Airspace 101 – Rules of the Sky
FAA rules apply to the entire National Airspace System -- there is no such thing as "unregulated" airspace. Drone operators should be familiar with the difference between controlled and uncontrolled airspace, and where you can legally fly. Controlled airspace is found around some airports and at certain altitudes where air traffic controllers are actively communicating with, directing, and separating all air traffic. Other airspace is considered uncontrolled in the sense that air traffic controllers are not directing air traffic within its limits.
In general, you can only fly your drone in uncontrolled airspace below 400 feet above the ground (AGL). Commercial drone operators are required to get permission from the FAA before flying in controlled airspace. Learn more about the rules for Certificated Remote Pilots and commercial operators on Flying Drones Near Airports (Controlled Airspace) – Part 107.
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Read more about controlled and uncontrolled airspace, as well as the different classifications of controlled airspace in the Pilot's Handbook of Aeronautical Knowledge (PDF) in chapter 15 (see page 377).
Remember, there are thousands of private pilots who fly in both controlled and uncontrolled airspace at various altitudes, and they usually cannot see your drone until it's too close for comfort. Drone operators are responsible for staying away from manned aircraft, not the other way around!