What happened
On December 1, 2022, an Airbus A330-202, registration EC-MLP, was operating a scheduled commercial flight from Carrasco International Airport (SUMU) in Uruguay to Madrid-Barajas (LEMD) in Spain. Approximately 30 minutes after takeoff, while climbing through the Uruguayan airspace, the flight crew observed a pulsing 'CAB ALT' advisory on the cabin pressure system page.
As the aircraft continued its ascent, the cabin altitude rose uncontrollably, eventually exceeding 9,550 feet. This triggered a 'CAB PR EXCESS CAB ALT' ECAM warning, accompanied by a Master Warning and a continuous aural alarm. In response to the rising cabin altitude, the crew donned their oxygen masks. After entering Brazilian airspace, the crew issued a 'PAN PAN' message to Curitiba Air Traffic Control to request an immediate descent.
The crew attempted to climb on two occasions, but found that the cabin altitude continued to track with the aircraft's altitude, preventing stabilization. Consequently, the crew coordinated a descent to FL120 and a return to the origin airport. The aircraft landed at SUMU at 20:09 UTC. Although the aircraft landed with a weight exceeding the maximum landing weight, the arrival was completed without further incident, and passengers disembarked normally.
The investigation
The CIAIA investigation focused on the failure of the cabin pressurization system. Investigators examined the aircraft's maintenance history and discovered that the aircraft had experienced a similar pressurization failure the previous day, which had forced a diversion to Santo Domingo, Dominican Republic. Despite maintenance efforts following that first event, the root cause had not been identified.
Technical inspections of the air conditioning and pressurization systems were conducted. The maintenance organization eventually utilized a specialized method, involving covering connections with aluminum foil, to identify air leaks. This process revealed that a cylindrical bellows, responsible for connecting the condenser outlet to the duct leading to the mixer unit, had been improperly installed.
Findings
- The primary cause of the incident was the inadequate installation of the bellows and clamp connecting the condenser outlet to the mixer unit duct, which resulted in a partial air leak within a non-pressurized area.
- A maintenance error occurred during the installation of the component, and existing quality control measures failed to detect the improper placement.
- The crew's response was influenced by the rapid onset of the event, which left insufficient time to consult the Quick Reference Handbook (QRH).
- The flight crew failed to record the use of oxygen masks in the flight log.