Why you have to maintain "S" speed in a320 circuits after a climb?

Question

I've been told that in circuits, after reaching thrust acceleration altitude , you should maintain S speed, after a flaps 1 take off? why is it so? why that particular speed. please explain

Answer 1

When flying in clean configuration, the Green Dot (GD) speed is the recommended speed for approach and holding¹ because

1. it provides sufficient maneuvering margin:

   Deviating below GD involves an increase in the drag on the aircraft and would eventually undermine the aircraft’s ability to continue a climb. [...]

   Therefore in the clean configuration, the crew should not fly below GD in order to avoid degrading climb performance.

   ^{(Airbus Safety First - Control your speed… during climb)}

2. it minimizes fuel consumption:

   GD speed is the managed speed target in CONF CLEAN when the FMS approach phase is activated. It is also the recommended speed to extend flaps to CONF 1 and for a holding in clean configuration. [...]

   In some phases of flight, GD is computed to minimize drag and thus, the fuel consumption (for example during the HOLD phase).

   ^{(Airbus Safety First - Control your Speed… During Descent, Approach and Landing)}

However, you said you are flying the circuits in CONF 1 (slats extended). GD speed is replaced by S Speed in CONF 1 (and 1+F). Similarly, F Speed is used for CONF 2 and 3.

In approach phase, S speed is the managed speed target, when in CONF 1 or 1+F. It is the recommended speed to select CONF 2.

^{(Airbus Safety First - Control your Speed… During Descent, Approach and Landing)}

The FCTM also recommends flying holding patterns in CONF 1 at S Speed:

HOLDING SPEED AND CONFIGURATION

If a hold is to be flown, provided NAV mode is engaged and the speed is managed, an automatic speed reduction will occur to achieve the Maximum Endurance speed when entering the holding pattern. The Maximum Endurance speed is approximately equal to Green Dot and provides the lowest hourly fuel consumption.

If the Maximum Endurance speed is greater than the ICAO or state maximum holding speed, the crew should select flap 1 below 20 000 ft and fly S speed.

^{(Airbus A320 FCTM - Normal Operations - Holding, emphasis mine)}

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¹ Circuits aren't mentioned in the manuals I have, but they are operationally similar to holding and approach phase.