This mission unlocks after you take over the outpost Old mines.
Enter the house and listen to the story of the plane crash. Go to the marked area, where there is a hang glider. Try to land on the beach, or in its vicinity. Once there, you will find fragments of the plane. Approach them to examine them..
To the left of the shown fragments, you will find a shack made out of the plane fragments, which you need to enter. Talk to the owner to complete the mission.