Introduction
On Saturday, 3 March 2018, a Boeing 737-700, registration D-AGEU, operated by Germania, was involved in a cabin depressurization incident while in cruise over the Canaries airspace (Spain). The aircraft was en route from Fuerteventura Airport to Berlin Schönefeld Airport.
History of the Flight
During the cruise phase at flight level 380 (FL380), the cabin crew experienced intense and sudden pressure changes. After checking the cabin rate of climb indicator, they identified a pressurization problem. Shortly after, the visual and aural cabin altitude warning activated, prompting the crew to don their oxygen masks. The pilot monitoring observed that the flight altitude (FLT ALT) and landing altitude (LAND ALT) readings displayed dashes, leading the crew to suspect a dual failure of the cabin pressure controller (CPC).
Cabin altitude rose uncontrollably to 15,000 ft, triggering the automatic deployment of passenger oxygen masks, which occurs above 14,000 ft. The crew declared a MAYDAY and initiated an emergency descent. During the descent, they manually regained control of cabin pressure and subsequently cancelled the emergency declaration. The flight diverted to Málaga Airport, where it landed without further incident. Three passengers sustained minor injuries as a result of the event.
Findings
The investigation determined that the #2 cabin pressure controller (CPC2) commanded the outflow valve (OFV) to open fully. The signal was caused by corrupted data in the CPC, potentially from a single-event upset (SEU) or failing solder joints. Subsequently, due to stiffness in the OFV, the #1 cabin pressure controller (CPC1) was unable to return the valve to its closed position, preventing stabilization of cabin pressure. The cause of the OFV stiffness was identified.
Prior to the incident on the same day, two other cabin pressure anomalies had been reported on the same aircraft. The investigation noted that the lack of detailed analysis of those prior incidents by the operator's maintenance technicians may have been a contributing factor.
After the incident, both CPC units were replaced due to suspected simultaneous failure. However, subsequent incidents involving loss of cabin pressure occurred. The operator did not provide information on the cause of those later incidents, and it is unknown if they were analyzed in detail.
Conclusion
The investigation was unable to reliably determine a single definitive cause for the incident. Two possible scenarios were identified: corrupted data in CPC2 leading to valve opening, and OFV stiffness preventing closure. Additionally, the handling of prior pressurization events was noted as a potential contributing factor.