That depend on the plane and all kinds of external circumstances as well as how it is programmed etc. etc. But I can assure you even modern passenger plane on

That depend on the plane and all kinds of external circumstances as well as how it is programmed etc. etc. But I can assure you even modern passenger plane on autopilot do not avoid crashes with other planes, buildings or ground (mountains etc.) It also wont start or land a plane. In a worst case scenario the autopilot would crash the plane immediately because it was give the task to fly into something and in best case scenario the plane flies according to the programmed route and corrects for wind and all that and if nothing is in the way it eventually runs out of fuel and crashes. AFAIK it would not even slow down and certainly it would not do an emergency landing.