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The aviation phonetic alphabet

How pilots and controllers spell, count and talk — so that nothing is misheard.

Aviation radio is a hostile place for language. Transmissions are short, the channel is noisy, two stations can talk over each other, and the people speaking come from every country on earth. A single misheard letter or digit can send an aircraft to the wrong fix or the wrong level. The answer is a deliberately rigid way of speaking: a fixed alphabet, fixed pronunciations for numbers, and a small set of standard words that mean exactly one thing. The current ICAO spelling alphabet came into use in 1956, and it is the same one heard in every control room today.

The alphabet

Each letter has one code word, chosen to sound unlike every other word in the set — even through static, and even for speakers whose first language is not English. Note the ICAO spellings Alfa and Juliett: written that way so that French and other speakers read them with the right sound.

You hear the alphabet constantly: in registrations ("Golf Echo Zulu Alfa Bravo"), in taxiway names ("taxi via Alfa, hold short of runway two seven"), in the ATIS letter ("information Kilo"), and whenever a short navaid or fix name has to be spelled out.

Numbers: tree, fife and niner

Digits get the same treatment. Three of them are pronounced unusually on purpose: tree for 3 (many speakers struggle with "th"), fife for 5 (so it cannot blur into "nine") and niner for 9 (so it cannot blur into "five" — or into the German nein).

The general rule is to say numbers digit by digit. A few kinds of number have their own pattern:

• Headings always use three digits: 090° is "heading zero niner zero".
• Flight levels are digit by digit: FL350 is "flight level three five zero". Some services say the round hundreds as words — FL100 as "flight level one hundred".
• Altitudes in feet use thousands and hundreds: 4,000 ft is "four thousand", 2,500 ft is "two thousand five hundred", 11,000 ft is "one one thousand".
• Speeds: 250 kt is "two five zero knots".
• Frequencies use "decimal": 121.5 is "one two one decimal five".
• Squawk codes and times are digit by digit: "squawk four five two one", "one two three zero" (always UTC — "Zulu").
• Runways: 27L is "runway two seven left".

Callsigns

An airline flight is called by its telephony designator followed by the flight number, digit by digit: BAW117 is "Speedbird one one seven", UAE201 is "Emirates two zero one", SVA1021 is "Saudi one zero two one". The telephony word is often nothing like the airline's name, which is why controllers learn them by heart. A private or general-aviation aircraft uses its registration spelled phonetically, and once contact is established the controller may shorten it to the first letter and the last two — G-ABCD becomes "Golf Charlie Delta". Heavy and A380 flights add "Heavy" or "Super" on first contact, a reminder of the extra wake turbulence they leave behind.

Standard words

Everyday English is full of words that sound alike or mean several things. Radiotelephony replaces them with a short list of standard words, each with one meaning:

ROGER

"I have received all of your last transmission." It does not mean yes, and it is not a readback.

WILCO

"I understand your message and will comply with it."

AFFIRM

Yes. ("Affirmative" is avoided: clipped by a bad transmission, it can sound like "negative".)

NEGATIVE

No, or "permission not granted", or "that is not correct".

SAY AGAIN

Repeat all (or the named part) of your last transmission.

STANDBY

Wait and I will call you. It is not an approval or a refusal.

UNABLE

I cannot comply with your request, instruction or clearance.

CORRECTION

An error has been made; the correct version is…

READ BACK

Repeat all, or the specified part, of this message back to me exactly as received.

MAYDAY / PAN PAN

Distress (grave, imminent danger) and urgency calls — see the emergency squawk codes.

A typical exchange

Put it together and an approach instruction and its readback sound like this:

ATC:    Speedbird one one seven, turn left heading two seven zero,
        descend flight level eight zero.
Pilot:  Left heading two seven zero, descend flight level eight zero,
        Speedbird one one seven.

Notice the pattern: the controller starts with the callsign; the pilot reads back the safety-critical items — headings, levels, speeds, clearances — and ends with the callsign. If the readback is wrong, the controller corrects it at once. That loop of instruction, readback and check is the single most important safety habit on the frequency.

Hear it in the simulator

The ATC Radar Simulator speaks this phraseology aloud: every instruction you give is voiced as a controller transmission and read back by the pilot, with headings and levels digit by digit, "niner" for nine, airline telephony callsigns, and short fix names spelled with the phonetic alphabet. It is a painless way to get the rhythm into your ear. For the meaning of the terms you hear, keep the ATC glossary open alongside.