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An aircraft’s call sign is "ABC 312" and another aircraft comes on frequency with the call sign ECHO 312. What may the controller do?

  • A

    One of the aircrafts may be instructed to change the type of call sign temporarily.

  • B

    Both aircraft must start to use their registrations as their call signs.

  • C

    No action is required as the first part of each call sign is different.

  • D

    One of the aircrafts may be asked to make up a new call sign.

ICAO Doc 4444
15.7.6 Change of radiotelephony call sign for aircraft

15.7.6.1 An ATC unit may instruct an aircraft to change its type of RTF call sign, in the interests of safety, when similarity between two or more aircraft RTF call signs are such that confusion is likely to occur.

15.7.6.1.1 Any such change to the type of call sign shall be temporary and shall be applicable only within the airspace(s) where the confusion is likely to occur.

15.7.6.2 To avoid confusion, the ATC unit should, if appropriate, identify the aircraft which will be instructed to change its call sign by referring to its position and/or level.

15.7.6.3 When an ATC unit changes the type of call sign of an aircraft, that unit shall ensure that the aircraft reverts to the call sign indicated by the flight plan when the aircraft is transferred to another ATC unit, except when the call sign change has been coordinated between the two ATC units concerned.

15.7.6.4 The appropriate ATC unit shall advise the aircraft concerned when it is to revert to the call sign indicated by the flight plan.

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