The difference between “I’m going to fly to Sofia on Friday” and “I’m flying to Sofia on Friday” is all about your intention; before making the sentence, you should ask yourself if you really want it or you just have a vague intention, thinking about how determined you are and how far did you go with your plans.
Did you get the ticket?
Did you book the hotel?
Well, then you are enough determined to dare a present continuous tense.
But the English grammar doesn’t let you talk about your plans in the future tense: all you can say, in fact, is about your intention, in the present time, to do a certain something in the future but, even if you have an iper-detailed, present-continuous–worthy plan, there will be no certainty as to whether you will actually do what you want to do, because the only thing we know about the future is that it is uncertain.
At the check-in desk, even feeling lucky because there was no queue, I tweeted a present continuous tense and completed it with a double emoticon: airplane + smiling face.
And so I am still in Rome, blogging at my old desk, cynical like a future tense in the English grammar.