Having been thrown off a BA flight the other week which was chokka to give the jumpseat to some ratty BA engineer who was claiming 'mainline' priority, I decided to check out the rules.....
Surely you're not telling me you don't read BA News? There's been a long running saga about who gets to sit on the jump seat running in the letters page. In short it's entirely down to the Captain and nothing to do with seniority, date of joining or anything else (except perhaps how pretty you are).