Qantas A332 near Adelaide on Feb 4th 2021, loss of cabin pressure
Last Update: March 21, 2023 / 14:53:43 GMT/Zulu time
Date of incident
Feb 4, 2021
Loss of cabin pressure
ICAO Type Designator
On Feb 8th 2021 the ATSB reported the crew performed an emergency descent due to a depressurization indication. A short investigation into the occurrence rated a serious incident has been opened.
On Mar 21st 2023 the ATSB released their final report concluding the probable causes of the serious incident were:
- Due to a design limitation associated with the aircraft’s cabin pressure controller (CPC) systems:
+ The controlling CPC was unable to detect a fault with its pressure sensor, resulting in the loss of cabin pressure control and the subsequent increase in cabin altitude.
+ While the backup CPC triggered the CAB PR EXCESS CAB ALT alert as required when the cabin altitude exceeded 9,550 ft, the data presented to the flight crew indicated normal cabin altitude.
- The flight crew responded to the CAB PR EXCESS CAB ALT alert procedure by observing that there was no confirmatory data and, in seeking evidence to verify the failure, delayed executing the required procedure.
- A preamble to the CAB PR EXCESS CAB ALT alert procedure was introduced as a procedural mitigation for the CPC system design limitation. This required flight crew to rely on the alert even if not confirmed by other system data. This mitigation had significant potential for error as:
+ although a critical component to the procedure, the preamble requirement -was not part of the ‘read & do’ procedural steps and was reliant on memory recall
+ the required procedural action was contrary to both the operator and aircraft manufacturer’s procedural philosophy of confirming alerts with system data before executing abnormal procedures
- An Airbus service bulletin (SB), introduced as a mitigation to the CPC design limitation, would have prevented the loss of cabin pressure control from the pressure sensor fault. However, the SB had not been incorporated on this aircraft.
- The mitigations introduced by Airbus to counter the design limitation associated with the A330 cabin pressure control systems were ineffective because:
+ changes to the CAB PR EXCESS CAB ALT alert operational procedure did not ensure appropriate management of the fault
+ the service bulletin had very limited uptake in the A330/A340 global fleet. [Safety Issue]
- The operator’s training system did not adequately cover the unique requirements of the CAB PR EXCESS CAB ALT alert procedure, increasing the risk of an incorrect or delayed application of the required procedure. [Safety Issue]
Other factors that increased risk
- Contrary to the operator's Flight Administration Manual policy requirements, the flight crew referenced the Flight Crew Techniques Manual instead of the Flight Crew Operations Manual to resolve the conflicting data presented with the CAB PR EXCESS CAB ALT procedure.
- While determining the need to execute the CAB PR EXCESS CAB ALT alert procedure, the flight crew focussed on the need for confirmatory data to support an emergency descent and consequently, did not don their oxygen masks, increasing the risk of hypoxia.
The ATSB analysed:
During cruise with the aircraft’s cabin altitude at 7,100 ft, the controlling cabin pressure controller (CPC2) experienced an intermittent pressure sensor fault, resulting in inaccurate control of the outflow valves. This fault was not automatically detected by CPC2, as the sensor continued to provide credible data. As the automatic transfer of pressurisation control for a sensor fault is dependent on that fault being detected, pressurisation control was not transferred to CPC1, which was functioning correctly. As a result, the aircraft began to slowly depressurise.
About 15 minutes after the undetected pressure sensor fault occurred, the actual cabin altitude (as recorded by CPC1) reached 8,800 ft. However, since CPC2 was still sensing 7,100 ft, the Electronic Centralized Aircraft Monitor (ECAM) advisory message CAB ALT was not activated.
This alert was designed to notify the flight crew that the cabin altitude was outside normal operating parameters, but its triggering relied on the controlling CPC detecting this event. About 6 minutes later, the actual cabin altitude exceeded 9,550 ft, and CPC1 triggered the CAB PR EXCESS CAB ALT alert on the ECAM. This also resulted in the cabin pressure information being displayed to the flight crew on the CAB PRESS page on the System Display (SD). However, as CPC2 was still in control and sensing a cabin altitude of 7,100 ft, the cabin pressure indications on the CAB PRESS page appeared normal. Consequently, the flight crew were presented with conflicting information.
Cabin pressure controller fault
In 2006, Airbus became aware of a design limitation associated with the CPC systems where a faulty cabin pressure sensor in the controlling CPC could cause misleading information to be presented to the flight crew on the CAB PRESS system page. In response, Airbus introduced a preamble into the flight crew operating manual’s (FCOM’s) CAB PR EXCESS CAB ALT alert procedure. The preamble required an immediate response to the ECAM alert by executing the associated procedure displayed on the engine and warning display, irrespective of system data indications.
A critical function of an emergency procedure checklist, or the ECAM ‘read & do’ procedure, is to ensure that items necessary for the safe operation of aircraft are carried out. Airbus introduced the preamble to the FCOM’s CAB PR EXCESS CAB ALT alert procedure as a risk mitigator against the limitations in the CPC fault detection capabilities. The preamble was unique in that it required flight crews to execute the procedure even without confirmatory data. However, this was contrary to an Airbus operating philosophy for flight crew to seek confirmatory evidence of a fault before executing the associated abnormal procedure.
