Callsigns and setting up communication
A radio exchange begins with positive identification. Use the correct station and aircraft callsigns, establish the link, keep listening, and close or transfer the exchange without leaving either party uncertain.
Aeronautical station callsigns
An aeronautical station callsign normally combines the geographical location with a suffix identifying the service. The suffix tells the pilot which function is being contacted.
| Service | Callsign suffix |
|---|---|
| Area control | CONTROL |
| Approach control | APPROACH |
| Radar service | RADAR |
| Aerodrome control | TOWER |
| Surface movement control | GROUND |
| Clearance delivery | DELIVERY |
| Flight information | INFORMATION |
| Aeronautical station | RADIO |
| Apron control | APRON |
| Arrival director | DIRECTOR |
| Precision approach radar | PRECISION |
The three aircraft callsign types
| Type | Full form | Permitted abbreviated form |
|---|---|---|
| 1. Registration | Registration characters, optionally preceded by the aircraft manufacturer or type. | The first character and at least the last two characters, such as VTCDE becoming VDE. |
| 2. Operator plus registration | Operator radiotelephony designator followed by the last four characters of the registration. | The operator designator and at least the last two registration characters. |
| 3. Operator plus flight identification | Operator radiotelephony designator followed by the flight number or flight identification. | Never abbreviated. |
Use the full callsign on first contact. An aircraft may use an abbreviated callsign only after satisfactory communication is established, only when confusion is unlikely, and only after the ground station has first used the abbreviation. Return to the full callsign whenever similar callsigns could be confused or the station requests it. A temporary callsign change may be directed to remove ambiguity.
Call, reply, maintain and close
On the first call, say the station called and then the full aircraft callsign. The reply identifies the aircraft and the station answering. A general call is addressed to ALL STATIONS; a reply to a general call is made only when a reply is requested or operationally necessary. If the identity of the calling station is uncertain, use STATION CALLING followed by your own station callsign and SAY AGAIN YOUR CALLSIGN.
Listen before transmitting, maintain a continuous watch on the assigned frequency, answer promptly, and use the callsign often enough to keep the exchange identifiable. OVER means a reply is expected. OUT means the exchange is finished and no reply is expected. They are not used together. On transfer, acknowledge the instruction and establish contact with the next unit using both callsigns.
Message priority and transmitting technique
The radio channel is shared. Priority decides who must be heard first, while disciplined microphone technique makes every second of transmission useful.
Order of priority
| Priority | Message category | Purpose |
|---|---|---|
| 1 | Distress | Immediate grave danger requiring immediate assistance, normally introduced by MAYDAY. |
| 2 | Urgency | A serious condition concerning safety that does not yet require immediate assistance, normally introduced by PAN PAN. |
| 3 | Direction finding | Requests and replies used to obtain a bearing, heading or position by radio direction finding. |
| 4 | Flight safety | ATC clearances, instructions, position reports and other messages directly affecting safe flight. |
| 5 | Meteorological | Weather observations, forecasts and reports required for flight operations. |
| 6 | Flight regularity | Messages concerning safe and efficient operation, servicing, schedules and aircraft movement. |
| 7 | Other | Permitted operational traffic that does not belong to a higher category. |
A lower-priority transmission must not interrupt a higher-priority exchange. BREAK BREAK separates messages transmitted to different aircraft in a very busy environment; BREAK separates parts of one message.
How to transmit
Before pressing the push-to-talk switch, listen to avoid blocking another call and prepare the message. Hold the microphone at the correct distance, use a normal conversational tone, and speak clearly at about 100 words per minute. Keep the volume and rhythm even. Pause briefly before speaking so the first syllable is not clipped, and pause after the final word before releasing the switch. Avoid hesitation sounds, unnecessary words and turning the head away from a boom microphone.
Long or complex messages should be divided into natural phrases. Numbers and critical instructions may be spoken more slowly. If the receiving station requests SPEAK SLOWER or WORDS TWICE, adjust the delivery accordingly.
Letters, numbers, frequency and time
Prescribed spelling words and number sounds prevent similar characters from being confused on a noisy or weak channel.
ICAO spelling alphabet
| Letter | Word and pronunciation | Letter | Word and pronunciation |
|---|---|---|---|
| A | ALFA, AL-FAH | N | NOVEMBER, NO-VEM-BER |
| B | BRAVO, BRAH-VOH | O | OSCAR, OSS-CAH |
| C | CHARLIE, CHAR-LEE | P | PAPA, PAH-PAH |
| D | DELTA, DELL-TAH | Q | QUEBEC, KEH-BECK |
| E | ECHO, ECK-OH | R | ROMEO, ROW-ME-OH |
| F | FOXTROT, FOKS-TROT | S | SIERRA, SEE-AIR-RAH |
| G | GOLF, GOLF | T | TANGO, TANG-GO |
| H | HOTEL, HOH-TELL | U | UNIFORM, YOU-NEE-FORM |
| I | INDIA, IN-DEE-AH | V | VICTOR, VIK-TAH |
| J | JULIETT, JEW-LEE-ETT | W | WHISKEY, WISS-KEY |
| K | KILO, KEY-LOH | X | X-RAY, ECKS-RAY |
| L | LIMA, LEE-MAH | Y | YANKEE, YANG-KEY |
| M | MIKE, MIKE | Z | ZULU, ZOO-LOO |
Number pronunciation
| Character | Pronunciation | Character | Pronunciation |
|---|---|---|---|
| 0 | ZE-RO | 5 | FIFE |
| 1 | WUN | 6 | SIX |
| 2 | TOO | 7 | SEV-en |
| 3 | TREE | 8 | AIT |
| 4 | FOW-er | 9 | NIN-er |
| . | DAY-SEE-MAL | 100 / 1,000 | HUN-dred / TOU-SAND |
How operational numbers are sent
| Item | Rule | Example |
|---|---|---|
| Aircraft callsign, flight level, heading, wind, transponder code and runway | Transmit each digit separately. Headings use three digits. | Heading 110 is WUN WUN ZE-RO. FL85 is FLIGHT LEVEL AIT FIFE. |
| Altitude, height and cloud height | Use HUNDRED and THOUSAND for whole hundreds and thousands. | 2,500 feet is TOO TOU-SAND FIFE HUN-dred feet. |
| Visibility and RVR | Use whole hundreds or thousands as appropriate and include the unit or RVR context. | RVR 600 metres is RVR SIX HUN-dred metres. |
| Altimeter setting | Transmit the QNH or QFE digits separately. | QNH 1008 is QNH WUN ZE-RO ZE-RO AIT. |
| Frequency | Say DECIMAL between the whole and fractional parts. State enough digits to identify the assigned 25 kHz or 8.33 kHz channel unambiguously. | 118.125 is WUN WUN AIT DAY-SEE-MAL WUN TOO FIFE. |
For a 25 kHz channel, the assigned frequency may be stated with five or six figures according to whether the final digit is zero. With 8.33 kHz channel spacing, all six figures are used so the channel is not mistaken. Do not shorten an assignment if that could identify a different channel.
