Frequencies, Clearances and Readbacks

Each frequency has a job. CTAF is where you self-announce at non-towered airports, and UNICOM is a nongovernment station that may pass advisory information like the active runway and wind. At a towered field, ground control gives taxi instructions, tower handles the runway for takeoff and landing, and approach or departure control provides radar service in the surrounding airspace. An ATC clearance authorizes you to proceed under stated conditions, and you must understand it before acting. Some instructions must be read back so the controller can confirm you heard them correctly — above all, hold-short instructions and any instruction to enter, cross, or hold on a runway — and the readback ends with your call sign. A clearance is never an order to do something unsafe: if you cannot comply you say "unable" and ask for an alternative.