Incident Overview
On April 4, 2012, a Boeing 737-8AS (manufactured in 2003) operated by an Irish airline encountered a severe pressurization anomaly while climbing over the Alps. The aircraft was on a scheduled passenger flight from Bergamo, Italy, to East Midlands, United Kingdom. Approximately 13 minutes after takeoff, while flying at Flight Level 308 in Swiss airspace near Lugano, the crew detected a sudden change in cabin pressure.
Crew Actions and Emergency Descent
The flight deck instruments indicated a rapid rate of climb for the cabin altitude, exceeding 4,000 feet per minute, with the cabin altitude surpassing 10,000 feet. The pilots donned oxygen masks and initiated the Cabin Altitude Warning/Rapid Depressurization checklist. After determining that the cabin altitude was uncontrollable, they declared a Mayday and commenced an emergency descent.
The aircraft descended at rates exceeding 6,000 feet per minute initially, leveling off at Flight Level 100. During this maneuver, the Boeing 737-800 briefly approached an Airbus A319 flying below it, with a horizontal separation of 4.5 nautical miles and vertical separation of 325 feet. The aircraft subsequently diverted to Frankfurt-Hahn Airport, where it landed safely.
Investigation Findings
The investigation, delegated by the Swiss accident investigation authority (SUST) to the German Federal Aviation Office (BFU), revealed a critical mechanical failure. Data from the Cabin Pressure Controllers (CPC) showed that communication faults caused the active CPC #2 to switch to standby mode, transferring control to CPC #1.
Physical inspection of the aircraft identified the root cause: the static pressure sensor port on CPC #1 was blocked by a black plastic shipping plug. This obstruction prevented CPC #1 from accurately measuring ambient pressure, leading to erroneous control signals and the rapid loss of cabin pressure. The incident resulted in minor injuries to 13 passengers, including one ruptured eardrum, while no structural damage was reported.