(Bloomberg) -- US Secretary of State Antony Blinken was delayed on his return from Davos on Wednesday due to a critical malfunction with his aircraft.

After flying from Davos to Zurich on helicopters and boarding the modified Boeing Co. 737, Blinken and his party were informed that the aircraft had been deemed unsafe to fly. An oxygen leak detected previously could not be remedied.

A smaller jet was flown from Brussels to ferry the top US diplomat home, while many of his aides and members of the press pool had to travel to Washington commercially. 

While faults are not uncommon on aircraft, the glitch comes at a difficult time for Boeing. The company has had many of its 737 Max 9 aircraft grounded by regulators following an accident on an Alaska Airlines jet on Jan 5. 

The jet being used by Blinken is an older variant of the 737, and is designated as the C40 by the US Air Force.

Read More: Boeing Legacy Vanished Into Thin Air. Saving It Will Take Years

Last September, Canadian Prime Minister Justin Trudeau got stuck in India after a Group of 20 summit in New Delhi after a mechanical fault with his plane. Germany’s fleet of outdated government jets has also suffered a string of faults on long journeys, prompting an accelerated retirement of planes like the Airbus A340 and the purchase of new generation A350s jets. 

--With assistance from Victoria Cochrane.

(Updates to reflect Blinken was able to get back to Washington)

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