What happened
On October 22, 2008, a McDonnell Douglas DC-9-83 departed Toulouse for Hamburg-Finkenwerder with 107 passengers and a crew of two. Upon reaching a flight level of 280, the crew noticed an uncontrolled increase in cabin pressure, rising at approximately 1,500 ft/perm, despite the outflow valves being fully closed. In response to the pressure change, the crew donned oxygen masks and requested clearance to descend to FL100.
Shortly after 17:40 local time, a cabin low pressure warning activated, causing oxygen masks to automatically drop from the cabin ceiling. The crew initiated a descent to FL100, reaching the requested altitude six minutes later. At this level, the cabin pressure altitude stabilized at approximately 7,000 ft. The pilot, suspecting an issue with the cabin positive pressure relief valves but finding no other defects and having sufficient fuel, elected to continue the flight to Hamburg. The aircraft eventually climbed back to FL140 and landed at Hamburg-Finkenwerder at 19:34 without further incident.
The investigation
The BFU examined flight data from the flight recorders, though the cockpit voice recorder was not analyzed as the event duration fell outside its 30-minute recording window. The investigation focused on the aircraft's pressure regulation systems and the physical state of the relief valves.
During the inspection of the aircraft, investigators found that insulation material from the fuselage was lodged in the flaps of the cabin positive pressure relief valves. Once this debris was removed, the valves functioned normally. Additionally, the investigation of the aircraft's maintenance records and components revealed several discrepancies in the pressure control system.
Findings
- Debris in relief valves: Remnants of fuselage insulation were trapped in the cabin positive pressure relief valves, preventing proper operation.
- Improper component installation: An unauthorized part (identified by an incorrect serial number) had been installed in place of Cabin Pressure Controller 1.
- Malfunctioning hardware: Cabin Pressure Controller 2 was found to be malfunctioning.
- Mechanical setting error: The manual control system for the outflow valve was mechanically adjusted in a way that prevented proper functionality.
- Improper insulation work: The insulation debris found in the valves originated from incorrectly installed fuselage insulation.