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I heard a pilot replied "CC" to ATC instruction.

I can't remember the exact situation, but does anyone knows what it means?

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    $\begingroup$ Can you clarify did you hear "CC" or "Charlie Charlie" $\endgroup$ – RoyC Feb 1 '18 at 22:05
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Short answer

This wording comes from the common meaning of letter C to be Correct/Yes, early standardized and used at sea since 1857. Under the Commercial Code of Signals (1857-1902), the C flag, in addition of representing the letter itself, had the meaning of Yes/Affirmative. This code has been improved and extended into the International Code of Signals (ICS). While the visual representation changed, the meaning was preserved.

enter image description here
Today's version of the C flag (previous version)

From the 2003 ICS:

Signals for flags, radiotelephony, and radiotelegraphy transmissions.
C: Affirmative—YES or “The significance of the previous group should be read in the affirmative”.

The pilots you heard were saying affirmative.


Longer answer

Codes have their origin in the need to abbreviate everything at a time the communication channels were limited, as for nautical flags, Chappe, Morse, Aldis, etc. Nineteenth and early twentieth centuries saw appearance of codes (flags, lights, telegraphy, Q, X and Z) in order to federate many isolated practices, in particular in commercial telegraphy (Marconi) and railroads. Codes have many resemblances as they have borrowed good practices and common usages from each others.

In radio communications, letter C has been early used in consistence with the ICS, to mean correct, like when acknowledging a readback with a single C, meaning this is a correct readback.


Example

Discovered by @ymb1, this accident report includes a transcription of the pilot (CA) controller (MA) exchange:

  • CA: Roger, we will appreciate it if you can alert, er, fire, er, er, er.
  • MA: Do you request a full emergency please? A full emergency?
  • CA: Affirmative, that's Charlie Charlie
  • MA: Roger, I declare a full emergency.

later:

  • MA: Confirm runway one four?
  • CA: Charlie Charlie.

And this other document, still from @ymb1:

  • Do you have ATIS information Delta?
  • Charlie Charlie we have Delta.

...

  • Ok our stand 41 AF028.
  • Understand B 41.
  • Charlie, charlie.

Details

Charlie-Charlie is a fancy substitution for a standard affirmative and accepted non-standard words roger and wilco.

Standard phraseology is meant to prevent confusion, and has been improved after incidents/accidents where the communication between parties played a role. In a conversation listed above, we can imagine how information can be easily misunderstood:

  • Do you have ATIS information Delta?
  • Charlie Charlie we have Delta.

ATCO are not required to know the ICS, though in many countries, an important part of civil aviation ATCO have had a military career, and are likely to know the meaning of letter C in many codes.

More

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    $\begingroup$ Formally, "roger," "wilco," and "affirm/affirmative" are not synonyms. "Roger" means "I understand," "wilco" means "I will comply," and "affirmative" means "yes." Usage does differ from the formal meaning, however. $\endgroup$ – Wayne Conrad Feb 1 '18 at 14:50
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    $\begingroup$ Is this some kind of elaborately layered pun? Charlie -> C -> "C" is pronounced Si (spanish for yes) -> yes $\endgroup$ – BlackThorn Feb 1 '18 at 18:51
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    $\begingroup$ @BlackThorn: Yes, mnemonic, also No-vember. I'm a little unsure whether "I require medical assistance" being Whisky is a joke or based on reality :-) $\endgroup$ – mins Feb 1 '18 at 19:15
  • $\begingroup$ @WayneConrad: does "affirmative" mean "I understand", "I will comply" or what? $\endgroup$ – smci Feb 1 '18 at 21:21
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    $\begingroup$ @smci "Affirmative" is "yes" in response to a yes/no question. "Negative" is "no". $\endgroup$ – ChrisDevo Feb 1 '18 at 21:38
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CC is short for Click Click. In situations where a military operator was busy or where talking could prove fatal an incoming message would be acknowledged by a double click of the transmit switch on the radio producing a "click click" at the other end.

"CC" somehow got into informal usage rather than the more precise roger wilco or affirmative. In this context it would probably be used as an informal acknowledgement of a purely informational message.

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  • $\begingroup$ Second that, I remember being taught this method of "silent affirmation of receipt" in the US Marines FROC (field radio operators course). $\endgroup$ – Mike Devenney Feb 1 '18 at 19:41
  • $\begingroup$ Could you clarify what you mean when you say CC is short for Click-Click? Is Click-Click a voice code at the origin or the noise of the PTT? Because if Click-Click is a voice code, then why would it change to Charlie-Charlie, and if it isn't (just the noise) then what is CC the "short" for? $\endgroup$ – mins Feb 1 '18 at 21:59
  • $\begingroup$ The OP's question was about CC not Charlie Charlie. $\endgroup$ – RoyC Feb 1 '18 at 22:02
  • $\begingroup$ @RoyC: "The OP's question was about CC not Charlie Charlie": Hum, seems the question is "What does “CC (Charlie Charlie)” mean" and anyway how do you use "CC"? You say "CC" on the radio? or "Click-Click"? $\endgroup$ – mins Feb 1 '18 at 22:03
  • $\begingroup$ @mins Added quotes to clarify. $\endgroup$ – RoyC Feb 1 '18 at 22:17

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