EXECUTIVE SUMMARY
On 24 July 2004, the flight crew of a Boeing 737-838 aircraft, registered VH-VXF, received a terrain proximity caution from the aircraft's enhanced ground proximity warning system (EGPWS) while descending to the south-south-east of Canberra Airport. The aircraft was being operated on a scheduled fare-paying passenger service from Perth to Canberra with two pilots, five cabin crew and 80 passengers on board.
Due to staff shortages on the morning of the occurrence, the approach control services normally provided by the Canberra Terminal Control Unit did not become available until approximately 40 minutes after the scheduled unit opening time. This meant that the aircraft's descent below 9,000 ft was conducted without air traffic control radar assistance.
The aircraft departed Perth at 0211 Eastern Standard Time (EST) and the occurrence was at 0544 EST. The flight deck during the flight was abnormally hot because of a pre-existing air conditioning problem.
As the aircraft approached Canberra, the crew elected to track to Church Creek1 (CCK), to enter the holding pattern at that position and descended to 5,000 ft to intercept the instrument landing system (ILS) approach in accordance with Airservices Australia and Jeppesen published procedures for the approach for runway 35.
The published CCK holding pattern requires that aircraft holding at 5,000 ft observe a maximum indicated airspeed (IAS) of 170 kts and limit time outbound to either 1 minute or a distance measuring equipment (DME) limit of 14 NM from Canberra, whichever is reached first.
As the aircraft approached CCK, the copilot, under the direction of the pilot in command, entered the holding pattern details into the Flight Management Computer (FMC). In doing so, an erroneous entry was made, which resulted in the FMC computing a holding pattern with a leg length of 14 NM, instead of 1 minute or a maximum distance from Canberra of 14 NM.
By entering a leg distance of 14 NM, the crew inadvertently commanded the FMC to establish the aircraft in a holding pattern that would take the aircraft about 11 NM beyond the published holding pattern limit. The crew initiated descent to 5,000 ft after passing overhead CCK. As it descended, the aircraft proceeded outside the airspace specified for holding. Consequently, the aircraft was operated closer to the surrounding terrain than would normally occur.
The aircraft was fitted with an EGPWS, which detected the aircraft's proximity to the terrain and provided the crew with a 'CAUTION TERRAIN' message to which the crew responded by climbing the aircraft to 6,500 ft. Sixteen seconds before the message, the crew had commenced a right turn to intercept the inbound track to CCK. At the time of the message, the aircraft's height above terrain was 2,502 ft (radio altimeter indication).
During the turn, the aircraft passed 0.6 NM (1.11 km) north abeam and 810 ft higher than the closest terrain that had a spot height of 4,920 ft above mean sea level. It also passed 2.7 NM (5 km) north abeam Tinderry Peak. The aircraft climbed to 6,500 ft and subsequently joined the runway 35 localiser.
This occurrence was not simply a case of incorrect data entry, but was influenced by a number of events occurring prior to, and during the flight that affected the crew, the aircraft and the air traffic control system. Evidence suggests that the flight crew's operational performance was affected at a critical stage of the flight by fatigue, the late advice of the status of air traffic services and the crew's misinterpretation of the CCK locator holding pattern data on the runway 35 ILS approach chart.
The crew's ineffective contingency planning for a descent to Canberra without air traffic control support and the erroneous data entry in the aircraft's flight management computer (FMC) suggest that the crew was not functioning at an appropriate level of alertness.
It is likely that both the pilot in command and the copilot were experiencing fatigue due to the cumulative effects of ineffective sleep in the period preceding the Perth to Canberra night sector and the ongoing period of wakefulness during the flight. Additionally, as they approached Canberra, the crew was working at a low point in their circadian rhythms2. It is therefore likely that they were experiencing a decreased level of alertness. The application of the minimum equipment list on the flight deck air conditioning system allowed continued flight operation despite abnormally hot conditions, about 10 degrees Celsius above normal. While this may have had less impact on crew performance during a short daylight flight, it was of greater significance during a night flight of more than three hours. In combination, those conditions probably interacted to reduce the level of crew alertness, performance and attention. The crew's lack of recognition of the inaccurate entry in the FMC is consistent with the effects of fatigue, and it is likely that those effects were exacerbated by the excessive flight deck temperatures.
As a result of this occurrence, the aircraft operator has taken action to ensure earliest rectification of flight deck or passenger cabin temperature control problems and increased the minimum holding pattern altitude at Church Creek. Airservices Australia has issued a temporary local instruction detailing how the Canberra Terminal Control Unit staff shortage contingency plan should be activated. Additionally, Jeppesen Sanderson Inc. has advised the ATSB that they intend to include the DME identifier in the holding pattern limit notes on relevant charts.
Related Documents: | Media Release |
1 Church Creek is an instrument approach fix (locator) 10.9 NM from Canberra Airport.
2 Circadian rhythms refer to body functions (i.e. sleep/wakefulness, motor activity, hormonal processes, body temperature, and performance) that are controlled by internal biological clocks and that vary over a 24 hour cycle. As a result, levels of human performance also vary significantly during the 24 hour period.