The ICAO Phonetic Alphabet and Numbers

Radio turns speech into a narrow, noisy channel, shared by pilots and controllers who speak many first languages. Letters such as C, D and V sound alike when spoken as letters, A can be heard as 8, and a misheard digit can turn a heading into a level or a squawk into a frequency. The ICAO phonetic alphabet, formally the radiotelephony spelling alphabet, and the ICAO rules for transmitting numbers close that gap: every letter has one code word, every digit one pronunciation, and every kind of number one way of being said.

The rules are set in ICAO Annex 10, Volume II, applied in Europe through SERA and illustrated in the ICAO Manual of Radiotelephony (Doc 9432); the FAA publishes its version in the AIM. They are tested in communications exams from the PPL to the ATPL. How to deliver a message is covered in radiotelephony transmitting technique, and the standard words in radiotelephony phraseology.

Why a phonetic alphabet

A spelling alphabet replaces each letter with a word that cannot be mistaken for another, even through distortion and by a listener working in a second language. The words were chosen so that the sound stays the same whatever the speaker's native language, which prevents confusions such as A with 8, or C with D and V.

ICAO's language section worked on the alphabet in 1948 and 1949 with Professor Jean-Paul Vinay of the Université de Montréal, and ICAO adopted it in Annex 10 on 1 November 1951. The words for C, M, N, U and X were then replaced, and the final version was implemented on 1 March 1956; it is still in use. It is also known as the NATO alphabet, and the same words are used well beyond aviation, for example to spell a name over the telephone. Older national alternatives, such as Jig, Queen or Beta, and improvised words are not used on the radio.

The ICAO spelling alphabet

Two official spellings catch people out in exams: Alfa, not Alpha, and Juliett, with two t's, pronounced JEW-LEE-ETT. A registration is passed in the spelling alphabet, so G-BJQX is "Golf Bravo Juliett Quebec X-ray". Other words are spelled only when needed: proper names such as a small private strip, service abbreviations and words of doubtful spelling are spoken and then spelled out. Common abbreviations such as ILS, VOR, NDB, QNH and QDR are said as letters without spelling, and some are spoken as words: ATIS, CAVOK (KAV-OH-KAY), NOTAM, SELCAL, SIGMET, SID, STAR and VOLMET among them.

Letters of the alphabet also name things. Each ATIS broadcast carries a code letter, and a new letter is used whenever the information changes, so a pilot reporting "information Romeo" tells the controller exactly which version is held. In the United States SIGMETs are named November through Yankee, leaving out Sierra, Tango and Zulu, which name the three G-AIRMETs.

The FAA uses the same alphabet. The AIM asks pilots to use it when identifying their aircraft on initial contact with ATC, as in "Columbia Ground, Cessna Three One Six Zero Foxtrot", and for single letters, groups of letters or difficult words when reception is poor; ATC may ask for phonetic letters when aircraft with similar-sounding call signs share a frequency (see radiotelephony call signs).

Pronouncing digits

ICAO also fixes the pronunciation of the digits, the RTF number pronunciation. Several are modified so that they stay distinct through a noisy, distorted signal:

NIN-ER, the most familiar of them, is usually explained as keeping 9 distinct from the German nein, "no", on international frequencies. The basic ICAO rule for the transmission of numbers is that every digit is spoken separately: call signs, headings, wind direction and speed, altimeter settings, transponder codes, runway designators and frequencies are all passed digit by digit. Grouped forms such as "heading seventy" or "twenty-five hundred" are not used, because mixing the conventions is how a heading turns into a level.

Altitudes, levels and headings

One exception covers the vertical and visibility figures. Numbers used for altitude, height, cloud height, visibility and runway visual range that contain whole hundreds or whole thousands are spoken with the words HUNDRED and THOUSAND: the digits of the thousands, then THOUSAND, then the hundreds and HUNDRED. Flight levels are spoken digit by digit, except whole hundreds.

Headings are always given as three digits and without the word "degrees", so a vector of 100 is "heading one zero zero", never "heading one hundred". The HUNDRED rule does not apply to headings, wind, squawks or runway designators. Altimeter settings follow the digit rule with one exception, 1000 hPa, which is "one thousand"; when the QNH is below 1000 hPa the unit is said with the value, and it belongs in the readback too.

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Frequencies and 8.33 kHz channels

Under ICAO procedures a frequency is spoken digit by digit with the word DECIMAL, pronounced DAY-SEE-MAL, never "point"; the FAA's usage differs (see below). Under the current ICAO standard it is passed as all six digits of its channel designator, except that when the fifth and sixth digits are both zero only the first four are spoken:

The reason is 8.33 kHz channel spacing. The basic ICAO spacing for VHF voice channels is 25 kHz, still used in most regions outside Europe. Dividing it by three gives 8.33 kHz and three times as many channels, which European airspace needed. Commission Implementing Regulation (EU) No 1079/2012 required aircraft radios to be 8.33 kHz capable from 1 January 2018, with some exemptions, and voice assignments to be converted to the narrower spacing. The emergency frequency 121.500 MHz and the search and rescue on-scene frequency 123.100 MHz stay 25 kHz channels, so that any radio can use them.

An 8.33 kHz channel needs three digits after the decimal point, and dropping one can select a different channel, so these designators are always spoken in full. A 25 kHz radio shows five digits, and its pilot sets the first five of the six given. A newly assigned frequency is a mandatory readback item, read back with the call sign.

The phraseology for the capability itself keeps the word POINT: ATC asks "CONFIRM EIGHT POINT THREE THREE", and the pilot answers "AFFIRM EIGHT POINT THREE THREE" or "NEGATIVE EIGHT POINT THREE THREE"; an aircraft exempted from the carriage requirement says "AFFIRM EIGHT POINT THREE THREE EXEMPTED". The radios themselves are described in VHF, HF and satellite communications.

Squawk codes, time and runway designators

Squawk codes are spoken digit by digit, with no HUNDRED or THOUSAND: 6500 is "six five zero zero". When winding in a new code the pilot avoids passing through 7500, 7600 or 7700, which could trigger a false alert (see transponder codes and squawk phraseology). Runway designators are also digit by digit, "runway two seven".

For the transmission of time, aeronautical communications use Coordinated Universal Time on the 24-hour clock, sometimes called Zulu. Normally only the minutes are transmitted, each digit separately, so passing a reporting point at 1447 is reported as "four seven" and 1352 as "five two", never "fifty-two". The hour is added whenever confusion is possible, for example close to the hour or for an estimate well ahead. A time check given by an ATS unit is to the nearest half minute.

FAA and ICAO differences

The FAA uses the ICAO alphabet and digit pronunciations, but the AIM differs in several places that exams test:

The AIM also gives airway numbers in group form, "Victor Twelve" for V12 and "J Five Thirty-Three" for J533. The quarter-minute rule has its own rounding: fractions of a quarter minute under 8 seconds go to the preceding quarter, 8 seconds or more to the next, so 0929:10 is "time, zero niner two niner and one-quarter". As in ICAO practice, the AIM allows time in minutes only when no misunderstanding is likely.

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.