What happened
On December 18, 1997, an MD-82, registration HB-INW, departed Shannon Airport on a ferry flight to Zurich following heavy maintenance. While climbing to FL 370, the crew experienced a loss of cabin pressure control near the STRUMBLE VOR. Despite attempting to switch to manual mode, the cabin altitude rose rapidly, forcing the crew to perform an emergency descent. Although the system appeared to stabilize at lower altitudes, a second loss of control occurred during a subsequent climb to FL 310, necessitating another emergency descent. The aircraft returned to Shannon to undergo technical troubleshooting.
On December 19, a test flight was conducted in the Shannon area to verify the serviceability of the systems. During the climb to FL 310, the crew switched the pressure control to manual, at which point the outflow valve became completely stuck in the open position. This triggered a third emergency descent. The aircraft returned to Shannon, where engineers continued investigations.
The investigation
The investigation focused on the intermittent failure of the cabin air outflow valve. Engineers initially checked the air conditioning system and found no obvious restrictions. After consulting with the manufacturer, McDonnell Douglas, investigators learned that similar issues in other aircraft had been linked to worn valve bearings and nicotine buildup.
Following this lead, the crew disassembled the valve, replaced the bearings, and cleaned the component. However, subsequent tests indicated the valve remained difficult to operate. Further inspection revealed a discrepancy between the operator's maintenance manual and the manufacturer's manual regarding critical clearances for the outflow nozzle valve and its body. A replacement valve was eventually flown in from Austria and installed according to the correct manufacturer specifications.
Findings
- The primary cause of the pressure control failures was the malfunction of the cabin air outflow valve.
- The valve became stuck, preventing the crew from maintaining cabin altitude.
- A discrepancy existed between the operator's maintenance instructions and the manufacturer's manual regarding the required clearances for the valve assembly.