What happened
On September 29, 2013, a Boeing 737-400 operating out of Warsaw Chopin Airport (EPWA) encountered a cabin altitude irregularity during its cruise at flight level 270. After reaching cruise altitude, the crew observed a 'RAM DOOR FULL OPEN' indication and noted that the cabin altitude had risen to approximately 9,000 feet.
As the flight was over the Atlantic Ocean with no nearby diversion airports, the crew decided to delay attempting to close the valve until they reached a more suitable distance from an alternate airport. While passing the VALDI waypoint, a cabin altitude warning was triggered. The crew donned oxygen masks and executed the necessary checklists, eventually restoring normal cabin altitude by operating the pressurization system in manual DC mode. The remainder of the flight proceeded without further issues.
The investigation
The investigation, conducted by the operator, focused on identifying the source of the pressure regulation failure. Following a second occurrence of the malfunction on September 30, 2013, a troubleshooting plan was developed. Due to the aircraft's continuous operational schedule, extensive testing was difficult to perform during short ground stays.
On October 4, 2013, the aircraft was finally removed from service for technical inspection. Detailed analysis and maintenance procedures revealed that the faulty outflow valve was the source of the leak. The valve was subsequently sent to the manufacturer for repair and was reinstalled on the aircraft on February 21, 2014.
Findings
- The primary cause of the incident was a malfunctioning outflow valve within the cabin pressurization system.
- The aircraft's valve had only 2,299 flight hours since its last overhaul, despite a maintenance program requirement for a full overhaul every 12,000 flight hours.
- The investigation noted that failures of outflow valves are a recurring issue, with three similar instances involving manufacturer repairs documented over the preceding three years.