Incident Summary
On 29 January 2017, a Boeing 737-800 (registration not specified) was conducting a descent procedure for landing at La Palma Airport (GCLA) when the flight crew heard an intermittent cabin altitude warning horn at Flight Level 280. The crew immediately performed the memory items from the 'Cabin Altitude Warning' checklist and initiated an emergency descent. By adjusting the outflow valve, they reduced the cabin climb rate from 1,200 ft/min to 800 ft/min. The cabin altitude remained around 10,000 ft, so passenger oxygen masks were not deployed; however, the captain informed the cabin crew of the situation and the possibility of mask deployment if cabin altitude increased further.
Communication with air traffic control was challenging during the descent due to the controller's continuous exchanges with other aircraft. After reaching 10,000 ft, the captain reduced the descent rate and continued to the destination airport. The landing and taxi to the parking position were uneventful. Following landing, the captain made an entry in the technical log.
Technical Findings
Maintenance personnel performed a system check according to DCPC#1 procedure but found no fault. The aircraft was then ferried without passengers to Katowice Airport (EPKT). During the ferry flight, the same warning recurred. After a safe landing, the issue was reported to maintenance. Further tests during a subsequent technical flight confirmed the recurrence. Post-flight inspections revealed a damaged High Stage Valve on engine number 2 and a leak on two sense lines on engine number 1. After replacing the faulty components and resolving the leaks, system tests were successful, and the aircraft was returned to service. As of 13 February 2017, the aircraft had completed 14 flights without recurrence of the issue.
Weather conditions were not considered a factor in the event.
Conclusion
The incident was caused by a technical malfunction involving the engine's pneumatic system. No recommendations were issued by the investigation authority.