Express Freighters B733 at Melbourne on Jul 6th 2021, uncontrolled pressurization change
Last Update: March 2, 2022 / 15:17:28 GMT/Zulu time
On Mar 2nd 2022 the Australian ATSB released their final report concluding the probable cause of the incident was:
An insulation blanket in the aft cargo bay was incorrectly secured leading to it coming adrift and inhibiting the main outflow valve operation. This resulted in uncontrolled pressurisation changes in the aircraft.
The ATSB analysed:
The flight crew reported that passing through 8,000 ft on descent into Melbourne, the AUTO FAIL and STANDBY annunciator lights illuminated and they felt a rush of cold air and a soreness in their ears. The flight crew also observed the cabin differential pressure gauge reading zero and the main outflow valve position indication gauge reading approximately 50 per cent open.
During descent, the main outflow valve modulates increasing cabin pressure as ambient air pressure increases. It is possible that the failure of the main outflow valve heater gasket, identified by engineering personnel after the occurrence, resulted in the valve freezing in the closed position. As the aircraft descended into warmer air, the valve may have thawed resulting in its sudden opening and observed ingestion of the unsecured insulation blanket.
However, with the main outflow valve operation inhibited, it was unable to modulate allowing an unrestricted venting of pressurised cabin air. This resulted in the inability to control the aircraft’s cabin pressure reported by the flight crew.
One of four conditions is required to illuminate the AUTO FAIL annunciator on the pressurisation panel. The pilots reported a rush of air and a soreness in their ears, indicating that an ‘excessive rate of cabin pressure change (1,890 sea level ft/min)’ was probably detected by the cabin pressure control system resulting in the illumination of the AUTO FAIL annunciator. The STANDBY annunciator illuminated due to the pressurisation system unable to automatically operate and switching to the STANDBY mode of operation.
The ATSB assessed that the decision by the flight crew to suspend the non-normal checklist at 4,000 ft was appropriate due to the aircraft being already depressurised and on approach to land.
During post occurrence troubleshooting and inspections, the engineers discovered multiple aft cargo bay insulation blankets either installed incorrectly, damaged, unsecured or missing. This included an insulation blanket protruding from the main outflow valve, inhibiting its operation. As part of the post occurrence rectifications, the engineers correctly orientated and installed new blankets where required.
The incorrect installation of some of the blankets along with an inadequate inspection of their condition occurred at some point prior to the occurrence. A heavy maintenance check was carried out on 26 April 2020, 15 months prior to the occurrence. This inspection and maintenance activity included multiple tasks within the aft cargo bay which necessitated the disturbance and partial or complete removal of the insulation blankets. After this date, and prior to the occurrence on 6 July 2021, there was no documented evidence of maintenance carried out in the aft cargo bay which would require access or disturbance of the insulation blankets.
It is unlikely that undocumented maintenance occurred in this area due to the requirement to remove multiple interior linings and the cargo floor to gain access to the insulation blankets. Therefore, it is probable that during the heavy maintenance check that was completed on the 26 April 2020, the insulation blankets were installed incorrectly, and an inadequate area inspection was carried out.
Aircraft Registration Data
This article is published under license from Avherald.com. © of text by Avherald.com.
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