Military / Non-Standard Codes and Callsigns
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According to the Ministry of Defense AIP (MIAM), non-standard callsigns used by French military aircraft should adhere to a standard Mode S coding system of the first letter of the callsign followed by the last three letters. The example given in the MIAM being callsign COTTON abbreviated to CTON. The reality appears to be that this convention is not strictly followed.
Additionally, French military aircraft frequently utilise an alphabetic callsign system sequence, instead of the standard numerical i.e. the callsign followed by a single letter. Two commonly seen examples are the callsigns XENON and ULYSSE. These would normally be expected to be XENON01 or ULYSSE03 however, aircraft are instead XENONA and ULYSSEC.
Further information is required for both why this is the case and what the significance of the choice of letter is. The most obvious explanations would be mission type, flight priority or the number of that flight in the day.
RAF Air Experience / University Air Squadrons use a Uniform callsign. The first letter is always Uniform followed by 2 letters unique to the unit. On first contact with an air traffic authority the aircraft will use the full phonetic call sign. In subsequent transmissions, unless instructed by the controller, the aircraft will revert to a Uniform only callsign.
13 Air Experience Flight is assigned UAI.
- Flight ID – UAI27
- First contact callsign – Uniform Alpha India Two Seven
- Subsequent callsign – Uniform Two Seven
United States ADS-C (MC) Callsigns
Information broadcast via ADS-C instead of ADS-B can be displayed with an IATA code instead of an ICAO or non standard code. For some reason, the IATA code for the USAFs Air Mobility Command, MC, is used for all US military aircraft on ADS-C. That’s Air Force, Navy, Army and others.
It appears that if the ADS-C code (IATA) is replacing an official ICAO code, it will do so completely. Meaning RCH177 will become MC177 and CNV4636 will become MC4636. Unfortunately it has been noted that some or all of the callsign numbers may be changed i.e. the previous example of RCH177 may become MC177, MC17, MC77 or something completely different.
Additionally, if the ADS-C code replaces a non-standard code the last 2 letters of that code will remain leaving a 4 letter code. Meaning MDUSA82 becomes MCSA82. The one exception to this rule appears to be Reach flights. As mentioned above they will generally convert to MC only. Occasionally it appears the system thinks the RCH is non-standard and it gets converted to MCH. Using the example of RCH177 one last time in this case it becomes MCH177.
This issue of “satellite callsigns” (referring how ADS-C functions differently to ADS-B) can cause a lot of confusion. Where possible we are correlating these MC callsigns with known callsigns, but the dynamic/haphazard nature of them makes this tricky. We are definitely grateful for this excellent post on the FighterControl forums which first steered our understanding of them.