VLS : Lowest Selectable Speed.
Computed by the FAC, based on aerodynamic data, and corresponds to 1.13 VS during takeoff, or after a touch and go.
Becomes 1.23 VS, after retraction of one step of flaps.
Becomes 1.28 VS, when in clean configuration.
Note : If in CONF 0 VLS were 1.23 VS (instead of 1.28 VS), the alpha protection strip would hit the VLS strip on the PFD.
Above 20 000 feet, VLS is corrected for Mach effect to maintain a buffet margin of 0.2g.
F : Minimum speed at which the flaps may be retracted at takeoff.
In approach, used as a target speed when the aircraft is in CONF 2 or CONF 3.
Represented by "F" on the PFD speed scale. Equal to about 1.23 VS of CONF 1 + F.
S : Minimum speed at which the slats may be retracted at takeoff.
In approach, used as a target speed when the aircraft is in CONF 1.
Represented by "S" on the PFD airspeed scale.
Equal to about 1.29 VS of clean configuration.
just a question of how much margin one requires or is comfortable with.
1.13=20%;1.23=30%;1.28=36+%;and1.29=37+% above
vs