AIM Text
A clearance, control information, or a response to a request for information originated by an ATC facility and relayed to the pilot through an air-to-ground communication station will be prefixed by “ATC clears,” “ATC advises,” or “ATC requests.”
Source: FAA Aeronautical Information Manual · current edition · paragraph 4-4-2.
The short answer
AIM 4-4-2 is FAA advisory guidance on the clearance prefix. It explains that a clearance, control information, or response to a request originated by an ATC facility and relayed through an air-to-ground communication station will be prefixed by “ATC clears,” “ATC advises,” or “ATC requests,” signaling the message comes from ATC rather than the relay station.
Research Notes
Common Questions
Why do some relayed ATC messages begin with “ATC clears” or “ATC advises”?
AIM 4-4-2 explains that when a clearance, control information, or response originated by an ATC facility is relayed to the pilot through an air-to-ground communication station, it is prefixed by “ATC clears,” “ATC advises,” or “ATC requests” so the pilot knows the content originated with ATC, not the relaying station.
What are the three clearance prefixes used by ATC?
Per AIM 4-4-2, the three prefixes are “ATC clears,” “ATC advises,” and “ATC requests,” each used when an ATC-originated message is relayed to the pilot through an air-to-ground communication station.