Airwork B733 at Darwin on Nov 4th 2021, cabin did not pressurize
Last Update: May 16, 2023 / 08:17:08 GMT/Zulu time
Incident Facts
Date of incident
Nov 4, 2021
Classification
Incident
Cause
Cabin did not pressurize
Airline
Airwork
Flight number
TFR-51
Departure
Darwin, Australia
Destination
Brisbane, Australia
Aircraft Registration
ZK-FXK
Aircraft Type
Boeing 737-300
ICAO Type Designator
B733
The ATSB reported: "After take-off, the flight crew identified that the aircraft was not pressurising as expected. As a precautionary measure, the flight crew levelled the aircraft at 11,000 ft and were cleared to descend to 10,000 ft. As the aircraft descended through 10,300 ft the cabin altitude warning presented and the flight crew donned oxygen masks. On return to Darwin, the flight crew identified that the cargo smoke depressurisation switch had been selected. The incorrect configuration was not identified prior to departure.", rated the occurrence an incident and opened a short investigation estimated to conclude in 2nd quarter of 2022.
On May 16th 2023 the ATSB released their final report concluding the probable causes of the incident were:
Contributing factors
- While preparing the aircraft for flight, the engineer selected the aircraft’s cargo/depress switch to ON then omitted to switch it off prior to leaving the aircraft.
- During their pre-flight activities, neither of the flight crew identified that the cargo/depress switch had been selected ON. Although the aircraft operational manual supplement required this switch to be checked, neither pilot was aware of this requirement.
- During the aircraft’s climb, the cargo/depress switch was in the ON position. This prevented the aircraft from pressurising as expected and the cabin altitude subsequently rose above 10,000 ft, triggering the cabin altitude warning.
- The aircraft system to be used in the event of a main deck cargo smoke event on the operator’s B737 fleet was being routinely used by the operator’s engineering personnel in Darwin as a means to cool the flight deck. This practice had become normalised as a result of the perceived benefit of doing so, but there were insufficient risk controls in place to ensure that the aircraft would be returned to the correct configuration prior to departure. (Safety issue)
- The operator did not provide sufficient training during the introduction of the B737-36E SF to its fleet to ensure its personnel understood the differences between these aircraft and the rest of its B737 fleet.
- The operator’s flight crew operating manual for the B737-36E SF aircraft had not been fully amended to incorporate all revisions as detailed in the cargo conversion operational manual supplement.
Other findings
- The flight crew were accustomed to checking cabin pressurisation during climb to ensure the aircraft was pressurising as expected. As a result, the flight crew identified the pressurisation problem involving ZK-FXK early, which enabled prompt action and prevention of a more serious incident.
The ATSB analysed:
Introduction
During pre-flight preparation, the engineer turned on the cargo/depress switch in an attempt to cool the flightdeck of ZK-FXK. The engineer omitted to turn the switch off prior to completing their duties and this was not identified by the flight crew. This prevented the aircraft from pressurising as expected and the cabin altitude subsequently rose above 10,000 ft.
The use of the cargo/depress switch in this manner was not authorised but had become normalised by the operator’s staff in Darwin.
The analysis will examine the issues related to unauthorised procedures and how documentation and training are essential for correct aircraft operations.
Normalised, unauthorised procedure
‘Normalisation of deviance’ was a process defined by Dianne Vaughan (1996) during the Space Shuttle Challenger investigation whereby unacceptable practices become accepted as the norm. The unacceptable practice is repeated without catastrophic results, reinforcing its normalisation.
Although the occurrence involving ZK-FXK did not have the same potential for a catastrophic outcome, it was an example of normalised deviance. The operator’s staff were using an aircraft system in a manner for which it was not designed (that is, using the cargo/depress switch on the ground). This practice was not authorised but had become accepted because of the perceived benefit of cooling the flight deck of its B737 aircraft in Darwin while working on the aircraft.
The engineer believed that in doing so they would be forcing air into flight deck but did not realise that this would not occur on ZK-FXK. It was identified that limited training on the B737-36E SF aircraft’s differences with the operator’s other type meant that operator’s staff were not aware that the desired result would not be achieved.
There was no evidence to suggest that anyone conducting this practice had undertaken a formal assessment of its efficacy or its potential for unintended consequences. The absence of formal documentation, procedures or training meant there was no assurance that the practice would be carried out consistently or safely. This was demonstrated by the engineer forgetting to deselect the switch, which is likely to have been a result of their normal routine being interrupted by the earlier than expected arrival of the flight crew. Lapses are common when interruptions occur and the absence of controls such as a documented procedure meant that the lapse was not recognised.
The absence of ground support equipment to provide external cooling appears to have instigated the unauthorised practice and it is likely that the practice may have continued given the frequently hot conditions in Darwin.
Aircraft documentation
Although the cargo conversion had taken place prior to the operator acquiring the aircraft, the operator did not ensure that all the aircraft documentation was adequately reviewed prior to entry into service. As a result, the flight crew operating manual (FCOM) had not been amended to include all changes detailed in the operational manual supplement (OMS), notably the requirement to check the main deck cargo smoke detector panel. The pilots were not aware of this requirement, thus removing a defence against the unauthorised use or incorrect position of the cargo/depress switch. Not checking the system also increased the risk of not detecting potential issues in the system.
The B737 is a very common aircraft but can be operated in various configurations which may differ between numerous operators. It is essential that aircraft documentation adequately reflect the correct aircraft configuration and procedures to prevent the aircraft being operated incorrectly.
Training on aircraft differences
Although the pilots had some training on the newly introduced aircraft, it was focused on the cargo door itself and not on all of the new procedures or systems following the cargo conversion. The engineer did not receive any formal training on the differences between the operator’s B737 aircraft. As such, the pilots and engineer were not provided with the opportunity to become fully aware of the aircraft they were required to operate.
In this occurrence, the limited training on aircraft differences reinforced the unauthorised use of the cargo/depress switch. Had the correct system knowledge been provided, it may have discouraged its use if it was known it would not work in the desired manner (at least on the B737-36E SF aircraft). The absence of training on required procedures also removed a defence against departure with an incorrect configuration.
Pilot vigilance
The pressurisation problem was identified early, enabled by the flight crew having developed the habit of monitoring pressurisation during their previous B737 experience. As the FCOM did not require a specific check of pressurisation during the after take-off checks or climb phase, the pressurisation problem would still have triggered the cabin altitude warning albeit later in the climb. The crew’s heightened vigilance of pressurisation allowed them to identify and monitor the situation, take appropriate action promptly and thus avoid a more serious pressurisation incident.
Incident Facts
Date of incident
Nov 4, 2021
Classification
Incident
Cause
Cabin did not pressurize
Airline
Airwork
Flight number
TFR-51
Departure
Darwin, Australia
Destination
Brisbane, Australia
Aircraft Registration
ZK-FXK
Aircraft Type
Boeing 737-300
ICAO Type Designator
B733
This article is published under license from Avherald.com. © of text by Avherald.com.
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