The F-35 Joint Strike Fighter is the U.S. Air Force's infamous trillion-dollar weapon system. So many millions were poured into making the airframe one of the stealthiest fighters on the planet, it might surprise aviation fans to know it comes with an option to totally kill its own stealth capabilities.
With every nook and cranny of this aircraft precisely engineered to make it invisible to enemy radar, it comes with these tiny bolts that are fashioned onto the top of its fuselage, ensuring every radar watcher and SAM battery knows exactly where it is.
There are actually a few great reasons for making the aircraft more visible to radar. The use of these devices, called Luneberg Reflectors, amplify the stealthy craft's radar signature to make it visible because not every mission is a combat mission. Troops require training with their weapons and the F-35 and its pilots are no different. Just flying an invisible plane in an area close to air lanes used by aircraft from around the world would be an incredibly dangerous venture.
Think about Area 51 in the Nevada desert, the site where the Air Force tests its combat aircraft, is just over a hundred miles from Las Vegas' McCarran Airport, where thousands of tourist flow in and out every day. Invisible airplanes would create a slow hell for the Air Traffic Controllers over those skies – and if you think U.S. pilots won't do something crazy over a civilian area, I invite you to google "Sky Penis."
An F-35B without reflectors.
So flying over friendly areas on non-combat missions would obviously be the first safety goal for such an aircraft. But a more military reason for keeping the F-35 visible is that the United States doesn't want to give the enemy any practice in looking for the F-35 on their radar. If the Russians don't know what it looks like on radar during peacetime, they won't be prepared to track it during wartime – whether in Syria or Eastern Europe, where Russian anti-air capabilities are the same.