What happened
On December 22, 2008, an Airbus A319-111, registration CS-TTA, operated by TAP Air Portugal, was performing a scheduled flight from Copenhagen to Lisbon. While cruising at flight level 390 near the RALUS waypoint, at the boundary of the Madrid and Lisbon Flight Information Regions, the aircraft experienced a loss of cabin pressure.
The aircraft's pressurization system failed in both automatic modes (System 1 and 2). The crew attempted to control the cabin altitude manually, but the outflow valve failed to respond to manual inputs. During this period, the cabin altitude fluctuated between 1,000 feet of descent and 1,600 feet of climb, eventually exceeding the 10,000-foot safety threshold and reaching 10,800 feet.
Due to the rapid change in altitude, the crew donned full-face oxygen masks. After an initial PAN PAN declaration to Madrid ATC went unanswered, the pilots declared a MAYDAY and initiated an emergency descent. The descent was temporarily interrupted at flight level 350 by a TCAS alert indicating traffic below the aircraft. The crew eventually established contact with Lisbon ATC, cancelled the emergency, and completed the flight to Lisbon International Airport without further incident.
The investigation
The investigation focused on the failure of the cabin pressure control system. Maintenance inspections performed by the operator revealed that the Outflow Valve (OFV) was mechanically blocked and unresponsive to both the Cabin Pressure Controllers (CPCs) and manual pilot commands.
Flight data from the DFDR confirmed that the OFV remained stationary throughout the duration of the flight. Subsequent analysis by the manufacturer, Airbus, of the removed component revealed bearing defects within the gearbox. These defects caused the gearbox to seize, preventing the valve from modulating.