I believe that a pilot using TCAS for separation is like a controller using the STCA for the same purpose. TCAS is a back-up safety tool that hopefully alerts us to an error in time to correct the situation.
There are strict reqirements for the mode C transmitted by transponders to be reguluarly checked. Have to check but I don't think that the UK LAMS requires full altimeter and encoder calibration at all.
The most interesting point about TCAS not being set up to check mode A is that if a controller finds mode C out of limits and the pilot can not turn off mode C, that flight is instructed to set code 0000.
Try explaining to a pilot receiving a RA from a aircraft dead ahead showing the same height that the aircraft is squalking 0000 and is 7000ft below the airway!
Perhaps we will eventually have a requirement for an annual calibration certificate to be given to all transponders and encoders
Regards,
DFC