Selected speed guidance
 

I have seen pilots select speeds below green dot and "S" speed without extending slat or flap. I know this is acceptable within certain limits, but I cannot find any reference to what those limits might be. I have looked in the Airbus FCOM's and a couple of company's SOP's and training material to find some guidance on the use of this procedure and can't find anything written anywhere.

Does anyone here know of Airbus guidance on this practice?

thanks a lot.

I too ofen see the other pilot get a bit phased by selection of a speed below GD or F or S. As far as I understand it, as long as you are above VLS, you are safe. GD is just min drag and F or S are just selection references.

From A320 FCTM 02.080 - Holding Speed and Configuration:

'If green dot speed is greater than the ICAO or state maximum holding speed, the crew should select flap 1 below 20,000' and fly S speed. Fuel consumption will be increased when holding in anything other than clean configuration and green dot speed'.


From A320 FCTM 02.100 - Intermediate Approach:

'If selected speed is 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'.

Thanks for the two references Dan. I don't have access to the actual Airbus FCTM and none of my other references mention this practise. It's interesting that the two references you gave me contradict each other. One advocates configuring the plane (holding) and the other says it's fine to select down to VLS (intermediate approach). It seems to be vary according to comfort level and phase of flight because you just don't see people flying around a VLS. While it may be "safe", it's not comfortable, and I was wondering what different flight departments recommend.

Thanks again

I do not think Dan's references contradict. GD will be above ICAO max holding only in high altitudes where difference GD <> Vls is quite small (say 5 kts - speaking A320). So there is not much room to go below GD and that leaves slats (conf 1) as the only option.

On the other hand, at 60 tonnes the G/D would be 205 kts, asked to fly 190 ? No problems if above Vls as per the book.

Why? Because the question is, with OEI, flaps /slats locked at zero, what speed would you fly ? VLS cf 4 + 45 kts + 5 = 180 kts, so one-ninety when slowing down on both engines and flaps available is really not a big deal.

Then 185 is S speed - optimal for flaps 1. When asked to fly 180 - no need for clean conf when optimal is conf 1. Fly 160 to OM on conf 1? Again, no problemo (provided you can slow her down that is) because it is still well above Vls and Vapp conf 1 is the precise 160!

Well done airbus. :ok:

FD.

Forgot to add: Flying one one engine would have me feeling uncomfortable, no doubt. Furthermore, I do not see reason to go 20+ knots below anything when there's more flaps available, however there is no reason to feel uneasy either. We fly Vls +15 at cruise if required, we fly Vls + 5 on every approach. Flying Vls + 20 should be no scare even if it is below optimum speed for the given conf. I like the approach speed similarity, noticed it on pprune. Normal operation Vls+5, so I will go as far as, say Vls +10 if deemed feasible.

VLS : Lowest Selectable Speed.
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 feet, VLS is corrected for Mach effect to maintain a buffet margin of 0.2g.
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 1.23 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 1.29 VS of clean configuration.

just a question of how much margin one requires or is comfortable with.:)
1.13=20%;1.23=30%;1.28=36+%;and1.29=37+% above vs