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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".


Some vital checklist actions from the PRE TAKE OFF checklist and the AFTER TAKE OFF checklist were not completed by the pilot. Oppressively hot and humid conditions on the ground would have been very uncomfortable and likely to encourage the pilot to hasten his departure. Any haste during departure would have increased the risk of omitting a checklist item.

The non-standard clearance instruction, received soon after take-off, required re-programming of the GPS. That action captured his attention during the climb, and distracted the pilot from performing parts of the AFTER TAKE OFF checklist and the Transition Altitude Procedure.

The pilot had expected the routine illumination of the green auto feather advisory annunciators during the takeoff and for part of the climb. Consequently he did not identify that additional green annunciators, in the form of the bleed air off indications, were illuminated.

The pilot only noticed that the cabin altitude warning lights were illuminated after the flight nurse had alerted him to the automatic deployment of the passenger oxygen masks. Because of the separate pressure switches involved, it is possible that this deployment occurred slightly before the cabin altitude pressure warning. Alternatively, or in addition, the sun's relative position to the aircraft may have partially occluded the master warning light, making it difficult for the pilot to detect. The inclusion of an aural warning to operate in conjunction with the visual cabin altitude warning annunciator would have provided the pilot with an additional warning during a period of high workload. Desirably, the aural warning would be triggered by a different pressure switch than the visual warning.

The operator's instruction that permitted completion of the AFTER TAKE OFF check "as workload permits", allowed for postponement of a critical check on cabin pressurisation until well above 10,000 ft. Postponement of the AFTER TAKE OFF check also maintained the Auto Feather in an active state, and kept the green annunciator lights illuminated.

The pilot chose not to put on the oxygen mask, as required by the operator's Emergency Procedures, when alerted to the lack of pressurisation. That action resulted in a risk of the pilot suffering from hypoxia had the aircraft continued to climb in an unpressurised state.

  1. The pilot did not complete the Pre Take Off and After Take Off cabin pressurisation checks.
  2. The pilot became pre-occupied with programming the GPS after receiving a track change instruction.
  3. The aircraft was allowed to climb above 10,000 ft in an unpressurised state.
  4. The effectiveness of the aircraft's cockpit warning system was reduced by the operator's practice of allowing postponement of the After Take Off check.


ATSB safety action

An Australian Transport Safety Bureau investigation into a Beechcraft King Air 200 depressurisation incident, BO/199902928, issued three recommendations on the subject of cabin alert aural warning systems. The final report contained an additional recommendation on the same subject. Recommendation R20000288 stated:

"The ATSB therefore recommends that CASA mandate the fitment of aural warnings to operate in conjunction with the cabin altitude alert warning systems on all Beechcraft Super King Air and other applicable aircraft".

The Civil Aviation Safety Authority's response dated 2 February 2001 stated:

"The Civil Aviation Safety Authority accepts this recommendation and will move to prepare a regulatory amendment to make it mandatory for pressurised aircraft to have aural cabin altitude alert warning systems. This amendment will follow the normal regulatory development process which, in the first instance, will lead to the circulation of a Discussion Paper. It is anticipated that the paper will be released this month".

On the 2 February 2001, the Civil Aviation Safety Authority (CASA) also issued a Draft Discussion Paper, DP 0102CS, to the Australian aviation industry. The discussion paper was titled Proposal for Aural Warning to Operate With Cabin Altitude Alert Warning Systems. The discussion paper indicated that it was CASA's preferred option to mandate requirements to modify the aircraft concerned to install an audible warning to complement the existing cabin altitude alert warning system. Responses to that paper were to be provided to CASA by the 12 March 2001.

In April 2002 CASA issued a Notice of Proposed Rule Making (NPRM) on the fitment of aural warnings to pressurised jet and turboprop aircraft.

Investigation report BO/199902928 and the resultant recommendations are available on the Australian Transport Safety Bureau's Website, www.atsb.gov.au or from the Bureau on request.

Local safety action

The operator conducted its own investigation into the issues surrounding this incident. As a result of that investigation a number of changes have been made to the company's operational procedures. Those include a reassessment of company pilot training and check-to-line requirements. Greater emphasis is now being placed on adherence to checklists and occupational health and safety issues relating to operations in hot and humid environments.

The operator actively commenced correspondence with the Civil Aviation Safety Authority to enable the re-installation of the aural warning device kits. On the 15 January 2002, the Civil Aviation Safety Authority responded to the request. The response indicated that the operator could manufacture the system under the operator's "current certificate of approval, as manufacture in the course of aircraft maintenance". The CASA letter stated:

"Following a review of matters associated with the original warning kits and their installation, it is considered there are a number of matters you need to address to accomplish these modifications, they are:

"1.The draft NPRM 0116CS "Proposal for Aural Warning to Operate with Cabin Altitude Warning Systems" should be considered as much as possible, to avoid having to make future design changes to the system.

"2. A design advice on the modification should be submitted to CASA by the CAR35 Authorised Person to save possible rework. It would be expected that there would be a FAR 23.1309 hazard analysis carried out on the system as part of the design justification.

"3. The design needs to incorporate a backup sensor for cabin pressure in addition to the basic sensor fitted. Any failure in the backup system should not disable the warning from the prime system and vice versa. (ie FAR 23.1309 analysis)

"4. The design will call up parts and components by specification for installation in the modification."

On the 20 February 2002, the operator advised the ATSB that it had commissioned a CAR 35 engineer to draft a proposal for the design and approval of audible warning devices using the Civil Aviation Safety Authority's guidelines. Once that has been accomplished and a CAR 35 Engineering Order has been raised, the devices will be manufactured by a sub-contractor and installed in the operator's fleet of King Air aircraft.

General details
Date: 24 October 2001 Investigation status: Completed 
Time: 1330 hours CST  
Location   (show map):22 km SSE Timber Creek, Aero. Investigation type: Occurrence Investigation 
State: Northern Territory Occurrence type: Incorrect configuration 
Release date: 17 May 2002 Occurrence class: Operational 
Report status: Final Occurrence category: Incident 
 Highest injury level: None 
Aircraft details
Aircraft manufacturer: Beech Aircraft Corp 
Aircraft model: 200 
Aircraft registration: VH-SWP 
Serial number: BB-529 
Type of operation: Aerial Work 
Damage to aircraft: Nil 
Departure point:Timber Creek, NT
Departure time:1333 hours CST
Destination:Tindall, NT
Crew details
RoleClass of licenceHours on typeHours total
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Last update 13 May 2014