Uncontrolled Cabin Pressure Rise on a McDonnell Douglas DC-9-83

Incident Overview and Flight Details On 22 October 2008, a McDonnell Douglas DC-9-83 (MD-83) departed Toulouse at 17:25 local time with 107 people on board, bound for Hamburg-Finkenwerder. The aircraft was operated by an Austrian company and was certified for traffic in Austria. It was built in 1986 in the United States with serial number 49448. At the time of the incident, the aircraft had accumulated 41,833 flight hours and 31,900 flights since manufacture. ## Events During the Flight Upon reaching flight level 280, the crew observed an uncontrolled rise in cabin pressure of approximately 1,500 feet per minute, despite the outflow valves being fully closed. The cockpit crew donned oxygen masks and requested clearance from air traffic control to descend to flight level 100. At 17:40, the cabin low-pressure warning activated, and oxygen masks automatically deployed from the cabin ceiling. Clearance to descend was granted, and the descent began. Six minutes after the warning, the aircraft reached flight level 100, and the cabin pressure altitude stabilized at approximately 7,000 feet. The pilot in command suspected a problem with the cabin positive pressure relief valves. Finding no other defects and having sufficient fuel, the decision was made to continue the flight to Hamburg-Finkenwerder. Approximately ten minutes after reaching flight level 100, the aircraft climbed to flight level 140. The aircraft landed at Hamburg-Finkenwerder at 19:34 without further incidents. ## Personnel The 63-year-old pilot held an Airline Transport Pilot Licence issued by the U.S. Federal Aviation Administration on 1 March 2008, with type ratings for DC-9, A-320, and L-382. He also held a recognition certificate from Austro Control GmbH and a valid Class 1 medical certificate under ICAO and JAR-FCL regulations, with a vision correction requirement. His total flight experience was approximately 16,500 hours, including about 6,700 hours on the type. The 23-year-old co-pilot held a Commercial Pilot Licence (CPL(A)) issued by Austro Control GmbH on 31 May 2005, with a co-pilot type rating for the DC-9. She held a valid Class 1 medical certificate under ICAO and JAR-FCL regulations, with a vision correction requirement. Her total flight experience was approximately 900 hours, including about 500 hours on the type. ## Aircraft and Systems The DC-9-83 is a twin-engine transport aircraft with a maximum takeoff weight of 72,575 kg, powered by two Pratt & Whitney JT8D-219 turbofan engines mounted at the rear. The cabin pressure control system includes two cabin pressure controllers with a control unit, an outflow valve, a manual control unit, and two cabin positive pressure relief valves to protect the fuselage from overpressure. ## Investigation Findings The BFU had access to raw flight data recorder data. Cockpit voice recorder data were not read because the recorder had only a 30-minute recording duration, and no relevant information was expected. During the investigation, it was found that insulation material from the fuselage was jammed in the flaps of the cabin positive pressure relief valves. After removal, the valves functioned without issues. Oxygen masks in the cabin were hanging from the ceiling, and the connected oxygen generators were mostly discharged. Mask containers over seats D, E, and F in row 9 and in the forward and aft left toilets were closed. During unscheduled maintenance after the incident, the following was found: - A device with an unapproved serial number was installed in place of Cabin Pressure Controller 1. - Cabin Pressure Controller 2 was malfunctioning. - The manual control system of the outflow valve was mechanically adjusted in a way that did not ensure proper function. - The insulation remnants in the cabin positive pressure relief valves originated from incorrectly installed fuselage insulation.