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A Boeing 747-400 (B747-400) was en route from Singapore to Sydney at flight level (FL) 330 on air route A585, and a B747 was tracking in the opposite direction at FL350 from Melbourne to Jakarta on air route G222. The routes A585 and G222 converged at SAPDA, a position approximately 720 NM north-west of Broome on the boundary of the Australian and Indonesian flight information regions (FIRs). Route G222 was north-east of route A585 and the prescribed lateral separation point between the routes was 272 NM south-east of SAPDA. The aircraft were under the control of the Kimberley sector controller of the Brisbane centre.

The B747-400 was estimating SAPDA at 1811 Universal Coordinated Time and the controller approved that crew at 1747 to climb to FL350 at SAPDA. The Kimberley controller was advised at 1806 the B747 was at METUM, a position 252 NM south-east of SAPDA, at 1805 maintaining FL350 and estimating SAPDA at 1836. At 1812, the Kimberley controller received a SAPDA position report from the B747-400 crew and shortly after recognised there was no separation being applied between the aircraft. At 1813, the Kimberley controller instructed the B747-400 crew to descend to FL330, however there was an infringement of separation standards. The B747-400 crew subsequently reported maintaining FL330 at 1819.

The estimated time of passing of the aircraft was 1824. The Kimberley controller needed to establish the vertical separation standard of 2,000 ft between the aircraft before 1809 to ensure separation.

The Kimberley sector was a procedural sector operated by a single controller. Communication between controllers and flight crews could be by either:
- very high frequency (VHF) radio,
- controller pilot data link communication (CPDLC), or
- high frequency (HF) radio through a third-party radio operator.

The controller was communicating with the B747-400 crew using CPDLC and with the B747 crew by third-party HF radio. There was no VHF radio coverage in the portion of the sector in which the aircraft were operating.

The controller commenced the shift at 1300 (2300 Eastern Standard Time) and worked till about 1530 when he had a break. He returned to the position at 1610 and remained there until the occurrence. Normally during the period from 1400 to 1800, the number of aircraft in the sector gradually increased and after that time, traffic numbers decreased. The controller reported that there had been a busy period that had finished just before the occurrence. That was the controller's first shift following a 2-day break.

When the B747-400 crew requested approval to climb to FL350 via CPDLC, the controller checked for conflicting aircraft on the air situation display. At that stage, the B747 on G222 was over Western Australia and the controller reported that he might have perceived that aircraft as part of a group of aircraft on a more northerly route. Consequently, he coordinated a change of level with the adjacent Indonesian sector controller, and then instructed the B747-400 crew to climb at SAPDA.

The air traffic system had a flight plan conflict probe (FPCP) function to assist in the early identification of conflicts outside radar coverage. However, the FPCP had not been included in the commissioning of the new air traffic control system during 1998-99 and was not active.

The Embraer Brasilia aircraft was being operated on a Regular Public Transport flight from Dili, East Timor to Darwin NT and was levelling off to cruise at Flight Level (FL) 210.

Shortly after, and when the aircraft was approximately 90 NM SW of Dili, the flight crew realised that the cabin air pressure was changing. The change was confirmed by reference to the cabin altitude and cabin rate of climb indicators. The cabin altitude was rising at an increasing rate, with the rate of increase quickly exceeding the instrument's full-scale deflection of 2,000 ft/minute.

The crew immediately commenced a high-speed descent and, because they were reacting to the increasing cabin altitude ahead of the aircraft's warning systems, did not don the supplemental oxygen masks. The aircraft was descending at a rate of 3,000 ft/min. While on descent, the crew continued to monitor the cabin altitude indicator.

As the cabin altitude exceeded 10,000 ft, the aircraft master caution warning and the cabin altitude warning chimes activated. At that stage, the aircraft was passing FL140 and continuing to descend at about 3,000 ft/min. With approximately one minute remaining before reaching 10,000 ft, the crew again decided against using the supplemental oxygen masks.

The crew continued the descent and appeared to regain some control of the cabin altitude by using the cabin pressurisation controller in the "manual" mode which maintained a cabin altitude of approximately 8,000 ft. They levelled the aircraft at FL112, the lowest safe altitude for this route segment, and continued to their planned destination. The flight attendant informed the pilots that during the descent the supplemental oxygen masks in the main cabin had automatically deployed.

No injuries were reported as a result of the incident.

Maintenance staff inspected the aircraft and replaced the pressurisation controller. The pressurisation system performed normally on the next scheduled flight. However, on the following sector, the system again malfunctioned. ATSB investigation report BO/200003725 refers.

