Green Dot F and S speed...

[TACHO]

Hello all,

Just looking for a bit of help in the following areas. Could someone please explain the concept of green dot, S and F speed for the airbus A320. As I understand it green dot is clean maneuovering speed, and S speed is the lowest speed at which flaps 1 can be retracted? However what is F speed and how does this relate to flying an actual approach?

thanks in advance .

[Field In Sight]

During acceleration the speeds can be considered minimum retraction to next config speeds. Aircraft should show +ve speed trend.

On deceleration they are manoeuvring/managed target speeds. You can always still reduce to VLS but lose the manoeuvring margins.

For normal approaches.
Config 1 = Slats 1 only. Target speed is S
Config 2 = Full slats, Flap = 2. Target speed is F. (This is a big change and the acft balloons).
Config 3 = Flap = 3. Target speed is still F
Config Full = Flap = Full. Target speed reduces to Vapp.

Hope that helps.

[TACHO]

Brilliant, just what I was after

[airseb]

and to remember it easily you can think of it as:

speed S = speed at which you should have Slats
speed F = speed at which you should have Flaps

easy enough

[tom775257]

From a novice POV... if you get this in the sim:
Master warning: overspeed during accel / when retracting flap: Pull speed and reduce below the previous Vmax. A gotcha on the bus (especially when sim time looms)
The limits on the PFD speed scale are produced with reference to the lever position, not actual slat/flap position. Whereas the CRC/ Master warning are produced with regards to actual position/ vmax.
Enjoy!

BTW Green dot equals max lift/drag hence if you press expedite (climb) the a/c will pitch for green dot = high R.O.C. etc.

[OPEN DES]

You can always still reduce to VLS but lose the manoeuvring margins
Where did you find this? The so-called margin you speak about is significantly different at S and F speed respectively. Many companies recommend against/restrict their pilots to fly below GD, F, S, however the aircraft can be flow between Vls and Vmax anytime. (example: V2+10 is a lot lower than S speed on an 1+F takeoff)
Also I wouldn't call S and F maneuvering speeds as they do not share the same characteristics as GD and do not have any fixed relationship to Vs1g or equally Vls and as such do not represent specific points on the L/D curve.
S and F speed are minimum slat/flap retractionspeeds! Airbus chose them as managed speed targets during deceleration because of the simple fact that they will always be between Vls and Vmax; so flyable.
Doesn't Airbus Industrie only talk about maneuvering margin (max bank) in the OneEngineInop case? The AEO scenario vs. max bank below S,F,GD remains untouched by Airbus.
Anyway these are my thoughts. Would appreciate any input.

[J.O.]

S and F speeds are minimum safe flap/slat retraction speeds. On approach, they are managed target speeds, but they are not a manoeuvring speed and they are not the minimum operating speed for the current configuration. To restrict pilots from operating at speeds below these values on approach is to display one's ignorance of the aircraft's capabilities.

Originally Posted by Field In Sight

For normal approaches.
Config 1 = Slats 1 only. Target speed is S
Config 2 = Full slats, Flap = 2. Target speed is F. (This is a big change and the acft balloons).
Config 3 = Flap = 3. Target speed is still F
Config Full = Flap = Full. Target speed reduces to Vapp.

A slight correction to the above is required, according to FCOM 1.27.50 pg 5

Config 1 = Slats 1 (18 degrees) only. Target speed is S
Config 2 = Slats 2 (22 degrees), Flap = 2. Target speed is F. (This is a big change and the acft balloons).
Config 3 = Slats 2 Flap = 3. Target speed is still F
Config Full = Slats Full (27 degrees) Flap = Full. Target speed reduces to Vapp

[FlapsOne]

On approach, they are managed target speeds, but they are not a manoeuvring speed

Well FCOM 1.22.40 says:
The FAC computes :
The minimum and maximum speeds :
o VSW (stall warning)
o VLS
o VFE and VFE for the next configuration
o VLE
o VMO/MMO
The maneuvering speeds :
o Green Dot Speed
o S speed
o F speed

[J.O.]

Fair enough, but it seems there is a confusion of terminology in the FCOM (unfortunately, not an uncommon Airbus manual characteristic). Maneuvering speed (Va) for all aircraft is defined as "the maximum airspeed for full control deflection" (see FCOM 3.04.10 pg 3). For the Airbus fly-by-wire fleet, this only applies when the aircraft is in alternate law or direct law, where the ECAM tells us to "maneuver with care". To find the correct value for Va, you have to refer to the table found in FCOM 3.01.20 pg 4a. The lowest value shown on that chart is 245 kts, considerably higher than your typical "S", "F" or Green Dot speed for the A320.

To find the proper definitions for "S", "F" or "Green Dot", look in FCOM 3.04.10 pg 2.

