A shortfall of air traffic controllers in Canada is at times forcing airport towers in Kelowna, B.C., and Winnipeg to close when the one controller on duty goes on a break, says a memo to Air Canada pilots seen by Reuters.

In the Oct. 14 memo, Air Canada advised its pilots to avoid the two airports when staff shortages are severe enough to shut their air traffic control towers.

It was unclear how often the airports have been closed and their representatives did not immediately respond to requests for comment.

The memo said Nav Canada , which manages the country's civil air navigation system, has advised airlines that air traffic control shortages will occur at Winnipeg and Kelowna, "but this could expand to other airports."

Nav Canada said in a statement on Thursday that closures in

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