Taiwan's Aviation Safety Council (ASC) has recommended that China Airlines reinforce its flight and maintenance crew training in several areas after one of its Airbus A330-300 aircraft had to make an emergency landing because of excess cabin altitude.
The aircraft, with registration B-18353, was flying from Kaoshiung to Hong Kong when it encountered the pressure anomaly on 24 August 2012. The jet was about 155nm northeast of Hong Kong International Airport and at a cruising altitude of 34,000ft (10,400m) when a "CAB PR EXCESS CAB ALT" warning was triggered. The flightcrew donned oxygen masks, declared a Mayday to air traffic control and landed 30min later at Hong Kong without further incident.
None of the 248 passengers or 14 crew on board were injured. There was also no damage to the aircraft.
Investigations by the ASC found that there was an unknown leakage in the no. 1 air conditioning pack outlet, which degraded the performance of the air conditioning system and cabin pressurisation capability before the flight. Both the aircraft's no. 2 engine bleed system high pressure valve (HPV) and pressure regulating valve (PRV) had also failed on two previous flights, but were not properly rectified.
The crew operating the previous flights on the aircraft reported that although they had seen alert messages, they thought that it was triggered by their procedure and system checking. Hence, they did not note down the bleed fault message on the technical logbook after the flights. The technical personnel who performed maintenance for the previous flight, meanwhile, could not recall seeing the bleed fault message. The proper minimum equipment list (MEL) operation procedures for engine 2 bleed failure were thus not performed.
The ASC also notes that the pilot had reset the engine 2 bleed system, which cleared the fault message and led them to think that the system had returned to normal, and thus closed the bleed valve. The failed bleed system therefore could not supply pressure for the no. 2 air conditioning pack outlet, while the no. 1 pack leak meant that it could not provide enough compressed air for cabin pressurisation. This led the cabin altitude to increase continuously after the closure of the bleed valve.
Five defects related to the control of the cabin temperature was also recorded on the jet's technical logbook in the month prior to the incident, with symptoms indicating a broken sleeve at the no. 1 air conditioning pack outlet.
With accumulated flight hours of 170 and 245 on the HPV and PRV respectively, considered low for the inspection interval of pack outlets, the sleeve defect could not have been discovered in a timely manner.
The short flight duration, coupled with bad weather, also increased the pilots' workload, and they did not notice the cabin altitude advisory message and bleed system page display that popped up before the excess cabin altitude warning was triggered.
The cabin pressure anomaly may also have been avoided had the pilots checked the aircraft’s bleed system page before the engine start or after resetting the engine 2 bleed and proper MEL actions could have been taken. The flightcrew also did not follow the abnormal procedure prescribed on the aircraft's operation manual to verify the abnormal condition shown on the display.
The ASC has since recommended that China Airlines reinforce its flightcrew’s discipline of logging alerts on the technical logbook, system training and operation procedures related to the engine bleed air system and cabin pressurisation system.
It recommended that the maintenance team be disciplined in using alert message and MEL. It should also reinforce the system training related to the engine bleed air system and air conditioning system, and keep a keen watch on repeated defects to reduce trouble shooting time.