View Full Version : A320 CG

Haroon

5th May 2019, 12:07

https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/1211x449/cg_2fb649c329ced5cba434bc670f945bdad0f9e6f0.png

Just want someone to check if I have understood it correctly. According to FCOM, if we are using the RTOW charts based on CG being at 25% then we adjust the weight (1 ton) and speeds (1 knot) if CG falls between 25 and 27%. If CG is between 15 and 25% then we use the OCTOPUS software and not the RTOW charts for takeoff performance. Thanks.

safelife

5th May 2019, 14:23

25% is the limit. Less and you have to account for forward CG for T/O performance.

Because load sheet is never 100% accurate, and passengers may change seats, a 2% operational factor is used. Hence if your CG on the Loadsheet is less than 27%, you use fwd CG.

For you as a pilot 27% is the relevant limit.

Haroon

5th May 2019, 19:01

Just want to confirm if the RTOW charts become invalid with CG between 15 and 25% and you have to use the OCTO software for weight limit and speeds. Thanks.

FlightDetent

5th May 2019, 19:30

No, not at all. Hope I can explain in simple, understandable language.

The basic envelope per AFM (flight test) is 25% or above. The extended envelope is below 25%, down to the most forward limit of 15% - again, AFM data.

There are no performance charts / tables anymore in AFM for long time. The aerodynamical model (two actually: low speed for TKOF and LAND, and high-speed for CLB, CRZ, and DES) are coded into a software equation / algorithm called OCTOPUS / SCAP Module.

If you want the low speed dataset / results for BASIC, you keep "basic" in the SW package and run the calculations.

If you want the low speed dataset / results for EXTENDED (also called alternative forward CG sometimes), you need to select "extended" in the SW package and run the calculations.

-->> this is what the AFM qoute tells you.

Now there is no SW called OCTOPUS, which is just a runtime library, or a calculation core. The SW package is called P.E.P (think mainframe computer at the Airbus' Perf. Department / your airline Flt Ops Engineers office). Or for line operations "FlySmart w/ Airbus", previously FOVE or LPC "NG". All of them run the OCTOPUS core. You can even purchase the OCTOPUS core in the form of standardized SCAP library (for extortionate amount money) and built it into your own performance software - many do.

The answer you look for is here: The RTOW charts are printed from the P.E.P software bundle. Normally they come out for the BASIC CoG value - thus in case you find yourself with a forward CoG, you need to apply a correction (degradation) to the values from those tables. If you had access to any SW running the OCTOPUS, you could just switch to EXTENDED mode and get the result straight away.

The FCOM you quote tells you, that if you have RTOW chart (for standard CoG - BASIC) in hand, to do those corrections. On the operational side, a 2% margin is applied to make sure you do not go below 25% unknowingly. Hence the 27 as explained, and also 17 you'd find on a normal load-sheet instead of the 15% AFM envelope limit.

Haroon

5th May 2019, 20:12

Other than OCTO being a component of the performance software rather than being a software itself, I think I am also saying the same thing unless I am missing out something. Let me put it this way:

Case 1) You have only RTOW charts with you in the cockpit and the load sheet says CG is 26%. As per FCOM you apply the corrections (i.e. 1 ton and 1 knot). Is this correct?

Case 2) You have only RTOW charts with you in the cockpit and the load sheet says CG is 20%. Now instead of RTOW charts you call up your flight ops and ask them to use the software for calculating the takeoff performance. Is this correct?

Thank you

FlightDetent

5th May 2019, 20:25

No, not at all.

If the loadsheet says less than 27 you need to apply the "below 25" method. Two ways to do that

1) Use RTOW chart (based on 25+) and apply the conservative decrement (2 deg FLEX, 1000 kg IIRC)

2) Recalculate using the OCTOPUS core in whatever software form you have access to

such as

- PEP to print new RTOW charts based on the forward "extended" envelope

- Mainframe via ACARS to make new calculations with EXTENDED mode

- iPad / PC based FlySmart SW - make selection "< 25%"

You are reading too much into it. Forget about the difference between 27 and 25, inside it is the same number and references the same limit

Your AFM quote explains how to obtain certified figures for extended CG envelope range.

The FCOM note advises how to deal with <27 loadsheet in case you only had RTOW charts for 25% or more aft (while you do not have the tools at hand to follow the AFM guidance and recalculate with the EXTENDED option).

-->> One is the certification and flight test language of AFM, the other is the operational fuzzy measurement of FCOM. Safelife explains above well.

(normally there are no RTOW charts for the extended envelope, of course).

One is the certification and flight test language of AFM, the other is the operational fuzzy measurement of FCOM. Safelife explains above well.

Haroon

5th May 2019, 20:45

hmm ok (disregarding the software and considering just the RTOW charts) it means a 1000 Kg correction in weight would be required for a CG anywhere between 15 and 27%. Do I get it or still not?

FlightDetent

5th May 2019, 21:31

Yes, perfect. It is there in the FCOM quote (which is itself coming from a section on how to calculate using 25+ RTOW charts):

- the certified limit is 25, and to make sure you observe it

- anytime in daily ops when CG shows less than 27, please use those corrections for less-than-25% CG.

---------

THE AFM note, on the other hand, tells you what is the way to obtain performance data directly and exactly for CG positions in front of 25%. If you did that and printed an RTOW chart with a forward CG setting, comparing to the normal RTOW values the difference would equal to 1000 kg / 2 deg FLEX in a conservative average.

Haroon

6th May 2019, 05:53

Thank you very much FlightDetent and Safelife.