The mitigations introduced by Airbus to counter the design limitation associated with the A330 cabin pressure control systems were ineffective, because:
Response by Airbus
Airbus advised that the 2 main aspects of the system design safety issue have been assigned to the Product Safety Enhancement Committee. That committee has been tasked to:
This comprehensive analysis will be completed in the first half of 2023.
ATSB comment
The ATSB welcomes and acknowledges the intention by Airbus to evaluate the mitigations used to address the design limitations associated with the cabin pressure control system in the A330 aircraft. However, as proposed safety action and a timeline were not provided, the ATSB remains concerned with the resolution of this safety issue. Accordingly, the ATSB issues the following safety recommendation to support Airbus’ proposed product enhancement.
The Australian Transport Safety Bureau recommends that Airbus takes safety action to address the effectiveness of the mitigations to the design limitation associated with the A330 cabin pressure control systems.
The origin of the event encountered by A330 VH-EBK was a wrong pressure measurement within the Cabin Pressure Controller (CPC) in control.
The wrong measurement, stuck at a plausible cruise cabin pressure, led the CPC to incorrectly control the cabin altitude, while the incorrect cabin altitude was displayed in green in the SD page.
The CPC in standby, permanently monitoring the cabin altitude using its own sensor independent from the CPC in control, triggered the red ECAM warning CAB PR EXCESS CAB ALT when the cabin altitude exceeded 9550ft.
Comparable failure scenario was met in the past and a design modification was developed to eliminate this scenario: the Cabin Pressure Control system now considers the lowest cabin pressure value of both CPCs sensor reading for the cabin altitude control.
This design modification was introduced in CPC P/N 20793-42BD. A Vendor Service Bulletin and an Airbus Service Bulletin (SB A330-21-3163) were published in 2012 to address this modification.
These Service Bulletins received the “Recommended” status, which is the highest compliance allocable by Airbus, and were also promoted to customers as one of the Enhancements in the annual "Digest of Available Enhancements".
To reinforce the information to the operators about the risk of inadvertent cabin depressurization and its subsequent operational consequences, and to encourage them to implement the Service Bulletin, a Service Information Letter (SIL) was published in 2014 by the vendor Nord-Micro. This SIL was also supported by an Airbus Operators Information Transmission.
The Nord-Micro Service Information Letter was updated in 2020 for a renewed promotion.
Regarding the operational documentation, the FCOM, as well as the FCTM which is the manual used to build the training, explain and emphasize the need to apply the ECAM procedure associated to the red ECAM warning CAB PR EXCESS CAB ALT as it is displayed, even with incorrect cabin altitude displayed in green in the SD page.
Nonetheless, Airbus and Nord-Micro have started to review additional means to increase the flight crew awareness and the actions required for this specific case of CAB PR EXCESS CAB ALT warning with nominal cabin altitude displayed on the SD page.
Airbus has also started to review potential additional levers to increase the fleet penetration of the existing SB. Airbus will complete this answer with the available results of these two reviews by the beginning of 2024.
The ATSB acknowledges the intention of Airbus and Nord-Micro to review additional means to increase flight crew awareness and actions in response to the CAB PR EXCESS CAB ALT warning, and to increase fleet penetration of the Airbus Service Bulletin SB A330-21-3163.
Therefore, the ATSB will continue to monitor the safety issue and expect the reports of the proposed safety action in January 2024. All safety action taken will be assessed to determine the residual risk from this safety issue and its status updated on ATSB's website.
In April 2024, Airbus released Operational Engineering Bulletin (OEB) 57 to increase A330 flight crew awareness of the operational requirements for the CAB PR EXCESS CAB ALT procedure application. This 'white' OEB directed flight crews to apply the CAB PR EXCESS CAB ALT procedure even if not confirmed by data presented on the CAB PRESS systems display page. Additionally, Airbus has proposed training sessions for all affected operators to provide details on this OEB.
A ‘white’ OEB is issued by Airbus in circumstances where non-compliance may have a significant impact on the efficient operations of the aircraft. OEB's are published in the Quick Reference Handbook (QRH) for the A330. When preparing for a flight, the flight crew must know what OEBs are applicable to that aircraft, the specific entry conditions for applying these OEBs, and the cockpit and systems effects of the OEB when applied to the aircraft. An OEB is applicable until a permanent corrective solution is installed on the aircraft.
The corrective solution for OEB57 is the implementation of Service Bulletin (SB) A330-21-3163 (a ‘recommended’ upgrade to A320, A330 and A340 aircraft pressurisation systems, designed to prevent a cabin depressurisation from a similar CPC pressure sensor fault). Airbus have commenced a number of programs designed to increase fleet penetration of this SB including commercial incentives for implementation of the SB by operators, as well as other supporting and monitoring programs. When an aircraft's cabin pressure controllers (CPCs) have incorporated the SB, OEB57 will be removed from that airframe.
The ATSB acknowledges the safety action taken by Airbus to address the effectiveness of mitigations to the design limitation associated with the A330 CPCs and considers that the stated safety action adequately addresses the identified CPC design limitation.
