During the climb, while passing through flight level (FL) 180 for FL190, the flight crew of the Beechcraft 1900D aircraft observed the illumination of the CABIN ALT HI warning annunciator and the flashing Master Warning light. The cabin altimeter was indicating 9,800 ft and gradually increasing, with a 500 ft/min cabin rate of climb noted on the cabin vertical speed indicator. The pilot in command contacted air traffic control (ATC) and requested an immediate descent to FL 140. An initial clearance was received to descend to FL160. As the aircraft reached that level, the crew noted that the indicated cabin altitude was approaching 12,500 ft. The crew then advised ATC that they required a further descent to FL140.
During the descent, the crew carried out the quick reference handbook CABIN DECOMPRESSION emergency procedures. Those checks were carried out when the CAB ALT HI annunciator illuminated, indicating that the cabin altitude had exceeded 10,000 ft. The crew reported that they had donned oxygen masks as detailed in the `immediate actions' part of the checklist and had closed the door between the cockpit and the cabin so the passengers would not become alarmed at seeing the crew wearing the masks. The crew also reported that as the indicated cabin altitude was below 14,000 ft, they had decided not to deploy the passenger oxygen masks as specified in the `immediate actions' part of the checklist.
At FL 140 the cabin altitude was still over 10,000 ft and further descent to FL120 was requested, and carried out. At that altitude, the CABIN ALT HI warning annunciator light extinguished.
The Beech 1900D aircraft was certified under Part 23 of the US Federal Aviation Regulations (FARs). The type certificate data sheet for the aircraft type noted that the Airplane Flight Manual (AFM) formed part of the aircraft equipment. Australia recognised FAR23 certification of the Beech 1900D Airliner under the provisions of Civil Aviation Regulation (CAR) 22A.
Civil Aviation Orders (CAO's) provided information on the requirements for the provision and use of oxygen for the crew and passengers. In particular CAO 20.4.5.1, paragraphs (a) and (b) stated that:
'An operator must include in the operations manual required under regulation 215 of the Civil Aviation Regulations 1988 to be provided by the operator, information relating to the following matters:
'(a) the procedures to be followed in the operation of the oxygen systems in the aircraft to which the operations manual relates;
'(b) the methods of administering oxygen to passengers.'
CAR 215 required that the operator have an operations manual and stated that:
'(1) An operator shall provide an operations manual for the use and guidance of the operations personnel of the operator.
'(2) An operations manual shall contain such information, procedures and instructions with respect to the flight operations of all types of aircraft operated by the operator as are necessary to ensure the safe conduct of the flight operations...'
In addition, CAR 232 detailed the requirements for Flight Check Systems and stated, in part, that:
'(1) The operator of an aircraft shall establish a flight check system for each aircraft, setting out the procedure to be followed by the pilot in command and other flight crew members prior to and on take-off, in flight, on landing and in emergency situations.' and
'(4) The pilot in command shall ensure that the flight check system is carried out in detail.'
The operator's approved Emergency Procedures, as detailed in Section III of the CASA approved AFM, detailed the action to be taken in the event of a CABIN DECOMPRESSION. The actions were required following the illumination of the CABIN ALT HI warning annunciator at an approximate cabin altitude of 10,000 ft. A note at the beginning of Section III stated that:
'Immediate action procedures are delineated by bold type with the remaining procedures following.'
The 'bold type' immediate actions for a CABIN DECOMPRESSION required, amongst other mandatory actions, that the pilot `PULL ON' the cabin oxygen control. That action deployed the passenger oxygen masks. The remaining, non-bold type, text of the emergency procedures required that the passengers are then instructed to activate and don their oxygen masks.
A maintenance investigation, carried out by the operator, found that the in-flight depressurisation had been the result of a failed cargo door pressurisation seal. The seal was replaced and the aircraft returned to service.
The aircraft also had an existing Minimum Equipment List (MEL) restriction, MEL 21.4, that was invoked on 8 October 2000. That MEL related to the illumination of the L ENVIR FAIL annunciator and required that the pilots select the left bleed air switch off for the duration of the application of the MEL. That rendered the left environmental bleed air system unserviceable and resulted in the pressurisation air being sourced from the right engine only. A maintenance investigation into that issue, following the occurrence, had been unable to fault the system and MEL 21-4 was removed. Subsequently, the environmental air problem re-appeared on 18 October 2000. However, examination of the system by maintenance personnel again found no problem.
