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The Beechcraft Super King Air 200 aircraft had arrived at Timber Creek NT to conduct an aeromedical flight to Tindal NT. The aircraft had the pilot, a flight nurse and one patient on board.

While on the ground at Timber Creek, the environment had been oppressively hot. Both the pilot and flight nurse reported feeling extremely uncomfortable and had both been perspiring profusely. The aircraft's airconditioning system was not operating properly and had offered little respite from the conditions, either on the ground or in the air. Due to the type of injuries that the patient had suffered, the flight nurse had requested that the pilot maintain "sea level" cabin pressure for the return flight. The flight nurse had also fitted an oxygen mask to the patient. The pilot recalled setting the pressurisation controls to suit the request from the flight nurse.

On the inbound flight, the pilot had been advised by Air Traffic Control to expect a non-standard clearance for the return flight due to RAAF aircraft activity in the area.

Following take-off, at about 2,000 ft, the air traffic controller instructed the pilot to intercept the 173 radial at 120 NM from Tindal, and then to track along that radial to Tindal. That had been necessary to avoid the now active Military restricted area R248(B). The pilot reported that he had then become occupied with re-programming the aircraft's Global Positioning System (GPS). During the climb to the cleared level, Flight Level 130, the pilot reported that he believed that he had actioned all the required checklist items.

As the aircraft climbed through FL125, the flight nurse noticed that the passenger oxygen masks had deployed and conveyed that fact to the pilot. The pilot was unaware of the deployment and had immediately turned around to assess the situation. When he turned his attention back to the instrument panel, the pilot noticed that the cabin ALT WARN caption positioned on the glare-shield mounted Master Warning panel was illuminated. Both Master Warning captions were also flashing. The pilot then contacted Air Traffic Control and received a clearance for an immediate descent to 10,000 ft.

The flight nurse donned the nearest available passenger oxygen mask and re-checked the flow of supplemental oxygen to the oxygen mask worn by the patient. The pilot did not don an oxygen mask during the incident.

Once established at 10,000 ft, the pilot discovered that both the left and right bleed air OFF green advisory annunciators were illuminated, and that both bleed air switches were in the ENVIR OFF position. In that position, no bleed air was available for aircraft pressurisation. The pilot had then selected both bleed air switches to OPEN, and restored normal pressurisation.

The flight was then continued to Tindal at the lower altitude.

The pilot was appropriately licensed for the flight and had approximately 3,600 hours total flying experience, of which 90 hours were on King Air 200 aircraft, with about 50 hours as pilot in command.

The Operator's Pre-Take Off Procedures required the bleed air to be selected to ON (OPEN). When the three-position bleed air switches were selected to EVIR OFF or INST & ENVIR OFF, a green advisory light L or R BLEED AIR OFF annunciator was illuminated. The pilot reported that he could not remember having selected the switches to OPEN prior to take-off.

The pilot indicated that he had not noticed the green L or R BLEED AIR OFF annunciators during the climb. He reported that that was partly due to him being accustomed to seeing the green L and R AUTOFEATHER advisory captions illuminated on the lower centre instrument console during the takeoff. The pilot was also unsure if the ALT WARN and Master Warning caption had been illuminated prior to him being aware of the passenger oxygen mask deployment.

The operator's After Take Off Procedure included a requirement to turn the Auto Feather "OFF not below 1500ft". The pilot reported that the task was often left until after the transition altitude, when the cockpit routine was "less busy". That meant that the green L or R AUTOFEATHER advisory captions would remain illuminated until the checklist was completed, sometimes up to FL 150. The operator's After Take Off Procedure included a note, which indicated that the checklist only needed to be completed when workload permitted.

The After Take Off Procedures also required the pressurisation to be checked. That task involved the pilot checking that the bleed air valve switches were OPEN (up) position. The pressurisation gauges were also to be checked to ensure that the aircraft was pressurising normally. The Transition Altitude Procedures stated that "pressurisation checks should be made at least every 10,000 ft during climb and again when stabilised in the cruise".

The operator's Phase One Emergency Procedures, for a loss of pressurisation with the cabin altitude above 10,000 ft, directed the pilot to don the crew oxygen mask. The pilot reported that he had not performed that task as he had quickly descended the aircraft to 10,000 ft.

The aircraft's air conditioning system had a history of operating problems, with six instances of maintenance recorded since January 2001. The flight nurse said that the airconditioning system had been malfunctioning for some time prior to the incident, and that the aircraft had been to Darwin several times for repair. The crew also indicated that on the flight from Tindal to Timber Creek the interior of the aircraft had been hotter than normal and that it had not operated at all on the incident flight.

The maintenance record entry following the flight indicated that the airconditioning system high-pressure switch had tripped. Maintenance troubleshooting found that the system gas pressure was incorrect and the pressure had been subsequently adjusted.

The aircraft's cabin altitude warning system and the passenger emergency oxygen mask system were both designed to operate at a cabin pressure altitude of 12,500 ft. The two systems were separate and operated in response to electrical signals received from individual pressure switches. The cabin altitude warning system illuminated both the glare-shield mounted flashing red Master Warning annunciators and the red ALT WARN annunciators on the warning annunciator panel. The passenger emergency oxygen mask deployment system activated a green PASS OXY ON annunciator on the aircraft's Caution/Advisory panel.

The operator had installed an aural warning device that operated in conjunction with the cabin altitude warning annunciator, into this aircraft on the 30 November 2000. That device had been installed under a CAR 35 engineering approval. On the 23 February 2001, following an instruction from the Civil Aviation Safety Authority that the device had to be removed because it had not been manufactured in accordance with current legislation, the unit was removed from the aircraft.

The sun's azimuth was 282 degrees true, which meant that the sun's light came from behind and to the left of the aircraft. The sunlight glare from the West may have occluded the red master warning and the altitude alert on the pilot's side and centre of the instrument panel glareshield.

A text by Thomas Turner, titled "Checklists & Compliance", written on the use of checklists in aircraft operations, pointed out the problem with operating equipment such as global positioning system units in "altitude critical areas". Such areas were defined as within 1,000 ft of the ground or within 1,000 ft of levelling off from a climb or descent. The textbook went on to say "Concentrate on what it takes to establish the new level flight attitude, trim the aircraft for level flight, and check to make sure critical items are complete before turning to less crucial tasks".

A Civil Aviation Safety Authority booklet "The Global Positioning System" stated that:

"GPS may relieve the mental task of computing the aircraft's position and speed in relation to maps of the terrain, but it may increase the workload of programming and accessing the desired information from the machine and interpreting it.

"Piloting an aircraft requires continuous monitoring and reacting to events both inside and outside the cockpit.

"The amount of information we can deal with at any one time is limited....

"Don't allow the operation of the GPS to interfere with your primary task of flying the aircraft".

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