VS: Stalling Speed
For a conventional aircraft (not Airbus), the reference stall speed is based on a load factor that is less than 1g. This gives a Stall Speed that is lower than the stall speed at 1g (1 Gravity). See Figure B6. All operating speeds are expressed as functions of this speed (for example, VREF = 1.3 VS).
VREF: Reference speed, It's the steady landing approach speed required at a height of 50 feet above runway threshold for a defined landing configuration. It's used as a reference for calculating the final approach speed and the landing distance.
Coefficient of Lift Vs Angle of Attack
Because the A320 family aircrafts has a low speed protection feature that the flight crew can't override, (This is true meanwhile the aircraft remains in Normal Law) Airworthiness Authorities have reconsidered the definition of stall speed for the A320.
All the operating speeds must be referenced to a speed that can be demostrated by flight tests. This speed is designated VS1g (Stall Speed at 1 Gravity).
Coefficient of Lift Against Speeds Representation during A320 Normal Law
High Angle of Attack Protection
Airworthiness Authorities have agreed that a factor of 0.94 represents the relationship between VS1g for A320 Family Aircraft and VS for conventional aircrafts.
As a result, Authorities allow A320 family aircraft to use the following factor:
VREF = 1.3 x 0.94 VS1g = 1.23 VS1g
As a conclusion every time we refered to the stall speed on A320 family aircraft we use VS1g instead of VS.
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