Green dot and Vls speed on Airbus
 

I'm not current on airbus anymore, can't remember, but had a discussion lately about green dot and Vls speed with a mate.
Do they come up in a turn, banking the aircraft?
Thx

Green dot, F speed, S speed etc are fixed and do not (they move with weight only). Vls is dynamic, and moves with bank, g loading, atmospheric conditions, speedbrake deployment, and configuration.

Green dot is a function of weight - and altitude. Usually double the weight, plus a constant (85-90kt), plus 1kt per alt over 20K. That's for the A32F anyway.

Vls is a margin over the VS from about 1.1 to 1.3 depending on the flight phase at lower altitudes, and 0.2g buffet margin at high alts.

VLS doesn’t move with bank or load either. Only VAprot, VAmax and Vsw.
VLS does go up with speedbrakes deployed and the margin on VS changes with different configurations.

VLS : Lowest Selectable Speed. Represented by the top of an amber strip along the airspeed scale on the PFD. Computed by the FAC based on FMS weight data, and on aerodynamic data as a backup, and corresponds to 1.13 VS during takeoff or following 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 ft, VLS is corrected for Mach effect to maintain a buffet margin of 0.2 g. In addition, VLS increases when the speedbrakes are extended.


Since VLS is 1.x X VS and VS for Airbus = VS1g, VLS doesn’t change with load factor.

Posted before
​​​​From Airbus document
Vls is
• Lowest selectable speed
• Represents minimum regulatory speeds depending on flight phase (can be VMCA, VMCL or Vs1g limited)
• Provides appropriate margin versus buffet at high
altitude
• Provides appropriate bank angle maneuverability
(defined by regulation) in a stabilised turn.

I stand corrected. The VAprot and stall strips can move above Vls, bringing the “foot” of the airspeed tape up, but Vls itself does not move. It DOES move with speedbrake use and configuration change.

https://cimg0.ibsrv.net/gimg/pprune....96491fd0f7.png
From Airbus

Yes, this is a common misconception*. VLS does not move during manoeuvres, but it does move (gradually) with changing aircraft mass and (quickly) with changes in the wing shape, i.e. speed brake deployment and flaps/slats.

Valpha prot does move during AoA changes, e.g. manoeuvres. Sometimes it can climb above, and hide VLS.


*Possibly some do not clearly understand the terminology: VLS is the thinest amber outlined strip against the lower segment of the speed tape. Valpha prot is a wider amber and black "tiger's tail". Normally seen below VLS, but as I say, it can climb above it.

That's very interesting, thx all for your input.
It's a little bit weird, because on the mother Airbus (A300) the Green Dot speed did change with bank!

But there was a known software bug. The change was in the wrong direction. Lower GD speed with increasing bank and vice versa.

Green Dot still doesn’t change with bank angle. Alpha prot speed does.

On A300 GD speed varies with load-factor, but in the wrong direction. At least it did in our ex-fleet with more than thirty A300.

That’s quite a software bug. I assume it was quickly fixed?

No.
Lasted about thirty years until phase out.
But of course we were informed by fleet management.
The interesting point however is, that on later airbus types GD speed is not a dynamic speed anymore.
If the definition is still best Lift/Drag ratio it sholud of course alter its value depending on load-factor IMHO.

The definition of GD on the FBW Airbii is
- the optimum engine-out speed.

My guess is that applies for flight with Beta target centered.

From the FCOM (A321NEO):

Engine-out operating speed in clean configuration. (Best lift-to-drag ratio speed).
Also corresponds to the final takeoff speed. Represented by a green dot on the PFD scale. Below 20 000 ft equal to 1.5 × weight (tons) +110 Above 20 000 ft, add 1 kt per 1 000 ft

Hhm, if GD-speed is best lift-to-drag ratio speed, shouldn't it change with load factor?

It probably means "best lift-to-drag ratio at 1G" and the last part is assumed.

Yep.
That way it makes sense.

PFD
 

Perhaps. But its not displayed.. At Config 1 Green Dot is replaced by S speed on the Speed Tape.
Beta Target is only displayed with Config 1 and greater; therefore Green Dot & Beta target will never be displayed together. (A32x)

Both (+hikoushi) corrections appreciated, gents.