Airplanes can be seen flying over the world at several times of the day. They appear at random intervals and simply fly over the player most likely to be realistic. You cannot interact with them as of yet but in a future update then they will probably be used to get rescued and then end the game.
When the airplanes do get a purpose then you might have to use flares found in shipwrecks to get their attention. It is not known yet when this will happen.