Characteristic Speeds:

F - minimum speed at which the flaps may be retracted at takeoff. In approach, used as a speed target 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 - minimum speed at which the slats may be retracted at takeoff. In approach, used as a speed target when the aircraft is in CONF 1.

Represented by "S" on the PFD speed scale. Equal to about 1.22 Vs to 1.25 Vs of clean configuration.

O - Green Dot Speed.

Engine out operating speed in clean configuration. (Best lift to drag ratio speed).

Corresponds also to the final takeoff speed.

Represented by a green dot on the PFD airspeed scale.

Below 20,000 ft equal to 2 x weight (in tonnes) + 85

Above 20,000 ft, add 1 kt per 1,000 ft.

[Max Angle]

I have never really understood why Airbus complicated the issue with F and S speed. The flap/slat settings are 0,1,2,3 and Full so why not refer to and display the speeds using the same numbers?.

[SIDSTAR]

It needs to be understood that these speeds are used by the autothrust system as a reference on the approach. Select flaps 1 (slats only) and S speed appears on the PFD speed scale and the autothrust system will provide a thrust setting to maintain S speed. However, you can fly slower than S speed without taking Flaps 2 but you'll have to use selected speed to do so with autothrust. As mentioned earlier, the aircraft is not going to fall out of the sky just because you fly slower than these speeds but the margin over the stall speed in that configuration is reduced. They are useful reference speeds to use in configuring the aircraft for the approach and this is what Airbus and most operators teach their pilots. Similarly, on takeoff or go-around they are used as a reference for the clean-up - at F speed select Flaps 1 and at S speed select Flaps Zero.

[J.O.]

Sorry to be technical, but a thorough understanding of how the automation systems work and interact is important.

The autothrust does not "control" the speed. When active, the autothrust adjusts the thrust to achieve the desired speed as ordered by signals from the FMGS / FMGEC, which is controlling the speed. When in Managed Speed mode, the FMGS determines the appropriate speed depending on the active PERF mode, thrust lever position, cost index, aircraft weight and flap / slat setting. In Selected Speed mode, the FMGS takes orders from the pilot's selections on the FCU and sends those orders to the autothrust to achieve the desired airspeed as "controlled" by the pilot.

Also, after takeoff, F and S speed are not just reference speeds, they are minimum safe flap / slat retraction speeds, i.e. they provide the safe airspeed margin for retraction of flaps and slats. Part of the "logic" of the system is that the FMGS will maintain S or F when in Managed Speed in the APPR phase so that if the crew elects to retract a previously selected flap setting (change of runways, additional maneuvering required, etc.), it would be safe to do so immediately as S or F is already being maintained.

[nonemmet]

'green dot' with gear down

If operating a ferry flight with the gear down, is there any way of knowing the best lift to drag ratio speed? I assume that green dot will still indicate the engine out operating speed (Best lift to drag ratio speed) in clean configuration?

[Captain_Scooby]

Hey all,

Had a quick question regarding green dot, and although I note that this thread has been dormant for 5 years, I felt it was appropriate.

In the 'Characteristic Speeds' section of the FCOM, as quoted above, it states that green dot corresponds to the final takeoff speed. However, my understanding is that at acceleration altitude, the managed speed target changes to either a pre-selected value (for, say, a wraparound sid) or 250kts. Where does green dot come into the takeoff?

[MD83FO]

VFTO is used in the engine failure scenario,

acceleration altitude to final takeoff segment.

and that will become your default managed speed in this scenario also.

[vilas]

Open Des
There is an Airbus presentaion on this subject. Which gives following information. I had quoted this before. This is from Airbus.
Flap maneuvering speeds are not limitations:
It is safe to fly between VLS and VMAX (VMO, VLE, VFE) :
The required aerodynamic margin is properly ensured
.The bank angle at which alpha protection will trigger are as follows:
V2 OEI 30 degrees.
V2+10 AEO 40 degrees.
GD 45 degrees
Vls T/O to Landing 40 degrees.
As you can see on Auto pilot/FD will not give you these banks anyway

[vilas]

Captain Scooby
In a go around also the target is GD.

[MD83FO]

Hello Vilas which presentation is this?
thanks

[vilas]

MD83FO
!7th Performance and Operations conference Dubai 2011 Charecteristic speeds.

[Altcrznav]

Originally Posted by TACHO

Hello all,

Just looking for a bit of help in the following areas. Could someone please explain the concept of green dot, S and F speed for the airbus A320. As I understand it green dot is clean maneuovering speed, and S speed is the lowest speed at which flaps 1 can be retracted? However what is F speed and how does this relate to flying an actual approach?

thanks in advance .

http://www.blackholes.org.uk/PP/oeb-819-1.pdf