I had trouble understanding the UK AIC 105/2004, at least so far as it applied to me. I phoned the CAA as follows:
I said: 1. I fly a C172 IFR and VRF (aircraft less than 5,700kg and max cruise less than 250kt); 2. I sometimes fly in the LTMA but for less than 30 hrs per year (I do fly in "major TMA and en-route airspace"); 3. I fly in other class A and class D UK airspace fairly frequently; 4. I fly in Europe from time to time, in all classes of airspace.
I asked: 'Do I need to fit Mode S before 31st March 2005?'
The answer was "No, and you will not need it before March 2007, at the earliest."
I asked: 'Do I need to apply for exemption?'
The answer was "No."
However, I was advised informally, that if I needed to replace my transponder for reasons other than Mode S, then I would be wise to replace it with a Mode S type.
AA.