Airbus minimum selected speed (a320-340s)

[jtr]

Did the obligatory search but didn't find what I was after.

From a company a few years ago we had it beaten into us that on approach one was not to select below the relevant maneuvering speed (e.g. not select below green dot if clean, S speed if in flap 1, F speed if flap 2)

This "makes sense" given A/Thr wont do it (i.e. Le Bus doesn't think it is smart)

Now said company where I first flew the 330 and 340 it was in the manuals, this may have been a company addition as they were bastardising the manuals a lot at that time.

Slide forward to the present where I fly a 320 and went looking in the (new) manuals for this point and could find no reference.

Can anyone help me on this or is it not an Airbus method?

Thanks.

[rudderrudderrat]

Hi jtr,

It's hidden in FCTM: NO -100 P3/10

"If selected speed is to be used to comply with ATC, the requested speed should be selected on the FCU. A speed below the manoeuvring speed of the present configuration may be selected provided it is above VLS. When the ATC speed constraint no longer applies, the pilot should push the FCU speed selector to resume managed speed."

[jtr]

Thanks, that was actually kind of contrary to my impression and in fact means <if you take it literally> you can select something like 25kt below green dot and not take any flap.

Less than desirable but the OEM seems to think ok...

[FlightDetent]

The way I was trained it is safe to fly below target manouvering speed. What benefit is there from doing it remains to be seen and is the pilot's choice.

Instructed to fly 180, -S at 184: easy decision. OTOH 25 kts below green dot would put you well below Vls and maybe below Aprot (my guess), so quite a different cup of tea.

[Oceanic815Pilot]

[FONT="Comic Sans MS"][/FONTThis "makes sense" given A/Thr wont do it (i.e. Le Bus doesn't think it is smart)


The last time I checked the A/THR system will allow you to select a speed all the way down to Vls....Hence why it's called "V Lowest Selected" speed.

[Checkboard]

That's because manoeuvring speed isn't the minimum speed for a configuration - it's a speed which allows excursions above and below without infringing minimum or limit speeds. VLS gives you your 1.3 buffer above the stall for the configuration, and is thus your minimum "safe" speed.

[jtr]

"The last time I checked the A/THR system will allow you to select a speed all the way down to Vls....Hence why it's called "V Lowest Selected" speed. "

Sorry oceanic, I failed to point out I meant with managed speed, i.e. "normally" To wit, A/Thr and FMGC wont let you do it when they are flying the jet.

Flightdetent, "OTOH 25 kts below green dot would put you well below Vls and maybe below Aprot (my guess), so quite a different cup of tea." was pulling numbers from out of nowhere but a green dot of say 210 usually shows a Vls of about 180, i.e. still gives you a 5 knot margin above Vls and is "ok" as far as bus is concerned.

Thanks for the replies folks.

[Microburst2002]

Anyone has the precise definition of maneuvering speed?
I once was told it gave margin for a 30 deg bank, but I am not sure.
I thought that VLS is in fact maneuvering speed, since it provides a good margin

[Last Ditch]

Any views on.. which confg-speed delivers lesser fuel consumption-
1.Clean confg, speed 180 (well below best L/D) or
2.Flaps1, speed 180
Thanx

Also, views on..
1.Flaps1, speed 160 or
2.Flaps2, speed 160

[tubby linton]

I think that you will find that the flight guidance will only command 15 degrees of bank if you are flying below the relevant maneuvering speed

[Alexander de Meerkat]

Tubby - I think that constraint only applies to engine-out (EO) operations. When the FMGC detects an EO condition, the AP & FD limits bank angles for t/o & app to 15° when the a/c speed < maneuvering speed (F,S or Green Dot speed) –10 kt. Then it has a linear increase to 25° up to maneuvering speed -3 kt. I hope that makes sense.

Regarding the manoeuvring speed limit, different companies often impose higher limits. The problem comes with definitions. FCOM PRO SUP 10 has some good stuff on this. On conventional aircraft, VA (known as design manoeuvring speed) is the speed above which it is unwise to make full application of any single flight control (or "pull to the stops") as it may generate a force greater than the aircraft's structural limitations. Clearly that is not possible on the Airbus. Therefore Airbus define VA: Maximum design manoeuvring speed. This corresponds to the maximum structural speed permitted for full control deflection, if alternate or direct law is active. In other words, it is not really relevant in normal law. Therefore Airbus talk of 'characteristic speeds' (F, S & O). They also say, the A318, A319, A320, and A321 have exactly the same manoeuvre margin that a conventional aircraft would have at its reference speeds. My own company, however, imposes the limit that we are not to fly below reference speeds to ensure an extra margin of safety. You can argue the ins and outs of that, but that is the 'official' view of many operators. I hope that is helpful.

[Speed Freak]

Have gone down to 150 kts in conf 1 on a very light 319. At 9 degrees pitch up attitude it is not pretty at all.

Have generally noticed below green Dot speed , you can only safely reduce speed by about 5 knots for the weights we fly. However taking flaps 1 gives a very big range of speeds and it makes sense to keep flap 1 for 180 or 170 and in some cases 160 also. However on a 3.3 degree glide path at my home base 160 in conf 1 is difficult and just about manageable in conf 2.

