WestJet spokesman Robert Palmer says the Boeing 737-700 was on its way from Phoenix to Winnipeg on Friday night when the trouble developed.
The pilots declared an emergency and made a rapid descent which took the plane from 12,000 metres to below 3,000 metres in just a few minutes.
Palmer says the plane landed safely at the nearest available airport — Rapid City, South Dakota.
Passengers were put up in a hotel in Rapid City for the night and were due back in Winnipeg Saturday evening.
There's no word yet on what caused the pressurization problem but Palmer says the airline is investigating.
There were 38 passengers, two pilots and a flight attendant on board.
Palmer said such incidents are uncommon. The last time a WestJet plane experienced a pressurization problem that required a rapid descent, he said, was three or four years ago.
"This is a descent that is faster than normal, than say, when you're landing." Palmer said.
"It took a few minutes. It's not like in the movies where the aircraft does a nose dive."
He says both the National Transportation Safety Board in the U.S. and the Transportation Safety Board in Canada are satisfied with WestJet's response and won't be investigating.
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