Aviation safety investigations & reports

Uncontrolled pressurisation change involving Boeing 737, VH-XMO near Melbourne Airport, Victoria, 6 July 2021

Investigation number:
Status: Completed
Investigation completed
Phase: Final report: Dissemination Read more information on this investigation phase


What happened

On 6 July 2021, a Boeing Company 737-376SF (B737), registered VH-XMO and operated by Express Freighters Australia, was operating a freight flight from Perth, Western Australia, to Melbourne, Victoria. While passing 8,000 ft on descent the flight crew received pressurisation system cautions and the cabin depressurised. The flight crew performed the non-normal checklist and continued to Melbourne for an uneventful landing.

What the ATSB found

The ATSB found that an aft cargo bay insulation blanket had not been secured correctly during a previous maintenance activity. During the descent, the unsecured blanket became lodged in the main pressurisation outflow valve, inhibiting its ability to control aircraft pressurisation.

What has been done as a result

The operator conducted an inspection of aft cargo compartment pressurisation components and insulation blankets in their B737 fleet resulting in multiple unserviceabilities being discovered and rectified. Following these inspections, Express Freighters Australia implemented a 4,000 hour or 36 month insulation blanket visual inspection task for the B737-300 and a 4,000 hour or 18 month visual inspection task for the B737-400.

Safety message

Aircraft periodic inspection tasks often require equipment or covering removal to access inspection areas. Maintenance crews are reminded of the importance of ensuring that any items removed for access are thoroughly inspected for serviceability and securely reinstalled. Items must be refitted in accordance with the maintenance manual to prevent unsecured items inhibiting flight critical systems such as the aircraft pressurisation system.

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The investigation


The ATSB is investigating the depressurisation of a Boeing 737, VH-XMO near Melbourne Airport, Victoria, on 6 July 2021. 

While passing 8,000 ft on descent, the crew received pressurisation system warnings and the cabin depressurised. The engineering inspection revealed an insulation blanket had detached and lodged in the aft outflow valve.

The evidence collection phase of the investigation will include a review and examination of maintenance procedures, maintenance records and the gathering of any other relevant evidence.

A final report will be released at the conclusion of the investigation. Should a critical safety issue be identified during the course of the investigation, the ATSB will immediately notify relevant parties, so that appropriate safety action can be taken.

General details
Date: 06 July 2021   Investigation status: Completed  
Time: 0104 EST   Investigation level: Short - click for an explanation of investigation levels  
Location   (show map): Ballarat Aerodrome, 161° T 37Km (FR24)   Investigation phase: Final report: Dissemination  
State: Victoria    
Release date: 02 March 2022   Occurrence category: Incident  
Report status: Final   Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer The Boeing Company  
Aircraft model 737-376SF  
Aircraft registration VH-XMO  
Serial number 23488 LN:1352  
Sector Jet  
Damage to aircraft Nil  
Departure point Perth, Western Australia  
Destination Melbourne, Victoria  
Last update 02 March 2022