OTTAWA—An air traffic controller’s ambiguous instruction and a flight crew’s mistake contributed to a close call at Pearson International Airport involving two Air Canada jets.

The nighttime incident in 2016 unfolded when the crew of a departing Air Canada Embraer 190 misunderstood a controller’s instruction and taxied onto a runway for take-off — into the path of another Air Canada jet that was just 30 seconds from landing on the same runway.

The arriving jet, an Airbus A320, was less than 100 metres above the ground when its pilots spotted the aircraft on the runway and aborted their landing.

In a report released Wednesday, the Transportation Safety Board of Canada lays out several factors it says contributed to what it deemed to be a risk of collision between the two passenger jets.

The ground controller, responsible for directing movements on the apron and taxiways, had told the pilots of the Embraer jet to “go to the right side.”

The ground controller meant the right side of a holding bay near the runway, but the pilots misinterpreted this as clearance to taxi directly onto Runway 24 Right.

The report says the pilots were mentally primed to receive a take-off clearance and therefore didn’t request clarification.

The fact that the perceived instruction to enter the runway came from the ground controller, not the tower controller, who typically issues such clearances, was not enough to prompt the crew to question their understanding of the situation, the investigation found.

Indeed, safety investigators say the pilots overlooked several clues that would have alerted them to the mistake, including the sight of the arriving aircraft less than a kilometre away that they believed was landing on a parallel runway.

“The expectations of both flight crew members, combined with ambiguous phraseology employed by the ground controller, likely contributed to the flight crew’s misunderstanding of the taxi instructions,” the report found.

“Although there were available cues that could have alerted the flight crew to the misunderstanding, the cues were either not sufficiently compelling or were considered and explained away,” the report found.

The safety board investigation found that controllers at Pearson use differing phrases to direct aircraft to the holding bays located by the runways, increasing the risk of miscommunication and further runway incursions.

An automated system meant to detect runway incursions did provide an alert, but the controller, focused on the arriving aircraft, did not notice.

The system could have provided an earlier warning, but, because it’s used only in low-visibility operations, it wasn’t operating on the night in question, as the weather was good.

None of the controllers were aware of the circumstances until the pilots of the arriving jet radioed that they had aborted their landing because of the other aircraft on the runway.

The danger of such runway incursions is on the safety board’s watch-list of most serious threats to aviation safety.

There were 416 runway incursions at Canadian airports in 2015 and the board has warned that serious incidents will continue to occur unless better precautions are put in place.

In the wake of the incident, Nav Canada, the agency that operates the air traffic control system, says it has taken steps to optimize the automated alert system, to provide alerts sooner.

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“This will increase the . . . warning time to the air traffic controller when a departing aircraft enters the area without authorization,” spokesperson Jonathan Bagg said in an email.

He said the agency is also reviewing communications procedures to “ensure language is explicit and effectively supports that common understanding.”

Air Canada convened a working group to review runway incursions. It recommended that pilots seek clarification of “critically important” clearances that were issued using non-standard phraseology.

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