On 321's generally vls is more than 160 for our average weights and conf 2 is an acceptable config for a wide range of speeds.

End of the day it all depends on your personal comfort factor.

[Yurich]

We know that VLS=1.28VS1g in clean configuration, so if FAC computes VLS and represent it on PFD will VLS be increased during maneuvering with bank or not?

[vilas]

Microburst
I am unable to paste so quoting from Airbus presentation:
Maneuvering speeds- Approach
Flap maneuvering speeds are recommended to fly the aircraft in a given flaps configuration for:


Comfort in aircraft maneuver:
Green Dot, S, F are comfortable
Particularly in turbulent conditions,or
Turns at high bank angles

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.
At V2+10 AEO and VLS stabilised turn at 40 degrees before alpha floor and at green dot 45 degrees.

[Rocket3837]

Yrich,
VLS is fixed and does not change when aircraft turns.
However, the alpha prot & max will increase during turns due to increase in G ( G dependent).

[Yurich]

Agree, thanks a lot!

[Microburst2002]

The "margin" is the gap between VLS and V alpha max (most of the times out of view but you can get an idea if you see V alpha prot).

VLS does not change with load factor, while V alpha max and V alpha prot will increase with load factor. In a pull up or a steep turn the gap narrows as V alpha prot gets closer to the fixed VLS.

I think it would be interesting to see a definition of maneuvering speed from other manufacturers such as boeing.

[halfofrho]

The last time I checked the A/THR system will allow you to select a speed all the way down to Vls....Hence why it's called "V Lowest Selected" speed.

Actually the lowest speed you can select on the FCU is 100kts, but with autothrust engaged it will not let the speed fall below VLS even if you have a lower speed set in the window.

[Chris Scott]

Quote:
"VLS is fixed and does not change when aircraft turns.
However, the alpha prot & max will increase during turns due to increase in G ( G dependent)."

Agreed. Just a reminder to novices that VLS increases progressively as the "Speedbrakes" (why do they call them that?) are extended. On the other hand, V[alpha-prot] and V[alpha-max] are unaffected, if memory serves.

[Goldenrivett]

Hi Chris,
On the other hand, V[alpha-prot] and V[alpha-max] are unaffected, if memory serves.
Although I can find no reference to your statement in FCOM, I think they must be affected in the same way they move with changes in delta g. Use of speed brakes will result in increased AoA.

[Microburst2002]

More precisely, I would put it
speedbrakes reduce stall AoA

therefore V alpha max should increase accordingly, and VLS, too

[Chris Scott]

Hi Goldenrivett,
"Although I can find no reference to your statement in FCOM, I think they must be affected in the same way they move with changes in delta g."

It seems logical that they would, so my memory may well be wrong. Perhaps vilas, or someone else with access to the FCOM, etc., will find a reference.

Quote from Microburst2002:
"...speedbrakes reduce stall AoA
therefore V alpha max should increase accordingly, and VLS, too"

Yes. My tentative understanding of aerodynamics suggests that the airbrakes would, as you say, reduce the stall AoA because of the disruption to airflow. That would obviously increase the stall IAS/EAS. But the loss of lift resulting from the disrupted airflow would increase the stall IAS still further for a given weight.

The combination of these two factors may explain the great effect that use of "speedbrakes" has on VLS, which IIRC is simply a function of Vs-min.

[vilas]

Goldenrlvett
Speed brakes effect incidence opposite of flaps so AoA actually reduces. VLS can be seen increasing with SB. When flying manually reversion to speed mode can occur at VLS-2 KTS but with full speed breaks it occurs at VLS-19(increased VLS). FCOM doesn't say much on this.

[Goldenrivett]

vilas
Speed brakes effect incidence opposite of flaps so AoA actually reduces.
Please explain why speed brakes actually reduces AoA.

I thought speed brakes spoil the lift over part of the wing effectively reducing the lifting Wing Area, thus requiring an increase in AoA to maintain the same lift.

[Chris Scott]

Quote from Goldenrivett's last-but-one post:
"Use of speed brakes will result in increased AoA."

Did you mean that use of speedbrakes will require an increase in AoA? If so, that would explain your misunderstanding with vilas...

[vilas]

Goldenrivett
Microburst is talking about stalling AoA. With flaps that reduces. You are talking about actual angle AoA. Flaps being under the wing the change in effective chord will increase the AoA. Speed brakes are above the wing the chord will reduce the angle of attack. However the pitching moment with flaps is down and speed brakes is up.

[Microburst2002]

With spoilers deployed, the airflow separates at a lower AoA (in those sections where there are spoilers deployed).

Not the whole wing, but if you are extending speedbrakes you can have a significant part of the wing in a stalled condition at speeds well above clean VS1G. Those stalled sections will give control problems and the margin to a totally unacceptable situation is reduced. I guess that is why the VLS increases with the speedbrakes extended.

There was an incident of a 320 where a spoiler remained deflected due to a mechanical failure. The VLS remained the same, but the AP had some trouble maintaining lateral control during approach, when speed was VAPP. If I recall correctly. It was that small section of the wing that was creating the problem, probably.