Flight data recorder

Analysis of the flight data recorder indicated that the aircraft reached top of climb at 0930 UTC (Coordinated universal time) and was maintaining FL 210. At 0944, the aircraft began to descend. The descent profile indicated that the descent was conducted at a rate of about 2,100 ft/min. At 0949, the aircraft levelled off at FL113.

Cabin altitude warning system

The aircraft was equipped with a cabin altitude warning system. In the event of the cabin altitude exceeding 10,000 ft, a 3 chime aural alert and a voice "cabin" warning sounded. In addition, a red "cabin alt" warning light would illuminate on the main annunciator panel and the red "master warning" light would flash. The system activated as designed.

Supplemental oxygen system

A supplemental oxygen system was installed for use by the crew and passengers in the event of a failure of the cabin pressurisation system. It was a conventional high-pressure gaseous storage system, which distributed low-pressure oxygen to the crew and passenger breathing masks. The passenger masks were stored in overhead dispensing units. The supplemental oxygen system operated normally and the masks automatically deployed when the cabin altitude exceeded 14,000 ft.

Hypoxia

Hypoxia describes the physiological condition where insufficient oxygen is available to meet the needs of the body. The condition is particularly significant because of the rapid rate at which symptoms can manifest themselves and the variation in the onset of symptoms between individuals. A person suffering the effects of hypoxia could experience a range of symptoms capable of adversely affecting their ability to safely operate an aircraft. These symptoms include impairment of mental performance, loss of judgement, vision impairment, memory loss, reduced levels of awareness and muscular impairment.

The effects of hypoxia may be such that the person is unable to recognise the symptoms or identify that their level of performance has been impaired.

Individuals experiencing the effects of hypoxia can have difficulty in completing even simple tasks. The severity of these symptoms depends on many factors, including the altitude to which the individual is exposed, the duration of the exposure and individual physiological differences. Untreated, hypoxia can result in loss of consciousness and death.

Depressurisation events and response procedures

The US Federal Aviation Authority Civil Aeromedical Institute (CAMI) classified any occurrence of decompression as significant if the cabin altitude exceeded 14,000 ft, the cabin masks were deployed, or if the occurrence resulted in injuries. The incident satisfied two of those criteria.

A rapid depressurisation, as defined by CAMI, occurs when the cabin altitude increases by more than 7,000 ft/min. There was no evidence that during the incident the change in cabin altitude exceeded that rate.

The crew had planned to cruise at FL210. For flights in "pressurised aircraft engaged in flights not above FL250", the Civil Aviation Safety Authority required that supplemental oxygen be used by all flight deck crew "at all times during which the cabin altitude exceeds 10,000 ft". "A crew member (not being a flight crew member on flight deck duty) ... must use supplemental oxygen at all times during which the cabin pressure altitude exceeds Flight Level 140." CAO 20.4 refers.

The aircraft must also carry sufficient supplemental oxygen for passengers as specified in CAO 20.4 Subsection 7.5.

"Supplemental oxygen for passengers

7.5 A pressurised aircraft to which this subsection applies that is to be operated above 10,000 feet flight altitude must carry sufficient supplemental oxygen:

(a) where the aircraft can safely descend to Flight Level 140 or a lower level within 4 minutes at all points along the planned route and maintain Flight Level 140 or a lower level for the remainder of the flight - to provide 10% of the passengers with supplemental oxygen for 30 minutes or 20% of the passengers with supplemental oxygen for 15 minutes; and

(b) where the aircraft cannot safely descend to, or maintain, Flight Level 140 or a lower level in accordance with subparagraph (a) - to provide each passenger with supplemental oxygen for so much of the flight time above Flight Level 140 that exceeds 4 minutes duration and to provide 10% of the passengers with supplemental oxygen for 30 minutes or 20% of the passengers with supplemental oxygen for 15 minutes."

The Quick Reference Handbook (QRH) used by the crew included a checklist for use following an illumination of the cabin altitude warning light. That checklist did not include a requirement for the crew to don oxygen masks but contained checklist items to establish control of the cabin pressure utilising the manual pressurisation controller. The QRH also included a checklist for rapid depressurisation that required the crew to immediately don oxygen masks and commence an emergency descent.

The Boeing 767 had just reached cruise altitude at flight level 330 approximately 40 minutes out of Singapore enroute to Perth, when the flight crew noted smoke and electrical fumes on the flight deck. The source of the smoke and fumes could not be readily identified. The pilot in command elected to have the flight crew don oxygen masks, and diverted to Jakarta.

