The ATSB has issued a safety recommendation to AirAsia Indonesia calling for the airline to review its passenger safety briefing material to ensure instructions on how to activate passenger oxygen masks are clear and effective.

The formal recommendation follows an ATSB investigation into a pressurisation event on board one of the airline’s Airbus A320 aircraft about 30 minutes after departure from Perth on a scheduled passenger flight to Bali, on 15 October 2017.

During the climb, while passing through flight level* 340, the crew initiated an emergency descent to 10,000 feet in response to a master caution warning of a malfunction of the aircraft’s pressurisation system.

The captain made a passenger announcement for the emergency descent and manually deployed the passenger oxygen masks. During the emergency descent, some oxygen masks did not deploy or passengers felt they were not receiving oxygen. Consequently, some passengers then moved around the cabin to find a functioning oxygen mask unit. Passengers later surveyed by the ATSB recalled that the cabin crew shouted commands such as ‘BRACE’, ‘GET DOWN’ and ‘CRASH POSITION’, increasing their level of confusion and panic.

After reaching 10,000 feet, the flight crew announced the aircraft was at a safe altitude for oxygen masks to be removed. The aircraft landed safety at Perth Airport, with no injuries to passengers and crew, and no damage to the aircraft.

The ATSB’s subsequent investigation into the incident found that AirAsia Indonesia’s pre-flight safety briefing and safety information card did not include a clear instruction on how to activate the flow of oxygen from the passenger oxygen masks, and that the bag may not inflate when oxygen is flowing. This may have resulted in some passengers not understanding whether or not there was oxygen flowing in the mask.

Further, cabin crew provided additional commands to passengers that were inappropriate for a depressurisation, which had the potential to increase confusion in the cabin and likely increased the level of panic experienced by some passengers.

An important aspect of managing abnormal passenger responses is the cabin crew’s ability to recall and use appropriate standard commands.

ATSB Transport Safety Director Dr Stuart Godley said in an emergency, cabin crew are required to perform a safety leadership role for passengers.

“This incident highlights that an important aspect of managing abnormal passenger responses is the cabin crew’s ability to recall and use appropriate standard commands,” Dr Godley said.

“Passengers generally responded well when appropriate commands were used, but incorrect commands resulted in some confusion and panic.”

Dr Godley said that inclusion of information highlighting that oxygen is flowing through the mask even though the bag may not inflate will improve passengers' knowledge and reduce anxiety and their susceptibility to a hypoxia-related event.

“Cabin crew emergency procedures training that includes role-playing of the full range of expected passenger behaviour, including panic and confusion, can better prepare cabin crew when exposed to more complex real-world scenarios,” Dr Godley said.

The ATSB found the intermittent pressurisation system faults that led to the emergency descent were likely due to an intermittent incorrect calculation of cabin pressure by one of the aircraft’s two independent cabin pressure controllers (CPCs). During the incident, the affected CPC provided 12 intermittent fault messages before the crew began the emergency descent.

Read the report: Pressurisation event involving Airbus A320, PK-AXD, 160 NM (300 km) north of Perth, Western Australia, on 15 October 2017 

* At altitudes typically above 10,000 feet commercial aircraft operate to and are separated by ‘flight levels’. Flight levels are measured in units of 100 feet with the aircraft’s altimeter set to a standard air pressure of 1013hPa (rather than the local barometric air pressure). This ensures safe vertical separation between aircraft that might otherwise have altimeters set to different barometric air pressures. Aircraft set their altimeters based on local air pressure (known as QNH) below the ‘transitional level’, which in Australia is 10,000 feet.

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