Uncontrolled Cabin Pressure Change During Descent into Melbourne

No fatalities • Ballarat Aerodrome, 161° T 37Km (FR24), Victoria

A flight crew experienced a sudden loss of cabin pressure and an influx of cold air while descending through 8,000 feet due to an improperly secured insulation blanket.

What happened

While descending through 8,000 feet toward Melbourne, the flight crew of the aircraft experienced a sudden rush of cold air and physical discomfort, specifically ear soreness. During this descent, the AUTO FAIL and STANDBY annunciator lights on the pressurisation panel illuminated. The crew noted that the cabin differential pressure gauge had dropped to zero, while the main outflow valve position indicator showed the valve was approximately 50 per cent open.

The investigation

Investigators examined the pressurisation system and the physical state of the aircraft's interior. Post-occurrence inspections by engineering personnel revealed that several insulation blankets in the aft cargo bay were either damaged, missing, or improperly installed. Specifically, one insulation blanket was found protruding into the main outflow valve, which prevented the valve from functioning correctly.

Engineers also identified a failure in the heater gasket of the main outflow valve. It is suspected that this failure caused the valve to freeze in a closed position during the descent. As the aircraft entered warmer air, the valve likely thawed and opened suddenly. This sudden movement, combined with the presence of the unsecured blanket, led to the unrestricted venting of cabin air. The investigation traced the origin of the improperly installed blankets back to a heavy maintenance check performed on 26 April 2020, as no other documented maintenance in the aft cargo bay occurred between that date and the incident on 6 July 2021.

Findings

  • An unsecured insulation blanket in the aft cargo bay drifted and obstructed the operation of the main outflow valve.
  • The failure of the main outflow valve heater gasket likely caused the valve to freeze and then suddenly open during descent.
  • The improper installation of cargo bay blankets occurred during a heavy maintenance check approximately 15 months prior to the event.
  • The flight crew's decision to stop the non-normal checklist at 4,000 feet was deemed appropriate given the aircraft was already depressurised and approaching landing.

Probable cause

The primary cause of the uncontrolled cabin pressure change was an insulation blanket in the aft cargo bay that had been incorrectly secured during a previous maintenance check, which subsequently moved to obstruct the main outflow valve.

Frequently asked questions

What happened in the 2021-07-06 The Boeing Company 737-376SF accident near Ballarat Aerodrome, 161° T 37Km (FR24), Victoria?

A flight crew experienced a sudden loss of cabin pressure and an influx of cold air while descending through 8,000 feet due to an improperly secured insulation blanket.

Were there any fatalities in the 2021-07-06 The Boeing Company 737-376SF accident?

No fatalities were recorded in this accident.

What aircraft was involved and where did it happen?

The accident on 2021-07-06 involved a The Boeing Company 737-376SF, registration VH-XMO, operated by EXPRESS FREIGHTERS AUSTRALIA PTY LIMITED, at Ballarat Aerodrome, 161° T 37Km (FR24), Victoria.

What was the probable cause of the accident?

The primary cause of the uncontrolled cabin pressure change was an insulation blanket in the aft cargo bay that had been incorrectly secured during a previous maintenance check, which subsequently moved to obstruct the main outflow valve.

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