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The facade of the headquarters of the Department of National Defence is pictured in Ottawa, on April 3, 2013.Adrian Wyld/The Canadian Press

Since 2018 a Canadian Armed Forces passenger jet has repeatedly broadcast a transponder identification code that could mislead flight trackers into thinking it was a British aircraft.

All aircraft use transponder codes to identify them in flight, both to each other and to air traffic control. The International Civil Aviation Organization or ICAO assigns ranges of code numbers to each country.

Flight data records indicate that during at least 80 flights since 2018, the identifier for one particular CC-144 Challenger jet with the Royal Canadian Air Force has switched, while flying, to a numerical range reserved for aircraft originating in Britain. The Challengers are used to transport government official and foreign dignitaries and support military operations.

Steffan Watkins, an Ottawa-based research consultant who tracks aircraft and ships worldwide and noticed this transponder behaviour, likened it to “changing licence plates midtrip.”

The Royal Canadian Air Force, when contacted about the transponder code transmissions, at first denied anything was wrong.

“A hex transponder code, as confirmed with Transport Canada and the RCAF aircraft certification team, is hardwired into the aircraft and cannot be changed in flight on CC-144 Challenger aircraft,” spokesman Lieutenant-Colonel Adam Thomson said in a initial statement.

"The RCAF does not hide the identify of its aircraft and always respects civilian transportation procedures and regulations when it comes to the operation of military aircraft in domestic civilian airspace.”

One week later, however, following an investigation, the military acknowledged something was amiss.

But it said this was because of an electrical glitch rather than deliberate obfuscation, and it blamed faulty connections on one of the two transponders on this Challenger aircraft.

“Upon testing of the transponders ... it was confirmed that the No. 2 transponder was indeed transmitting the wrong ICAO code which actually indicated a British aircraft,” Lt.-Col. Thompson said in a follow-up e-mail.

“On behalf of the RCAF leadership, we sincerely appreciate your educated interest in this and thank you for reaching out to us so that we could quickly fix this."

The practice of broadcasting bogus transponder codes is sometimes used around the world to hide the true identity of fighter jets, mercenaries or smugglers, Mr. Watkins said.

For instance, this month, a Chinese university think tank, the South China Sea Strategic Situation Probing Initiative, publicly accused the U.S. military of such subterfuge, saying an American spy plane over the South China Sea switched the identifier code it was transmitting to one that would normally be assigned to a Malaysian aircraft. The U.S. government never responded to the allegation.

Mr. Watkins, who consults flight-tracking websites ADSBexchange, RadarBox24, FlightAware, and Flightradar24, said he finds the Chinese allegation credible given the data from those same sources.

In the case of the RCAF Challenger jet, tail number 144615, however, the Canadian military says a loose plug was to blame. The passenger jet has two identical transponders so that there is a backup available if one fails.

The Air Force said the Challenger flight crew normally use the No. 1 transponder during takeoff and then switch to No. 2 transponder “when the co-pilot takes over flying in the cruise and prior to the landing portion.” It was the No. 2 transponder where the fault lay, the military said. The plug responsible for programming the transponder code “had loosened, likely due to vibration over time." This poor connection between the plug’s pins and the transponder “most likely induced the wrong code,” the RCAF said.

Mr. Watkins said he finds the explanation plausible after examining installation and maintenance manuals for the transponder. And since The Globe and Mail raised the matter with the RCAF, the Challenger jet in question has not been broadcasting a British transponder code.

He said it would not make sense for the Challenger jet to switch to the exact same British transponder code each flight if the military really wanted to obfuscate the aircraft’s movements because anyone paying close attention would notice the same disguise was used repeatedly. “If I can see them, the Russians, Chinese, Iranians or anyone, really, could see them, too, making their defence compromised,” Mr. Watkins said.

The RCAF said the transponders on all its Challenger jets have been checked. It said these transponder codes are normally checked during tests every 24 months.

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