US Secretary of State Antony Blinken was stranded in Zurich city of Switzerland on Monday after his modified Boeing 737 was grounded due to a critical failure, NBC News reported.

Blinken first boarded the US military service’s Boeing C-40, but that aircraft sustained a critical failure related to an oxygen leak.

The aircraft is a modified Boeing 737 used by the vice president, first lady or secretary of state of the United States.

A new smaller plane was arranged for Blinken after his aircraft was deemed unsafe to fly and the press pool travelling with him was advised to make commercial flight arrangements.

The secretary of state was in Davos to attend the World Economic Forum, a summit of government and business leaders.

Boeing’s production processes have been under scrutiny since a panel tore off the Alaska Airlines jet while in flight this month, leaving a hole on the side of the plane.

The incident rekindled worries about Boeing’s jets a few years after a pair of crashes killed 346 people. Investors are also worried about potential delays to aircraft deliveries.