Modern ATC phraseology (or radiotelephony) uses spelling alphabets (the best-known of which is the NATO Phonetic Alphabet), in which the letters of the English alphabet are arbitrarily assigned words and names in an acrophonic manner to avoid misunderstanding. Though often called “phonetic alphabets”, spelling alphabets have no connection to phonetic transcription systems like the International Phonetic Alphabet (IPA).
The ICAO (International Civil Aviation Organization) spelling alphabet was the product of extensive research to choose a set of words which would sound as different from each other as possible when spoken by people whose native language was not English over noisy and degraded communications channels (ICAO 1993).
The ICAO/NATO phonetic alphabet is a list of words used to identify letters in a message transmitted by radio or telephone. For example, the word “Tower” would be Tango-Oscar-Whiskey-Echo-Romeo when spelled in the phonetic alphabet. This practice helps to prevent confusion between similar sounding letters, such as “m” and “n”, or “b” and “d”, and to clarify communications that may be garbled during transmission.
For instance, the message “proceed to map grid DH98” could be transmitted as “proceed to map grid Delta-Hotel-Niner-Ait”.
The ICAO phonetic alphabet is used when communications conditions are such that the information cannot be readily received without their use. Pilots should use the phonetic alphabet when identifying their aircraft during initial contact with air traffic control facilities. ATC facilities may also request pilots to use phonetic letter equivalents when aircraft with similar sounding identifications are receiving communications on the same frequency. Additionally, the phonetic equivalents are used for single letters and to spell out groups of letters or difficult words during adverse communications conditions. For example, a pilot may say he is in Visual Meteorological Conditions, or VMC. But if the air traffic controller has trouble hearing him, he may say he is Victor Mike Charlie.
A careful study of the words will reveal that they are words which if spoken aloud are not easily confused with other words. The words are distinctive in their sounds, so that they can be given over the radio to another person without confusion.
The ICAO Phonetic alphabet is becoming a world standard but is not compulsory, so many nations can use their own versions and spelling variations occur within different languages. In most versions of the alphabet, the non-English spellings Alfa and Juliett are found. Alfa is spelled with an f as it is in most European languages. The English and French spelling alpha would not be properly pronounced by speakers of some other languages whose native speakers may not know that ph should be pronounced as f. Juliett is spelled with a tt for native French speakers because they may otherwise treat a single final t as silent.
The IPA version prescribed by ICAO usually has a non-rhotic accent (‘r’ pronounced only before a vowel), as in /tʃɑ:li/, /ʃɑ:li/, /no’vembə/ and /’junifɔ:m/. The IPA from ICAO form of Golf implies it is pronounced /ɡʌlf/ which perhaps occurs in some English dialects, but not in either British RP or General American English. Furthermore, a nasal and velar phoneme/ŋ/ in the IPA forms of Tango and Yankee is shown as a nasal and alveolar phoneme /n/.
In the NATO/ICAO scheme it is not only the letters that are supposed to be pronounced in specified ways, but also the numbers. The two special pronunciations are fife /faɪf/ for ‘five’ and niner /naɪner/ for ‘nine’ since ‘five’ and ‘nine’ can sound the same over the radio. The numbers three /θri:/ and thousand /θaʊznd/ are pronounced as tree /tri:/ and tousand /’tauznd/, i.e. dental and fricative /θ/ is replaced by alveolar and plosive /t/. /θ/ is a ‘marked sound’, that is to say, it is relatively rare in the languages of the world. It seems to be a natural tendency for languages to tend to move away from marked sounds, changing them into less marked ones. /θ/ is pronounced in different ways even by native speakers of English, and it is difficult to pronounce for many foreign speakers who often mistake it for /s/ or even /t/.
When transmitting messages containing call signs, altimeter settings, flight levels, altitudes, wind velocity, heading, frequencies etc. each letter and digit should be pronounced clearly and accurately in order to avoid misunderstanding or confusion. All numbers should be transmitted by pronouncing each digit separately.
Numbers used in the transmission of altitude, cloud height, visibility and runway visual range (RVR) information, which contain whole hundreds and whole thousands, should be transmitted by pronouncing each digit in the number of hundreds or thousands followed by the word HUNDRED or THOUSAND as appropriate.
I would like to point out that some aspects of ATC phraseology should be improved, especially the imperfections of the ICAO/NATO phonetic alphabet which can cause misunderstandings and confusion in pilot/controller communications.
In order to eliminate wide variations in pronunciation, the ICAO prescribes a kind of IPA pronunciation, but, many versions freely available on the Internet labelled (IPA) are not officially sanctioned and may contain errors.
The ICAO, NATO, and FAA use the common English number words (with stress), but not always pronounced the same. Also, the pronunciation of the words in the alphabet as well as numbers may vary according to the language habits of the speakers.
The pronunciation of numbers is indicated by respellings in order to avoid any confusion, but the respelling of some numbers (e.g. wun, too, ait) is not always clear, and we are not sure if they are pronounced as is usually the case: /wʌn/, /tu:/ and /eɪt/. The most confusing is fower for ‘four’ and it could be pronounced as /fǝʊǝ/ or /faʊǝ/. On the Audio CD produced by Oxford University Press 2008 (Aviation English for Pilots and Air Traffic Controllers – Sue Ellis & Terence Gerighty) the number four is pronounced /faʊǝ/ only when its pronunciation is introduced in isolation, but then speakers go on to pronounce four in the usual way /fɔː/ in examples of actual transmissions.
Phonetic transcription was originally devised to remove ambiguities that conventional spelling systems could not cope with. Thus, the IPA alphabet may be a good solution, in my opinion, and ICAO should ask a phonetician to advise on enhancing the ICAO phonetic transcription and making respellings unambiguous.