The Transportation Safety Board is recommending that the layout of two runways at Canada's busiest airport be changed to lessen the risk of collisions between aircraft.

That recommendation was one of three included in a report from the independent safety watchdog after its investigations into 27 runway incursions at Toronto's Pearson International Airport between June 2012 and November 2017.

The TSB defines a runway incursion as an incident where an aircraft or ground vehicle "mistakenly occupies an active runway." A worst-case scenario would be a direct collision between two planes as a result.

According to investigators, all of the Pearson incidents occurred between two "closely spaced parallel runways" at the south end of the airport grounds. The two runways are connected by several "rapid-exit taxiways" — small stretches of runway aircraft can use to move from one to the other.

Both runways are used at the same time during peak hours at Pearson, and each can see hundreds of take-offs and landings in a single day.

"That means that when an aircraft lands, it needs to move out of the way as soon as possible because the next approaching aircraft might be only seconds behind," Ewan Tasker, a TSB investigator, said at a morning news conference in Richmond Hill, Ont.

Planes use rapid-exit taxiways, shown here in red, to move between the parallel runways in the 'south complex' at Pearson International Airport. Both runways are in use during peak hours at the airport. (Transportation Safety Board)

The problems arise when a plane has landed on the most southerly runway and attempts to take one of the taxiways to the adjacent runway, the TSB said.

Tasker notes that the layout of the taxiways is "different from almost every other major airport in North America" and has several "uncommon" design features. This has resulted in confusion among flight crews and increased the risks of a major collision, the report says.

"All 27 incursions examined involved flight crews who understood they needed to stop, and that they were approaching an active runway," Kathy Fox, chairwoman of the board, said.

"Despite all the visual cues, including lights, signage and paint markings, professional crews were not stopping in time as required, thereby risking a collision with another aircraft on the other runway."

Fox said that in at least five instances, last-second interventions by air traffic controllers — she dubbed them the "first and last line of defence in these incursions" — prevented potentially serious collisions between planes.

The red line in this diagram shows where taxiing planes are supposed to stop to ensure the parallel runway is clear. Due to "uncommon" design features at Pearson, some flight crews get confused about where they need to stop the aircraft. (Transportation Safety Board)

The Greater Toronto Airport Authority, which oversees operations at Pearson, has several options to deal with the ongoing problem of incursions, Fox said. Ultimately, one of the following will need to happen:

Change the design and positions of the taxi-ways that connect the two runways.

Build a "perimeter taxiway" that goes around the other active runway.

Construct an entirely separate "intermediate taxiway" between the two parallel runways.

In a statement, the GTAA said it is reviewing the TSB's recommendations.

"Safety is our top priority, and we will continue to make improvements that enable continued safe operations for the surrounding communities and the nearly 50 million people who use Toronto Pearson on an annual basis," the organization said.

"In addition to specific enhancements to our runways and taxiways, new lighting systems and mandatory LED backlit signage, we have also provided up-to-date safety information and educational outreach" to various airlines.

The TSB's short-term recommendations include making changes to the language air traffic controllers use to convey safety-critical commands. That would put Pearson in line with international guidelines, Fox said.

Further, the board says Transport Canada and the U.S. Federal Aviation Administration should amend standard operating procedures so crews only begin post-landing checks after a landing aircraft has cleared all active runways.

"This would keep flight crews' attention focused outside the cockpit when approaching and crossing active runways, and reduce work-related distraction at critical moments," Fox said.