The operator's engineering personnel examined the aircraft and found the right DME (Distance Measuring Equipment) circuit breaker open. Technicians isolated the problem to the right DME interrogator unit. The malfunctioning DME unit was disabled in accordance with the MEL (Minimum Equipment List) guidelines to allow the aircraft to continue to Perth. Following arrival in Perth, the unit was replaced.

Examination of the unit by the manufacturer revealed that the DME unit's A5 modulator had overheated. This failure mode was similar to two other units, which had overheated on a different aircraft in January 2000 (see Occurrence 200000055). The failure mode of those units was such that the A5 modulator had overloaded the positive 86 volt DC power supply and overheated the power transformer. Compliance with service bulletins recommending product improvements to this unit were not mandatory, and the recommended modifications had not been incorporated into this unit, or the previous two units that had sustained failures.

The Embraer Brasilia aircraft was being operated on a Regular Public Transport flight from Dili, East Timor to Darwin NT. Shortly after the aircraft had levelled off at FL230, the pilot in command became aware of cabin air pressure changes. That was verified by reference to the cabin rate of climb indicator, which indicated that the cabin altitude was increasing at a rate of approximately 500 ft/min.

The crew reported that they attempted to regain control of the cabin pressurisation system by switching from the automatic to the manual pressurisation controller. When that action did not rectify the problem, the crew immediately commenced a descent. The rate at which the cabin altitude was increasing was not excessive and the crew elected to perform a normal descent. While on descent, the cabin altitude rate of climb suddenly increased to about 1,000 ft/min. The rate of pressure loss was not uncomfortable and the aircraft was rapidly approaching FL140. The crew did not don supplemental oxygen masks during the initial descent.

The flight attendant was at the rear of the aircraft when the passenger oxygen masks deployed. At almost the same time the interphone sounded and she moved to the front of the aircraft to answer the call (there was no cabin interphone at the rear of the aircraft). The flight attendant did not use any of the spare passenger masks as she moved forward through the cabin and did not don her mask after arriving at her crew station. The pilot in command advised her that there was a slight depressurisation problem and that they had commenced a descent to 10,000 ft. The pilot warned her that the passenger masks may deploy and she advised him that they already had. The flight attendant was then instructed to get the Emergency Procedures card and perform the emergency briefing. The flight crew were not wearing supplemental oxygen masks when she spoke to them at that stage of the descent.

The crew reported that they donned oxygen masks later during the descent after the passenger oxygen masks automatically deployed. The flight attendant did not don a mask at any stage of the descent. She reported that some passengers also did not use the masks after they deployed, or after being instructed to do so during the emergency briefing.

The crew established the aircraft in visual meteorological conditions, with fine weather forecast for the planned route. The aircraft landed with about 1,100 lb of fuel, which was within the standard company reserves for depressurised operations. No injuries were reported as a result of the incident.

The aircraft sustained a similar pressurisation problem 12 hours earlier. Following that incident maintenance crews replaced the pressurisation controller and returned the aircraft to service. The aircraft then completed one sector without incident.

In response to the second occurrence, the rear cargo compartment door seal was inspected and found dislodged from its retaining rail. The damage was assessed to have most likely occurred during the loading and unloading of passenger baggage and freight.

After reinstallation of the door seal in the retaining rail, the aircraft was test flown and returned to service without re-occurrence.

Recorded information

Analysis of the flight data recorder indicated that the aircraft reached top of climb (FL231) at about 2347 UTC (Coordinated universal time) and almost immediately commenced a descent. The recording indicated that the descent was conducted at a rate of approximately 2,600 ft/min and that the aircraft reached FL140 at 2351. The aircraft maintained that altitude for approximately 5 minutes before continuing a 500 ft/minute descent to maintain a cruise altitude of 10,000 ft.

Cabin altitude warning system

The aircraft was equipped with a cabin altitude warning system. In the event of the cabin altitude exceeding 10,000 ft, a 3 chime aural alert and a voice "cabin" warning would sound. In addition, a red "cabin alt" warning light would illuminate on the main annunciator panel and the red "master warning" light would flash. The system activated as designed.

Supplemental oxygen system

A supplemental oxygen system was installed for use by the crew and passengers in the event of a failure of the cabin pressurisation system. It was a conventional high-pressure gaseous storage system, which distributed low-pressure oxygen to the crew and passenger breathing masks. The flight crew masks were of the quick donning type. The passenger masks were stored in overhead dispensing units positioned in the ceiling of the passenger cabin. An altimetric switch ensured automatic deployment of the oxygen masks whenever the cabin altitude exceeded 14,000 ft. Manual deployment of the masks could also be performed from the cockpit. The supplemental oxygen system operated normally during the incident flight and the masks automatically deployed when the cabin altitude exceeded 14,000 ft.

