This is just for my information really. Occasionally, mid-flight, RC will give us a new squawk code to enter. Presumably this is like RL and I just wondered why it happened?Iain Smith

Essentially, Iain, it comes down to the fact that there are only 4096 codes available for all uses and a significant number of those are reserved for special purposes, further reducing the number available for general use - i.e. there are more aircraft flying around at any one time than there are codes for them to use.To help with this problem, ICAO has set up a scheme called the Originating Region Code Assignment Method (ORCAM) which divides the world (other than the US which does not participate in ORCAM as far as I know) into regions, each of which has its own allocated block of codes. There are so few codes and so many regions that codes have to be repeated in different regions but, wherever possible, adjacent regions have different codes for their own use. So, in theory, you can make a flight within a single region and, often, in 2 adjacent regions without having a squawk change but for longer distance flights which take in 3 or more regions, at least one squawk change may be necessary.There are occasionally other (usually ATC operational) reasons for a squawk change becoming necessary even for a flight within a single region but, in essence, it all comes back to my opening statement that there are more aircraft around than codes available under the present Modes A and C system. With introduction of Mode S, which has 16,777,214 individual 'addresses' as they're known, this problem will disappear.Pete

Essentially, Iain, it comes down to the fact that there are only 4096 codes available for all uses and a significant number of those are reserved for special purposes, further reducing the number available for general use - i.e. there are more aircraft flying around at any one time than there are codes for them to use.To help with this problem, ICAO has set up a scheme called the Originating Region Code Assignment Method (ORCAM) which divides the world (other than the US which does not participate in ORCAM as far as I know) into regions, each of which has its own allocated block of codes. There are so few codes and so many regions that codes have to be repeated in different regions but, wherever possible, adjacent regions have different codes for their own use. So, in theory, you can make a flight within a single region and, often, in 2 adjacent regions without having a squawk change but for longer distance flights which take in 3 or more regions, at least one squawk change may be necessary.There are occasionally other (usually ATC operational) reasons for a squawk change becoming necessary even for a flight within a single region but, in essence, it all comes back to my opening statement that there are more aircraft around than codes available under the present Modes A and C system. With introduction of Mode S, which has 16,777,214 individual 'addresses' as they're known, this problem will disappear.Pete

Thanks Pete for your excellent explanation. All clear now.Iain Smith