On 6 April 2005, at 1253 Eastern Standard Time, a de Havilland
Canada DHC-8-102 (Dash 8) aircraft departed Mackay for Townsville,
Qld. The aircraft was being operated under the instrument flight
rules (IFR) and was climbing to flight level (FL) 160. At 1254, a
Boeing Company 737-800 (737) aircraft departed Proserpine for
Brisbane, Qld. The aircraft, with two pilots and a company approved
observer, was being operated under the IFR. The crew's intention
was to climb the aircraft to 8,000 ft above mean sea level (AMSL)
pending a clearance from air traffic control (ATC) to enter
controlled airspace (CTA) on climb to the planned FL410.
The airspace in the Proserpine area was classified as class G
(non-controlled) airspace from ground level to 4,500 ft and class C
(controlled) airspace from 4,500 ft to FL180.
Within class C airspace, air traffic controllers are required to
separate IFR aircraft from other IFR aircraft.
Figure 1: Extract from Mackay Terminal Area
Chart
At 1256, the sector controller issued the crew of the 737 with a
clearance to enter CTA on climb to 5,000 ft to establish the
minimum vertical separation standard of 1,000 ft with the Dash 8,
prior to conducting a step climb1.
The 737 crew reported they were approaching 6,000 ft and commenced
a descent to 5,000 ft.
Recorded data later showed that the 737 reached a maximum
altitude of 6,400 ft with a minimum vertical spacing between the
two aircraft of 430 ft and it was calculated that the aircraft were
approximately 45 NM apart laterally. At 1257, the vertical
separation standard of 1,000 ft was established and, at 1300 when
the two aircraft were radar identified, they were about 25 NM apart
laterally.
At the time of the incident, the radar that normally provided
low-level coverage within the area had been temporarily removed
from service, restricting coverage to above 8,000 ft. A notice to
airmen (NOTAM) had been issued, which detailed the planned outage,
the restricted radar coverage, and possible delays in CTA.
The crew of the 737 later reported that they had obtained and
read briefing material, including NOTAMs, but did not recall any
information relating to the radar outage. They first became aware
of the outage at about 9,000 ft during the previous flight on the
inbound descent to Proserpine, when they were instructed by the
controller that the radar was off and radar services were
terminated. At that point, they mistakenly confused the termination
of radar services with a change in the base of CTA to 9,000 ft.
Prior to departure from Proserpine, the crew briefed and set 8,000
ft as an initial level for climb, believing this level to be
outside CTA.
The published minimum sector altitude (MSA)2 around Proserpine was 4,500 ft within 10 NM
and 5,100 ft within 25 NM. The lowest safe altitude (LSALT) for the
departure track of the 737 was 5,500 ft. As a result, the cleared
level of 5,000 ft was below the LSALT for the aircraft.
The forecast cloud at Proserpine was scattered at 2,000 ft and
broken at 4,000 ft and the 737 crew later reported entering
instrument meteorological conditions (IMC) when passing about 2,500
ft on the departure climb.
The Manual of Air Traffic Services (MATS) section 6.1.2.1
specified that clearances issued shall enable the pilot to comply
with Civil Aviation Regulations (CAR) 157, relating to minimum
heights for aircraft operations. CAR 178 specified that a pilot
must not fly an aircraft at a height lower than the published
lowest safe altitude, 'and on departure this means the time during
which an aircraft is climbing after take-off at a rate that is
reasonable under the circumstances'. MATS 6.1.2.5 specified level
assignment shall take into account terrain clearance and MATS
6.1.7.1 specified that a pilot may [only] be assigned a level below
the LSALT provided that the pilot has reported 'visual' and
'visual' is appended to the clearance.
The air traffic controller later stated that he understood that
the responsibility for terrain clearance on departure was a pilot
responsibility and there was a published LSALT on the departure
track for pilot reference. As he did not expect the aircraft to
have to maintain 5,000 ft in the step climb, he did not issue a
'visual' instruction with the level assignment.
MATS 4.1.1.4 provided guidance to controllers relating to
tactical separation assurance, which:
'places greater emphasis on traffic planning and conflict
avoidance rather than conflict resolution. This is achieved through
the proactive application of separation standards to avoid rather
than resolve conflicts; planning traffic to guarantee rather than
achieve separation; executing the plan so as to guarantee
separation; and monitoring the situation to ensure that plan and
execution are effective.'
- Step climb is a procedure used to
simultaneously climb aircraft to vertically separated levels. - Minimum sector altitude (MSA) and
lowest safe altitude (LSALT) are calculated to provide 1000 ft
obstacle clearance for IFR flights, and are published on
aeronautical charts and in the Aeronautical Information Publication
(AIP) for pilot and controller reference.