Hypoxia

Hypoxia describes the physiological condition where insufficient oxygen is available to meet the needs of the body. The condition is particularly significant because of the rapid rate at which symptoms can manifest themselves and the variation in the onset of symptoms between individuals. A person suffering the effects of hypoxia could experience a range of symptoms capable of adversely affecting their ability to safely operate an aircraft. Those symptoms include impairment of mental performance, loss of judgement, vision impairment, memory loss, reduced levels of awareness and muscular impairment.

The effects of hypoxia may be such that the person is unable to recognise the symptoms or identify that their level of performance has been impaired.

Individuals experiencing the effects of hypoxia can have difficulty in completing even simple tasks. The severity of those symptoms depends upon many factors, including the altitude to which the individual is exposed, the duration of the exposure and individual physiological differences. Untreated, hypoxia can result in loss of consciousness and death.

Depressurisation events and response procedures

The US Federal Aviation Authority Civil Aeromedical Institute (CAMI) classified any occurrence of decompression as significant if the cabin altitude exceeded 14,000 ft, the cabin masks were deployed, or if the occurrence resulted in injuries. The occurrence satisfied two of those criteria.

A rapid depressurisation, as defined by CAMI, occurs when the cabin altitude increases by more than 7,000 ft/minute. There was no evidence that during the occurrence the change in cabin altitude exceeded that rate.

The crew had planned to cruise at FL230. For flights in "pressurised aircraft engaged in flights not above FL250", the Civil Aviation Safety Authority required that supplemental oxygen be used by all flight deck crew "at all times during which the cabin altitude exceeds 10,000 ft". "A crew member (not being a flight crew member on flight deck duty) ... must use supplemental oxygen at all times during which the cabin pressure altitude exceeds Flight Level 140." CAO 20.4 refers.

The aircraft must also carry sufficient supplemental oxygen for passengers as specified in CAO 20.4 Subsection 7.5.

"Supplemental oxygen for passengers

7.5 A pressurised aircraft to which this subsection applies that is to be operated above 10,000 feet flight altitude must carry sufficient supplemental oxygen:

(a) where the aircraft can safely descend to Flight Level 140 or a lower level within 4 minutes at all points along the planned route and maintain Flight Level 140 or a lower level for the remainder of the flight - to provide 10% of the passengers with supplemental oxygen for 30 minutes or 20% of the passengers with supplemental oxygen for 15 minutes; and

(b) where the aircraft cannot safely descend to, or maintain, Flight Level 140 or a lower level in accordance with subparagraph (a) - to provide each passenger with supplemental oxygen for so much of the flight time above Flight Level 140 that exceeds 4 minutes duration and to provide 10% of the passengers with supplemental oxygen for 30 minutes or 20% of the passengers with supplemental oxygen for 15 minutes."

The Quick Reference Handbook (QRH) used by the crew included a checklist for use following illumination of the cabin altitude warning light. That checklist did not include a requirement for the crew to don oxygen masks, but contained checklist items to establish control of the cabin pressure utilising the manual pressurisation controller. The QRH also included a checklist for rapid depressurisation that required the crew to immediately don oxygen masks and commence an emergency descent.

During the descent, the aircraft cabin altitude exceeded 14,000 ft and the passenger oxygen masks automatically deployed. It was not possible to estimate the maximum cabin altitude experienced, nor the length of time that the cabin altitude exceeded 10,000 ft. The passenger oxygen masks probably deployed during the first 4 minutes of the descent from FL 230.

History of the flights

A Metroliner had departed Darwin on a flight to Dili, East Timor. The crew had climbed to flight level (FL) 180 and were maintaining that altitude. They had been under the control of Darwin Air Traffic Control for the first 60 NM and had transferred to Brisbane Control in accordance with normal procedures. They had established two-way radio communication with Brisbane sector control.

The Airbus A330 (A330) was on a flight from Kuala Lumpur, Malaysia to Darwin and the crew had commenced descent to FL210 in accordance with instructions from Brisbane Control.

When the A330 was approximately 97 NM from Darwin, the crew requested further descent. The Brisbane controller immediately activated the intercom line to Darwin Approach and informed the Darwin controller of the position of the A330. The Metroliner was 81 NM from Darwin at that time.

The Darwin controller, who was under training, only had access to a 60 NM radius display screen. He had already handed off the outbound Metroliner to the Brisbane controller and authorised descent to FL 120 for the A330 in the belief that the Brisbane controller would separate the two aircraft. The trainee was not immediately aware of the relative positions of the aircraft as the conflict was not shown on his display.

