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Yet there’ve been U.S. cases in which droplets of aviation fuel have landed on people, the ocean and farm crops. As a precaution, Transport Canada encourages airlines to “dump fuel over unpopulated areas and areas clear of heavy traffic,” Anyk said.

According to Canadian Aviation Regulations, it is illegal to jettison fuel from an aircraft in flight unless it is necessary to ensure aviation safety and all appropriate measures are taken to minimize danger to human life and damage to the environment, as circumstances of the emergency allow.

Pilots must notify air-traffic control and fuel is dumped over designated areas, Anyk said.Pilots must also notify environmental regulatory authorities of fuel dumping events, she added.

Transport Canada does not keep statistics on fuel dumping, including exactly how much is dumped every year.

A Postmedia News review of 27 Transport Canada incident reports for dumping in 2017 showed just one case in which the amount of fuel was revealed.

On Oct. 13, 2017, near Thunder Bay, Ont., a Delta Air Lines Boeing 777-200 flying from Atlanta to Tokyo advised it was diverting to Minneapolis/Saint Paul because of a medical emergency. Two aircraft required diversions to avoid the area around the dumping of about “120,000 pounds of fuel,” the report stated.

That’s about 55 tonnes of fuel — from just one aircraft.

Jason Kobi, acting regional manager of the federal transportation safety board, said that large airliners have a maximum landing weight that is lower than the maximum take off weight, in order to protect landing gear and aircraft structure during landing.