Devonport. The presence of the Shrike was established when the pilot of that aircraft advised the pilot of a third aircraft, that was east of King Island en route to Wynyard at 7,000 ft, of his position and altitude of 6,500 ft. The pilot of the third aircraft (tracking to Wynyard) had broadcast his intention to descend from 7,000 ft and that radio transmission alerted the Shrike pilot to a possible conflict. The pilots agreed to maintain their respective altitudes until the aircraft had passed. Neither the pilot nor the passenger in the Shrike saw the Chieftain.

At 1001, the Chieftain was 61 NM from Devonport and the pilot advised the controller of the occurrence and requested traffic information on any other aircraft in the area. The controller replied that there was no observed traffic [displayed on the radar].

Both pilots reported operating their aircraft transponders, including the Mode C altitude function, as required by the Aeronautical Information Publication (AIP) procedures. A review of the recorded air traffic control radar data confirmed that both aircraft were cruising at their reported altitudes about 20 minutes before they passed. However, the area where the aircraft passed was not within radar coverage and the investigation could not confirm the altitudes of the aircraft when they passed.

The Chieftain was fitted with two altimeters and the pilot reported that he had set both subscales to the area QNH and had engaged the autopilot. The Shrike was fitted with two altimeters and the pilot reported that he had set both subscales to either the local QNH or, when outside the mandatory broadcast zone (MBZ), the area QNH. The pilot had also engaged the auto-pilot. Both aircraft were maintained to IFR equipment requirements.

The AIP detailed altitudes to be used for aircraft on IFR/VFR flights in an easterly or westerly direction. The altitudes reported by the pilots were in accordance with the AIP.

Both aircraft were fitted with dual very high frequency radios. From about 0954, both pilots were simultaneously monitoring the Melbourne Centre area frequency on 122.6 MHz and the Devonport/Wynyard MBZ frequency on 126.9 MHz. At that time, the Chieftain pilot reported his Devonport estimate of 1020 to the Melbourne Centre controller. The pilot of the Shrike was monitoring Melbourne Centre on 122.6 MHz, but could not recall hearing the position report by the Chieftain pilot to Melbourne Centre. The pilot of the Shrike was not required to make any radio broadcasts. Also, pilots were not required to make any radio broadcasts when leaving an MBZ. A replay of the Melbourne Centre on 122.6 MHz revealed some interruptions by other pilots on that frequency.

Apart from the mandated broadcasts, the procedure in MBZ and Class G airspace is for pilots to listen on the appropriate frequency and to make a radio broadcast if there is the potential for aircraft to come into conflict.

In Class G airspace, air traffic control (ATC) provides traffic information to pilots of aircraft operating an IFR flight about other aircraft operating as IFR flights and military jet aircraft. Pilots operating a VFR flight may request traffic information from ATC, but it is provided subject to workload at the time. Pilots may make additional radio broadcasts at their discretion.

Neither aircraft was fitted with a traffic alert and collision avoidance system, nor was there any legislated requirement to do so.

While the investigation was unable to establish the actual altitudes the aircraft were maintaining, due to the lack of radar coverage, both aircraft were reported to be at altitudes that would enable 500 ft spacing. However, even had that spacing been achieved, an unalerted aircraft suddenly appearing in a pilot's vision might be perceived to be closer than it actually is, leading an observer to think that there is a collision risk. It is also possible, given the error margins of altitude equipment on each aircraft, that the actual spacing may have been less than 500 ft.

It is possible that the pilot of the Shrike did not hear the Chieftain pilot's report because it was mixed with radio transmissions from other pilots on the frequency at the time. Given the reported altitude of the Chieftain of 7,000 ft and the Shrike's reported altitude of 6,500 ft, even if the Shrike pilot had heard that report there would have been no requirement for him to make a radio transmission and the situation would have remained unalerted for the Chieftain pilot.

Radio broadcasts can enhance a pilot's situational awareness when used in conjunction with maintaining a look out to see and avoid other aircraft. Flying in Class G airspace often involves the simultaneous monitoring of two radio frequencies, such as the area frequency and the MBZ frequency. If a radio transmission is not heard, a pilot has to rely on segregation from other aircraft through use of appropriate cruising altitudes and seeing other aircraft.

The limitations of see and avoid as a sole means to maintain awareness are well known (see ATSB website).