Despite the importance of the preamble, it was not part of the ‘read-&-do’ procedural steps presented on the ECAM, and thus not presented to the flight crew during the execution of the required procedure. Instead, the immediate actioning of the procedure in response to the alert (as required by the preamble) was reliant on flight crew memory recall of the preamble’s requirement.
Following previous CPC faults, Airbus released service bulletins (SB) for the A320, A330 and A340 aircraft types that included CPC software logic updates. For the A330, one of those software changes allowed the CPC in control to use the lowest cabin pressure value of the two CPC pressure sensors if there was a large enough difference between the values. The aircraft did not have this SB incorporated at the time of the incident.
While the SB would not have prevented the intermittent pressure sensor fault observed on VH-EBK, its incorporation would have resulted in automatic identification of the pressure difference when the cabin altitude was about 8,000 ft, resulting in CPC2 using the CPC1 pressure sensor for cabin pressure control. This would have prevented the depressurisation event and activation of the CAB PR EXCESS CAB ALT alert.
When the red CAB PR EXCESS CAB ALT warning alert was triggered, the flight crew immediately sought confirmatory data consistent with both the Airbus philosophy and the Qantas internal memo dated June 2020. The CAB PRESS page on the SD, however, indicated that the system was operating normally. The captain also identified that there had been no 8,800 ft CAB ALT advisory message preceding the red warning alert, raising questions about whether the controlling CPC was functioning correctly. As previous simulator depressurisation training had reinforced the need to obtain confirmation of a depressurisation before conducting the required emergency descent, the flight crew continued to seek further evidence of depressurisation. Discussion with the customer service manager further delayed the required response.
The first officer retrieved the Flight Crew Techniques Manual (FCTM) for guidance. The captain reported that, upon review, they found the information to be inconsistent and confusing, although the first officer assessed that the information instructed an immediate emergency descent. The confusion over the FCTM information resulted in the flight crew further delaying the execution of the alert’s procedure.
Emergency oxygen When the aircraft’s cabin altitude exceeds 10,000 ft, the occupants’ risk of hypoxia is significantly elevated. Therefore, the loss of cabin pressurisation notified by the CAB PR EXCESS CAB ALT is a red warning alert, where the immediate imperative for the flight crew is the use of supplemental oxygen.
On activation of the ECAM alert on this occasion, the flight crew focussed on acquiring data to support the cabin altitude warning alert, and in particular the requirement to conduct an emergency descent for a depressurisation. Consequently, they overlooked the first line of the ECAM procedure, which was for both crew members to use oxygen. The risk from hypoxia is relevant in any cabin pressurisation issue, whether through explosive or slow depressurisation. Although the outflow valves would automatically limit the cabin altitude to 15,000 ft, this is still within the hypoxic range.
Use and content of manuals
When presented with conflicting data for the CAB PR EXCESS CAB ALT alert, the flight crew referenced the FCTM and not the FCOM. While the operator’s Flight Administration Manual (FAM) stated that the primary references for operational matters were the ECAM/Quick Reference Handbook (QRH), and then the FCOM and the FAM, the flight crew’s use of the FCTM was probably influenced by the first officer’s recollection that it contained information relevant to depressurisation/emergency descent.
Although actioning the CAB PR EXCESS CAB ALT alert was critical to flight safety and could potentially present a unique situation for flight crew, knowledge of the preamble’s unique requirement was not well known among the operator’s flight crew. Further, the depressurisation training in the simulator potentially created a bias as flight crews became accustomed to always having supporting evidence of a cabin depressurisation alongside the CAB PR EXCESS CAB ALT alert. As such, the operator’s training system did not adequately cover the unique requirements of the CAB PR EXCESS CAB ALT alert procedure that may be encountered in flight, increasing the risk of an incorrect or delayed application of the required procedure.
Mitigations to CPC design limitation
In response to the possibility of cabin depressurisation events resulting from the CPC design limitation, Airbus introduced two specific mitigations for the A320, A330 and A340 fleets. These mitigations were:
- to remove the design limitation through the application of improvements to the CPC through application of SBs released in 2012 and 2014
- procedural changes, which included the incorporation of a preamble to the CAB PR EXCESS CAB ALT abnormal procedure around 2006.
As the CAB PR EXCESS CAB ALT alert is a red warning alert, being the highest priority alert, the required procedural response should be designed to ensure a correct application every time with no exceptions. However, since 2016 there have been 7 events of a similar nature to the VH-EBK according to Airbus. Of these 7 events, 2 had indications of the incorrect application of the procedure, where the preamble requirement did not appear to have been applied. The VH-EBK event was a further example.
These events, as well as the potential failure points of the reliance on memory recall and the various operational philosophy aspects, identified that the procedural mitigation did not ensure appropriate management of aircraft experiencing a cabin depressurisation due to the CPC design limitation. Further, while the SB is the Airbus preferred method of addressing the design limitation, there has been very limited uptake in the pre-2012 A320 and A330/340 fleets. Overall, the mitigations introduced to counter the CPC design limitation have not been sufficiently effective.
Aircraft Registration Data
Date of incident
Feb 4, 2021
Loss of cabin pressure
ICAO Type Designator
This article is published under license from Avherald.com. © of text by Avherald.com.
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