Abstract: FIG. 1 shows airframe 10 with electromagnetic field sensor 12, adjustable reference electromagnetic field strength 14, comparator 16, parachute 18, parachute trigger 19, and inspection camera 20 inspecting a transmission line corridor containing towers 40, 42, and 44, phase conductors 46, 48, and 50, and shield wires 52 and 54. Reference electromagnetic field strength 14 is adjusted before the flight to set the minimum electromagnetic field strength before parachute trigger 19 deploys parachute 18. The reference electromagnetic field strength 14 corresponds to a radius, and thus virtual tunnel 22, outside of which airframe 10 cannot fly without deploying parachute 18, regardless of the state of the autopilot, GPS signal, or radio link.