ICAO Annex 11 Appendix 1
4. Use of designators in communications
4.1 In printed communications, the designator shall be expressed at all times by not less than two and not more than
six characters.
4.2 In voice communications, the basic letter of a designator shall be spoken in accordance with the ICAO spelling
alphabet.
4.3 Where the prefixes K, U or S specified in 2.3 are used, they shall, in voice communications, be spoken as follows:
K — KOPTER
U — UPPER
S — SUPERSONIC
The word “kopter” shall be pronounced as in the word “helicopter” and the words “upper” and “supersonic” as in the English
language.
4.4 Where the letters “F” or “G” specified in 2.4 are used, the flight crew should not be required to use them in voice
communications.
So they should be spoken as Upper and not Uniform