What happened
On July 13, 2018, a Boeing 737-8AS operating a scheduled flight from Dublin, Ireland, to Zadar, Croatia, experienced a severe loss of cabin pressure while cruising at FL 370 in French airspace. The incident began when the Outflow Valve (OFV) moved from an 18° opening to a fully open position of 104° over a nine-second period.
The flight crew noticed the pressure drop within seconds, and the cabin altitude warning activated as the altitude climbed through 9,470 ft. The crew donned their oxygen masks and began executing emergency memory items. During the event, the OFV position oscillated, causing the cabin altitude to continue rising to 14,639 ft.
To manage the situation, the crew switched the OFV to manual control mode and partially closed the valve. The pilot flying then initiated an emergency descent to FL 100. During this descent, the cabin pressure built up to the maximum differential pressure, causing the pressure relief valves to trigger. The crew subsequently manually fully opened the OFV, which led to a second rapid depressurization as the cabin pressure equalized with the outside altitude. The aircraft eventually landed at Frankfurt-Hahn Airport. The pressure fluctuations resulted in 33 passengers sustaining minor injuries, primarily related to ear, nose, and throat issues.
The investigation
The BFU investigation focused on the malfunction of the Environmental Control System (ECS) and the subsequent crew response. Investigators examined the Cabin Pressure Controller (CPC) and the behavior of the Outflow Valve. The investigation also reviewed the crew's use of oxygen masks, the execution of emergency checklists, and the effectiveness of the ground-based emergency response at Frankfurt-Hahn Airport.
Findings
- The primary cause of the depressurization was a Single Event Upset in one of the Cabin Pressure Controllers, which led to a miscalculation of the OFV reference position and commanded the valve to open fully.
- System redundancies within the pressure control system were insufficient to prevent the rapid loss of pressure in this specific instance.
- The crew's mental model of the situation was complicated by the difficulty of monitoring the ECS status in manual mode, which prevented them from correctly interpreting that pressure was building up during the descent.
- The crew's training, which standardizes an emergency descent for unrecoverable pressure loss, may have influenced their expectation and decision-making.
- Ground response efforts were hindered by discrepancies between the emergency plans of the airport and the local district, as well as communication gaps between the handling agent and the airline's operations center.