Let’s understand the various operating speeds in the Airbus A320.
CHARACTERISTIC SPEEDS
The characteristic speeds displayed on the PFD are computed by the Flight Augmentation Computer (FAC), according to the FMS weight data (for PFD/MCDU display consistency and accuracy purposes), and aerodynamic data as a backup.
VLS (of normal landing configuration: CONF 3 or FULL), F, S, and Green Dot speeds are also displayed on the MCDU TAKEOFF and/or APPR pages. These values are computed by the FMS, based on the aircraft gross weight (which is computed according to the entered ZFW and the FOB), or the predicted grossweight (for approach or go-around).
VS: Stalling speed
Stalling speed and it is not displayed. For a conventional aircraft, the reference stall speed, VSmin, is based on a load factor that is less than 1 g. This gives a stall speed that is lower than the stall speed at 1 g. All operating speeds are expressed as functions of this speed (for example, VREF = 1.3 VSmin). Because aircraft of the A320 family have a low-speed protection feature (alpha limit) that the flight crew cannot override, Airworthiness Authorities have reconsidered the definition of stall speed for these aircraft. All the operating speeds must be referenced to a speed that can be demonstrated fight tests. This speed is designated VS1g. Airworthiness Authorities have agreed that a factor of 0.94 represents the relationship between VS1g for aircraft of the A320 family and VSmin for conventional aircraft types.
As a result, Authorities allow aircraft of the A320 family to use the following factors:
- V2 = 1.2 × 0.94 VS1g = 1.13 VS1g
- VREF = 1.3 × 0.94 VS1g = 1.23 VS1g
These speeds are identical to those that the conventional 94 % rule would have defined for these aircraft. The A318, A319, A320, and A321 have exactly the same maneuver margin that a conventional aircraft would have at its reference speeds. The FCOM uses VS for VS1g.
VLS: Lowest Selectable Speed
VLS is the lowest selectable speed for the autopilot and the autothrust. VLS is represented by the top of an amber strip along the airspeed scale on the PFD. VLS (of selected landing configuration: CONF 3 or FULL), is also displayed on the FMS APPR page. VLS is computed by the FAC, based on aerodynamic data, and corresponds to 1.13 VS during takeoff, or after a touch and go. It becomes 1.23 VS, after retraction of one step of flaps. It 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 ft, VLS is corrected for Mach effect to maintain a buffet margin of 0.2 g. In addition, VLS increases with speed brakes extension.
F Speed
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 1.18 VS to 1.22 VS of CONF 1+F.
S Speed
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. This is represented by “S” on the PFD airspeed scale. Equal to about 1.22 VS to 1.25 VS of clean configuration.
Green Dot Speed
Greeen Dot (GD) speed is the engine-out operating speed in clean configuration. It provides an estimate of the speed for best lift-to-drag ratio. Also corresponds to the final takeoff speed. The Greeen Dot (GD) speed is computed by Auto Flight System (AFS) based on the Zero Fuel Weight, aircraft weight, and pressure altitude. This will provides the best lift-to-drag ratio for a given altitude and aircraft weight when one engine is out in clean configuration.
Represented by a green dot on the PFD scale.
– Below 20 000 ft equal to 2 × weight (tons) +85.
– Above 20 000 ft, add 1 kt per 1 000 ft.
PROTECTION SPEEDS
Vα PROT, Vα MAX and VSW are computed by the FAC, based on aerodynamic data. They are only used for display on the PFD, and not for flight control protection (the activation of the protections is computed by the ELAC).
Vα PROT
Angle of attack protection speed. Corresponds to the angle of attack at which the angle of attack protection becomes active. Represented by the top of a black and amber strip along the PFD speed scale, in normal law.
Vα MAX
Maximum angle of attack speed. Corresponds to the maximum angle of attack that may be reached in pitch normal law. Represented by the top of a red strip along the PFD speed scale, in normal law.
VSW
Stall warning speed. Represented by a red and black strip along the speed scale when the flight control normal law is inoperative.
VMAX
Represented by the bottom of a red and black strip along the speed scale. Determined by the FAC according to the aircraft configuration. Is equal to VMO (or speed corresponding to MMO), VLE or VFE.
LIMIT SPEEDS
- VA : Maximum design maneuvering speed. This corresponds to the maximum structural speed permitted for full control deflection, if alternate or direct law is active.
- VMCG : Minimum speed, on the ground during takeoff, at which the aircraft can be controlled by only using the primary flight controls, after a sudden failure of the critical engine, the other engine remaining at takeoff power.
- VMCA : Minimum control speed in flight at which the aircraft can be controlled with a maximum bank of 5 °, if one engine fails, the other engine remaining at takeoff power (takeoff flap setting, gear retracted).
- VMCL : Minimum control speed in flight, at which the aircraft can be controlled with a maximum bank of 5 °, if one engine fails, the other engine remaining at takeoff power (approach flap setting).
- VFE : Maximum speed for each flap configuration. (See Limitations).
- VLE : Maximum speed with landing gear extended. (280 kt IAS / M 0.67).
- VLO : Maximum speed for landing gear operation. (220-250 kt IAS).
- VMO : Maximum speed. (350 kt IAS / M 0.82).
- VFE NEXT : Maximum speed for the next (further extended) flap lever position.
OTHER SPEEDS
V1
The highest speed, during takeoff, at which the flight crew has a choice between continuing the takeoff or stopping the aircraft. Represented by “1” on the airspeed scale (or the V1 value when it is off the airspeed scale). Inserted manually through the MCDU by the flight crew at the latest. Displayed on the MCDU TAKEOFF page.
VR
The speed at which the pilot rotates in order to reach V2 at an altitude of 35 ft at the latest after an engine failure. Inserted manually through the MCDU by the flight crew. Displayed on the MCDU TAKEOFF page.
V2
Takeoff safety speed that the aircraft attains at the latest at an altitude of 35 ft with one engine failed, and maintains during the second segment of the takeoff. Represented by the SPEED SELECT symbol on the speed scale. Minimum value equal to 1.13 VS for the corresponding configuration. Inserted manually through the MCDU by the flight crew. Displayed on the MCDU TAKEOFF page.
VREF
Reference speed used for normal final approach. Equal to 1.23 × VS of CONF FULL. Displayed on the MCDU APPR page, if landing is planned in CONF FULL (VLS CONF FULL).
VAPP
Final approach speed. Displayed on MCDU APPR page. Calculated by the FMGCs.
Represents : VAPP = VLS + wind correction.
The wind correction is limited to a minimum of 5 kt and a maximum of 15 kt.
The flight crew may modify VAPP through the MCDU.
‐ During autoland or when A/THR is on or in case of ice accretion or gusty crosswind greater than 20 kt, VAPP must not be lower than VLS +5 kt.
VAPP TARGET
Represented by a magenta triangle. Calculated by the FMGCs Gives efficient speed guidance in approach during various windy conditions.
Represents : VAPP TARGET = GS mini + actual headwind (measured by ADIRS).
GS mini = VAPP – TOWER WIND (headwind component along runway axis calculated by FMGC from tower wind entered on MCDU).
Disclaimer: The above information are personal notes for educational purposes only, compiled from various sources. Do not use it for flights.
Manually changing VAPP on the MCDU affects GS mini?