Circuit training question

[Dazmundo]

I'm trying to understand how they developed the "3 second per 100 feet" rule for timing the turn to base from passing the runway threshold on the downwind leg. I'm finding that I'm way too high once I turn to final.

Flying the downwind leg at 1500 AGL, the formula tells me I turn after 45 seconds. Considering that I need to be at 4.5 miles DME turning to final at 1500 AGL, this would mean I need to be flying at 313 kts (or 360 MPH) if I'm to be 4.5 miles beyond the threshold in 45 seconds. But my average gear down flaps 15 speed is probably 150 kts, so it feels like I should be applying a 6 second per 100 feet rule, no?

Appreciate the help!

[eckhard]

I think that you're supposed to start descending at the 45 second point, not fly level around the base turn.
At 45 sec you select gear down, flap 'y' (where 'z' is landing flap), start turning and establish in a gentle descent as speed reduces.
When visual with PAPIs, select flap 'z' and adjust the flight path and speed as required.
Seems to work for me (most of the time).

[KayPam]

It depends if you start your descent when turning in base (like a light aircraft) or when you 're established on final (like a larger aircraft performing an ILS approach)
A large aircraft would require more seconds per 100 feet.

[ChimneySweep]

On the 737 circling approach we start a shallow descent while on base and then descend visually on to the 3 degree path.

Intercepting final should be around 2 or 3 DME, not 4.5.

[4Screwaircrew]

I assume you are simulating a 737.

Down wind flap 5 note thrust.

Abeam threshold gear down flap 15, watch for the balloon leave the thrust set and allow deceleration to manoeuvre speed, which should get you there as the time expires.
Start your turn, flap 25 pitch 4 nose up, approaching the extended centre line flap 30 2 1/2 nose up and that should put you in the right place at the right speed, from there make small adjustments to pitch power and heading as required to keep yourself in the slot.
Flap 40 will require about 1/2 degree pitch up and about 5% more N1.

It's a long time since I used to do base training on the 737 but this should improve your chances.

[Dazmundo]

Thanks all. So, if I'm flying 45 seconds past the threshold and aiming to turn to final at about 2.5 miles, doesn't than mean I'm flying a 45 degree intercept (base leg) to final? I estimate that 45 seconds takes me 4.5 miles past the threshold, so I need to l make up 2 miles on the base, to turn final at 2.5 miles, right?

[4Screwaircrew]

Don't look at distance, start the turn and the descent based on the time.

[eckhard]

I estimate that 45 seconds takes me 4.5 miles past the threshold, so I need to l make up 2 miles on the base, to turn final at 2.5 miles, right?

Given a typical downwind speed of 180KIAS, the 45 seconds would take you 2.25 nm.
To travel 4.5 nm, you would have to be doing 360, which is a tad fast in the circuit!