The case of two Canadian pilots arrested in Scotland on suspicions of being drunk in the cockpit has highlighted the strict “bottle-to-throttle” rules around drinking before operating an aircraft -- a set of country-specific restrictions rarely broken by pilots.

The two Air Transat pilots, both of whom are from Ontario, were arrested in Glasgow and charged with “being impaired through alcohol” on Monday. They have been identified as Jean-Francois Perreault, 39, and 37-year-old Imran Zafar Syed.

The arrests came as a major surprise to Canadian aviation expert Phyl Durdey, who called instances pilots drunk on the job “very rare.”

“Pilots are true professionals, and it’s the one rule that should not be broken because the repercussions can be serious,” Durdey told CTV News Channel on Tuesday.

“I don’t know what the situation was or what their excuse was, but the rules and regulations are quite clear.”

Each country sets its own rules around how soon a pilot can fly after having a drink, and rules enforced based on the flight’s departure destination.

In Canada, the Canadian Aviation Regulation outlines that no one can fly if they’ve consumed alcohol in the last eight hours or if they are “under the influence” of alcohol.

Pilots are also barred from flying “while using any drug that impairs the person's faculties to the extent that the safety of the aircraft or of the persons on board” is endangered.

The rules apply to all aircrafts, including hot air balloons and helicopters. Some commercial airlines push the limit further and insist that pilots extend the “bottle-to-throttle” gap to 12 or 14 hours.

U.S. Federal Aviation Administration regulations also include an eight-hour “bottle-to-throttle” rule, and pilots leaving the U.S. cannot fly if their blood-alcohol level reaches 0.04% or higher. (For a 180 lb. man, it would take about three standard drinks to reach that level in the first two hours after consumption.)

Pilots are responsible for “self-mentoring” how much they drink, Durdey said, but flight crews are trained to speak up if they think a pilot is beyond the legal limit.

“The thing is that the crews work together so if something is not good or there is a safety concern or an issue, they will bring it to light,” Durdey said.

If the two Canadian pilots are found guilty of attempting to fly a plane drunk, they could face serious consequences, Durdey said.

“They’ll probably lose their licences and lose their jobs. It has serious ramifications,” he said.

But in some cases, pilots have been given a second chance. In 1990, three American pilots were found guilty of having been intoxicated for an hour-long domestic flight. Two of those pilots -- Norman Lyle Prouse and Joseph Balzer -- were later allowed to fly again after undergoing rehabilitation.

Canadian aviation authorities say the rules are quite clear, but that pilots should be aware of how their unique body type metabolizes alcohol.

“A single beer consumed eight hours or more before a flight will not yield the same hangover as multiple servings of beer, wine or liquor,” wrote Michel Treskin, a civil aviation safety inspector for Transport Canada, in a 2005 report.

“What we need to remember is that if we decide to have an alcoholic beverage the night before a flight, we must ensure that we are completely clean before we start that flight. Otherwise, we might find ourselves without a licence.”