Originally Posted by
Gonzo
LHR has a ‘spare’ VHF for use if one of the others gets jammed/too much interference etc.
At one time all NATS airports had a 'spare' which coud be used for either ADC or APP/APS and yes it was seldom used but 'deemed' necssary in case of main failure or other reasons and likewise there was often a GMC frequency at many NATS and non NATS airports which was seldom used but if it was desgnated as GMC, the DOC was usually very small and it was not supposed to be used for aircraft in the air..