Ottawa has added a twist to its long-running effort to buy new fighter jets, opening the door to new spending on upgrades that would prolong the lifespan of its fleet of CF-18s while further delaying a decision on the controversial process according to government documents and sources.

A change in the procurement document that is made available to potential suppliers invites manufacturers to propose new scenarios for maintaining and renewing Canadian air power.

Sources said the changes in the eligible options make it easier for manufacturers to propose a "mixed fleet" of upgraded CF-18s and other fighter jets, or a later delivery of new jets as the CF-18s fly beyond their planned phase-out.

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The new proposal is the latest shift of position by the Harper government after last year's highly critical report by Auditor-General on a planned sole-sourced purchase of 65 F-35 fighters.

The government has hit the "reset button" on that troubled, $40-billion file, but it remains a moving target – an ongoing irritant to a Conservative government that prides itself on fiscal responsibility.

The government – through the National Fighter Procurement Secretariat, created in the aftermath of the Auditor-General's report – is going through an "options analysis" to determine which scenarios meet its needs.

The changes are among a number of new elements in the final version of the secretariat's questionnaire to five aerospace firms, which is designed to obtain technical information on the aircraft in the running for the contract. Another questionnaire is in the works to obtain financial information on all fighter jets, but that one has also undergone significant changes that stand to further delay the process.

There is no firm timeline for the acquisition, but there is speculation in Ottawa that the government will want to delay the final announcement until after the 2015 general election.

The firms in the running for the contract have launched lobbying and public-relations campaigns to position themselves in the event that Ottawa launches a full-blown open competition for new fighters. The most vocal manufacturers have been Lockheed Martin, which is touting its F-35 as a full technological generation ahead of its rivals, and Boeing, which argues that its Super Hornet offers better overall capabilities than the F-35 at half the cost. The other firms in the running are Saab (Gripen), Dassault (Rafale) and Eurofighter (Typhoon).

The CF-18s first entered into service in 1982, and are currently programmed to be phased out between 2017 and 2023. A source who has been briefed on the ongoing replacement process said Ottawa could push back the acquisition of new jets as it contemplates the "capabilities, costs and risks" of all of its options, including "extending the CF-18."

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"It could provide the government with the opportunity of delaying the acquisition. If the government decides that it prefers to wait, it will have that option," the source said.

The government said that the upgrades could allow manufacturers to prolong the lifespan of the CF-18s as a "bridge" to the delivery of new fighters later in the decade.

"Extending the CF-18 is consistent with the commitment to examine all options, including bridging options," Public Works spokeswoman Lucie Brosseau said.

The government has added a new line in the questionnaire that will seek to gauge whether the aircraft capabilities that are being promoted by the various manufacturers are based on "test flights, simulators or past deployments." The question is particularly relevant in terms of the F-35, which is still in development, while the other aircraft in the running are already operational.

The government has also made it clear that the fighter jets are being purchased "first and foremost" to defend Canadian soil. In the draft version of the questionnaire, Ottawa called for the new aircraft to contribute to the efforts of the Canadian Forces to defend Canada and North America, and contribute to international peace and security, without clearly prioritizing Canadian sovereignty.

The new emphasis on Canadian sovereignty could have a greater impact on the F-35, which is deemed by some experts to be the most effective fighter in international coalition alongside the American military. However, rival manufacturers have been boasting that a twin-engine jet such as Boeing's Super Hornet offers superior performances on Canadian soil, such as Arctic patrols.

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The procurement secretariat has yet to state when it will send out a second questionnaire seeking financial information from the five manufacturers.

The draft version of the document called on the firms to provide "estimates of the full life-cycle cost of the aircraft," which would have entailed a thorough estimate including acquisition, sustainment and operations budgets for four decades. However, the government will now only seek "cost estimates of the aircraft," which will not allow for a full financial comparison of the rival fighters.

The financial questionnaire will also not include any questions on the regional benefits that would flow from the purchase. Instead, the government said it will seek the information "later in the process."