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Published: 22 November 2017

Factual information

On 15 October 2017, at about 1152 Western Standard Time,[1] the flight crew on board an Air Asia Airbus A320-216, registered PK-AXD, initiated an emergency descent from flight level (FL) [2] 330 to 10,000 ft.

Earlier at about 1122, the aircraft departed Perth Airport, Western Australia, for the planned destination of Bali, Indonesia, with two flight crew, four cabin crew and 146 passengers on board. On departure, the flight crew were issued with a clearance from air traffic control to climb to FL 340. At about FL 250, the crew noticed a system 1 (SYS 1) fault for the pressurisation system appear and then disappear. At about 1148, just after passing FL 300, the master caution activated for the opening of the pressurisation system safety valve(s)[3] (SAFETY VALVE OPEN). The flight crew selected manual control (MAN) for the pressurisation system and, noting a cabin pressurisation rate of climb[4] of 1,100 ft per minute, they attempted to close the outflow valve to reduce the cabin rate of climb. The first officer also noted the cabin differential pressure[5] was 8.3 pounds per square inch and the cabin altitude was 8,900 ft. Before the checklist actions could be completed the master caution cleared.

At about 1151, the master warning activated intermittently for a high cabin altitude. The excess cabin altitude warning (EXCESS CAB ALT) initially activated intermittently and then remained on.

At about this time, the cabin crew noted intermittent activations of the cabin seat belt signs and emergency lighting. The flight crew informed the cabin crew manager that they were going to conduct an emergency descent, donned their emergency oxygen masks and started the descent. Shortly after, the cabin crew manager reported to the flight crew that the emergency oxygen masks in the cabin were not deployed and the flight crew then manually deployed them. The cabin crew reported that they secured the cabin and instructed the passengers in accordance with their emergency descent procedures.

The flight crew informed air traffic control that they were conducting an emergency descent with pressurisation problems and requested a clearance to return to Perth Airport at 10,000 ft. After some initial communications difficulties,[6] air traffic control issued the respective clearances for a descent to 10,000 ft and return to Perth Airport where the aircraft landed safely at about 1248.

Some of the crew members reported some ear discomfort just prior to the emergency descent, but none reported any indications of hypoxia. During the descent there was an intermittent activation of the master caution for cabin low differential pressure (LO DIFF PR). Figure 1 below depicts the activation of the alerts, and the descent from FL 330 to 10,000 ft, which was at an average rate of about 2,300 ft per minute. There were no reported injuries and the aircraft was not damaged. The post-flight maintenance tests identified the cabin pressure controller 1 as at fault (see Aircraft pressurisation).

Figure 1: Activation of alerts and start of descent

Figure 1: Activation of alerts and start of descent

Source: ATSB

Preliminary observations

Aircraft pressurisation

Cabin pressure can be set by the flight crew to operate in automatic, semi-automatic or manual mode. In automatic and semi-automatic mode, it is maintained by two independent cabin pressure controllers, which communicate with each other via a cross-channel link. One controller is active and the other is on standby. The active controller operates the cabin outflow valve by means of its associated motor to control the difference in pressure between the cabin and atmosphere.

If there is a problem with the automatic or semi-automatic mode of control, the flight crew can select manual mode and signal the outflow valve to open or close as desired to manage the differential pressure. In addition to the outflow valve, there are two independent pneumatic safety valves to prevent excessive excursions in differential pressure. They operate in the range of 8.2‑8.9 pounds per square inch differential pressure.

Incident flight faults

The SYS 1 fault triggers when the respective cabin pressure controller has failed. The alert is for crew awareness only and no action is required.

The SAFETY VALVE OPEN fault triggers in-flight if either valve is not fully closed for more than 1 minute. If an overpressure is confirmed when the safety valve opens, manual control of the outflow valve may be employed by the flight crew to reduce the differential pressure. The target cabin pressure rate of climb in manual mode is 500 feet per minute and the target cabin pressure altitude for FL 300 is 5,500 ft and 7,000 ft at FL 350.

The EXCESS CAB ALT fault triggers if the cabin altitude is above 9,550 ft. If the aircraft is above 10,000 ft, the flight crew are required to use their emergency oxygen masks and initiate an emergency descent to 10,000 ft. If the cabin altitude exceeds 14,000 ft and the passenger oxygen masks have not automatically deployed, then they must be manually deployed by the flight crew.

The LO DIFF PR fault triggers if the time for the cabin to reach a differential pressure of zero is less than 1.5 minutes and the aircraft is at least 3,000 ft above the landing elevation.

Previous incidents

There have been previous isolated incidents of erroneous pressurisation management and indications associated with A320 aircraft cabin pressure controllers.

Investigation progress

Under Annex 13 to the Convention on International Civil Aviation, the ATSB have appointed as Accredited Representatives to the investigation:

  • the Indonesian National Transportation Safety Committee, for liasion with the aircraft operator, Air Asia (Indonesia)
  • the French Bureau of Enquiry and Analysis for Civil Aviation Safety, for liasion with the aircraft manufacturer, Airbus.

At this stage of the investigation the ATSB have:

  • collected and reviewed the aircraft flight data recorder and cockpit voice recorder
  • interviewed the flight crew and cabin crew
  • reviewed the basic operation of the aircraft pressurisation system
  • reviewed the operator’s abnormal and emergency procedures for the pressurisation system
  • reviewed previous investigations of A320 cabin pressure controller incidents.

Safety issues and actions

No safety issues have been identified to date.

Direction of the investigation

The ATSB has requested further documentation from the aircraft operator and manufacturer via the Accredited Representatives. Further investigation by the ATSB will include examination of:

  • aircraft maintenance records
  • results of pressurisation equipment tests and inspections
  • training records
  • cabin safety documentation and procedures.



The information contained in this web update is released in accordance with section 25 of the Transport Safety Investigation Act 2003 and is derived from the initial investigation of the occurrence. Readers are cautioned that new evidence will become available as the investigation progresses that will enhance the ATSB's understanding of the accident as outlined in this web update. As such, no analysis or findings are included in this update.



  1. Western Standard Time (WST): Coordinated Universal Time (UTC) + 8 hours.
  2. Flight level: at altitudes above 10,000 ft in Australia, an aircraft’s height above mean sea level is referred to as a flight level (FL). FL 330 equates to 33,000 ft.
  3. There are two safety valves fitted, but it only takes one to activate the master caution.
  4. The aircraft cabin pressure climbs and descends on a schedule, as the aircraft climbs and descends, to minimise the pressure difference across the cabin wall and to limit the maximum cabin altitude to a level that is acceptable for passenger comfort.
  5. Cabin differential pressure is the difference in pressure between inside the aircraft cabin and local atmosphere (i.e. across the cabin wall).
  6. The flight crew were communicating with air traffic control through their emergency oxygen masks, which contributed to the communications difficulties.
General details
Date: 15 October 2017 Investigation status: Active 
Time: 11:55 WST Investigation phase: Evidence-Collection 
Location   (show map):300 km north of Perth Investigation type: Occurrence Investigation 
State: Western Australia Occurrence type: Air/pressurisation 
 Occurrence class: Operational 
Report status: Pending Occurrence category: Serious Incident 
Expected completion: 2nd Quarter 2018 Highest injury level: None 
Aircraft details
Aircraft manufacturer: Airbus 
Aircraft model: 320-216 
Aircraft registration: PK-AXD 
Serial number: 3182 
Operator: Indonesia AirAsia 
Type of operation: Air Transport High Capacity 
Sector: Jet 
Damage to aircraft: Nil 
Departure point:Perth, WA
Destination:Denpasar, Indonesia
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Last update 28 February 2018