Radiotelephony Phraseology

Standard RTF phraseology is the controlled vocabulary of aviation radio. Words such as ROGER, WILCO, AFFIRM and UNABLE, and phrases such as SAY AGAIN or CLEARED TO LAND, each have one defined meaning, and the messages built from them follow fixed patterns. Plain language is used only when no standard phrase fits. The aim is a message that means the same to the station sending it as to the station receiving it, over a noisy channel, often between people speaking a second language, and under time pressure.

This article covers the general vocabulary: acknowledgements, corrections, the words that end or divide transmissions, cleared versus approved, the terms that grant or withhold discretion, and readback. How to speak numbers and letters is in radiotelephony transmitting technique, call signs in radiotelephony call signs, and the phraseology of particular phases in ATC clearances and aerodrome control phraseology.

Why standard phraseology matters

Voice radio strips out much of what makes face-to-face speech robust. There is no expression or gesture, only one station can talk at a time, and part of a transmission can be clipped or blocked by another. Listeners also fill gaps with what they expect to hear. This expectation bias is strongest when a message is routine and workload is high: a pilot always cleared to 3,000 ft on a given arrival may hear, and read back, 3,000 when cleared to 5,000. Standard phraseology narrows the ways a message can be misunderstood, and readback and hearback catch much of what still gets through.

The deadliest accident in aviation history shows the cost of ambiguity. At Tenerife on 27 March 1977 a KLM Boeing 747 began its take-off roll without a take-off clearance and collided with a Pan Am 747 still on the runway; 583 people died. Among the factors were non-standard phrases about the take-off that the crew and the controller understood differently, and a crew so ready to expect its clearance that an unclear reply was taken for it. Since then the word TAKE-OFF is used only in the take-off clearance itself and in its cancellation. At every other moment pilots and controllers say DEPARTURE or AIRBORNE, so a crew at the holding point reports "ready for departure".

Reference manuals and glossaries

ICAO sets the baseline. Annex 10, Volume II, contains the radiotelephony procedures and the standard words and phrases, and Doc 4444 (PANS-ATM) the air traffic control phraseologies for each phase of flight. In the EU, SERA includes the voice communication procedures, with phraseology in its acceptable means of compliance; France, for example, publishes its English phraseology as an alternative means of compliance to that material. The UK CAA's CAP 413, the Radiotelephony Manual, is the reference for UK airspace.

In the United States the pilot's references are the AIM, chapter 4 section 2, and the Pilot/Controller Glossary; controllers work from FAA Order JO 7110.65. FAA usage differs from ICAO in places. The FAA's word for yes is AFFIRMATIVE rather than AFFIRM, phrases such as "at pilot's discretion" and "cleared for the option" are FAA usage, and several transponder instructions are worded differently (see transponder codes and squawk phraseology). Pilots flying abroad use the phraseology of the State they are in, as published in its AIP.

Acknowledging: ROGER, WILCO, AFFIRM, UNABLE

AFFIRM / NEGATIVE answer questions. A controller who asks "confirm QNH 1013" or "confirm established on the localiser" needs a direct answer; "roger" says only that the question was heard. UNABLE is the word for declining a clearance or instruction, normally followed by the reason and, where possible, an alternative: "unable FL60, icing, request FL80". Reading back a clearance that the crew does not intend to fly is the worst possible reply, because the controller will separate other traffic on the assumption that it is being flown.

STANDBY is neither approval nor refusal: whatever clearance was in force remains in force, and if nothing follows within a reasonable time the request is repeated. In US airspace a reply that uses the aircraft's call sign, even "(call sign), standby", establishes the two-way communication needed to enter Class C or D airspace, unless the pilot is told to remain outside; "aircraft calling, standby" does not.

Correcting and repeating

A speaker who makes an error says CORRECTION, repeats the last correct group or phrase and then gives the correct version, so the listener knows exactly where the correction starts; if it is easier to send the whole message again, the form is "CORRECTION, I SAY AGAIN". DISREGARD cancels a transmission altogether. A controller who hears an incorrect readback does not say "correction", which is for one's own errors, but "NEGATIVE" followed by the correct version.

Say again / I say again are mirror images. SAY AGAIN asks the other station to repeat all, or a named part, of its last transmission: "say again squawk", "say again all after QNH". I SAY AGAIN is the speaker repeating their own words for clarity or emphasis, as in "go around, I say again, go around". The word "repeat" is not used. WORDS TWICE has two uses: as a request it means communication is difficult, please send every word or group twice; as information it means that, since communication is difficult, every word or group in this message will be sent twice.

