A Swiss International Air Lines Airbus A330-300 was forced to make an emergency return to Zurich on Sep 13th, 2024, after a subtle but dangerous loss of cabin pressure occurred during the climb, triggering the release of oxygen masks and a rapid descent back to safe altitude.
The aircraft, registered HB-JHI and operating flight LX-18 from Zurich to Newark, was carrying 205 passengers and 12 crew members when the incident unfolded. Shortly after departure from runway 32, while climbing through flight level 250, the flight crew detected abnormal cabin pressure indications. As the cabin altitude continued to rise, the crew initiated an emergency descent, donned their oxygen masks, and released passenger oxygen masks throughout the cabin.
After leveling off at around 10,000 feet, the aircraft turned back toward Zurich and entered a holding pattern to burn off fuel before landing. The A330 touched down safely but overweight on runway 16 approximately 70 minutes after departure, with emergency services standing by due to the possibility of brake overheating. All passengers and crew disembarked normally, and no injuries were reported. The aircraft remained on the ground in Zurich for about 14 hours, and the scheduled rotation was canceled.
Swiss later confirmed that the pilots had followed standard operating procedures after detecting irregularities in the cabin pressurization system, emphasizing that the landing was conducted safely despite the higher-than-normal landing weight.
On Oct 6th, 2024, the Swiss Transportation Safety Investigation Board (SUST) classified the event as a serious incident and launched a full investigation. More than a year later, on Jan 19th, 2026, the SUST released its final report, detailing the technical chain of failures that led to the emergency descent.
According to the investigation, the cabin altitude exceeded 10,000 feet during the climb because a defective skin check valve remained stuck in the open position, preventing the pressurization system from building sufficient differential pressure. Although the aircraft’s outflow valves closed as designed, air continued to escape through the open skin check valve into the low-pressure manifold. Compounding the issue, a damaged sealing sleeve in the low-pressure system allowed additional air to escape, accelerating the loss of cabin pressure.
Investigators found that the aircraft manufacturer had issued a service bulletin in 2016 recommending the earliest possible replacement of the skin check valve with a modified version to prevent exactly this type of failure. That bulletin had not been implemented on HB-JHI, despite the valve being removed and reinstalled multiple times for routine inspections over the years. The SUST described this omission as incomprehensible from a safety perspective.
The report also highlighted human and operational factors. The gradual rate of cabin altitude increase—about 750 feet per minute—made the problem difficult to detect early, as neither the cockpit displays nor physical sensations provided clear warning signs. By the time the ECAM warnings appeared, the situation had already escalated, leaving the crew with little choice but to execute an emergency descent. Investigators noted that such “subtle decompression” events have historically been difficult for flight crews to recognize during initial climb.
The SUST concluded that the flight crew’s response was timely and correct, preventing the cabin altitude from exceeding 10,800 feet and ensuring a safe outcome for everyone on board. The incident has since been cited as a reminder of the critical importance of implementing manufacturer safety recommendations and maintaining rigorous oversight of pressurization system components.
