A Boeing 747-400 was enroute from Los Angeles to Melbourne,
cruising at FL320 in a south-westerly direction on airway R576 in
the Oakland Flight Information Region. A McDonnell Douglas MD-11
departed Honolulu and was climbing in an easterly direction to
FL350. The aircraft were on converging tracks.
The Honolulu Air Traffic Control Facility was responsible for
providing positive separation between the aircraft. The radar
separation standard was 5 NM lateral, or 1,000 ft vertical.
A controller subsequently recognised that, without intervention,
the tracks of the two aircraft would come within 5 NM when less
than the required vertical separation existed. The controller
instructed the MD-11 crew to "...fly heading 020 ..." to change the
aircraft's track. The crew responded by broadcasting "... Air 205,
now right, right heading...". The controller did not clarify the
required turn direction, and then instructed the B747 crew "...fly
180, vectors on traffic". Those instructions were passed 60 seconds
and 49 seconds respectively, prior to the aircraft passing each
other.
The B747 crew correctly read back the instruction to fly 180
degrees. Immediately after receiving that instruction, the crew
received a traffic advisory alert from the aircraft's traffic
alerting and collision avoidance system (TCAS). The alert was
followed by a TCAS annunciation of a resolution advisory (RA) to
descend. The crew advised air traffic control, "... heading 180 and
TCAS descent", as they descended the aircraft, in accordance with
standard operating procedures. The crew had the MD-11 in sight for
the duration of the incident.
Analysis of the continuous data recording plot indicated that
the MD-11 did not commence the left turn until after the aircraft
had passed. There was no indication that the B747 turned onto a
heading of 180 degrees. A minimum of 700 ft vertical separation
existed when there was less than 5 NM between the aircraft. An
infringement of separation standards had occurred.
The MD-11 crew apparently misunderstood the instruction to fly
heading 020 as "right" heading 020 degrees. A right turn would have
been confusing to the crew as the aircraft was tracking
approximately 090 degrees. The direction of turn onto a new heading
would normally be flown via the shortest arc distance; in this
case, a turn onto a heading of 020 degrees would normally involve a
left turn. Thirty-three seconds later the MD-11 crew sought
confirmation that a left turn to heading 020 degrees was
required.
The crew of the B747 may not have turned as instructed due to
their response to the TCAS alert, and their visual observation of
the conflicting traffic. It could not be established whether
standard separation would have been achieved if one or both crews
had turned their aircraft when instructed to do so.
There was no evidence that the controller applied the principle
of separation assurance in the control of these aircraft. Rather,
the controller had relied on aircraft performance to achieve
separation.
