OTTAWA—The Envoy Air jet slowed on Runway 24 Left at Pearson International Airport after an uneventful flight from Chicago’s O’Hare airport. It was 5:30 p.m., the height of the afternoon rush at Canada’s busiest airport.

As the Envoy pilots exited the runway, fast-talking controllers in the tower — moving metal at an industrial pace — issued instructions to them to stop on the taxiway, short of a parallel runway being used for departures.

The controller then cleared a Westjet Boeing 737 bound for St. John’s to depart on that parallel runway.

But worried that the Envoy flight, an Embraer 175 jet operated by a subsidiary of American Airlines, wasn’t stopping as instructed, the controller issued firm orders, according to a recording on the website LiveATC.net.

“Envoy 3765, stop, stop,” the controller said.

Tuesday’s incident was yet another close call at Pearson. The Transportation Safety Board of Canada has launched a special review of operations at Pearson, worried that the frequency of incursions — when an aircraft inadvertently taxies into the runway environment — could lead to disaster.

“We’re very aware of a broader systemic issue,” said Ewan Tasker, the regional manager for air investigations for the Transportation Safety Board of Canada.

The circumstances around Tuesday’s incident remain under investigation. In a statement to the Star, American Airlines, the parent company of Envoy Air, says the flight “turned onto the appropriate taxiway, held short of runway 24R, and then taxied to the gate as instructed.”

The pilots say they did not cross the active runway until directed by controllers.

Tasker says the Envoy Air flight crossed the hold short line, entering the protected runway environment, but stopped short of the runway itself. “It wasn’t as if the planes narrowly missed . . . the severity of this individual event, not that high,” Tasker said.

But a rash of similar incidents — almost two dozen dating back to June, 2012 — has safety board officials worried about the potential for a catastrophic incident.

“Some of the worst aviation disasters in history have been due to a runway incursion of sorts. So it’s a very serious issue,” Tasker said.

That’s why the board has taken the rare step of launching a special safety study to examine incursions that occur between the two parallel runways on the south side of Pearson airport. At busy times, aircraft land on the outer runway and then taxi across the inner runway to reach the terminal buildings.

“They generally land on the outer or southerly runway and are instructed to hold short of the inner runway. There’s been a whole bunch of incidents where air crews haven’t done this,” Tasker said.

As they exit the runway, pilots are performing post-landing checks, looking at airport diagrams to find their way to the terminal and it’s easy to get distracted and lose situational awareness of the approaching parallel runway.

“That’s something we need to look at in all these events, what the crews were doing, who was doing what, who was looking where,” Tasker said.

It’s already recognized as a potential hazard. When the south parallel runways are in use at Pearson, another controller position is opened in the tower, specifically to monitor communications and be on guard for potential incursions, Tasker said.

As well, ground radar can sound an alarm when an aircraft on the ground violates the protected space around a runway.

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The study, which the board hopes to finish by year’s end, is looking at issues like airline procedures, airport design, air traffic control and the automated warning systems. Investigators are also looking at airports around the world with similar runway configurations to see whether measures in those places have helped mitigate incursions.

It’s the second such incident for Envoy Air at Pearson this year. An Envoy flight had a close call on the same parallel runways in April.

But Tasker said the research so far shows a variety of airlines have been involved in the incursions.