What happened
On April 18, 2015, while operating at a cruising altitude of FL340, the crew of a Boeing 737-400 received both audible and visual alerts indicating that the cabin altitude had exceeded 10,000 feet. Approximately 25 minutes before the scheduled descent for landing, the crew initiated memory items from the "Cabin Altitude Warning or Rapid Depressurization" checklist. This included donning oxygen masks, verifying 10/100% oxygen flow, and establishing communication between the pilots.
Following these actions, the captain assumed control and monitored the cabin altitude. The crew requested a descent, and shortly after beginning the maneuver, the cabin altitude dropped below 10,000 feet as the pressurization system operated in automatic mode. The aircraft descended to FL290, where the crew continued the flight while monitoring the cabin altitude. The aircraft subsequently landed safely at Copenhagen Airport (EKCH).
Upon arrival, during the startup of the Auxiliary Power Unit (APU), an "OVERSPEED" alert occurred. While the captain recorded the APU overspeed in the technical log, the initial cabin altitude excursion was not documented in the aircraft's technical log immediately after landing.
The investigation
The investigation, conducted by the operator, focused on the crew's response to the depressurization warning and the subsequent technical issues. Investigators reviewed flight data and the crew's adherence to standard operating procedures. The investigation also examined the maintenance actions taken following the landing, including an APU computer reset and a pressurization system test performed by a licensed mechanic at the request of the Maintenance Control Center (MCC).
Findings
- The crew failed to correctly execute the required memory items from the depressurization checklist.
- The captain decided to discontinue non-normal checklist procedures because the cabin altitude readings appeared to be returning to normal.
- The captain failed to record the cabin altitude excursion in the technical log after landing, only noting the APU overspeed.
- The improper operation of the PRECOOLER CONTROL VALVE SENSOR was identified as the probable cause of the cabin altitude increase.
- The maintenance technician performed a pressurization system test with the APU running, but this specific test was not documented.