The training officer immediately realised that separation had not been assured by the coordination interaction and looked to one side where he had set up a second screen display on an adjacent console. He had selected a greater range scale on the console, that showed the position of the two aircraft. As he was briefing the trainee on urgent action to remedy the situation, the crew of the A330 reported on the Darwin Approach radio frequency that they were on descent to FL120. While that radio exchange was occurring, the trainee had selected the longer range scale on his console and the display had now appeared on his screen. It showed that the aircraft were approximately 9 NM apart in a nose-to-nose conflict. He immediately issued an amended instruction to the crew of the A330 to maintain FL190. The crew was able to carry out that instruction before the vertical separation standard of 1,000 ft was infringed.

A few moments later the captain of the Metroliner saw the A330 pass immediately overhead. He reported that there was insufficient time to draw the co-pilots attention to the aircraft before it passed. He was of the opinion that there would have been insufficient time to initiate an evasive manoeuvre if it had been required.

The A330 crew saw the Metroliner but took no action as the aircraft were obviously going to miss. The traffic alert and collision-avoidance system was serviceable but did not activate.

Radar analysis indicated that the aircraft passed with approximately 1,300 ft vertical separation but with no horizontal difference.

At the Brisbane sector console, the short term conflict alert activated approximately 20 seconds prior to the time of passing. The controller immediately checked the radar display and found that a separation standard was being applied by Darwin approach control and took no further action.

Darwin approach control

The Darwin approach unit controlled airspace to 60 NM, in the north-west quadrant, up to and including FL200. At the time of the coordination from Brisbane sector, both aircraft were outside those parameters and, therefore, not subject to direct control by Darwin.

Local procedures required that arriving aircraft be assigned no lower than FL210 by Brisbane Control. Therefore Darwin Control was required to approve any further descent despite the aircraft being in Brisbane's airspace. When the Brisbane controller coordinated the position of the A330, the trainee read into the tone of the voice that a request was being made for further descent even though the actual words used did not state that intent. The trainee was not immediately aware of the confliction because it was outside his area of jurisdiction and not within the range of his selected display. The trainee also had the impression that, as the aircraft were in Brisbane's airspace, he was only approving descent in Darwin's airspace and that the Brisbane controller was still responsible for the separation of the aircraft.

Radar hand-off was the subject of a specific instruction in the Manual of Air Traffic Services Supplement (MATS Supp) which stated: "Aircraft handed off by radar between Darwin ATC and Brisbane/Tindall Sectors shall be subject to `no restrictions' unless otherwise advised."

The adjacent console was not in use and had been positioned so that the controllers could view the display screen, albeit at a distance of 2 - 3 m. The range was selected at a longer distance than the 60 NM in use at their primary screen. That selection meant that the position of the aircraft could be seen but the details in the information blocks could not be read. A software change was scheduled for introduction the next day that would have allowed the trainee to view the longer range (second look) on his own console rather than having to look across to the second screen.

When the trainee had approved descent for the A330, the Brisbane controller de-selected the intercom line before the training officer could intervene to cancel the approval. The training officer immediately briefed the trainee to maintain the A330 at FL190 because he expected the crew to change frequency quickly as, in his experience, they normally did. The training officer was prepared to go back to Brisbane via the intercom line, but the broadcast by the crew of the A330 made that option redundant.

Brisbane sector control

The controller coordinated the position of the A330 with Darwin Control and expected an instruction from Darwin regarding descent. When they issued approval for FL120 he assumed that they were separating the aircraft, even though he thought that he may not have looked at his display at the time of carrying out the coordination and may have, momentarily, overlooked the actual position of the Metroliner. He then issued the descent instruction and told the crew to change radio frequency to Darwin Approach.

Shortly after, the short term conflict alert activated and the controller immediately checked the display. He saw that there was no infringement of separation standards and took no further action.

The controller had been rated on The Australian Advanced Air Traffic System (TAAATS) for 4 months but had just returned from a month's leave. He received 2 days familiarisation and had performed 4 active shifts prior to the occurrence. He stated that he was well rested and that the workload was moderate. However, he said that he was a bit `rusty' and that the 2 days familiarisation was a bare minimum for adequacy.

Short term conflict alert

The activation of the alert was to warn the controller that the aircraft were predicted to pass within 600 ft and 4.1 NM. Analysis of the performance of the alert indicates that it operated in accordance with the parameters and provided approximately 20 seconds warning of the conflict.