What happened
On 9 December 2000, a Boeing 737-500, registration EI-CDD, was operating a scheduled passenger flight from Dublin to Rome. While cruising at 37,000 feet north of Paris, the flight crew noticed a sudden change in cabin pressure. The captain observed the cabin rate of climb rising above 1,500 feet per minute, accompanied by a differential pressure of 6.9 p.s.i.
Following the appearance of "MASTER CAUTION" and "AUTO FAIL" warnings, the crew attempted to stabilize the cabin by switching to standby and manual modes. However, the cabin altitude continued to climb, prompting the crew to don oxygen masks and initiate an emergency descent. After declaring a "MAYDAY" to Air Traffic Control, the aircraft descended rapidly to 10,000 feet.
During the descent, passenger oxygen masks were deployed. The aircraft diverted to Paris Charles de Gaulle Airport, landing safely at 14:20 hrs. While there were no fatalities or serious injuries, 20 individuals—including passengers and cabin crew—sought medical attention for ear pain and nosebleeds caused by the rapid pressure change.
The investigation
The AAIU investigation focused on the mechanical failures within the pressurization system. Technical examinations of the aircraft upon its return to Dublin revealed several critical component failures. Specifically, investigators identified a fault in the Cabin Pressure Controller (CPC) regarding its ability to transfer to auto-fail mode, as well as a failure in the AC actuator of the outflow valve.
Additionally, the investigation looked into reports of loud bangs heard near the rear right-hand door. It was noted that the safety and relief valve could potentially open suddenly during rapid altitude changes, creating such noises. The investigation also identified an intermittent fault in a supply pack's bleed valve.