View Full Version : Airbus Approch Speed: VLS versus 'S' Speed

20th Aug 2007, 15:52
What is the lowest speed that can be safely flown on approach before taking flap2? Is it 'S' speed or VLS?
I have asked this question to a few other pilots and there seems to be two very distinct opinions:

1. NEVER reduce speed on approach below 'S' speed without taking flap2.

2. Generally take flap2 at 'S' speed but when operationally necessary you can safely reduce speed to between 'S' speed and VLS without taking flap2.

I personally agree with those in group 2 but I would be interested to hear were you, and your company SOP's, stand on this?

20th Aug 2007, 16:14
The orange company tells you not to reduce below S in flaps 1.

In my previous company option 2 was accepted. (they operated standard Airbus SOP)

In my understanding you can fly the airbus anytime between Vls and Vmax. S and F are retract speeds and in no way resemble an aerodynamic signifant speed with regards to maneuvering (both have entirely different margins, and are represented by different point on the L/D curve).
However they are used as managed speed targets by the FMGS, I guess for simplicity.
S and F are suitable as speed targets because they always are between Vls and Vmax and especially S is a good ballpark speed for the intermediate approach. Assuming that you would always fly a decelerated approach there would be no need for actual flap maneuvering speeds. But how different is reality. I would be happy to fly below S speed with flaps 1. (I am guessing that the delta Vref for flaps 1 would result in a Vapp below S anyway). However my company does not allow it.

20th Aug 2007, 16:27
1. If ATC want 180K, and 'S' is say 183K, F1 will give you lower fuel flow and acceptable. As you get further below 'S' I get uncomfortable with the high nose attitude. However, the min legal speed is VLS...
2. Getting near VLS in this scenario is going a bit too far - you cannot use S/B without it powering up against you etc.

I don't believe BA's SOPs are 100% prescriptive in this area, and that's probably the best way. Often I will fly a bit under 'S' @ 180K F1 but as the G/S approaches take F2 to use the drag down the slope (F1 it will accelerate), and avoid the F2 "balloon" while trying to hold the G/S...

20th Aug 2007, 16:30
There was a similar thrtead smtm ago, check the search.
Once we reduced speed very close to VLS with F1. You know, the bird becomes a bit shaky. You feel it. (Oh it was so nise morning appr to Amsterdaam!).
Since then I prefer to use F2 at low speeds.
Nothing at SOPs though.

20th Aug 2007, 17:45
The clue is in the definition .... Vls = lowest selectable speed. It is designated as such by Airbus and therefore is safe.

It is another matter altogether as to what is sensible when it comes to normal operations. I am (or was) of the brigade that flying below S speed [10kts or so] was fine on a short term basis in good (non-turbulent) conditions rather than take the extra drag associated with Flap 2.

25th Aug 2007, 19:07
VLS is the lowest SELECTABLE speed with the autothrust engaged. It is not the Alpha Floor or Alpha Prot speed. On the approach, S and F speeds are used by the autoflight system as target/minimum speeds for flap 1 and Flap 3 config. The aircraft can be safely flown below this speed all the way down to the Vref for your current configuration but you may encounter turbulence which could lower the buffet margin.

Operating below S speed in config 1 is not in itself dangerous and is often used by European and US operators to fly 180 when requested by ATC. However, many operators do not allow you to do this and use S speed as the minimum for Config 1. Use whatever your company SOPs say. If they are silent on this point it is probably because the management has decided to leave it up to the pilot's discretion.

On takeoff/goaround F and S speeds are the minimum speeds for retracting flaps to the next appropriate setting.

26th Aug 2007, 02:35
In the Airbus Flight Crew Training Manual, under Normal Operations, Approach General, 02.100....
The second paragraph says...A speed below the maneuvering speed of the present configuration may be selected provided it is above VLS.

Dixons Cider
26th Aug 2007, 08:11
It hasn't been mentioned so far so apologies if I'm telling you how to suck eggs -
as quoted already its fine to go below manuevring speed provided you stay above Vls, BUT be very cautious about getting too close to Vls if you have any turns to make eg intercepts. Vls will rise considerably in the turn when in this portion of the drag curve.

It may bite thee.

