"Voler" means "to fly" and "s'envoler" means the "action that represents the start of the flight".
L'avion s'envole = l'avion décolle = The plane takes off = the plane flies away.
And after than the plane has take off, L'avion est en vol = The plane is in flight.
So the "en" in "envol" is not the preposition in this case. And "envol" has not the same meaning as "en vol" in two words.
I hope that what I said makes sens, I am not sure I understand exactly your question.