Nature of ATC communications
ATC communications are voice only, that is controllers and pilots talk to each other at a distance, through radiotelephony communications. These communications are required to support coordination of aircraft movement in all phases of flight, to ensure aircraft separation, transmit advisories and clearances, and to provide aviation weather services.
It is of course important that radio equipment should be reliable and easy to use, and should be capable of conveying the spoken word clearly and without distortion over long distances. The process of pilot/controller communication is further complicated by environmental variables known as masking, clipping, and blocking/distortion.
My main aim in this post is to emphasize the importance of clear speech and good pronunciation in pilot/controller communications, and to explain some linguistic phenomena that can lead to misunderstanding and confusion.
In fluent speech words within a speech unit are usually said without a break. The sound at the end of one word is linked to the sound at the beginning of the next so that there is a smooth connection between them. For example:
a serious◡accident, far◡ away, pure◡oxygen, who◡ is it?, can you see◡it?
Languages modify difficult and complicated sequences in connected speech so as to simplify the articulation process. English tends to have distinguishable patterns according to which such modifications occur. In the multitude of phonemic processes, assimilation, coalescence, gradation, liaison (linking processes) and elision are phonemic variations which are more likely to happen in more rapid colloquial speech and can affect pilot/controller communication.
||ten miles||/’ten ‘maɪls/ > /’tem ‘maɪls/|
||I lost you||/aɪ ‘lɔstjʊ/ > /aɪ ‘lɔsʧʊ/|
||must||/mʌst/ > /mǝst, mǝs, mst, ms/|
||collision||/kə’lɪʒn/ > /’klɪʒn/|
Thus, flight deck becomes /flaɪ(t)dek/ or load sheet becomes /lǝʊ(d)ʃiːt/. Some pilots and controllers may find certain words difficult to enunciate, particularly when they are busy, e.g.: ‘Juliet Juliet Tango’ becomes ‘Jew Jew Tango’.
Apart from the fact that phonemic variations can cause misunderstandings in pilot/controller communications, the stress, the intonation, the speed of speaking and the placement and duration of pauses may affect the understanding of any communication, whether in abbreviated or plain language.
In standard RT phraseology there are a lot of abbreviations and acronyms. ICAO abbreviations are converted into the unabbreviated words or phrases (except for those which, in accordance with ICAO, need not be spelled out, e.g. ATC, ILS, QFE, QNH, RVR, etc.). In standard English two, three and four-letter abbreviations said as individual letters often have main stress on the last letter and secondary stress on the first (the ˌU’K, the ˌBB’C etc.). However, in ATC communications each letter should be stressed in order to avoid confusion.
Compared to many other languages, English has a high degree of differential stress, the application of which is determined by some fairly complex rules. It is a very difficult aspect of English for speakers of such languages as Spanish, French, Japanese or Serbian, where differential stress is much less marked.
Intonation is the ‘music’ of a sentence, the way the speaker makes his or her voice rise or fall or both, and is connected to the intended meaning or mood of the speaker. Pitch is important in English insofar as it combines stress to produce characteristic intonation patterns in order to make a statement, a question or an exclamation. Even an affirmative statement becomes a question if spoken with a rising intonation: e.g. You did? , The aircraft is taking off?
A simple, one-word exclamation – right! – can be understood as enthusiasm, resignation or sarcasm, depending on the intonation.
Stress on a particular word can radically alter the meaning of a sentence:
1. I’m LIStening.
|(What are you doing?)|
|2. I’M listening.||(Who is listening?)|
|3. I AM listening.||(Why aren’t you listening?)|
Homophony is also a linguistic phenomenon which can lead to confusion in pilot/controller communications. Homophones are words that sound the same but are spelt differently. For example: plane and plain, knot and not, there and their, wait and weight, brake and break, missed and mist, hear and here, right, write and rite, two, too and to, four and for etc.
It is recommended to avoid the use of a word in an instruction which could be misinterpreted as a digit (e.g. the word “to” could be confused with the digit “2”, or the word “for” with the digit “4”).The next example depicts the problem with the similar sounding of the number two and preposition to (Callback no. 126, 1989):
Controller clears the aircraft to descend “two four zero zero.”
Pilot reads clearance back as “OK. Four zero zero.
Aircraft descends to 400 feet rather than the appropriate altitude of 2,400 feet.
Controller: 2,400 ⇒Pilot: [To] 400
Good pronunciation means that people can understand what you say easily and that you do not create confusion.
Pronunciation of pilots and air traffic controllers needs to be sufficiently clear and assumes a dialect and/or accent intelligible to the aeronautical community. Pronunciation, stress, rhythm, and intonation are influenced by the first language or regional variation but only sometimes interfere with ease or understanding. (ICAO RATING SCALE, LEVEL 4: OPERATIONAL)
ICAO guidelines and techniques for radio transmission highlight the following objectives:
- Transmissions shall be conducted concisely in a normal conversational tone;
- Full use shall be made of standard phraseologies, whenever prescribed in ICAO documents and procedures; and,
- Speech-transmitting techniques shall be such that the highest possible intelligibility is incorporated in each transmission.
When talking over the radio, it is important to be familiar with the microphone operating techniques (particularly in maintaining a constant distance from the microphone).
It is extremely important to be clear and loud, and to enunciate, i.e. each word should be clear and distinct from the next one. Inadequate enunciation is the reason for unacknowledged instructions or requests for message repeats. A recommendation is to speak “staccato,” that is, to break the instruction up into its component words by inserting tiny pauses. It is particularly recommended to make a slight pause preceding and following numerals.
In ATC communications, it is important to speak at a moderate rate and to maintain an even rate of speech. The ICAO recommends not exceeding 100 words per minute. Speech rate should be adjusted to allow clearances etc. to be written down if necessary.
Speaking volume should be maintained at a constant level; High-pitched voices transmit better than low-pitched voices, and hesitation sounds such as er or um should be avoided.