Iva harden
26th Aug 2007, 08:32
I believe S speed indicates the lowest speed in config 1, Minimum speed with slats out. Below this speed you should have Flap 2 selected. Flying near VLS is dodgy and as has been noted before, on an intercept where the aircraft can command upto 30 degrees of bank to pick up the LOC, you could end up with egg on your face as the aircraft goes into Alpha Floor! If you fly 321 you will definitely need flap 2 at 180 knots as S speed tends to be around 200 knots, I have seen a number of pilots make this mistake.:ok:

27th Aug 2007, 09:43
I believe S speed indicates the lowest speed in config 1, Minimum speed with slats out. Below this speed you should have Flap 2 selected. With the utmost respect to your opinion, I am sure there is plenty of official reference that state the exact opposite. For one, after departure you will select flap 1(+F) while passing F speed, well below S speed, alebeit accelerating. Same applies for 1 > 0.

Right Way Up
27th Aug 2007, 10:53
Big difference between "conf 1" below S speed and "conf 1+F" below S speed.

Fly Ginger
27th Aug 2007, 10:59
As far as i understand it, and without going to the books (so prepared to be shot down!!), flying at s speed gives you the full flight envelope with which to manouvre, as garuanteed by toulouse. If you go below this without taking flap, you haven't got the full envelope to play with.

This is how it was described to me, i've only just finished my line training, so as ever would welcome any comments.............. clean though please!!

27th Aug 2007, 12:15
it's in the fcoms somewhere but Green dot, F and S speeds give you full 25 degrees bank protection to those speeds less 3 knots. This protection then decreases as you continue towards VLS.
Bottom line, i'd fly the Green dot, F,and S speed but at very heavy weights and when straight and level i've set below the S speed to get Flap 2 out without busting the Flap limit speed.

Iva harden
27th Aug 2007, 18:34
FLIGHT DETENT........ I think you had better start reading your manuals again, looks like you could do with a refresh. Have a think about what RIGHT WAY UP has written and go look in your manuals, you might be surprised. If you still haven't grasped it, I was discussing the approach not after take off.:ugh:

28th Aug 2007, 09:41
I am quite aware that it is approach speed discussion and also, that 1(+F) makes a huge difference and is not a very good example. No need to argue with RWU.

As you do fly the A321 you also know that at 89t the S speed is 220 kt and flaps would auto-retract at 210, so you are flying as designed 10 below S speed with slats only - and it is completely normal. Same applies when passing S speed to clean up, again you are flying below green dot with no LE or TE devices at all (I hope I am correct to assume that you would claim it is incorrect to fly below green dot in clean, just like you say CF-2 is required to go below S speed).

As far as the books go:
FCTM Ch2 Intermediate Approach / DECELERATION AND CONFIGURATION CHANGE A speed below the manoeuvring speed of the present
configuration may be selected provided it is above VLS.
FCOM 3.04.10 p2
S speed definiton: In approach, used as a target speed when the aircraft is in CONF 1 ..... Equal to about 1,22 to 1,24 Vs in clean configuration.
F speed definition: In approach, used as a target speed when the aircraft is in CONF 2 or CONF 3. .... Equal to about 1,18 to 1,22 Vs of CONF 1+F.
Now, everybody agrees that it is safe (nothing more than that) to fly V2 with engine out. V2 is: Minimum value equal to 1,13 Vs for the corresponding configuration.

Hence my assumption that lowest speed in config 1, Minimum speed with slats out. Below this speed you should have Flap 2 selected. is indeed a very good practice but not a reqirement as per original Airbus design.

Iva harden
28th Aug 2007, 18:43
FLIGHT DETENT.... I am glad you took my advice and got your head in the books, keep studying, one day you will get there.

Passing S speed cleaning up..... you do not fly below green dot without L/Edge devices as they take time to retract unless your Airbus has very very very quick retract on it, mine retracts flap etc over a period of time, by which time you have accelerated beyond green dot.

Blimey you must be hard work on the flight deck.:ugh:

29th Aug 2007, 09:44
No, I am downright dangerous, flying 15 knots above VLS and other stunts. :cool:

6th Feb 2017, 09:51
What about flying below the S speed with a selected speed and you have an engine fail? Do you have enough energy to fly?

Smokey Lomcevak
6th Feb 2017, 13:11
I like Flight Detent's referenced explanation of his opinion, and its one I share. Perhaps if Iva Harden would make a similar argument for his/her position, and focussed a little less on patronising, we might understand that point of view too.

For those who would choose not to fly slower than S speed - Would you fly below F speed in conf 3? If the answer is no - what would you use as Vapp on a conf 3 landing?

