What happened
On December 7, 2016, a BD700-1A10 (Global 6000), registration M-ARGO, was performing a positioning flight from Zurich (LSZH) to Basel (LFSB). The flight was operating under instrument flight rules (IFR) with a crew of two pilots and one flight attendant on board.
Shortly after departing from runway 28, while climbing through flight level 100, the crew heard a loud noise caused by an airflow within the cabin. Following this noise, the engine indicating and crew alerting system (EICAS) displayed a passenger oxygen advisory. As the aircraft continued its climb, the EICAS issued cabin altitude caution and warning messages, indicating a loss of cabin pressure.
In response to the deteriorating situation, the crew requested a descent to 6000 ft. After initially being told to wait for contact with Basel approach, the crew declared a "Mayday" at 10:37:16 UTC to demand an immediate descent. The crew initiated an emergency descent following the Quick Reference Handbook (QRH) procedures and donned oxygen masks before reaching flight level 100. During the descent, a passenger door warning also appeared on the EICAS. The aircraft landed safely in Basel without further incident.
The investigation
Following the event, technical inspections were performed on the aircraft according to the aircraft maintenance manual (AMM). Investigators examined the passenger door mechanism, the door latch, and the ventilation flap. While the functional tests of the door and its components met the required standards, a discrepancy was noted during a functional test of the external door handle's spring pot; the force required to unlock the handle was measured at 33 lb, significantly higher than the minimum requirement of 15 lb.
Findings
Technical investigations found no mechanical failure that would have prevented the cabin from maintaining pressure. However, the observation that the door's vent flap was partially open upon landing suggests that the passenger door was not properly closed and locked prior to departure. This improper closure explains both the audible airflow heard during the climb and the subsequent loss of cabin pressure. The crew's decision to declare an emergency and their execution of the emergency descent procedures were found to be appropriate and decisive.