The Australian Transport Safety Bureau did not conduct an
on-scene investigation of this occurrence. The report presented
below was prepared essentially from information supplied to the
Bureau.
REPORTED INFORMATION
At 0745 eastern summer time, on 12 January 2005, the pilot of
Aero Commander Division 500-B, (Aero Commander) registered VH-YJO,
was climbing to an assigned altitude of 5,000 ft enroute from
Essendon to Wangaratta Victoria. An Airbus A320, registered ZK-OJG,
was inbound to Melbourne on a crossing track at 6,000 ft. The
vertical separation standard required between the two aircraft was
1,000 ft. The Aero Commander was observed on radar by air traffic
control (ATC) to climb to about 5,300 ft. ATC requested that the
pilot clarify the aircrafts altitude and the aircraft was observed
to descend back towards the assigned altitude. The pilot reported
that, at the time ATC queried his altitude, the aircraft altimeter
read 5,100 ft. That was within the normal flight tolerance of plus
or minus 100 ft for IFR flight specified in Civil Aviation Order
40.2.1.
Prior to the flight, the pilot conducted an accuracy check of
the aircrafts altimeters as part of normal pre-flight checks. Those
checks indicated that both aircraft altimeters were within IFR
altimeter tolerances. On the return flight to Essendon later that
day, the pilot carried out a check of the aircraft transponder with
Melbourne Centre. That check indicated an aircraft altimeter
reading of 6,060 ft, and a corresponding radar-derived altitude of
6,200 ft.
After the aircraft landed at Essendon, the operator conducted a
maintenance check of the transponder encoder. That check determined
that the encoder was over-reading by 140 ft. The operator took
maintenance action to correct the anomaly.
While the aircraft altimeter confirmed the pilot was within
normal IFR flight tolerance, the encoder anomaly contributed to the
radar-derived apparent error in the pilots cruising level. Had the
anomaly in the encoder not been present, this incident would not
have occurred.