6th Feb 2017, 13:56
Well, what happens if you lose an engine at V2? Do you have enough energy to fly?

6th Feb 2017, 15:23
What is your call out for going around? "Go around, flaps", by any chance?
If you do that with flaps 1 and speed below S, you're in trouble.
For that reason in our outfit it's a no-no. Big German airline.

6th Feb 2017, 15:53
If you are going around in Conf1 or Conf2 there is no need to retract flaps. They will meet gradient requirements since they are supposed to meet in CONF3. Official airbus position is all speeds up to Vls are flyable but bank margin for triggering protection reduces. F and S are flap retraction speeds.

6th Feb 2017, 15:59
If you activate the approach phase and manage the speed will it go to VLS or will it target S F speed as appropriate? If you apply speed brake what happens to your VLS? Take flap 2. The fuel penalty is minor, the deck angle is better and the margin for manoeuvre higher. I don't fly at Vne nor do I fly at VLS.

6th Feb 2017, 16:08
What I stated is airbus answer. Obviously it is short term solution if you expect to fly below that speed for a considerable time one should reduce speed after going to higher configuration.

7th Feb 2017, 04:27
Represented by the top of an amber strip along the airspeed scale on the PFD.

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 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 : 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 : 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 1.22 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).

Also corresponds to the final takeoff speed.

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

The above is from Airbus. The reason you can use auto thrust and select and fly down to VLS is that is may be needed to non normal approaches (flap or slat problem etc). Or you may be heavy weight in some types (345/6, 380) and need to select below S or F speed to establish below Vfe.

7th Feb 2017, 05:36
Our SOP for going around is to call for flaps, whatever the setting was. Also you're prone to do that automatically, I don't believe Airbus says "only when conf 3 or full " in their SOP.
That exactly is the reason why Airbus uses S speed as the target with managed speed, to be in position to go around and retract flaps from there.

7th Feb 2017, 11:26
Horses for courses. If you had speed below S then don't have to retract. Below is from Airbus:

When landing configurationis not established (CONF1 or CONF2) there is no particular need to retract theflaps one step as there is no drag issue, and therefore, no performance impact.However, standard go around procedure (GO AROUND – FLAPS) could be applied,even when in CONF1, provided TOGA detent is reached prior flaps retraction (toensure GO AROUND phase engagement). A/THR and FD’s will maintain VLS, and SlatsAlpha / Speed lock function will inhibit slats retraction if necessary

Smokey Lomcevak
7th Feb 2017, 18:36
The other interesting situation relevant to this discussion would be an overweight landing that required conf 3 for MISAP perf restrictions. Our SOP is to go around in conf 1+F (despite the fact that planning tables exist for missed approach climb limiting weights for a go around in conf 2). If this were to happen in the later stages of approach - at or around Vapp, one will be going around well below S speed. Obviously a situation we'd like to avoid in real life, but its no dramas in the sim.

7th Feb 2017, 19:18
Well , I think everyone here understands that the lowest speed that can be flown is VLS and it has been properly quoted.
But here how it works: when someone from the management (pilot) is scared of something because of his own conception ,generally it will be prohibited for the whole community of pilots of that airline.

8th Feb 2017, 06:04
Sadly citation2, most who like to fly close to VLS have no idea of the implications of doing so, and can't quantify any gain in performance for being there. Just knowing it's the lowest selectable speed is not enough knowledge. Unless you are in a non normal approach, or heavy weight needing to get below Vfe, I can't see why you need to be there.

8th Feb 2017, 10:18
Good airmanship and sound judgment will help you understand the issue. The discussion is not to fly at VLS and flaps 1 during approach but merely close to S speed. So if ATC requires 180 kts while your S speed is at 183 kts , you won't fall off the sky because of 3 kts below S. Airbus confirms that the upper limit is Vmax and Lower limit is VLS , that does not mean you will fly at those limits on a routine basis, but only in case of emergency / abnormal.

But being close to S to satisfy an ATC requirement is a different story , again sound judgment is required , depending on weather, weights etc.., how far below S?

It is not clear cut , black or white . In gusty winds that would not be appropriate, in normal condition , few knots below S , will not hurt you.
By the way on final configuration , flaps full , how far are you from VLS?

8th Feb 2017, 12:45
With flaps full VLS is Vref. So you fly at Vapp which is the same as any other aeroplane. You see my point then.

