What happened
During pre-flight preparations in Darwin, an engineer engaged the cargo/depress switch on a B73/36E SF, registration ZK-FXK, in an attempt to cool the flight deck. The engineer failed to return the switch to its original position before completing their duties. This oversight was not detected by the flight crew during their pre-flight inspections. As the aircraft climbed, the incorrectly positioned switch prevented the cabin from pressurising as intended, causing the cabin altitude to rise above 10,000 ft and triggering a cabin altitude warning.
The investigation
The investigation focused on why the switch was left in the ON position and why the error went undetected. It was found that using the cargo/depress switch for ground cooling had become a normalised, though unauthorised, practice among staff in Darwin due to the lack of external cooling equipment. The investigation also revealed that the operator had not fully updated the Flight Crew Operating Manual (FCOM) to reflect changes from a recent cargo conversion, meaning the crew was unaware of specific check requirements. Furthermore, training regarding the specific system differences of this aircraft model was insufficient.
Findings
- The practice of using the cargo/depress switch for cooling was an unauthorised procedure that had become accepted among ground staff.
- The engineer's failure to deselect the switch was likely caused by an interruption in their routine due to the early arrival of the flight crew.
- The operator failed to provide adequate training on the specific system differences of the B737-36E SF compared to the rest of their fleet.
- Inadequate review of aircraft documentation meant the flight crew was unaware of certain required checks outlined in the operational manual supplement.
- The crew's habit of monitoring pressurisation during climb allowed for the early detection of the issue.