I have long wondered why, in the UK at least, abbreviated call-signs (on the radio) use the "G-" nationality identifier as the first letter.
Example: G-ABCD becomes G-CD
Practically every GA aircraft starts with G and as a result, call-signs do not really stand out.
Not too long ago, there were three G-CDs (figuratively - me being one of them) on frequency and while the controller and (worryingly) at least one of the other pilots of a G-CD seemed oblivious, I found it very confusing.
This was not the first occasion that such a situation has occurred to me and when it does happen, in most cases the controllers do not announce that multiple aircraft with similar call-signs are on frequency. When ATC does notice, the affected aircraft revert back to full call-signs which is a verbose solution to the problem.
I was thinking of scouring the register for an unused or lesser used combination and asking for a change of reg. ;-)
There are almost 200 aircraft that have the same abbreviated call-sign as mine. And if you actually fly a real G-CD, the situation is even worse... there are almost 800! To make a point, I might organise a fly in for all G-CDs. Hee hee.
If, instead of the nationality identifier, an additional letter of the registration mark was used (G-ABCD now becomes 'BCD), this would, in all probability, significantly reduce the likelihood of multiple aircraft with the same abbreviated call-sign appearing on a given frequency at once.
The only advantage that I can see with the current standard is that foreign registered aircraft call-signs really stand out, for example, N123A becomes N3A - very easy to hear in a sea of G-this and G-that. As this is to the disadvantage of the majority, I find it hard to believe that this could be the reason for doing it this way.
Why do we create problems like this in the UK? There must be a reason.
Is it the same elsewhere? I am almost certain that it's not in FAA-land, but what about Europe? I hope to find out for myself later this year.
In Australia it is the last three letters....in fact domestically you never include the VH country code...And since registrations are VH-xxx all call signs are unique
Surely this is ICAO regulated?
In my limited experience of flying in the USA, the international airports (e.g. Phoenix) used the full callsign, whereas the little ones (e.g. Chandler Municipal) used just the bit after the "N".
In the UK, G-reg, I have no idea why they do what they do, given that some combinations of the last 2 letters are a lot more popular than the others. For example I am ...AC and that's true for both the original G-reg and the later N-reg, and there is often another "AC" on the frequency.
But I would disagree that "Practically every GA aircraft starts with G" because the bulk of the IFR capable community is N-reg.
When there are two G aircraft with the same last two letters on freqency, I've been told to use complete callsign. Stated radio procedure is that initial call is GXXX, and only shortened to GXX by ATC.
When flying I the US I notice that Canadian aircraft are referred to by their last three letters...not C-xx
In the US, the full callsign (minus the N - this is never spoken) is used at initial contact, no matter size of airport or CAS. Further calls then use the last three. However, FAA controllers will always alert everyone to potential callsign conflicts. eg. xx33B / 33C or similar.
The US typically use the aircraft type and the last few letters/numbers of the callsign, i.e. N1234Y becomes Cessna 34Y. I like that, it's useful information for others. Well, not in some places where 90% are Cessnas 
achimha, correct - partly. This is mainly used at GA airfields, in CAS or busy terminal environments the controllers don't use it. Exception: when there is a potential conflict (as in my earlier post), they sometimes add the a/c type, if, indeed the two are different.
In the US, we are expected to use the full N-number, with or without the N. Once it is acknowledged by ATC we are expected to use the last three. ATC usually uses the aircraft type and last three after the initial communication. They will notify the pilot if there is a similar sounding identifier in the airspace and in some cases will request "use full call sign".
A typical communication exchange will go something like this:
Me: Charlotte, Bonanza 7083N, six thousand.
ATC: Bonanza 7083N, radar contact, altimeter 29.92.
Me: 83N
...
ATC: Bonanza 83N, turn right 200, vectors for Rock Hill.
Me: 200 heading, 83N.
A real pet peeve of ATC is when a pilot acknowledges a call but does not use their identifier. ATC will often repeat the communication. So the following is an example of bad form:
Me: Bonanza 7083N, level 6000 feet, with you (note: they hate "with you" as well as it serves no purpose)
ATC: Bonanza 7083N, radar contact, Charlotte altimeter 29.92, turn right heading 200, vectors for Rock Hill
Me: Roger
ATC: Bonanza 7083N turn right heading 200
Me: Wilco
ATC: Bonanza 7083N you need to acknowledge your transmissions with your N number
Me: OK
ATC: 7083N, I understand you are cancelling IFR
Me: Negative, turning right to 200, 83N
