OTTAWA -- The federal government could be forced to buy new executive jets to transport the prime minister and other VIPs because of changes to air-traffic control rules in the U.S. and Europe.

Two of the four Challenger jets currently used by the Canadian Forces for executive transport lack the equipment needed to comply with the new ADS-B system, which replaces radar-based air-traffic control with the transmission of GPS-based data.

That will curtail the aircraft's ability to fly in the U.S. and Europe beginning next year.

The U.S. Federal Aviation Administration has set Jan. 1, 2020, as the date aircraft will require ADS-B transmitters to operate legally in most airspace. Europe is implementing the system in June 2020.

It doesn't make sense to install the equipment on two of older 1980s-era Challenger 601 jets flown by 412 Squadron, the defence department says.

"Given the age of the fleet, investing in an ADS-B modification/upgrade would not be cost effective," the department said in an email.

"We are therefore looking at other mitigation options. Timelines and costs will be determined once options have been selected."

Buying new executive jets to replace the aging Challengers could be politically risky for the Trudeau government when it still hasn't acquired fighter jets to replace the aging fleet of CF-18s.

Opposition parties of all stripes have assailed governments for what they considered profligate use of government aircraft in the past.

"The executive fleet has been perennial issue where the country is penny-wise and pound-foolish," said David Perry, a defence procurement expert with the Canadian Global Affairs Institute.

"We don't want to spend the money to buy aircraft and we have this bizarre expectation where you're taking the people in government whose time is most valuable and making them fly economy, basically, around the world."

The continued use of older aircraft makes Candians look like "cheapskates," he said, comparing political opposition to the planes with the government's refusal to renovate 24 Sussex, the prime minister's official residence.

The defence department projects spending between $20 million and $49 million to "consolidate" the Challenger fleet, a figure likely based on acquiring used aircraft.

Sources familiar with the operation of 412 Squadron say it would be difficult to meet its mandate with only two aircraft available to travel outside the country. The jets are used to transport the prime minister and Governor General and cabinet ministers, as well as the chief of defence staff, and visiting members of the Royal Family.

But the jets are also on-call for medical evacuations, deploying advance teams with the Disaster Assistance Relief Team (DART) and, on occasion, covertly transporting Canadian special forces personnel.

With only four Challengers, the squadron is operating close to capacity and losing two aircraft could threaten its ability to perform these missions, the sources said.

If the government chose to acquire brand-new aircraft, it might be tempted to consider the Bombardier Global Express, which is faster and has a longer range than the smaller Challenger, making travel to Europe and Asia easier. Most of the aircraft is assembled in Canada.

The German government currently uses four of Bombardier's Global 5000 jets for executive transport.

Other documents show DND is projecting spending up to $249 million to extend the lifespan of the five larger Airbus 300-series jets beyond 2026, including one that prime minister and his staff fly on occasion.

Once dubbed "the flying Taj Mahal" by then-opposition leader Jean Chretien, the Airbus is antiquated compared to most modern commercial aircraft, though it does have a private room for the prime minister to sleep in.

Flight crews on the Airbus run extension cords and power bars down the aisles to allow passengers to run their laptop computers.