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Analysis

Summary

The RAAF pilot was not experienced on the type. The additional workload created by instructions from ATC, and from attempting to re-program the GPS at the time when he was completing his climb checks may have captured his attention, thereby reducing his capacity to notice deviations from normal procedure.

Normal procedures included re-positioning blower switches at this stage of the flight. These switches were located very near to the bleed air valve switches, and it is probable that the pilot inadvertently moved both bleed air switches to ENVIR OFF during the climb checks instead of moving the two blower switches. An inadvertent repositioning of the bleed air switches would not be detected by the sequenced monitoring of the pressurisation instrumentation in the climb checklist, as the pressurisation check was before the airconditioning and aft blower checks.

The pilot's performance was progressively degraded due to the effects of hypoxia as the cabin altitude increased.

Safety critical warning systems such as the cabin altitude warning system, need to be sufficiently effective to alert flight crews despite any distractions that may be present at the time. Visual indications supported by auditory alerts have been shown to be more effective in fulfilling this requirement (Refer to Attachment A). CAO 108.26 also strongly recommended an aural warning. The warning system fitted to the aircraft comprised red warning lights on the glareshield. While there was no evidence of failure of the cabin altitude warning system, it did not alert the pilot to the depressurisation in time for him to respond.

The aircraft type had been certified with a cabin altitude warning that was designed to activate at 12,500 ft. This warning system had also been in accordance with the requirements of the Australian CAOs 20.4 and 108.26 current at that time. During the climb there was a limited period when the cabin altitude warning system could have been expected to alert the pilot before the pilot's performance became significantly degraded. Amendment 92 of CAO 108.26 required the warning to activate at 10,000 ft. If the cabin altitude warning operated as required by the amended CAO, the window of opportunity for alerting the pilot would have been increased at a time when the pilot was most able to respond.

The pilot training syllabus was designed in part to meet the perceived needs of military operations. However, the aircraft was being used for operations that were nearer in type to civilian charter. The training syllabus did not provide the same degree of practical reinforcement of normal procedures as was found in the civilian contractor's normal training syllabus. The syllabus therefore did not provide the same tools to enhance resistance to error in normal procedures.

In this type of incident in which the depressurisation was not rapid, the effects of hypoxia gradually develop and difficult to notice. The pilot, having previously checked the cabin pressurisation, had no suspicion that the aircraft was depressurising, and did not associate his inability to master a simple GPS problem with any other aircraft or physiological abnormality.

The pilot and passengers had all undertaken regular hypobaric hypoxia training. Despite this training, they did not identify the onset of the symptoms of hypoxia until one person became unconscious. The training had not provided an effective defence by equipping the flight crew to recognise the onset of symptoms of hypoxia.

The initial factory fitment of the passenger oxygen system mask container doors incorporated short retaining lanyards for the doors. This would have prevented incorrect orientation of the doors during installation. The original lanyards however had been replaced by longer ones, which removed this designed safety feature.

The approved maintenance system only required regular testing of some parts of the automatic oxygen mask deployment system and the cabin altitude warning system. However, not all parts of the systems were required to be tested on a regular basis. A maintenance procedure for a test of the complete systems installed in the aircraft should have indicated that each system would work in flight, however, neither system was required to be completely tested for correct operation in the aircraft.

The maintenance problems found with the passenger oxygen system, and the lack of effective and timely detection of the cabin altitude alert system, were deficiencies that could have resulted in a more serious occurrence. This is especially significant in single-pilot operations. It is likely that the provision of an audible warning device as strongly recommended in CAO 108.26 would have alerted the pilot to the developing pressurisation problem.

FINDINGS

  1. The aircraft cabin altitude warning did not operate at an altitude of 10,000 ft as required by CAO 108.26.
  2. The modified pilot training syllabus did not give the same level of defence against human error.
  3. The maintenance systems for automatic oxygen mask deployment and the cabin altitude warning system did not ensure reliable operation.
 
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