What happened
On a flight from Calgary to Abbotsford, a Boeing 737-200 operated by WestJet experienced a significant loss of cabin pressurization while climbing to flight level 310. The flight began with a non-routine "bleeds-off" takeoff, a procedure necessitated by the aircraft's weight and ambient conditions at Calgary. This specific method requires closing engine compressor bleed valves, relying on the auxiliary power unit (APU) to provide initial cabin pressurization and air conditioning.
Shortly after takeoff, an air traffic control instruction interrupted the crew's After Take-off checklist. Following this interruption, the crew failed to reconfigure the pressurization equipment for normal flight or shut down the APU as required. Upon reaching cruise altitude, the crew shut down the APU without investigating why it was still running. This action triggered a rapid loss of cabin pressure. The cabin altitude rose to 24,000 feet, prompting the automatic deployment of passenger oxygen masks at 14,000 feet. The aircraft performed an emergency descent and landed safely in Kelowna, British Columbia, with no injuries reported.
The investigation
The investigation focused on the sequence of events following the takeoff interruption and the crew's subsequent management of the pressurization loss. Investigators examined the aircraft's mechanical condition, which was found to be normal, and reviewed the crew's adherence to standard operating procedures (SOPs). The probe also looked into the compatibility of onboard emergency equipment and the effectiveness of the aircraft's communication systems during the event.
Findings
- The primary cause of the depressurization was the failure to reconfigure the engine bleeds and pressurization system following a non-routine takeoff procedure.
- An air traffic control interruption during the After Take-off checklist led to the omission of critical steps.
- The crew failed to perform required memory checks at 10,000 feet and at cruise altitude, which would have identified the improper configuration.
- The crew's response to the cabin altitude warning was delayed because they consulted the incorrect checklist, focusing on an advisory light rather than the rapid depressurization procedure.
- A lack of redundancy in checklist execution, where the pilot not flying completed items unilaterally, reduced situational awareness and error detection.
- Incompatible fittings between different portable oxygen bottles and masks in the cabin caused a brief delay in assisting an infant passenger.
- The aircraft's public address system volume was insufficient for all passengers to clearly understand instructions during the descent.
Safety action
- The operator updated passenger briefings to explain that oxygen masks function even if the reservoir bag does not inflate.
- The operator implemented a maintenance order to standardize fittings for all portable oxygen equipment across the fleet.
- Boeing revised the Auto Fail or Unscheduled Pressurization Change checklist for all Boeing 737 models to ensure bleed-air and air conditioning switches are prioritized.