A boarding pass is a pass you show when boarding something. (Dictionary)
You board an aircraft when you step on board. (Homer Simpson, sorry)
So you should show your boarding pass when you do that and step on board an aircraft ? (Elementary Watson)
So you should not show your boarding pass before you step on board because the boarding pass should only be shown when stepping on board. (as above, logical deducytion)
So should the thread not read...
Grumpy anywhere other than aircraft when asked for boarding pass?
Do not the Grumpies get grumpy at the wrong grumpy point and not at all the right grumpy points?
Does that make the sense of the thing.