Because providing full radar separation (typically 3nm/1000') between IFR and VFR would be unnecessarily restrictive. With class D we can build in a safety margin where prudent, eg. so that an airliner does not have its TCAS triggered (the other side of the coin to you being happy to self-separate), while allowing far less than 3nm/1000' where possible. There is a passage in CAP 493 that deals with this, I'll look it up later when I have a minute.
Last edited by Roadrunner Once; 28th May 2012 at 17:15.
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