It's my understanding that W is a revenue class which is available to FB elites as an additional award booking class. Not just for intra-European flights but for transatlantic (I've booked these) and other intercontinental flights as well. As rcs85551 says, my guess is that AF will note on the ticket that you have an award and therefore cannot earn miles with another program on an award ticket. But I suppose one could always try, if one is so inclined, to cheat the system.