A320 CG correction when to apply?

[Flyman35]

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%.

[dgtl887]

Forward of 27%

[simyoke]

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.

[AerocatS2A]

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.

[simyoke]

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%.

[Denti]

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

[FlightDetent]

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.

[joe falchetto 64]

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.

[AerocatS2A]

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

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.

[simyoke]

Thank you very much, Captain.

[simyoke]

Thanks much for the clarification. Really appreciate.

[flyingbear88]

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

[FlightDetent]

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.

[sonicbum]

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%.

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 %.

[FlightDetent]

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

[sonicbum]

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

Ahah yes I believe so !