What happened
On April 4, 2012, a Boeing 737-800 operating a scheduled passenger flight from Bergamo, Italy, to East Midlands, United Kingdom, experienced a severe depressurization event while climbing through the Alps near Lugano, Switzerland. Approximately 13 minutes after takeoff, the crew noticed a sudden drop in temperature, airflow, and cabin pressure. The cabin altitude rose rapidly, exceeding 10,000 feet, with a rate of climb reaching over 7,000 feet per minute.
The flight crew immediately donned oxygen masks and initiated the rapid depressurization checklist. After attempting to manually control the outflow valve, the pilot determined the pressure could not be stabilized and declared a Mayday, initiating an emergency descent. The aircraft descended from FL308 to FL100, passing through airspace occupied by another aircraft during the maneuver. The flight eventually diverted to Frankfurt-Hahn Airport, Germany, where it landed safely. While there were no fatalities, 13 passengers sustained minor injuries, including one ruptured eardrum and several cases of ear pain.
The investigation
The investigation, conducted by the BFU in coordination with the Swiss authorities (SUST), focused on the technical failure of the cabin pressure regulation system. Investigators examined the non-volatile memory of the two Cabin Pressure Controllers (CPC) and analyzed flight data recorder (FDR) and cockpit voice recorder (CVR) playback.
Maintenance records revealed that the aircraft had experienced pressure regulation issues the previous day. During overnight maintenance, a technician replaced CPC#1. The investigation found that a black shipping plug had been left covering the static port of the newly installed CPC#1. This plug prevented the controller from receiving accurate ambient pressure readings, leading to the malfunction.
Findings
- The primary cause of the rapid depressurization was the failure to remove a shipping plug from the static port of the newly installed Cabin Pressure Controller #1.
- The maintenance technician, having performed similar tasks multiple times that night, experienced a memory lapse and failed to follow the full installation procedure, relying instead on memory.
- The shipping plug was black, making it difficult to distinguish from the surrounding hardware, and it lacked a prominent identification tag.
- The pressurized system ground tests performed after the installation were insufficient to detect the presence of the plug.
Safety action
Following the investigation, the BFU issued safety recommendation 24/2012, advising the FAA to require Boeing to redesign shipping plugs to be more recognizable, specifically through the use of distinct colors and attached, eye-catching tags. Additionally, the BFU recommended that maintenance organizations ensure all shipping plugs are clearly coupled with a visible tag.