Frequency Monitor Codes (FMCs) have now been in use in the UK for over 10 years and they’ve collected a range of names during that time such as Listening Squawks and Monitor Codes but they are all actually referring to the same thing.

Pilots are encouraged to use a FMC when they are flying outside controlled airspace, but close to controlled airspace boundaries, in order to increase situational awareness and help to combat infringements.