ATSB report....
FACTUAL INFORMATION
At 0211 Eastern Standard Time (0011 Western Standard Time), on 24 July 2004, a Boeing 737-800, registered VH-VXF departed Perth on a scheduled passenger service to Canberra. Due to an air conditioning system fault, the crew had difficulty controlling the flight deck temperature and experienced uncomfortably hot conditions throughout the flight.
At about 0544, while on descent to Canberra the crew received an alert from the aircraft’s enhanced ground proximity warning system (EGPWS).
As the aircraft approached Canberra, the Melbourne Centre air traffic controller advised the crew that due to staff shortages, Canberra Approach was not `able to be manned’ and `MBZ procedures’ applied. The Canberra Terminal Control Unit service usually commenced at 0530. Therefore, the controller guidance normally afforded by the radar equipped unit could not be provided.
The after-hours airspace classification therefore continued, remaining Class G below 8,500 ft within 30 NM of Canberra and Class E above 8,500 ft. The crew continued under MBZ procedures and requested tracking to the Church Creek Locator (CCK), with the intention of entering the holding pattern and descending to an altitude from which an instrument landing system (ILS) approach to runway 35 could be made.
As the aircraft approached CCK the copilot, under the direction of the pilot in command, entered the CCK holding details into the flight management computer (FMC). An erroneous entry was made, resulting in the FMC computing a holding pattern with an inbound leg length of 14 NM. In order to make good an inbound leg length of 14 NM, the FMC uses current wind data to adjust its outbound distance and rate of turn. An inbound leg length of 14 NM, based on the CCK locator, could allow the aircraft to operate in the order of 20 NM to 30 NM from Canberra.
The published holding pattern requires that aircraft holding at CCK at 5,000 ft observe a maximum indicated airspeed (IAS) of 170 kts and distance measuring equipment (DME) limit of 14 NM from Canberra. The FMC computed holding pattern extended beyond the required limits of the published holding pattern.
The leg length entered caused the aircraft to operate beyond the published holding pattern limit. Based on radar data, during the descent the EGPWS provided the crew with a ‘caution terrain’ alert when the aircraft was passing 5,800 ft and about 22 NM south of Canberra. The crew reported that they responded to the alert by climbing the aircraft to 6,500 ft and maintained that altitude until the runway 35 ILS glide slope was intercepted. This was confirmed from radar data.
The Minimum Sector Altitude between Canberra and 10 NM south of Canberra is 5,100 ft and between 10 and 25 NM south of Canberra is 7,400 ft. The Aeronautical Information Publication defines Minimum Sector Altitude as the lowest altitude which may be used which will provide a minimum clearance of 1,000 ft above all objects located in an area contained within a sector of a circle of 25 NM or 10 NM radius centred on a radio aid to navigation or, where there is no radio navigation aid, the Aerodrome Reference Point.
Data from the EGPWS computer indicated that at the time of the alert the aircraft was positioned approximately 2,500 ft above ground level.
The investigation is continuing and includes the analysis of available recorded data.
Source:
http://www.atsb.gov.au/aviation/occurs/occurs_detail.cfm?ID=659