Tiger B738 at Sydney on Jul 12th 2018, cabin pressurization not armed
Last Update: September 30, 2019 / 13:58:12 GMT/Zulu time
On Jul 20th 2018 the ATSB reported while climbing through about FL100 the crew notice the cabin did not pressurize. The crew subsequently detected that the pressurization system had not been correctly armed during pre-flight checks. The occurrence was rated an incident and is being investigated.
On Sep 30th 2019 the ATSB released their final report concluding the probable causes of the incident were:
- The aircraft did not pressurise due to an incorrectly configured pressurisation system.
- The incorrect configuration was the result of procedures and checklists not being managed appropriately.
The ATSB reported a trainee first officer (ATPL, 18,300 hours total, 27 hours on type) was pilot flying (he had been in command of Boeing 767s and Boeing 777s prior to converting to the 737), a training captain (ATPL, 21,511 hours total, 949 hours on type) was pilot monitoring.
The crew performed the checklists prior to engine start and switched both air conditioning packs OFF. Following engine start and prior to taxi the checklists required both packs to be switched to AUTO, however, the switches remained in the position OFF.
The ATSB summarized the sequence of events:
The aircraft took off at about 1136 Eastern Standard Time (EST). After flap retraction at approximately 1137, the crew completed the final component of the take-off procedure, followed by the after take-off checklist. At about the same time, air traffic control (ATC) issued several instructions to the crew. The take-off procedure and after take-off checklist both included a requirement to check the position of the air conditioning packs switches. However, during both the procedure and the checklist, the packs were not identified as being OFF.
At about 1142, while the aircraft was still climbing and passing 13,500 ft, the cabin altitude warning horn sounded. The First Officer identified that both air conditioning packs switches were in the OFF position. Both packs were then immediately switched to AUTO. The Captain took over control of the aircraft and selected altitude hold on the autopilot. The aircraft reached a maximum height of 14,900 ft (at about 1144), whereby the Captain disengaged the autopilot and commenced a descent to 10,000 ft. The warning ceased at about 1146. During this time, both pilots donned oxygen masks in accordance with the cabin altitude warning procedure.
Once level at 10,000 ft, the crew completed the remainder of the cabin altitude warning checklist. As the cabin pressure was now under control and operations were normal, the crew continued the flight to Melbourne.
The ATSB analysed:
Before Taxi Procedure
The First Officer did not turn the packs to AUTO during the before taxi procedure. This was contrary to the FCOM procedure but there was no evidence to suggest other deviations took place. The Captain did not notice this error.
The First Officer had significant experience on other aircraft types and his training records at Tiger did not show that an error of this type had occurred previously. The lengthy break in flying roles and significant gaps in the training program may not have allowed the First Officer sufficient time to consolidate the procedures to an intuitive level that was resilient to error. Acquired skills decay over time and consistent rehearsal and application are essential for long-term retention. Despite this, the First Officer had demonstrated satisfactory performance thus far in his training, having been assessed as no longer requiring a safety pilot.
The Captain’s discussion of the ground disconnect requirements may have served as a distraction for the First Officer. The Captain could not recall exactly when this discussion occurred, but the procedure position was approximately the same time as the First Officer’s required actions to reconfigure the pack switches following engine start. Distraction can interrupt the procedural sequence. As the before taxi procedure was conducted from memory, pilots were required to remember at what point the interruption occurred in order to recommence the sequence. This may lead to further error if there is not a conscious recognition of that distraction and interruption.
The ATSB noted that the before taxi checklist did not include checks for the correct configuration of pack switches. However, the Boeing 737 has supplementary procedures for a no engine bleed or unpressurised take-off, which required the crew to reconfigure the air bleeds and packs after take-off. If this was missed or done incorrectly, then the after take-off checklist was designed to capture the error. According to Boeing, the after take-off checklist is also in place to catch a bleed/pack configuration error made during the before taxi procedure. Consequently, absence of this step in the before taxi checklist was not considered as being contributory.
The ATSB also considered whether the First Officer’s previous experience as a captain, and the recent experience of the crew having flown together contributed to a relaxed level of supervision by the Training Captain. The Captain and First Officer had flown together on three occasions in the previous week. The Captain was well aware of the First Officer’s flying experience and had been satisfied with his previous performance such that he had a reasonable level of confidence in First Officer. The Captain explained that the First Officer had got it right before and that he (Captain) may have relaxed his supervision of the First Officer thus contributing to him not identifying the error at this time. Although a highly experienced pilot, the First Officer was still a trainee on the Boeing 737 and as such, required vigilant supervision of a training captain. This is a crucial defence against error by a trainee pilot.
The combination of relative inexperience, disjointed training, distraction and lapses in supervision appear to have contributed to the omission of the pack switches step during the before taxi procedure.
After Take-off Checklist
The take-off is a high workload phase of flight and Sydney is known to be a busy airport to operate from. The procedures to ensure that the air conditioning packs were correctly configured placed significant reliance on the after take-off checklist to achieve this. The high workload experienced by the crew included the procedural actions required by the departure flown and the training being undertaken. This workload was increased by the operational discussions that took place which appeared to have distracted the crew from correctly completing the procedure and subsequent checklist.
Whilst the Captain reported that he did look at the gauges and switches, he is likely to have had a low level of expectancy of error with regard to the pack switches. His confidence in the First Officer’s performance during training and not knowing the First Officer to have made that same mistake before is likely to have caused him to not perceive the error, despite looking at the switches. This would have been exacerbated by rushing the checklist as he reported.
In this case, the pressurisation controller was correctly set and functioned normally by closing the outflow valve to prevent the escape of cabin air. This likely resulted in a natural lag in cabin altitude rising since the controller closed the outflow valve to prevent the escape of cabin air, but additional air via the air conditioning packs was not available to pressurise the aircraft. The Captain specifically mentioned that when he looked at the gauges, the cabin differential and altitude were as expected. Considering the timing of the check (early stage of climb) and the gauges reading as expected, this probably indicated to him that the pressurisation system was correctly operating and gave him no reason to believe that the pack switches were OFF.
The investigation also considered the effect of viewing the air conditioning and pressurisation panels from the left seat. In this case, the Captain may not have moved enough, or at all, to allow a complete view of all gauges and switches, especially given he described rushing the checklist. It is likely that his low expectation of error was reinforced by the cabin differential and altitude appearing to be as expected.
Workload management and disruption also affected the First Officer’s ability to accurately conduct the checklist while flying the aircraft, especially considering the operational discussion that took place during that time. The operational requirements of ATC are considered normal but in light of the induced additional workload due to discussion and distraction, the First Officer did not check pressurisation as would normally occur.
This article is published under license from Avherald.com. © of text by Avherald.com.
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