Originally Posted by blue2002

It is all stipulated in their collective labour agreements, which probably most frequently translates to business class for flight crew and economy for cabin crew when deadheading (not commuting!).

If that were the case, airline lounges would be full of flight crews. But deadheading flight crews are NOT "normal business passengers". They are deadheading crew. No lounge access for them. In fact, many (or perhaps most?) airlines even forbid their flight crews from being "guested" into one of their lounges.