Warning device event, 46 km west of Mackay, Queensland, Boeing 737-7Q8, VH-VBC

On 2 December 2005, a Boeing Company B737-7Q8 aircraft, registered VH-VBC, was being operated on a scheduled passenger flight from Townsville to Brisbane Qld. While the aircraft was passing flight level 370 on climb, the crew heard a 'bang', which was closely followed by the annunciation of a Window Overheat master caution. After becoming aware that the outer layer of the pilot in command's L1 window was cracked, the crew followed the checklist for window damage. As a result, at about 2106 Eastern Standard Time, a cabin altitude warning horn sounded. The sounding of the warning horn was the normal result of the crew's implementation of the checklist for window damage. However, the flight crew believed that the aircraft was depressurising as a result of the window damage and responded to the cabin altitude warning by carrying out an emergency descent from 33,000 ft to 10,000 ft.

During the descent, the crew closed the valve that controlled the outflow of air from the aircraft. However, the pressurisation system was functioning normally and closing the outflow valve caused the aircraft to exceed its cabin pressure limit, as a result of which the over-pressure safety relief valves opened. The flight crew realised that the aircraft was not depressurising, but pressurising, and opened the outflow valve. The combined action of the crew and the automatic opening of the safety relief valves reduced the cabin pressure at a rate greater than that which passengers normally experience. As a result, 11 passengers sustained minor injuries.

Following a company investigation, the operator retrained the flight crew involved in the incident, audited its check and training system and modified the simulator programme to include operational issues identified in this incident.

The aircraft manufacturer has modified the checklist for window damage to minimise the possibility of a cabin altitude warning occurring when the checklist is used.

The operator, in conjunction with the manufacturer, is still investigating the cause of the window breakages.