737 Freighter Failed to Pressurize Due to Unauthorized Practice

A 737 freighter failed to pressurize due to normalized but unauthorized practice: ATSB final report.

The issue was cabin pressurization involving a 737 freighter (Boeing B737-36E SF, ZK-FXK), near Darwin Airport, Northern Territory, UK, November 4, 2021.

A 737 freighter aircraft failed to pressurize after a cargo depressurization switch was left on during pre-flight preparations, an ATSB (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 November 4, 2021.

After takeoff, the flight crew observed the aircraft did not pressurize 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 pressurization. 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 the freighter 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 depressurize 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 authorized, but it had become normalized 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 normalized, 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 unauthorized 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 unauthorized 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 unauthorized practice would be carried out consistently or safely.”

What has been done as a result

The operator issued communications to its staff to immediately cease the unauthorized practice and remind staff to only operate equipment in accordance with approved documentation. Additionally, they commenced a review of operational documentation and completed incorporating the requirements of the operations manual supplement.

The operator also commenced a review of its training and aircraft induction processes to ensure sufficient staff and documentation were available to conduct support these processes.

Safety message

This incident highlights the risks associated with undertaking unauthorized practices and using equipment in a manner other than for its intended purpose. Without formal assessment of its efficacy or its potential for unintended consequences, combined with no documentation or training, there is no assurance that an unauthorized practice would be carried out consistently or safely.

This incident also demonstrated how essential training and up-to-date documentation is in ensuring correct understanding and operation of an aircraft.

Read the final ATSB report: “Cabin pressurization issue involving Boeing B737-36E SF, ZK-FXK, near Darwin Airport, Northern Territory, on November 4, 2021.” Investigation number: AO-2021-047. Occurrence date: 11/4/2021. Report release date: 5/16/2023.

Featured image source/credit (Figure 1: Main deck cargo smoke detector panel): Source: Airwork.

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