Two planes approaching Montreal's Pierre Elliott Trudeau Airport came too close to each other while still aloft — an incident the federal Transportation Safety Board (TSB) blames on short-staffing of air traffic control and a deviation from standard procedures.
In a report released Wednesday, the TSB said in May 2018, an Air Transat Airbus A310 and a Cessna 421 light twin-engine aircraft were approaching the airport when they came within 500 feet (152 metres) vertically and 1.7 nautical miles (three kilometres) laterally of each other.
"Normally the aircraft should be separated by at least 1,000 feet [304 metres] vertically or three nautical miles laterally," read a statement issued by the TSB.
Although the aircraft got closer than Transport Canada minimums permit — a situation known as "loss of separation" — "there was absolutely no risk of collision," said Kevin Roach, a senior investigator at the TSB.
The vertical distance separating the planes works out to roughly one-and-a-half football fields.
The TSB report says normally seven controllers and a shift supervisor would have been scheduled to work, but "absences and illness reduced that number to three controllers and a supervisor."
That short-staffing meant that three controllers were responsible for six sectors of airspace, adding to their "workload and its complexity."
The planes were headed for different runaways, but while still 33 kilometres northeast of the airport, they came too close to each other, the report said.
The TSB's investigation found that as the Cessna was approaching from the northeast, "control responsibility for it was not transferred to the next sector according to standard procedure."
A controller-in-training responsible for that receiving sector "was not initially aware of the presence or intentions of the Cessna until it entered his airspace, and as a result did not have an opportunity to develop a plan to deal with the converging traffic."
The report goes on to say that the instructor, who was both the shift supervisor and responsible for the trainee, "was distracted by other tasks and wasn't able to accurately monitor the developing situation."
Ultimately the controller-in-training saw the Cessna plane on the display was able to divert the planes away from each other before they got any closer.
Both aircraft landed at Trudeau airport without incident.
The TSB said it's not aware of any safety action taken by the airport following their investigation.
A spokesperson for the Montreal airport told CBC News they are aware of the report and that "the safety of the public/travellers and the facilities at YUL is a priority and a concern at all times." (YUL is the three-letter airport code for Pierre Elliott Trudeau Airport.)
It said that any incident involving air navigation is the responsibility of Nav Canada and that the airport authority "will follow up with them to deal with this situation."