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.