The aircraft was on a scheduled flight from London Heathrow to New Delhi, India. The aircraft was dispatched in accordance with the Minimum Equipment List (MEL) with the left air conditioning (AC) system disabled. Shortly after reaching FL350 the crew were alerted by EICAS that the cabin altitude was increasing above normal, triggered at 8,500 feet. With no additional Environmental control system (ECS) actions available to control cabin altitude, the flight crew initiated a descent. During this descent the cabin altitude exceeded 10,000 ft and the crew completed the relevant emergency actions.
The loss of cabin pressurisation was caused by detachment of the lower right air conditioning recirculation fan duct on a sector where the left air conditioning system had been disabled before flight. As a consequence of this finding, the Aircraft Maintenance Manual has been amended to alter the process of replacing the relevant recirculation fan and maintenance procedures to react to a related Maintenance Alert Message have been altered.
The investigation also identified a software problem related to the volume of the cabin decompression pre-recorded announcement (PRA) in the passenger cabin which is being addressed by the Operator’s safety action. Three Safety Recommendations are made concerning the testing of the installed performance of CVR systems.
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