Airbus performance question

[Brotti]

Dear Friends ,
I am a little bit confused with a peformance issue which I encountered today regarding selecting the takeoff speeds in Hamburg :
Our actual takeoff weight was something around 72 tons ( A321 ) , and Config2 speeds were : 112*/134/136. Runway was reported as wet on the ATIS , so the wet corrections as for the speeds were : -8/-3/-3 .
Min. speeds at the right bottom of the RTOW chart were 112/../.. ( I cant not recall the exact V2 and Vr speeds atm which are not relevant with my question )
So , as we all know , asterix on V1 speed was for to draw our attention that mentioned speed is below min V1 value at the bottom of the chart , which is ok. As far as I know , airbus states that , there is no need to check speeds against min values at the bottom of the RTOW chart up to more than 1 corrections applied. So , when we deduct 8 knots from 112 , then we go to 104 knots , which seemed a bit unusual. And when I tried to enter this value for V1 on PERF page , MCDU gave a warning message that the speed was too low.
So , we decided to stick with the min 112 knot value.
The question is , why did the aicraft had generated that kind of warning message or have we missed something with the performance calculations ?
Thanks in advance for you comments.

[Fly3]

Maybe because you tried to enter a speed below Vmg.

[Brotti]

You mean Vmcg or is it another speed abb.? Even it is Vmcg , airbus does not mandate to check V1 against min Vmcg ? Am i right ?

[woodja51]

I think the previous post has it.

V1 must always be greater than vmcg... So as to be able to accel on one engine and not depart the strip before rotate..possible statement of the obvious sorry,

I could be wrong but are there not a couple of charts that require a check of
Vmu vs rotate and V1 greater than vMcg in the preamble pages?.

Has been a long time since used those charts as most operators have gone to lpc etc...

Still, could be something else??

Mw

[woodja51]

By the way, that is a massive v1 to vr split from any twin I have seen... Is along time to wait to get from 104 to 134 ( or 131) to rotate... Normally never see that except in quads in my experience.., but not flown 321s so could be right...maybe vr is limited to vmu given the geometry of the aircraft etc??

[9.G]

brotti, sorry but that just doesn't sound right. I'm not sure what the cause of such an enormous drop in V1 might have been but I haven't come across anything like this either. Very weird indeed. :ok:

[mutt]

Airbus use optimised V-speeds, so its normal to see such a speed spread. Remember that Vr is the speed required to ensure that you achieve V2 at 35 feet. V1 in this case is the slowest V1 that you can have and still accelerate to Vr in the available distance. (Note if it wasn't limited by the VMCG, it might even be lower)

Gone are the days when the V1/VR ratio was a fixed ratio.

Mutt

[guclu]

No speed check is required for the first correction. However, if the first influence
correction follows a conservative FCOM correction, a speed check is required.

I think you remember this information.


But in your case I think you are not using TOGA thrust and taking off with MTOW. Because if you carefully review your FCOM you will find out that the information above is ONLY VALID for MTOW calculation and not in FLEX situation.

With the speeds you mention I am sure you are making a flex take off and because of that you have to check the minimum speeds.


I hope it helped.

:ok::ok::ok:

[9.G]

Mutt, the fixed ratio is still there. 112 is exactly 84% of the 1 ratio V1/Vr.
More on optimization speeds can be found getting to grips to performance brochure from airbus. 1>V1/VR>0,84 is the manufacturer values.:ok:

[Brotti]

Dear guclu ,
Thank you very much for your clear and concise explanation. I havent realised that the speed check against minimums was kinda segregated as FLEX and MTOW calculations. But as i reviwed the FCOM in this perspective , it seems that you are right.
But , to me , Airbus must have distinguish these two seperate situations clearly enough in order to prevent any further misunderstandings in the future.
I would also like to thank all other posters...

[guclu]

Brotti my pleasure,

also I would like to remind another issue which is most of the time not known correct.

People usually think it is not necessary to make a speed correction for Qnh correction but if you are making a TOGA tafeoff for MTOW case then you need to make the speed correction.

Here is the extract:
No speed correction is required for QNH and bleeds influence (Not applicable to maximum takeoff
weight determination).

[South Prince]

Does the following shed some light?

CORRECTIONS PRODUCED ON THE RTOW CHART
Applicable to: All
For example: Refer to PER-TOF-TOC-10-30 EXAMPLE OF TAKEOFF CHART
A description of this correction is given on Refer to PER-TOF-TOC-10-20 DESCRIPTION OF THE
CORRECTIONS ON TAKEOFF CHART. The list of corrections is not exhaustive, however the most
commonly used corrections are wet runway, QNH, air conditioning and/or anti ice. A maximum of
three corrections can be produced on one chart.
To apply the corrections, proceed as follows:
1. Enter the chart with given OAT and wind to determine the maximum takeoff weight before
correction.
2. Apply the first correction:
If OAT is less than or equal to TVMC (line 3), apply W correction from line 1 and V1/ VR/ V2
corrections from line 2.
Else, (for OAT greater than TVMC), apply W correction from line 3 and V1/ VR/ V2 corrections
from line 4.
3. To combine a second (and third, as applicable) correction:
If OAT is less than or equal to TVMC (line 3), apply W correction from line 1 and V1/VR/ V2
corrections from line 2.
Check that the resulting speeds are higher than the minimum speeds displayed on the RTOW
chart and that V2 is higher than the VMU limited speed (Refer to PER-TOF-TOD-25-10 SPEEDS
LIMITED BY VMC).
If OAT is higher than TVMC (line 3) or if the above speed check is not fulfilled, apply W correction
from line 3 and V1/ VR/ V2 corrections from line 4. No speed check is required.
Note: ‐ QNH correction is given for ±10 hPa . It is allowed to extrapolate linearly for greater QNH
deviation.
‐ When using a takeoff chart with failure cases, it is not allowed to combine two failure
cases.
‐ Corrections from the chart must be applied from top to bottom, i.e. in the RTOW on Refer
to PER-TOF-TOC-10-30 EXAMPLE OF TAKEOFF CHART, apply the wet correction first.
‐ If asterisk or dotted lines appear in the correction boxes, refer to more conservative
corrections provided in the FCOM.
‐ No speed check is required for the first correction. However, if the first influence
correction follows a conversative FCOM correction, a speed check is required.