Loss of pressurisation, 15 km north-west of Jindabyne, New South Wales

On 9 November 2005 at 0834 Eastern Daylight-saving Time, a Boeing Company 737-7Q8 aircraft, registered VH-VBI, was being operated on a scheduled passenger service from Sydney, NSW to Melbourne, Vic. The aircraft was at an altitude of 40,000 ft (Fight Level 400) when the pilot in command (PIC) noticed the cabin rate of climb indicator suddenly indicate a maximum rate of climb. The PIC disengaged the autopilot and commenced an emergency descent to an altitude of 10,000 ft. When levelled at 10,000 ft, the crew observed that the cabin had repressurised, so they elected to maintain 10,000 ft and continue the flight to Melbourne. There were no reported injuries to passengers or crew.

The PIC reported feeling upset in the stomach and discomfort in the ears before noticing that the cabin rate of climb indicator showed a sudden increase in cabin altitude. Flight data recorder information showed that the aircraft descended to approximately 500 ft below the assigned altitude for 50 seconds. The co-pilot contacted air traffic control and requested an emergency descent to 10,000 ft; when the clearance was obtained the PIC continued the descent.

The PIC attempted to inform the cabin crew of the emergency descent but the announcement was not heard on the passenger address system in the cabin. The cabin crew reported that not all passengers had put their oxygen masks on after the masks were deployed.

The crew observed that at approximately FL300 the cabin altitude was stabilising. At approximately FL230 the cabin altitude was 9.000 ft and the cabin altitude warning ceased.  At approximately FL160 when the cabin altitude was 4.000 ft, the pilot elected to reduce the rate of descent and the cabin supervisor was advised by the copilot that the oxygen masks could be removed. The aircraft levelled off at 10,000 ft approximately 11 minutes after the descent was initiated.

The examination of the pressurisation system revealed that the sudden increase in cabin altitude was due to the positive pressure relief valves opening at a lower pressure than required.