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Airbus minimum selected speed (a320-340s)

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Airbus minimum selected speed (a320-340s)

Old 17th Jul 2012, 03:40
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jtr
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Airbus minimum selected speed (a320-340s)

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.
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Old 17th Jul 2012, 08:35
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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."
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Old 17th Jul 2012, 09:49
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jtr
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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...
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Old 17th Jul 2012, 11:06
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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.
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Old 17th Jul 2012, 11:11
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[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.
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Old 17th Jul 2012, 11:13
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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.
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Old 17th Jul 2012, 12:18
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"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.
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Old 18th Jul 2012, 05:49
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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
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Old 23rd Jul 2012, 11:17
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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
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Old 23rd Jul 2012, 12:20
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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
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Old 23rd Jul 2012, 13:08
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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.
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Old 24th Jul 2012, 15:39
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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.
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Old 15th Aug 2014, 08:42
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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?
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Old 15th Aug 2014, 10:31
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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.

Last edited by vilas; 15th Aug 2014 at 10:47.
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Old 18th Aug 2014, 04:22
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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).
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Old 18th Aug 2014, 09:23
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Agree, thanks a lot!
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Old 20th Aug 2014, 12:18
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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.
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Old 20th Aug 2014, 17:20
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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.
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Old 20th Aug 2014, 18:12
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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.
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Old 20th Aug 2014, 20:23
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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.
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