A320 CG correction when to apply?
 

Dear fellow pilots,I was wondering when to apply CG correction during takeoff speed calculation concerning (increasing v speeds by 1 kt and decreasing flex by 2 degrees) is it when below 25% or 27% because the FCOM states the following which is confusing:(please state the answer for both RTOW and LPC)
PER-TOF-TOC-14-10 P 1/10

CORRECTIONS DUE TO DIFFERENT TAKEOFF CONDITIONS
when the takeoff conditions are different from those provided on the chart,apply associated corrections.
Note: -if the RTOW chart is based on the CG being at 25%,the crew can determine the flexible temperature at a more forward CG by decreasing flexible temperature by 2 degrees.V speeds must be increased by 1 kt.
-25% is the basic certified limit,on which all takeoff computations are based.To take into account the operational margins,the above penalties must be applied when operational CG is forward 27%.

Forward of 27%

The note confuses me too; the second note says 25% is the basic certified limit, on which all takeoff computations are based. As the aforementinoed note says, if the computations are already based on 25%, why does the FCOM tell us to apply panelties when CG is forward of 27%? After all, 25% is worse than 27% in terms of performance isn't it?
Maybe, just maybe, FCOM dose not mean that RTOW chart for A320 is based on 25%; I am wondering what CG position does the RTOW chart use for calculations.

seems clear to me that you apply it if it is forward or 27% in order to maintain a margin, I.e, they don't want to be right on the 25% limit.

The muddling point is that if RTOW chart computations are done with 25% CG why bother with 27%? 25 is more forward than 27%.

Plug the numbers into the iPad and let it do its magic... Is there more to it?

Denti, depends on how your iPad (or whatever app you're using) is configured. The label is modifiable by the administrator, ours currently reads <= 27%.

Answer: according to Airbus guidance, the limit when to apply alternative forward CG lo-speed performance corrections is 27% indicated CG. (reference available).

The logic is, as I understand it: The aircraft is flown-certified with 25% CG position as the differentiating value. In controlled condtions, loading-wise, that is.

Because in real life operating scenario, the calculated / indicated CG postion is only approximation of actual CG - true passanger weight distribution being the most variable factor - a conservative margin of 2% is normally added to the limiting value.

It is easy to understand that RTOW charts show 25% because that's how they are calculated, but to use them without a correction for forward CG, one needs LDS figure of 27 or more.

Correct. Sometimes reading the notes on the FCOM is worth the time.
Let's check PER-TOF-TOC-10-20 TAKE OFF CHART DESCRIPTION , Corrections due to Different Take Off Conditions and check the "note" field.

Correct, RTOWs are for a 25% CG. If the CG is forward of 25% then the RTOWs aren't valid. However the calculated CG is not 100% accurate, so to allow for a small amount of error, the CG adjustment is made if the calculated CG is 27% or less. They are being conservative.

Thank you very much, Captain.

Thanks much for the clarification. Really appreciate.

what effect do these corrections have on the aircraft in practical terms though?

In practical terms, once you accept "almost ok" in the flight preparation stage you'll eventually end up with a smoking hole later. Sets the wrong example for the cannon fodder.

Smartarsing aside, I think the controllability may not be the direct victim but climb gradient would be (larger control surfaces deflection) The FCOM corrections are +1 on speeds and -2 on FLEX, IIRC.

PER-LOD-WBA-LTS

Note:When referring to CG lower than 27 %, an operational margin is taken into account. It is the reason why performance at forward CG (lower than 25 %) must be used for operational CG lower than 27 %.

@sonic: In practical terms, the OP probably goes under Flyman40 these days. You reckon? :}

Ahah yes I believe so !