Improperly-secured insulation blanket lodged in freighter aircraft’s pressurisation outflow valve
An improperly-secured insulation blanket became lodged in a 737 freighter’s main pressurisation outflow valve resulting in the aircraft cabin depressurising during descent, an Australian Transport Safety Bureau investigation report outlines.
On 6 July 2021, the Boeing 737-376SF operated by Express Freighters Australia – a subsidiary of Qantas – was operating a freight flight from Perth to Melbourne with a flight crew of two on board.
While descending through 8,000 ft, the flight crew received pressurisation system cautions and the cabin depressurised.
“In response, the flight crew performed the first four steps of the relevant non-normal checklist,” ATSB Director Transport Safety Stuart Macleod said.
“As they descended through 4,000 ft, and with the cabin already depressurised, they elected to suspend that troubleshooting process and landed the aircraft at Melbourne without incident.”
An engineering inspection determined that an insulation blanket from the aft cargo bay had been partially ejected from the aircraft’s main outflow valve, preventing it from closing.
“The engineering inspections revealed multiple aft cargo bay insulation blankets were either missing, installed incorrectly, or unsecured,” Mr Macleod said.
“A heavy maintenance check about 15 months prior to this occurrence necessitated the disturbance and partial or complete removal of the insulation blankets in the aft cargo bay, and it is probable that after this check, the blankets were installed incorrectly, and an inadequate area inspection was carried out.”
As a result of the incident, the operator conducted an inspection of all aft cargo compartment pressurisation components and insulation blankets in their 737 fleet, and discovered multiple unerviceabilities, which were all rectified.
Following these inspections, Express Freighters Australia implemented a 4,000 hour or 36 month insulation blanket visual inspection task for the 737-300, and a 4,000 hour or 18 month visual inspection take for the 737-400.
“Aircraft periodic inspection tasks often require equipment or covering removal to access inspection areas,” Mr Macleod explained.
“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.”