What happened
On September 28, 2002, a Ryanair Boeing 737-204 ADV, registration EI-CJE, departed Derry Airport for Stansted. The captain elected to perform a "Bleeds Off" takeoff, a procedure intended to demonstrate the technique to the relatively inexperienced first officer. During the pre-takeoff phase, the crew configured the air conditioning panel for this specific takeoff mode.
Following departure, while performing after-takeoff checks, the first officer inadvertently switched the air conditioning packs to the OFF position. This action was not verbalized and went unnoticed by the captain. As the aircraft climbed, the cabin altitude rose significantly, eventually reaching approximately 14,000 feet, which triggered the automatic deployment of passenger oxygen masks. The crew heard an aural warning horn but initially misidentified it as a configuration warning. The aircraft continued to climb to Flight Level 270 before the crew identified the pack switches were in the incorrect position and restored them to ON. The flight proceeded to Stansted without further incident.
The investigation
The AAIU investigation focused on the sequence of events leading to the depressurization and the crew's response. Investigators examined the cockpit procedures, noting that the captain's briefing on the reconfiguration process was interrupted by air traffic control clearances. The investigation also reviewed the discrepancy in terminology used during training versus line operations, specifically the difference between the "C" scan pattern and the "squeeze-spread-squeeze" method used in simulators.
Furthermore, the investigation looked into the physiological state of the crew, noting that the delay in identifying the system misconfiguration suggested the possibility of hypoxia. The investigation also assessed the impact of the locked cockpit door policy on the crew's ability to communicate with cabin crew during the emergency.
Findings
- The primary cause was the incorrect positioning of the air conditioning pack switches, which left the aircraft unpressurized during the climb.
- Conflicting terminology between simulator training and standard operating procedures contributed to the first officer's error during the reconfiguration process.
- The captain failed to monitor the pressurization system settings after takeoff.
- The crew misdiagnosed the cabin altitude warning horn as a configuration warning, leading to a period of ineffective troubleshooting.
- The delay in resolving the issue suggests that the crew may have been experiencing the effects of hypoxia.
- The decision to continue the climb to Flight Level 270 and the failure to notify air traffic control of the incident were contrary to standard operating procedures.