When two Air Canada pilots mistook a San Francisco taxiway for a runway and almost crashed their plane into four fully-fuelled airliners awaiting takeoff last summer, they told investigators they were fatigued at the time.

Being tired behind the controls of an aircraft is the number one safety concern pilots say they face on the job.

Next month, the federal government is expected to roll out new regulations to limit how long pilots can fly.

But the country's largest pilot union says the proposed limits on flight times are still too long and are not supported by the science. Under the draft regulations, a pilot could fly for 10 and a half hours through the night before breaking the rules.

Milt Isaacs, CEO of the Air Canada Pilots Association, said that's still too long to go without rest.

"The number one concern for Canadian pilots is fatigue," Isaacs said. "What our ask is of the government is that, (at) a minimum, just get us to science and that science is NASA."

Isaacs said NASA recommends a maximum of eight-and-a-half-hours of night flying.

"So the duty period at night time, basically, is no more than 10 hours," he said. "That's it. And that breaks down to, in terms of flying time, no more than 8.5 hours, because you have some prep time before the flight and what not."

Measuring impairment

As a former astronaut, Transportation Minister Marc Garneau is quite familiar with NASA's protocols. That doesn't mean he's budging on any of his proposals.

"Notwithstanding my great respect for NASA, we make our own decisions based on the best available science," Garneau told reporters in Ottawa Tuesday.

"We've done an enormous amount of consultation and of course our process is to make sure we're in accordance with the latest science as well as making sure that we are going to respect the international civil aviation organization standards."

Garneau said the final regulations will be out sometime in June and he'll have more to say then.

Clinton Marquardt, a human fatigue specialist, has used biomathmatical modelling software to study the fatigue effects of flying for eight hours and for 10.5 hours.

"If you were to compare them to how someone would perform under various levels of alcohol, they drop down, in both cases, to the level you'd expect to see in somebody who's got a legal limit, or has breached the legal limit, of 0.08 per cent blood alcohol concentration," said Marquardt.

Video shows Air Canada plane narrowly missing several planes as it attempts to land on taxiway instead of runway 1:16

He said governments should look at pilot fatigue much the way they look at blood alcohol content in motorists, although he cautions that "it's rare that we see a big occurrence due to fatigue."

Marquardt also said pilots can keep themselves mentally sharp on long flights through the use of caffeine, exposure to specific types of light, physical movement and sleep management techniques.