Radar vectoring charts and cleared altitudes?
Purely as a matter of interest, here's a question for anyone willing to shed light on the following:
(this really needs a picture to explain it, but here goes anyway)
Imagine an airfield with a 4000ft ridge 20 miles away. The radar vectoring chart shows a min alt of 2000ft inside 20 miles, rising to 5000ft outside it. An approaching aircraft is passing 9000ft for 5000ft with high speed towards the airfield.
At what point can you clear the aircraft to 2000ft? Is it when the actual a/c symbol on your radar screen is inside the 2000ft area, or can you employ some anticipation using the likely flightpath of the aircraft, taking its descent rate into consideration?
For instance, if the aircraft has reached 7000ft 25 miles from the airfield, can you clear it to 2000ft -at that point- in anticipation of the fact that (using the 3 x table) it's going to need at least 6 miles to descend another 2000' (before reaching 5000ft), thereby passing the critical 20 mile point at around 5330ft?
thanks in advance - sorry if that's a bit confusing.