The Royal Canadian Air Force is recommending that the government buy used aircraft to replace two of the planes in its aging VIP fleet, at a possible cost of $12 million to $18 million, CBC News has learned.

The prime minister and Governor General are not allowed to fly on commercial aircraft for security reasons. They use the RCAF's Challenger jets for most of their air travel.

The RCAF has flown Bombardier's Challenger business jets since the early 1980s for VIP transport and other special missions, including medical evacuations, disaster relief assessments and national security operations.

Only four aircraft remain in the fleet: two 1980s-era Challenger 601 models and two of the more modern Challenger 604 models purchased in 2002. Two older Challengers were retired from the Air Force in 2014.

The recommendation to buy used aircraft is part of a July, 2016​ RCAF study, prepared for the commander of the Air Force and released to CBC News under the Access to Information Act.

Cost estimates for replacement aircraft were deleted from CBC's copy of the report. A search of used aircraft websites show that several Bombardier 604s with comparable flight hours are available for roughly $6 million to $9 million each.

A brand-new Challenger 650 model can be purchased from the factory in Montreal for just over $32 million (the price can fluctuate depending on the buyer's choice of interior options).

One of the old Challengers is painted black while the newer aircraft are painted white with a red stripe 1:40

Half of the planes in the RCAF's VIP Challenger fleet are too old to be flown in some areas of the world.

"As new airspace regulations come into effect," the report says, "the CC144 fleet is becoming less operationally effective. In order to respond to expanding operational restrictions a number of avionics upgrades are required ..."

The report says the manufacturer, Bombardier, doesn't support upgrades to the cockpit instruments in the two older 601 models, and having the instruments replaced elsewhere "would be cost-prohibitive because no economically viable solution currently exists."

But the RCAF continues to fly the two Challenger 601s as it searches for a way to get replacement aircraft approved by the government.

The report doesn't look at the option of buying aircraft from another manufacturer. It says buying two used Challenger 604s would "fall within the current budget, and many used CL604 aircraft are available for purchase.

Then-prime minister Stephen Harper's security staffers look over the Challenger jet during his arrival at the Shell Aerocentre during his Western tour in North Saanich, B.C., Tuesday, January 7, 2014. (Chad Hipolito/The Canadian Press)

"This option consolidates the fleet to one aircraft type. Cost savings in the form of avionics upgrades, sparing and crew/maintenance training could be realized."

Right now, the Challenger fleet uses two different types of engines. The cockpits in the older aircraft feature analogue instruments, while the newer aircraft use digital instrumentation.

The report ruled out the option of leasing four new aircraft, stating that the cost of retrofitting them to suit the RCAF's needs — and then converting them back again at the end of the lease — would wipe out any cost savings.

Buying two new aircraft would break the fleet's budget. And the Challenger 604 has since been replaced with a much newer version by Bombardier, so its parts would not be interchangeable with those in the older aircraft.

The report mentions the possibility of a public-private partnership to modernize the fleet but the affordability of such a venture appears to have been redacted from the CBC's copy. Bombardier refused to comment on such a partnership, citing competitive reasons.​

Defence Minister Harjit Sajjan spoke with the CBC's Chris Rands 1:41

Asked about plans to replace the Challengers, Defence Minister Harjit Sajjan told CBC News the RCAF is working on "various options."

"And when I have those options I'll be able to present that to Canadians," he said.

RCAF policy is to have at least two aircraft in the Challenger fleet flight-ready at all times, so the other two can undergo maintenance or be used for training.

Typically, the prime minister and the Governor General use the newer Challengers while the older models are used by the Canadian Forces for medical evacuations and for trips that can't be completed by flying commercial.

Opposition defence critic James Bezan said he learned about the fleet's shortcomings firsthand in June 2016, when he and ​Sajjan shared a flight to Berlin — a flight which required two refuelling stops.

Defence Critic James Bezan speaks with the CBC's Chris Rands 1:06

"First of all, they are not very comfortable," he said. "Secondly, this plane, because of the old aeronautical system, had to fly under 10,000 feet all the way across the Atlantic, burned a lot more fuel in the process and required more stops."

New international civil aviation rules force the older Challengers to fly at lower altitudes, drastically reducing their range without refuelling.

Spending money on VIP aircraft is a politically challenging decision for any government. When Jean Chretien bought two Challengers in 2002 to refresh the fleet, the government released the news just before Good Friday to reduce its impact.

"They need to make a hard decision," said Bezan, "Kicking the can is not the answer and that's what it looks like the Liberals are going to do."

However, Bezan wouldn't say if the Conservatives would support replacing all or part of the Challenger fleet.

"Let's see what they come up with and see if they actually make a decision, because they haven't done one yet."

NDP MP Matthew Dube said he supports buying new Challengers instead of purchasing used jets that could wear out faster than expected — as long as they're bought through an open and transparent process.

"The price tag is acceptable, provided that the process leading to the use of that money is one that is appropriate for taxpayers," he told CBC News.