Has anyone ever sat down and drawn a scale diagram of a visual circuit? You would be surprised just what a profound difference the following has on a circuit.
True Air Speed.
TAS squared determines radius of turn. TAS increases with an increase in temperature and altitude.
150 kts IAS during a 1500 ft AGL circuit in Sydney in ISA gives a TAS of 154 kts. Radius of turn using 25 deg AoB is therefore 0.74 nm
A similar circuit with 150 kts IAS in Ayres Rock at 35C (ISA+26) gives a TAS of 165 kts and a radius of turn of 0.85 nm. Thats adds an extra 0.3 nm to your track miles during a 180 degree turn.
Wind.
It too has a profound effect on the circuit.
The downwind spacing from the runway especially when the wind is blowing you towards the runway and is up your tail on base leg needs to be sufficient to allow you to turn the heading more than 180 degrees (180 degrees plus 2 x drift) and not go through the runway centreline on final.
Downwind timing from abeam the runway threshold to turning base is also affected (assuming you want to keep the final approach length consistant).
Most circuits you do in the simulator are ones where you're turning onto finals at around 800 ft AGL at about 2.5 nm. But consider when else you might be required to make a visual manoeuvre to landing. A circling approach! There are still plenty of them out there. Ayres Rock RWY 13 is one that comes to mind.
The B737 is a catagory C aircraft so the Circling minima is often 4000m visibility. 4000m equals 2.1 nm. Have a think about how you are going to keep a B737 within 2.1 nm of the threshold. That's not a 2.1 nm final either. You need to keep the base turn within this distance if you are going to maintain visual contact with the runway. Draw a circle with a radius of 2.1 nm centred on the runway threshold and see what it takes to keep an aircraft with a TAS of say 140 kts and 25 deg AoB within the circle!
Clue: Turn Radius (nm) = TAS^2/(68,412 x tan
AoB).
Never mind when the TAS is 165 kts as in the example above!!
The point I want to make here is that the tracking distance to the runway can vary considerably. Therefore the altitude gates that you want to pass through along the constant 3 degree descent path to the runway (abeam the runway, starting descent from the circling minima, turning base, turning final) will vary considerably too.
Have you ever been turning base and not quite sure how it's going? (Am I high? Am I low?) I have. But ever since I sat down and nutted out all the various Circuits that I am likely to need ie:
a) 2 Engine fine weather circuit
b) 2 Engine low visibility (4000m) circuit
c) Engine out (FLAP 15) (TAS 170 kts) circuit
and noted down what effect winds of 10 kts, 20 kts, 30 kts from various directions can have on them, I've been able to fly my circuits with confidence and consistancy.
I hope this helps.
PS I can email you a copy if you like.