What happened
On 8 February 2022, a Boeing 767-332ER, registration N1602, operated by Delta Air Lines was performing a scheduled international flight from New York (JFK) to Madrid (MAD). During the cruise phase over the Atlantic Ocean, the flight crew received an indication that the automatic cabin pressurization system had failed. Following established procedures, the crew transitioned to manual pressurization control to continue the flight.
While flying over the Galicia region in Spain, the aircraft experienced a significant drop in cabin pressure due to a loss of manual control over the pressurization system. This triggered a cabin altitude warning, necessitating the deployment of passenger oxygen masks and the use of oxygen masks by the flight crew. The crew declared an emergency and initiated a descent from FL340 to FL120. After regaining manual control of the pressurization system, the aircraft climbed back to a stable altitude and eventually landed at its destination without further incident. There were no injuries to the 59 passengers or 9 crew members on board.
The investigation
The CIAIAC investigation focused on the mechanical failure of the pressurization system and the subsequent loss of control. Investigators examined the aircraft's technical logbook, which noted a previous failure of the automatic pressurization control on a prior flight leg, though ground maintenance had found no anomalies.
Post-flight inspections of the aircraft revealed that ice had formed on the outflow valve (OFV). The investigation identified a leaking tube near the valve and a broken plastic clamp that had failed to secure the tube. This leak allowed water to escape and subsequently freeze, which physically blocked the doors of the outflow valve.
Findings
- The primary cause of the incident was a water leak from a tube that, upon freezing, blocked the outflow valve doors.
/n- The use of plastic tube clamps in the vicinity of the outflow valve contributed to the failure; these clamps had a history of deterioration in similar incidents, though they should have been replaced by metal versions.
- The ground-based Built-In Test Equipment (BITE) failed to detect the fault prior to departure.
- Maintenance procedures (Task 803 of the FIM) did not require a visual inspection of the outflow valve when the automatic system was inoperative, meaning the ice buildup went undetected.