An analysis conducted for the military's second in command says two models of the executive jet, which has often been at centre of political storms, are "at the end of their upgradeability and would require additional capital investment" if they were to remain flying past 2014.
The 2013 documents, obtained by The Canadian Press under the Access to Information Act, laid out three different proposals, all of which called for the bulk of the fleet to be retired last year.
But a spokeswoman for the Royal Canadian Air Force said this week that all six of the Challengers remain in service with 412 Squadron in Ottawa and no changes have been made to their status.
"Options for the future of the C-144 Challenger fleet are still under consideration and so details are unavailable at this time," Capt. Holly-Anne Brown said in an email.
"No final decision has been made to retire aircraft within the C-144 Challenger fleet."
Among the options under consideration is an upgrade related to avionics, Brown confirmed. But that is mandated by air navigation regulations and she would not say whether an airframe extension is something the government might also consider.
The electronics are so old in the jets, known as CL600 and CL601 variants, that their analog systems prevent them from flying overseas, the documents said. Spare parts are also becoming an issue for the 30-year-old planes.
Brown would not say when a decision would be made, saying the jets are well-maintained and safe to fly.
A few years ago, National Defence imposed a moratorium for any new spending on the four aircraft.
The jets are typically used to carry the prime minister, the Governor General, cabinet ministers and the country's top military commanders. But they also have a storied history of being used as political footballs.
In a 1993 report, the federal auditor general blasted the Mulroney government's failure to disclose operating costs and questioned the need for executive jets. Nearly a decade later, the auditor revisited the subject when the Liberal government of the day purchased two additional Challengers to round out the fleet to six.
They were back in the news again in the fall of 2011 when questions were raised about the travel of both former defence minister Peter MacKay and the then-chief of defence staff, the now-retired general Walt Natynczyk.
MacKay logged more than $2.9 million worth of flights aboard the VIP jets between 2008 and 2011.
Aside from ferrying around dignitaries, the jets perform medical evacuations for injured soldiers; the question of how that service would be replaced has been central in the discussions about their future, the documents show.
The only other aircraft in the inventory suitable for the medical role would be the C-150 Polaris, but using for that purpose would "triple the cost per hour" when compared with the Challenger, according to the seven-page report, which was prepared for the vice-chief of defence staff.
Ditching the four outdated aircraft would save the air force about $16.1 million per year, including all maintenance and operational costs. But the analysis noted that going to a two-jet fleet would force commanders to juggle the schedule.
If put up for sale, the jets could net taxpayers up to $35.5 million, but the report notes that the "market for these aircraft has decreased steadily."
Follow @Murray_Brewster on Twitter