Crashed plane is found on a rare island that you have a small chance to find while drifting on your Raft. Some people will play for hundreds of hours and not find it and some people find it within their first day of playing the game. It is a smaller island with a plane sitting on top of a hill (cannot miss it). You only need to get near or step foot on the island to get its respective achievement.
Remember, you cannot find this island using the receiver, you have to encounter it randomly.