FlySafe is a safety system used in some Second Life aircraft to avoid Full parcels, mid-air collisions and other related hazards.
Background
By activating FlySafe mode, the plane and pilot become phantom and temporary. Making the plane temporary enables an aviator to fly through full parcels, as temporary prims do not get limited by the parcel's prim limit. So, if a parcel has a prim capacity of 400 prims, one can still have temporary prims which exceed this limit. Being phantom allows the pilot to fly through physical floating obstacles such as skyboxes.