Actually, the e-mail is
[email protected] <G>... But with that.....
Mode S is only required due to TCAS. If you are a commercial aircraft with 30 ore more pax ( I believe that is still the limit) then you must have TCAS on board. The more pax that you have the more advanced the TCAS system.
If you have a TCAS on board for your own feeling of bliss, but aren't required by regulation to have it, you will still have a mode S transponder so that it can talk to other TCAS units.
As to running out of codes. It happens from time to time, however, we don't really ever run out of codes for active aircraft. What we run out of codes for are those who are waiting to come into our airspace or those on the ground who haven't recieved clearances yet. What the causes, is the controller will not be able to allow an aircraft to use the same code that he / she was using when they were in the last facility due to someone already using it in your facility. You then assign another code. The problems that you will run into on the ground is that you may not have codes available to you until just a few minutes prior to departure, vrs. 30 min. to an hour.
regards