Incident Overview
On 8 November 2017, a Boeing 737-800 aircraft was in cruise at flight level 360 en route from Salalah Airport in Oman to Larnaca Airport in Cyprus. Just before reaching the PASAM waypoint, the right bleed trip off warning illuminated. The flight crew immediately performed the Bleed Trip Off non-normal checklist from the Quick Reference Handbook.
Approximately 15 minutes later, the cabin altitude increased to 9,000 feet, triggering the auto fail warning. The crew donned oxygen masks, established communication between the captain and first officer, and executed the corresponding non-normal checklist. The cabin altitude fluctuated between 9,500 and 10,500 feet. To maintain proper cabin pressure, the crew switched to manual control. Once the cabin altitude stabilized, they removed their oxygen masks. The remainder of the flight proceeded without further issues.
Post-Flight Actions
After landing at Larnaca, the captain made an entry in the aircraft technical log and notified the maintenance control center and operations department. Technical personnel authorized the aircraft for the return flight to Katowice, Poland, under Minimum Equipment List item 36-8, which required the flight to be conducted below flight level 250.
Maintenance and Resolution
Upon arrival in Katowice, maintenance personnel took corrective action. On 11 November, the high-stage valve and the precooler control valve sensor (390°F) were replaced. Engine ground runs were performed, and the malfunction did not recur. The aircraft was subsequently released for unrestricted service.
Probable Cause
The probable cause of the incident was a malfunction of the high-stage valve and the precooler control valve sensor.