Flight attendants for United Airlines are allowed to drink while on duty in certain circumstances. So are flight attendants for Air Canada.

• They are ‘deadheading’ – flying during their duty day from one city to another, but not working as crew. For instance, they might fly from their base to another city where they will pick up a flight to work, or fly back to base after working.

• And they aren’t expected to work anymore that day.

There are times when deadheading isn’t all that exciting… you fly as a passenger for a couple of hours and then immediately pick up another flight and work it as a flight attendant. In this type of situation, there would definitely be no excuse to drink.

But what if flight attendants are deadheading as the last duty of their day, especially on a long-haul flight? …[I]t turns out that out of the major US airlines, only flight attendants at United Airlines are allowed to drink when they are deadheading and not expected to work the same day.

There are, of course, some rules. For example, flight attendants can’t be uniform, and the door to the plane must be shut with everything ready to go before they take their first sip… just in case something crops up and they are suddenly expected to work on the flight.

One reason not to permit this is brand: there’s a long history of not wanting airline employees to be seen drinking. It’s obvious for pilots, and even photos of pilots mixing a drink in an inflight magazine caused a row, but it’s also true for flight attendants also. And alcohol wasn’t something airlines wanted reflecting on their brand.

Another is that with extensive route networks, and since they’re paying the flight attendant still anyway, why allow anything that would disqualify further work on the off chance it were needed?

Of course, airline crew are people. I’ve seen a flight attendant furtively drink an alcohol mini in the galley prior to boarding with an FAA inspector on board. I’ve seen a pilot take a bottle of wine with him from the galley after a flight. There are plenty of stories like this online. In the former case it’s a potential safety issue, in the latter it wasn’t but still a violation of policy.

Large airlines with tens or even over a hundred thousand employees have some alcoholics amongst their ranks, and some at times will also have lapses in judgment. That’s why airlines have safety programs that allow crew to seek help usually without jeopardizing their careers. But those programs aren’t always robust enough to keep crew from hiding substance use.