Thanks 900, I had a vision of junior barristers working for senior barristers who were the partners and bosses in a law firm, hadn't realised that they were mainly all self employed.
You say: "seniority is indefensible in law and sooner or later will be successfully challenged".
How would you go about proving your point? Take a typical airline situation, say three vacancies for captains and ten equally qualified and experienced first officers awaiting promotion? By equally qualified and experienced I mean holding the company's requirement for promotion to captain, such as licence and experience, not the oldest, the one with most hours, the one who has been a captain before, elsewhere etc. etc. all of which can be very subjective. Surely date of joining can be the only fair and non discriminatory form of selection for command training?