What happened
On 23rd August 2017, a Boeing 767-424ER, registered N68061, departed Zurich Airport for Washington Dulles International Airport. During the initial climb, the pilot in command noticed ear popping, which eventually led to the discovery of a rapidly increasing cabin altitude. While the crew initially managed to stabilize the pressure by switching a selector to standby, they later requested further climbs to higher altitudes.
While cruising at approximately 19,500 ft over Audincourt, France, the crew experienced severe ear popping and a significant rise in cabin altitude. The crew immediately initiated an emergency descent, donned oxygen masks, and declared a mayday. The aircraft eventually returned to Zurich, where the crew jettisoned fuel to reach a safe landing weight. There were no injuries to the crew or passengers, though the aircraft sustained minor damage.
The investigation
An investigation by the STSB revealed that the equipment cooling overboard exhaust valve was stuck in the fully open position. This occurred because the electrical connector for the valve had been unplugged. Post-flight inspections found that the air duct connected to this valve had burst due to the pressure differential.
The investigation traced the root cause back to pre-flight maintenance. Technicians had identified a defect in a different valve and intended to deactivate it according to the Minimum Equipment List (MEL). However, the technician performing the work deactivated the wrong valve and failed to apply the required warning sticker in the cockpit. The investigation also noted that the technician performed the task from memory without written procedures and highlighted inconsistent valve terminology within the airline's maintenance manuals.