Incident Overview
On Friday, 7 September 2012, at 07:43 local time, a Ryanair Boeing 737-800 with registration EI-EKV was climbing from Madrid-Barajas Airport (LEMD) to Flight Level 220 en route to Las Palmas de Gran Canaria, Spain. The aircraft carried 160 passengers and 6 crew members, all of whom were uninjured.
Sequence of Events
During the climb, the purser contacted the cockpit and informed the captain that cabin crew were feeling unwell. This confirmed the captain's own feeling of unwellness. Although no CABIN ALTITUDE WARNING had been received, the captain decided to declare an emergency, stop the climb, and initiate the CABIN ALTITUDE WARNING procedure. The aircraft returned to Madrid-Barajas Airport.
Aircraft Systems
As a consequence of executing the CABIN ALTITUDE WARNING checklist, the outflow valve remained closed for the remainder of the flight, leading to subsequent overpressurization and depressurization of the cabin. The investigation determined that the pressurization system on EI-EKV worked correctly during the incident flight. At the time of the incident, the cabin altitude was 3,774 ft.
Crew and Passenger Outcome
All 166 persons onboard were uninjured. The crew's symptoms could not be attributed to a specific cause by the investigation. Hypotheses considered included hypoxia from poisoning, motion sickness, suggestion among crew members, or a combination of the last two.
Contributing Factors
The investigation identified two factors that contributed to the incident and its handling: the crew's suspicion that the aircraft was malfunctioning, evidenced by constant problems since takeoff (especially the operation of the CABIN ALTITUDE WARNING, which had not worked prior to the flight), and possible confusion.
Investigation Conclusion
The Civil Aviation Accident and Incident Investigation Commission (CIAIAC) concluded that the cause of the symptoms reported by several crew members could not be determined. The pressurization system operated correctly, and no definitive causal link was established.