1. The semi-circular rule. Which is safer - to put all that traffic at a few levels (8 in my example - every 500 ft, even assuming they apply below 3000 ft), or to have them at all levels? I think I'd argue the latter - you're much safer with people climbing and descending. If the cross section of a couple of aircraft is say 20 feet (ie they have to be within 20 ft of height to hit each other), then you have 175 levels available if climbing and descending like gliders, only 8 if flying semi-circular. Much lower risk of collision with the glider like behaviour. (And I'm as guilty as anyone - I have a habit of flying around in power at 'exactly' 2000 ft or whatever - it's the instrument training Much safer to pick a random altitude, eg 3420 ft and stick to that. Or pretend to be a glider and go up and down).
In
"On the Use of Height Rules in Off-route Airspace" (Journal of Navigation, vol 36, issue 2, pp. 269-287, 1983), R L Ford of the Royal Signals and Radar Establishment analysed the effectiveness of semicircular and quadrantal height rules and concluded that
"The application of the standard rules can lead to a reduction [my emphasis]
in intrinsic safety unless significant height-keeping errors are present" - essentially for the reason that PaulisHome explains.
The Journal of Navigation - On the Use of Height Rules in Off-route Airspace - Cambridge Journals Online (Apologies if you need a subscription for access.)
This was previously discussed on Pprune here
http://www.pprune.org/atc-issues/419...ml#post5780485
Windrusher