Transmission of time
Use Coordinated Universal Time. Normally transmit only the minutes past the hour when there is no possibility of confusion. Give all four figures when the hour must be clear, such as TIME ZERO NINE TWO ZERO. A time check is given to the nearest half minute. If a date boundary matters, distinguish the beginning and end of the UTC day clearly.
Standard words and phrases
Standard words replace conversational language with one recognised operational meaning. Learn the meaning, not merely the sound.
| Word or phrase | Exact operational meaning |
|---|---|
| ACKNOWLEDGE | Let me know that you have received and understood this message. |
| AFFIRM | Yes. |
| APPROVED | Permission for the proposed action is granted. |
| BREAK | Indicates separation between portions of a message. |
| BREAK BREAK | Indicates separation between messages transmitted to different aircraft in a very busy environment. |
| CANCEL | Annul the previously transmitted clearance. |
| CHECK | Examine a system or procedure. No answer is normally expected. |
| CLEARED | Authorised to proceed under the conditions specified. |
| CONFIRM | Have I correctly received, or did you correctly receive, the following? |
| CONTACT | Establish radio communication with the stated station. |
| CORRECTION | An error was made in this transmission. The correct version follows. |
| DISREGARD | Ignore the previous transmission. |
| HOW DO YOU READ | What is the readability of my transmission? |
| I SAY AGAIN | I repeat for clarity or emphasis. |
| MONITOR | Listen out on the stated frequency. |
| NEGATIVE | No, permission not granted, that is not correct, or not capable. |
| OUT | This exchange is ended and no response is expected. |
| OVER | My transmission is ended and I expect a response. |
| READ BACK | Repeat all, or the specified part, exactly as received. |
| RECLEARED | Your previous clearance is changed, and this new clearance replaces it or the stated part. |
| REPORT | Pass me the specified information. |
| REQUEST | I should like to know, or I wish to obtain. |
| ROGER | I have received all of your last transmission. |
| SAY AGAIN | Repeat all, or the specified part, of your last transmission. |
| SPEAK SLOWER | Reduce your rate of speech. |
| STANDBY | Wait and I will call you. |
| UNABLE | I cannot comply with your request, instruction or clearance. |
| WILCO | I understand your message and will comply with it. |
| WORDS TWICE | Communication is difficult, so every word or group of words is to be spoken twice. |
Read-back, radio checks and transfer
The final safety barrier in voice communication is the hear-back loop: the pilot reads back critical information, the controller listens, and any error is corrected before action.
Items read back in full
| Category | Read-back requirement |
|---|---|
| Route clearance | ATC route clearances, including the clearance limit, route and assigned level. |
| Runway | Runway in use and clearances or instructions to enter, land on, take off from, backtrack, cross or hold short of any runway. |
| Surveillance | SSR transponder codes. |
| Pressure | Altimeter settings, including QNH or QFE. |
| Flight path | Level, heading and speed instructions. |
| Communication | Frequency changes when instructed. |
Read back the complete safety-critical content and finish with the aircraft callsign. The controller must listen to the read-back and correct a discrepancy. ROGER or WILCO cannot replace a required read-back.
Radio contact and test transmissions
A test call contains the station called, the aircraft callsign, the words RADIO CHECK, and the frequency being used. The reply identifies the station and aircraft and reports readability. Keep a test transmission short enough to avoid blocking operational traffic.
| Scale | Readability |
|---|---|
| 1 | Unreadable. |
| 2 | Readable now and then. |
| 3 | Readable but with difficulty. |
| 4 | Readable. |
| 5 | Perfectly readable. |
Corrections and repetitions
If you make an error, say CORRECTION, repeat the last correct word or group, then give the corrected version. If the best repair is a complete restart, say CORRECTION, I SAY AGAIN and repeat the whole message. To request repetition use SAY AGAIN. Narrow the request with SAY AGAIN ALL BEFORE, SAY AGAIN ALL AFTER, SAY AGAIN WORD BEFORE, or SAY AGAIN WORD AFTER, followed by the relevant word or phrase.
Frequency change and communication transfer
CONTACT instructs the aircraft to establish two-way communication on the named frequency. MONITOR instructs it to listen for calls or broadcasts on that frequency. Read back the assigned frequency, change at the instructed point or time, listen first, then call the new station with its callsign and the full aircraft callsign. If contact fails, check the selection and volume, return to the previous frequency when appropriate, and report the failure. Close the old exchange only when transfer is clear.