Ending and interrupting transmissions

Over / out: OVER means my transmission is ended and I expect a response; OUT means this exchange is ended and no response is expected. Neither is normally used on VHF, where the end of each transmission is obvious. BREAK separates portions of one message where the text would otherwise run together. BREAK BREAK separates messages to different aircraft sent in one transmission on a busy frequency: "...cleared ILS approach runway 26, break break, (call sign) squawk ident".

GO AHEAD and PASS YOUR MESSAGE invite the other station to transmit its message and grant nothing else, least of all a clearance to move. Three instructions about frequencies are also easily confused. CONTACT (unit) (frequency) means change and make an initial call; MONITOR (unit) (frequency) means change and listen, without calling; and STAND BY (frequency) FOR (unit) means wait on that frequency for the unit to call.

Cleared versus approved

Cleared vs. approved is a distinction exams test often. CLEARED means authorised to proceed under the conditions specified; it belongs to ATC clearances, such as a route, approach, take-off or landing clearance. APPROVED means permission for a proposed action has been granted, and answers requests such as "start-up approved" or "pushback approved". There is no combined "cleared for start and taxi": taxi is a separate request.

Each clearance covers only what it says. A route clearance does not permit entry onto a runway or take-off, and an approach clearance does not include the landing clearance. RECLEARED means a change has been made to the last clearance and the new clearance supersedes it or the part concerned; CANCEL annuls the clearance just given, as in "hold position, cancel, I say again, cancel take-off". MAINTAIN means continue in accordance with the specified condition and is never used in place of CLIMB or DESCEND when a level change is intended. In US airspace, radar contact and a squawk are not a clearance into Class B: the words "cleared into the Class Bravo airspace" are.

Urgency and discretion terms

Some words tell the pilot how quickly to act:

• EXPEDITE asks for the fastest safe execution: "expedite taxi" means taxi at the maximum normal speed, answered "expediting", and "expedite descent" asks for the best practicable rate.
• IMMEDIATELY marks an instruction to be acted on at once, such as avoiding action or "cleared for immediate take-off", which means lining up and taking off in one continuous movement. A pilot who cannot do so declines the clearance.
• WHEN READY gives the pilot the timing: "when ready descend FL80" is already a clearance to FL80, but the pilot chooses when to leave the present level.
• Own discretion appears in ground phrases such as "start-up at own discretion" and "pushback at own discretion".

Pilot's discretion is the FAA term for a climb or descent clearance whose start and rate the pilot chooses: the aircraft may level off at intermediate altitudes, but once it has vacated an altitude it may not return to it, and any crossing restriction in the same clearance still applies.

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Readback and hearback

A readback is the procedure in which the receiving station repeats a message, or the appropriate part of it, back to the sender to confirm that it was received correctly. Under SERA.8015 and Doc 4444 the flight crew reads back the safety-related parts of clearances and instructions:

• ATC route clearances;
• clearances and instructions to enter, land on, take off from, hold short of, cross, taxi on and backtrack on any runway;
• the runway in use, altimeter settings, SSR codes and newly assigned frequencies;
• level, heading and speed instructions;
• transition levels, whether passed by the controller or taken from the ATIS.

The readback ends with the aircraft's call sign, without which the controller cannot tell which aircraft accepted the clearance. ROGER and WILCO never replace it. HOLD, HOLD POSITION and HOLD SHORT OF are answered HOLDING or HOLDING SHORT, and a conditional clearance is read back with its condition, as in "behind the landing A320, lining up runway 24 behind, (call sign)". Information such as wind, traffic and RVR is only acknowledged, and a request to report something is answered WILCO. The FAA's AIM asks for readback of altitudes and altitude restrictions, vectors, runway assignments, hold-short instructions and, in general, any part of a clearance containing numbers.

Hearback is the controller's half of the loop: listening to the readback and correcting any discrepancy at once. It is the defence that remains when expectation bias makes a pilot read back, with complete confidence, the clearance expected rather than the one given. An uncorrected wrong readback, a hearback error, can go unnoticed until the aircraft deviates. The pilot's part is to say "say again" rather than guess, to query anything unexpected, and never to treat a readback as a substitute for listening.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.