Why does the ICAO alphabet use "Charlie" for C? Specifically, why choose "Charlie", which has a "Ch" sound, rather than a word with the hard "C" such as "Carl"?
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ICAO is a phonetic alphabet, so it's all about sounds. English language doesn't have a distinct sound for singular letter "C". You're proposing "Carl", but it's pronounced kɑɹl̩ - with K. Other option would be "cent", but this one is pronounced sɛnt - with S.
"Ch" as in Charlie (ˈtʃɑːli) is the only C that sounds (tʃ) distinctively and can be recognized as "C" without any doubt. The fact that's not a singular C but a part of digraph is not relevant. The clarity of the message is.
Hard C sounds too much like K. Ch (Charlie) will not be confused with K (Kilo). And soft C sounds too much like S (Sierra).
For the same reason we use niner for nine. The phonetic alphabet took a lot into consideration when they were choosing words including how words are pronounced with different accents. "Charlie" like all the other words was likely chosen due to its unique pronunciation across dialects. It is also a nice short two syllable word.