As I waited idly at the airport yesterday I watched the fans on the front of an aircraft’s engines (an A340). They were turning around slowly - I always assumed that they were sort-of on “tickover” when this was happening (I can see that’s stupid because they’re not turning fast enough to compress anything).
But then it occured to me that they were turning backwards, too. So are they just windmilling in the breeze when they’re doing this? Or is there some active energy input to make them turn? And if so why?