What happened
On December 31, 2008, at approximately 21:00 UTC, a Fokker 100, registration CS-TPE, operated by PGA – Portugália Airlines, was performing a scheduled passenger flight from Lisbon, Portugal, to Toulouse, France. While cruising near waypoint ZMR at flight level 340, the crew received a cabin altitude alert. The cabin altitude was recorded at 10,000 feet and was increasing at a rate of 1,000 feet per minute.
The flight crew immediately initiated the appropriate descent procedures. To comply with safety altitude requirements for the area, the aircraft descended to 14,000 feet. Air Traffic Control was notified, and the crew requested coordination to return to Lisbon. During the descent, the cabin altitude continued to rise until it equaled the aircraft's altitude of 14,000 feet. Once the aircraft stabilized at 14,000 feet, the pressurization system began to recover, eventually stabilizing the cabin altitude at 3,500 feet. The aircraft landed safely at Lisbon Airport without further incident.
The investigation
The GPIAAF investigation focused on the malfunction of the aircraft's pressurization system. Maintenance inspections conducted by the operator following the event identified a piece of speed tape debris lodged within the outflow valve mechanism. The investigation determined that this debris had been dislodged from a previous temporary repair and was subsequently moved by the airflow within the pressurization system until it became stuck in the valve's seat.
Findings
- The primary cause of the depressurization was the malfunction of the outflow valve caused by a piece of adhesive speed tape obstructing its movement.
- The debris originated from a temporary repair on a compartment grille, which had been patched with speed tape to protect passenger luggage from potential sharp edges until a permanent repair could be performed.
- Because the outflow valve constantly modulates to maintain the required pressure differential, the presence of the tape prevented the valve from closing completely.
- The recovery of cabin pressure at 14,000 feet occurred because the pressure differential required at that lower altitude was small enough that the valve could still maintain a 3,500-foot cabin altitude despite the obstruction.
Safety action
Following the incident, the operator, PGA, implemented measures within its maintenance training and refresher programs. These updates emphasize the necessity of inspecting work areas for foreign object debris, such as tools or leftover materials, following the completion of maintenance tasks.