What happened
On 3 March 2018, a Boeing 737-700, registration D-AGEU, operated by Germania, was cruising at FL380 in the Canaries airspace while en route from Fuerteventura to Berlin Schönefeld. During the flight, the cabin crew experienced sudden, intense pressure changes. Shortly after, visual and aural cabin altitude warnings activated, prompting the crew to don oxygen masks.
The pilot monitoring observed that flight and landing altitude readings were displaying erroneous dashes, suggesting a dual failure of the cabin pressure controllers. The cabin altitude rose uncontrollably to 15,000 feet, triggering the automatic deployment of passenger oxygen masks. The crew declared a MAYDAY to Casablanca air traffic control and initiated an emergency descent. After manually regaining control of the cabin pressure during the descent, the crew diverted to Málaga Airport, where they landed without further incident. The event resulted in 3 minor injuries to passengers.
The investigation
Investigators analyzed the non-volatile memory of the cabin pressure controllers and the physical components of the pressurization system. The study of the outflow valve (OFV) revealed worn bearings and a damaged electrical connection. Analysis of the cabin pressure controllers (CPC) indicated that the incident was driven by two independent faults.
Technical examinations of the #2 cabin pressure controller suggested that the valve was commanded to open fully due to either corrupted data—potentially caused by a single-event upset (SEU) from atmospheric radiation—or failing solder joints. While the #1 controller attempted to close the valve, the mechanical stiffness of the OFV, caused by worn bearings, prevented the valve from returning to the closed position to stabilize the pressure.
Findings
- The primary cause of the initial depressurization was a command from the #2 cabin pressure controller to open the outflow valve completely, triggered by corrupted data or faulty solder joints.
- The severity of the depressurization was increased because the #1 cabin pressure controller could not overcome the stiffness of the outflow valve to close it.
- A lack of detailed analysis of two prior pressurization incidents reported on the same day may have acted as a contributing factor.
- Maintenance ground tests performed prior to the incident were unable to detect these specific flight-only malfunctions.