And what if you've to go to the loo?

Same as with a car. You find someplace to stop.
Otherwise, use a bottle

Kev

Uh... about that bottle... don't ever forget to empty it during a stop and
decide to pour it out of the window at the run-up area prior to the next
t/o... bad for the side of the plane! Not to mention the side of your face
when some gets blown back in by the prop! A friend of mine did that once!
LOL!

-Frank