“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.

Envoler does not mean to fly into something, it means to go into a state of flying. This is a common meaning of the prefix en- (§A.2, B, C): s’endormir (to enter a state of sleeping, i.e. to fall asleep), s’endeuiller (to enter a state of mourning), s’enrichir (to enter a state of being rich(er), i.e. to become rich(er)), … Without the reflexive pronoun, the verb means to cause something to enter the corresponding state (endormir: to make someone sleep, enrichir: to make someone richer, …).

S’envoler is a pronominal verb. Envoler is never used without a pronoun. The etymological idea is presumably that the subject is taking flight under its own power, even if this does not have to be the case (“Il ouvrit la fenêtre. Le vent fit s’envoler les papiers.” = “He opened the window. The wind sent the papers flying.”)