What happened
On November 8, 2017, a Boeing 737-800 was operating a flight from Salalah, Oman (OOSA) to Larnaca, Cyprus (LCLK). While cruising at flight level 360, the crew observed a "Right Bleed Trip Off" indication. In response, the crew immediately initiated the appropriate Non-Normal Checklist (NNC) procedures as outlined in the aircraft's Quick Reference Handbook (QRH).
Approximately 15 minutes after the initial indication, the cabin altitude rose to 9,000 feet, accompanied by an "AUTO FAIL WARNING." Following established protocols, the crew donned oxygen masks and established clear communication between the Captain and the First Officer to continue executing the NNC. During this period, the cabin altitude fluctuated between 9,500 and 10,500 feet. To maintain stable cabin pressure, the crew transitioned to manual pressure control. Once the cabin altitude was stabilized, the crew removed their oxygen masks, and the remainder of the flight proceeded without further incident.
The investigation
Following the landing at Larnaca, the commander recorded the event in the aircraft technical log and notified both the Maintenance Control Center (MCC) and the Operations Department. Maintenance personnel evaluated the aircraft under the Minimum Equipment List (MEL) 36-8, which permitted the return flight to Katowice (EPKT) provided the aircraft remained below flight level 250. Upon arrival in Katowice, maintenance crews performed repairs, which included the replacement of the High Stage Valve and the Precooler CTRL Valve Sensor (390°F). Subsequent engine tests confirmed that the malfunction had been resolved, and the aircraft was returned to service without restrictions.
Findings
- The primary cause of the incident was a malfunction of the High Stage Valve and the Precooler CTRL Valve Sensor.