The Department of National Defence predicts that its fleet of CF-18s will be able to fly into the next decade, even as Defence Minister Harjit Sajjan ramps up his calls for an urgent purchase of new fighter jets to ward off an eventual "capability loss."

"Some aircraft could begin to be retired beginning in 2023," said a document provided last week to the five aircraft manufacturers in the race to offer new fighter jets.

Over all, the Canadian Armed Forces are working to "extend the fleet to 2025" as part of a series of upgrades, said the document, which was also provided to The Globe and Mail.

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As he announced a two-month-long consultation with the defence industry last week, Mr. Sajjan doubled down on talk of a disparity between the current availability of the CF-18 fleet and Canada's commitments to NORAD and NATO.

"I can't be any more clear. A capability gap will lead to a capability loss if we do not address it," Mr. Sajjan told reporters.

He added that the Canadian Forces have adopted a risk-management strategy to deal with the scarcity of available aircraft, and he wants to put an end to the practice. Because of maintenance issues, he said, only about half the fleet of 77 fighter jets is available at any given time.

"The previous government has found it acceptable; I do not," Mr. Sajjan said.

Still, critics and experts are raising the possibility that the Liberal government is trying to justify a future purchase of Boeing Super Hornets – an aircraft currently flying primarily with the U.S. Navy – instead of the brand-new Lockheed-Martin F-35 that was favoured by the previous Conservative government.

Military analyst David Perry said the government's talk of a capability gap came "out of the blue," and seems related to the Liberals' election promise not to buy F-35s. "It's pretty hard to divorce the politics of this from the actual operational requirements," said Mr. Perry, a defence analyst with the Canadian Global Affairs Institute.

Conservative MP James Bezan said the government seems to be looking for a way to fulfill its promise to buy an aircraft other than the F-35. "This is about setting the narrative to go to a sole-source [acquisition of Super Hornets]," he said.

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Mr. Bezan said the government could simply launch an open competition and quickly obtain the best product for the Canadian Forces at the best price for taxpayers.

"There's no question that planes are available," he said, explaining that both Super Hornets and F-35s are currently in production. "They can go through this very methodically and still come to the right decision, rather than try to rig the process."

Asked to expand on the capability gap, Mr. Sajjan's office said the 2023 timeline for the retirement of the first aircraft "doesn't take into consideration any of the issues that could come up with our fleet," such as crashes, being used for parts, or other commitments for deployment to NATO or peacekeeping missions that would add flying hours.

The government sent a 38-page questionnaire last week to five aircraft manufacturers that are in the running to replace the CF-18s: Boeing, Lockheed-Martin, Saab, Dassault and Eurofighter.

Ottawa is seeking a wide range of information from the firms by the end of July in order to design an acquisition process that will be unveiled as early as September. The questions include acquisition and life-cycle costs, current and planned production numbers, and potential industrial and technological benefits for Canada.

The government is also asking the various manufacturers how their aircraft would perform on a series of missions, such as flying from the fighter base in Cold Lake, Alta., to Inuvik, NWT, about 2,000 kilometres to the north.

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The introduction to the questionnaire states that the government has not decided on a future course of action. "All procurement options are being considered," the document said.