"Peter" wrote in message
...

wrote

Ah, but the main advantage (AIUI) is that the IFR clearance "VFR on top"
gives
you the flexibility of chosing your own routing while still staying in the
system. In
congested airspace, it's often unlikely they'll give you a routing at IFR
altitudes
due to conflicting traffic. With VFR on top they're more likely to let
you through
without a big diversion.

Why would this be?

Forgive me for looking at this from the Euro POV, but it is true that
if outside CAS one can fly anywhere one likes if VFR, whereas if IFR
one is supposed to fly on ATS routes (published airways).

However, there are countries (e.g. Greece) where *all* traffic (incl.
VFR) is supposed to be on airway routes, and one can also get DCT
clearances when IFR.

Moreover, I can be flying VFR in CAS (say Class C) and be talking to
some IFR (approach or departure) ATC unit, and they will be watching
me (with a Mode C squawk) and in effect separating me and other
traffic, occassionally giving me vectors. This is VFR, but it makes
perfect sense.

So I can't see why ATC would allow more routing leeway if VFR than if
IFR. Does the minimum separation change?

Yes, you are looking at it from a euro-POV and I think that's the
disconnect.

In US Class D & E VFR are not separated from IFR.

In the US virtually all airspace is some class of controlled airspace; not
just airways and terminal areas. There is some Class G (just enough to
prompt some gotcha test questions or usenet replies) but for most
discussions it's N/A.

Within all that controlled airspace only one controller provides ATC Service
within any one particular chunk of airspace.

Since there is (virtually) no uncontrolled airspace and only one controller
is responsible for the controlled airspace (and thus should have his Big
Picture) there are no provisions (or really any need) for the ATSORA (RIS
and FIS but especially RAS) that you find in the UK.

When an aircraft in probably 90% of US airspace is VFR or in this discussion
VFR-On-Top, ATC is not responsible for separation.

The two main reasons for IFR re-routes or request denials are separation and
traffic flow/sequencing into the terminal area. Since VFR-On-Top
eliminates separation as a reason and sequencing only matters in the
terminal area the result is more flexibility when it comes to routing.