As the Boeing 747-400 was approaching flight level 360, the flight crew observed a cabin altitude warning message and an increase in cabin altitude. As the crew carried out the non-normal checklist, the cabin altitude continued to rise and could not be controlled. The pilot in command decided to carry out an emergency descent and deploy the passenger oxygen masks. As the aircraft descended, all the electronic flight data display screens went blank. The crew continued the descent to an altitude of 10,000 ft using standby instruments. One passenger reported ill effects as a result of the descent. The aircraft was then turned back to Sydney where an uneventful, but overweight, landing was conducted.

An inspection by ground engineers revealed that the upper cabin pressure relief valve had operated, indicating that at some time during the flight the aircraft's internal pressure had exceeded its maximum differential with the ambient pressure. After a number of system

 circuit breakers were cycled the aircraft's display screens returned to normal operation. An engine ground run was carried out, but the inflight faults could not be reproduced. The primary components of the aircraft's pressurization and flight deck display systems were removed for further analysis.

The investigation is continuing.

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