A 737 freighter aircraft failed to pressurise after a cargo depressurisation switch was left on during preflight preparations, an Australian Transport Safety Bureau report details.

The Boeing 737-36E SF (an aircraft built as a passenger airliner but subsequently converted to a freighter) was being operated by Airwork on a flight from Darwin to Brisbane on 4 November 2021.

After take-off, the flight crew observed the aircraft did not pressurise as expected. After stopping the climb at 11,000 ft, the flight crew began to descend to 10,000 ft, during which time a cabin altitude warning alert occurred.

Once at 10,000 ft, the crew completed required checklist actions, but were unable to establish control of the pressurisation. Subsequently the equipment cooling fan failed, the electronic flight information system reverted to a monochrome display output, and the weather radar failed.

The crew made the decision to return to Darwin for an uneventful landing.

“On arrival it was identified that the guarded cargo/depress switch was on,” ATSB’s Dr Michael Walker said.

“This switch was normally only used in the event of a main cargo deck smoke event, when it will depressurise the aircraft to assist smoke removal.”

The ATSB found the switch had been turned on by a maintenance engineer during pre-flight preparation, in an attempt to cool the flight deck. The engineer omitted to turn the switch off prior to completing their duties, and this was not identified by the flight crew.

“Using the switch in this manner was not authorised, but it had become normalised by the operator’s staff in Darwin, where there was no ground support equipment to provide external cooling,” Dr Walker said.

“Even though this practice had become normalised, there were insufficient risk controls in place to ensure that the aircraft would be returned to the correct configuration prior to departure.”

Additionally, the ATSB investigation found a pre-flight check of the switch was not incorporated into the operator’s flight crew operating manual, despite the aircraft’s cargo conversion operations manual stipulating it as a requirement.

Since the incident, the operator issued communications to its staff to immediately cease the unauthorised practice, and remind staff to only operate equipment in accordance with approved documentation.

Additionally, the operator commenced a review of operational documentation and completed incorporating the requirements of the operations manual supplement.

“This incident highlights the risks associated with undertaking unauthorised practices and using equipment in a manner other than for its intended purpose,” Dr Walker said.

“Without formal assessment of its efficacy or its potential for unintended consequences, combined with no documentation of training, there is no assurance that an unauthorised practice would be carried out consistently or safely.”

Read the report: Cabin pressurisation issue involving Boeing B737-36E SF, ZK-FXK, near Darwin Airport, Northern Territory, on 4 November 2021

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