After you call for a heavy drop or an extraction, a big helicopter should fly over. When you see it, fire your grapple gun at the helicopter. If you hit, you should be attached. Pull yourself in. You must get very close then switch weapons to let go. You should land next to the helicopter door. Once done, press Triangle to get in. This vehicle is indestructible, but you are not. You can still die but the helicopter will always stay intact.