Why did Trudeau's plane get stuck in India? It has to do with the purchase of new fighter jets.

The long process of replacing Canada's fleet of C-150 Polaris transports and air tankers — one of which broke down spectacularly this week, stranding Prime Minister Justin Trudeau in India — has been hung up in part by the desire of the federal government and the air force not to put the cart before the horse.

For years, senior air force and federal procurement officials tied the fate of the five aging aircraft to the Liberal government's much-delayed plan to buy new fighter jets.

The grounding of the prime minister's flight in New Delhi following the G-20 Summit because of a mechanical issue has turned the spotlight on the Department of National Defence's (DND) often-overlooked Strategic Tanker Transport Capability Project (STTC) — which, like many other air force projects, has languished on the books for years.

The question of why the nearly four-decade old aircraft haven't been replaced has become far more pressing since the prime minister's travel troubles generated media interest around the world.

The $3.6 billion tanker and transport replacement program, involving nine Airbus 330s, was announced quietly over the summer. It came several months after then-defence minister Anita Anand confirmed that the federal government had decided to buy the Lockheed Martin-built F-35 stealth fighter.

The opposition has long accused the Liberal government of slow-rolling the purchase of new fighter jets for political purposes. The government took over seven years to run a competition before settling on the F-35 — the warplane Prime Minister Trudeau pledged not to buy during the 2015 election.

Often overlooked has been the air force's insistence — outlined in a September 2016 House of Commons defence committee report — that the purchase of new tanker aircraft remain on hold "pending the result of the evaluation to replace the [CF-18s], due to different fuel receiving systems in use by various fighter aircraft."

That committee took an exhaustive look at the state of the air force's fleets and concluded the replacement of the transport and tankers was the third most important purchase facing the newly-elected Trudeau government.

"Once a decision is made on the next fighter aircraft, the next decision will be the tanker replacement … The plan all along was to choose a fighter and then make sure that the tanker capacity was there," said the commander of the air force in 2016, the now-retired lieutenant-general Michael Hood.

The Polaris fleet was "coming to the end of its life" and needed to be retired, he told MPs on the four-party committee. 

"We're waiting for the decision to be made on the future fighter aircraft, and that will determine the requirements of the next tanker aircraft," Hood testified.

He said that since the refuelling technology used by the new fleet — "whether it is a probe-and-drogue, as we use right now, or a boom that flies into a refueling receptacle" — would depend on the design of the new fighter plane, the purchase of a new fleet would have to wait until the federal government picked a replacement for the CF-18s.

His testimony was later echoed by the Canadian deputy commander of NORAD at the time, the now-retired lieutenant-general Pierre St-Amand, and by Scott Bishop, DND's director general of international security policy, now a vice-admiral and Canada's military representative at NATO.

On Tuesday, DND downplayed the sequencing of the programs, suggesting that technical fixes had eliminated the necessity of buying the fighters first and the tankers second.

"Manufacturers of air-to-air refueling aircraft integrate both refuelling mechanisms on the same platform," DND head of media relations Dan Le Bouthillier said in a media statement.

But industry publications noted earlier this year that the Airbus 330s, in their current configuration, do not have the boom required to refuel the F-35 in flight. Those articles speculated that booms would have to be added or the Canadian fighters would have to be modified for the hose and drogue system.

Technical debates aside, DND made it clear to the Commons committee in 2016 that it didn't expect to sign a contract for Polaris replacements until 2022.

Even though replacements weren't a priority, the committee noted a sense of urgency because "a number of witnesses stressed the need to move forward with the replacement of Canada's air tanker fleet as soon as possible."