Opposition parties say the Harper government should exclude the F-35 from its search for replacements for Canada's CF-18 jets after a new Pentagon report found more flaws with the stealth fighter.
A leaked evaluation in Washington criticizes the visibility in the cockpit of the multi-role fighter, and contains blunt comments from test pilots that suggest the shortcomings could get planes shot down in combat.
The design prevents pilots from looking behind them.
"The head rest is too large and will impede aft [rear] visibility and survivability during surface and air engagements," one test pilot was quoted as saying in the U.S. Defense Department's directorate of operational test and evaluation report.
A second pilot reportedly said visibility is crucial and any disruption "will get the pilot gunned" down in dogfights.
The document was leaked and posted online Wednesday.
Pilots have also cited concerns about the sophisticated helmet that's supposed to display data, saying there are flickering and non-existent readings.
Vocal critics, such as aviation expert Winslow Wheeler, say the F-35 is not ready for combat training, let alone combat.
Both the New Democrat and Liberal defence critics say the stealth fighter should be dropped from consideration.
The Harper government has asked a panel of independent experts to evaluate all of the options on the market to replace the 1980s vintage CF-18 fighters.
The experts' report, expected in the spring, will help the government determine whether to hold a full-fledged competition.
The Public Works secretariat overseeing the fighter replacement recently sent a questionnaire to the five companies that could be bidding, asking for detailed information and plans, including costing and availability.
The Lockheed Martin-built F-35, with a history of development delays and cost overruns, is still considered a strong contender. The Harper government initially chose it in 2010, and has expended much political capital defending the decision.
Last year's auditor general report, which accused National Defence of hiding the full cost and Public Works of not following the proper process, forced a reset of the program.
Liberal MP John McKay, the party's defence critic, said Lockheed Martin has a brochure while the other competitors have real planes.
"I think the challenge for the government now is: How do you run a legitimate competition that pits a paper fighter against [other] real, operational fighter jets that can be flown and tested?" he said.
One of the options, long considered, is extending the life of the CF-18s beyond 2020 until the stealth fighter is fully tested and ready for operations.
Engineers would have to find a way to extend the CF-18 airframe life, technically complex and expensive.
New Democrat defence critic Jack Harris says that given the latest report by the Pentagon, an extension is unrealistic.
"There are planes that are already available and flying," he said.
"The government needs to have a good, hard look at what they've committed themselves to and say this plane is not going to be available to us when we need it."