In the United States, all unmanned aircrafts need to be registered before being operated regardless of weight. However, there are exceptions to this condition.
Unmanned aircrafts can be operated in an unregistered condition when the aircraft has a takeoff weight that is more than 0.55 pounds, but less than 55 pounds without including fuel and necessary attachments. In another case when the aircraft weighs less than 0.55 pounds of takeoff, including everything that is on-board or attached to the aircraft, it is allowed to run unregistered. An unmanned aircraft, popularly known as a drone refers to an aircraft that runs without any pilot, crew or passengers but is administered by a ground based controller and a system of communications.
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