8th Feb 2017, 13:49
It hasn't been mentioned so far so apologies if I'm telling you how to suck eggs as quoted already its fine to go below manuevring speed provided you stay above Vls, BUT be very cautious about getting too close to Vls if you have any turns to make eg intercepts. Vls will rise considerably in the turn when in this portion of the drag curve.

No it won't. I think you meant Alpha Prot speed and Alpha Max speed

This seems to be a common confusion: There are three low speed warning strips at the bottom of the speed scale, one on top of the other. The top one (usually) is VLS and it only changes with a change of wing shape - i.e. Slats, flaps and speed brakes.

V alpha prot and V alpha max are usually below VLS and they do change with wing loading. Alpha Prot speed can get above VLS sometimes during manoeuvres.

From FCOM DSC-31-40:

1)Minimum Selectable Speed (VLS)
The top of the amber strip [- they mean a hollow rectangle outined in amber] along the speed scale indicates this speed. It represents the lowest selectable speed providing an appropriate margin to the stall speed.

(2)Alpha Protection Speed
The top of a black and amber [- horizontal stripes] strip along the speed scale indicates this speed.
It represents the speed corresponding to the angle of attack at which alpha protection becomes active.

(3)Alpha MAX Speed
The top of a red [-solid red] strip along the speed scale indicates this speed. It represents the speed corresponding to the maximum angle of attack that the aircraft can attain in pitch normal law.

8th Feb 2017, 14:32
I am sorry Call it VREF or XYZ your margin to VS1g is shorter in final configuration than with Flaps 1 . Well initially just look at your PFD and watch how far is VLS with Flaps 1 and compare it to final configuration VLS.

Now let me Help you with this :

S : Equal to about 1.22 VS to 1.25 VS of CLEAN configuration

I would like to emphasize on "Clean " because it means that your margin to VS1G is based on a higher speed ( green dot ) and not actual configuration , hence greater margin to VS1G . That is why VLS is so low in Flaps 1 . Notice how VLS goes down when you select Flaps 1.

From Supplementary - Definition:
At Green dot speed VLS= 1,28 VS1G
S speed : 1,22 to 1,25 VS1g of CLEAN. Let's say an average of 1,23

So your margin to VS1G in S speed is 1,23X1.28=1,57 VS1g

Now let's compare this value to the tables available on Limitations :

For a weight of 60 T , VS1G is 120 kts in Flaps 1+F and 125 in Flaps 1
For a weight of 60 T , S speed is 185 kta


Close enough isn't it ? With the table I found a margin of 54% to VS1g and with the calculation I found 57% of margin to VS1g.

Let's say 55%. S speed is 55% greater than VS1g.

Now let's come back to final configuration . What is your actual margin to VS1g? 23% right? Vref= 1,23 VS1g

So where do you think you are closer to a stall ? You accept to fly with 23 % of margin to VS1g and not 55% , just because they call it VREF?

8th Feb 2017, 18:12
Seems not much has changed over the last 9,5 years. :hmm:

9th Feb 2017, 06:50
In airbus we don't think of stall but triggering protection. Quoted below is from airbus presentation on the subject:

Manoeuvring capability

At V2, one engine out: stabilised turn at 30 degrees bank before alpha max.
At V2+10 all engines operative: Stabilised turn at 40 degrees before alpha floor.
At green dot: Stabilised turn at 45 degrees bank before alpha prot.
At Vls (final take off up to landing):Stabilised turn at 40 degrees before alpha floor.
General rule: fly approach manoeuvres at the manoeuvre speed minimum.
However if ATC speed constraint:

Selected speed
Possible to fly below manoeuvring speed down to Vls. It enables standard manoeuvres.
Margin versus alpha prot is reduced.
Margin could become small in turbulent conditions, or with high bank angles.
If such conditions are anticipated consider CONF change.

Resume to managed speed when out of ATC constraint.

9th Feb 2017, 09:34
How much manoeuvring are you doing when at Vref? Hmmm I wonder if that's why there is a difference in the margin.

10th Feb 2017, 16:29
How much are you supposed to manoeuvre with Flaps 1 and speed 180 ? If it is just to satisfy an ATC speed restriction when S speed is few kts higher?

VFe flaps 3 is 185 kts . Take flaps 3 and fly 180 kts , you will have less chance to fall off the sky.

11th Feb 2017, 10:10
Citation, so you see my point then. Some pilots don't understand what they are doing, and don't understand why you can only fly selected speed when below manoeuvre speed.