Advising/Requesting speed changes

[soaringhigh650]

If you are given clearance for a VFR transit across the terminal area at some altitude, assuming your Cessna is flying at 120kts, is it a requirement to advise or request ATC for any speed changes, for e.g. to set up slow flight at 70kts to have a longer look at the airport beneath?

[samotnik]

Well, in the ICAO world you should report any speed changes of more than +-5% of FP speed. Annex 2. I don't know how do you deal with this in the US.

[Touny]

I am not sure that it is required for 10/20 kts (though the 5% rule makes sense), but I would say it is common sense to advise ATC if you want to remain over the airfield for 5 min :}

About your example,
a VFR traffic transiting via overhead might not always be easy to handle.
Sometimes you might want to make the most of a "gap" inbetween 2 inbounds to let the VFR cross the extended centre line (or axis, I never know how to say it), so that no one is concerned about separation (even in class D airspace... I've already had an airprox filed "against" me because a fast VFR had an airbus on final in sight and crossed just behind, a little too close... even though I was not supposed to ensure them radar spacing...)

Anyway, if you want to have a "longer look at the airport beneath", best thing to do is to advise ATC who will probably make its most to satisfy you, if traffic permits...
Don't try that kind of stuff without warning us first...