What happened
On 13 September 2024, an Airbus A330-343, registered HB-JHI, was climbing through flight level 220 en route from Zurich to Newark when the flight crew received cabin altitude warnings. Despite the outflow valves being fully closed, the aircraft was unable to maintain sufficient differential pressure. The cockpit crew received a master warning indicating excessive cabin altitude, prompting them to don oxygen masks and issue a 'MAYDAY' call. The pilots initiated an emergency descent, manually activating oxygen masks for the 205 passengers and 12 crew members on board. The aircraft eventually returned to Zurich, landing safely without any 0 injuries reported.
The investigation
Investigators examined radar recordings, flight data from the FDR and QAR, and cockpit voice recordings. The technical inspection of the aircraft revealed damage to the cabin pressure system components near the emergency ram air inlet. Specifically, a sealing sleeve was found badly damaged inside the fuselage, and one of the screw clamps was missing. Further examination of the skin check valve showed that one of its flaps was stuck in an open position due to a broken hinge and a bent hinge pin. The leaf spring intended to keep the flap closed had also snapped.
Findings
- The primary cause of the pressure loss was a defective skin check valve located between the mixer unit and the low-pressure manifold.
- This defect allowed compressed air to escape from the cabin into the low-pressure manifold and exit the aircraft through a damaged sleeve at the emergency ram air inlet.
- A critical contributing factor was that the airline had not implemented a 2016 service bulletin from the manufacturer, which recommended replacing the existing skin check valve with a modified version to prevent such failures.