A Boeing 747 (B747) had departed Avalon, Victoria on a track
that passed over Melbourne and then to the northeast. The crew had
been issued with a requirement to initially maintain flight level
(FL)200. The departures controller had imposed the limitation in
accordance with standard operating procedures that required the
"cap" to be placed on all aircraft that had planned to a higher
flight level. Airspace above FL200 was under the jurisdiction of a
sector controller. In addition, the horizontal boundary between the
two controllers' airspace below FL200, was 30NM from Melbourne
airport.
The departures controller had a Piper Navajo aircraft tracking
ahead of a sequence of three jet aircraft departing from Melbourne
airport. That situation required that the jet aircraft be radar
vectored around the slower aircraft before they could be placed on
their flight planned tracks. The first of the jet aircraft was a
Boeing 737 (B737) for Brisbane, which departed approximately the
same time as the B747.
The tracks of the aircraft were such that the B737 was initially
to the left of the B747, but at approximately 30NM northeast of
Melbourne they crossed and thereafter diverged.
The sector controller had noticed that the B747 would probably
reach FL200 while still in departures airspace and, as a
consequence, be forced to maintain FL200. In order to provide the
crew of the B747 with an unrestricted climb profile, he coordinated
with the departures controller to authorise the crew of the B747 to
climb to FL370.
At that time, the B737 was below and approximately 30NM ahead of
the B747. However, the ground speed of the B747 was approximately
70 knots faster than the B737, and the B737 had a greater rate of
climb.
As the B737 approached the horizontal airspace boundary, the
departures contoller handed over the aircraft to the sector
controller while it was passing FL170. The sector controller then
approved the crew to climb to FL370, as he believed there was
sufficient distance between the aircraft to maintain separation. A
short time later, the B747 was also handed over to the sector
controller as it was passing FL200 and approximately 8 NM behind
the B737. At that moment, The Australian Advanced Air Traffic
Control System (TAAATS) Short Term Conflict Alert (STCA) activated
and the controllers immediately attempted to prevent an
infringement of separation standards. However, the crew of the B747
was on his frequency and asked the departures controller to
maintain that aircraft at a lower level. The departures controller
issued an instruction for the crew of the B737 to maintain FL190
but did not receive a reply, because that crew was on the sector
frequency as instructed.
At that moment, the crew of the B747 made radio contact with the
sector controller who immediately issued an instruction for them to
turn the aircraft. There was a delay as the crew questioned the
instruction, but they commenced the manoeuvre when the controller
issued the instruction a second time using the word "immediately".
He then instructed the crew of the B737 to level out, which they
did.
The aircraft passed approximately 2.5NM apart while the vertical
separation standard of 1,000ft did not exist. The required radar
separation standard was 5 NM.
