What happened
On December 7, 2000, a Boeing 737-548, registration EI-CDB, was operating a scheduled passenger flight from Cork Airport to Amsterdam. During the initial climb-out, the flight crew noticed an "Auto Fail" warning light on the pressurization panel. Despite this alert, the crew continued the climb, eventually reaching an altitude of approximately 14,144 feet while the cabin remained unpressurized.
As the aircraft ascended, the cabin environment became increasingly unstable. The First Officer began experiencing lightheadedness and was forced to use an oxygen mask. Simultaneously, cabin crew members reported physical symptoms, including ear pain and a sensation of extreme cold, and noted that misting was occurring in the cabin. Eventually, the passenger oxygen masks deployed automatically.
Recognizing the deteriorating situation, the Captain decided to level the aircraft and initiate an emergency return to Cork. The aircraft landed safely at 0755 hours with no injuries to the 90 passengers or 6 crew members.
The investigation
The AAIU investigation focused on why the pressurization system failed to maintain a safe cabin altitude. Investigators examined the aircraft's technical records, which showed that the pressurization system was fully functional and properly configured when tested after the incident. The investigation also reviewed the flight crew's actions, noting that several distractions had occurred prior to departure, including a passenger boarding error and a smoke alarm in the forward galley.
Findings
- The primary cause of the incident was the failure of the flight crew to complete and verify the "After-start" and "After take-off" checklists, which would have ensured the pressurization system was correctly set.
- The crew did not perform a thorough analysis of the "Auto Fail" and "Cabin Altitude" warnings when they first appeared.
- The Captain failed to fully grasp the severity of the situation as the aircraft continued to climb into an unpressurized state.
- The First Officer recognized the onset of hypoxia and utilized oxygen, whereas the Captain did not use oxygen, creating a risk of pilot incapacitation.
- The continuous communication from the Senior Cabin Crew Member regarding the cabin's condition was vital in preventing a more serious outcome.
Safety action
Following the investigation, several safety recommendations were made to the operator, including the need to reinforce the requirement for pilots to fully don oxygen masks when cabin altitude exceeds 10,000 feet or when system integrity is in doubt. Additionally, the operator was advised to improve recurrent training regarding insidious pressurization failures and to align company operating procedures with the Quick Reference Handbook (QRH) regarding oxygen usage.