What happened
On 7 September 2012, a Ryanair Boeing 737-800, registration EI-EKV, was climbing through FL220 en route from Madrid-Barajas to Las Palmas de Gran Canaria when the flight crew declared an emergency. During the climb, the purser notified the cockpit that the cabin crew were feeling unwell, a sensation the captain also began to experience.
Prior to the emergency, the aircraft had experienced several technical warnings. Shortly after takeoff, the MASTER CAUTION light activated multiple times, and the crew found they were unable to engage the autopilot. Following the report of crew illness, the captain stopped the climb, initiated the CABIN ALTITUDE WARNING procedure, and returned to Madrid. During the descent and subsequent taxiing, the cabin underwent significant pressure fluctuations, including an overpressurization and a rapid depressurization, which caused barotrauma and ear discomfort for several passengers and crew members.
The investigation
The CIAIAC investigation examined the aircraft's maintenance history, the flight data, and the crew's execution of emergency procedures. The investigation found that the aircraft had undergone unrecorded maintenance the previous night, which left a circuit breaker and a trim switch in incorrect positions. This led to the activation of the MASTER CAUTION and prevented the use of the autopilot.
Investigators also analyzed the symptoms reported by the crew. While the symptoms were consistent with hypoxia, the investigation could not definitively confirm the cause, considering hypotheses such as motion sickness, suggestion, or carbon monoxide poisoning. Furthermore, the investigation reviewed the effectiveness of the airline's emergency checklists during the event.