Radiotelephony and communications
Where radio fits
Radio is a safety tool used to make your intentions known and to hear others' intentions. At non-controlled aerodromes you make standard broadcasts on the CTAF, including position and intentions; in a mandatory broadcast zone the broadcasts are required. In controlled airspace you need a clearance and a readback, and you will need the appropriate radio operator qualification. Radio never transfers responsibility for see-and-avoid.
Structure and phraseology
Learn the phonetic alphabet, number pronunciation and standard phrases. A good broadcast answers: who you are, where you are, what you are, what you intend. Use plain, standard wording; avoid slang and avoid "take-off" unless a clearance is being issued (use "departure" or "airborne" when describing your own movement). Read back clearances, hold-short instructions and runway assignments. If you do not understand, say "say again".
Emergencies
MAYDAY three times for distress (grave and imminent danger); PAN PAN three times for urgency. Guard frequency 121.5 is monitored for emergencies. If radio fails in controlled airspace, follow the published lost-communication procedure and leave the airspace as cleared or as published.
Worked example / rule of thumb
CTAF broadcast: "[Location] traffic, [callsign], remotely piloted aircraft, [position], [altitude], [intentions], [Location]." Short, ordered, repeated at the correct points in the circuit or operating area.
Traps
Assuming a radio call creates separation; using non-standard phrasing; forgetting a readback; talking over a manned-aircraft transmission; neglecting the listening watch.