You have no idea how debillitating ( I hope that is the word) a moving kidney stone can be. Without any warning or signs it begins and grinds a way down the tubes, bleeding, scraping and suffering until if you are lucky it comes out. If you are not so lucky it gets blocked and you cannot p properly.
Mine was lucky, not in the air but on a desert island far away from anything like a lithotripsy kit or a urologist. You do not want to play games of chance like marbles with those stones boyos!
Flushing with water or beer is not always going to dislodge a stone stuck either in the kidney or in a tube. It may dilute the blood trace for a medical but you can still carry the problem with you which, I think, can be made worse or started up by the pressure changes in an aircraft making the tubes dillate and contract - like windy passangers perhaps?