What happened
On January 29, 2017, a Boeing 737-800 operating in the Canary Islands UIR airspace experienced a cabin altitude warning during its descent toward La Palma Airport (GCLA). While cruising at flight level 280, the crew heard an intermittent cabin altitude horn. In response, the crew performed memory items from the cabin altitude warning checklist and initiated an emergency descent procedure. To manage the pressure differential, the crew adjusted the outflow valve to reduce the cabin climb rate from 12 and 200 ft/min to 800 ft/min.
As the aircraft descended, the cabin altitude reached approximately 10,000 ft. While this altitude did not necessitate the deployment of passenger oxygen masks, the captain briefed the cabin crew on the situation and the potential for mask deployment should the cabin altitude continue to rise. Although the crew declared an emergency requiring immediate descent, communication with Air Traffic Control (ATC) was initially difficult due to the controller managing high levels of traffic.
Once the aircraft reached 10,000 ft, the captain reduced the rate of descent and continued the flight to the destination. The landing and taxiing at La Palma were completed without further incident, and the captain recorded the event in the technical log.
The investigation
Following the initial event, maintenance personnel performed inspections according to standard procedures, but no faults were identified. The aircraft was subsequently repositioned without passengers to Katowice Airport (EPKT). During this repositioning flight, the cabin altitude issue recurred. After a safe landing, technical staff were notified, and subsequent flight tests also resulted in the recurrence of the warning.
Further intensive testing and inspections eventually identified the mechanical source of the problem. The investigation established that the High Stage Valve on engine number 2 was damaged, and leaks were discovered on two Sense Lines on engine number 1.