737 CRZ CG

[SR71]

Can anyone shed any light on the suggestion that CRZ CG is MACTOW CG - 4% MAC?

My loading manaul suggest the CG location does no such thing as fuel is burnt off?

:confused:

[B737MRG]

Hi SR71,

I daily put the correct CG in the PERF INIT page to obtain a higher Max Cruise Altitude. Depending on the FMC software version installed, the CG in the FMC is default set to 5 or 15 (or perhaps other values).

Changing the default 5 value to the actual value (for example 18), will increase your max ceiling by a few hundred feet, just what you need when ATC asks you "are you able to climb Level ...". Changing from 15 to 18 won't give you much.

You can enter the Takeoff CG for a start, but you must update this CG during flight. Use the fuel curve printed on the manual loadsheet to verify how CG moves when you burn fuel (this is not a straight line from TOW to ZFW in the W&B enveloppe).

Hope this makes sense
Pat

[SR71]

G'day Pat,

I'm aware of what should be entered in this field but I was wondering where the anecdotal(?) -4% correction on TO originates from?

Its even in the FMC Guide although FAA and CAA corrections differ?

During burn off from full to empty tanks, my loading manual suggests the CG moves aft, then forward, then aft again....

[BOAC]

IIRC the classic has c of g on the trim wheel scale?

[CaptainSandL]

The -4% correction to MACTOW is to allow for the worst case C of G change due to fuel burn from the auxiliary fuel tank (when fitted). There are also small MAC changes for fuel burns from the other tanks but these are much less with a max change of 2%, see any manual loadsheet for details.

The maneuver altitude capability is increased by approximately 100ft for every 2% the C of G moves aft, so the technique is used to find the actual maneuver altitude.

The default C of G values (UK CAA all types 4%; FAA, -300/18.5% -400/15% -500/23% -700/22.2%.) particularly the UK CAA values are conservative and may be used safely without entering the actual MAC from the loadsheet. However if you want to find your actual altitude capability then use loadsheet MAC -4%.

I suppose it would also be correct to simply read the actual MAC from the trimwheel when in the cruise, but I have never seen this done. Not an option on the NG's as they removed the MAC scale from the stab trim indicator.
S&L

[SR71]

CaptainSandL,

Yours is the traditional answer to the question.

But if you produce a loadsheet which uses figures for an aircraft with no auxiliary fuel tank fitted, why is it necessary to use this correction?

Your loadsheet will give you a figure for the MACTOW CG which you may then correct for the CRZ using your knowledge of what happens to the CG as you burn fuel from the centre and wing tanks.

Personally I don't use the 4% correction as I fly a -300 with no auxiliary tank. I merely enter the appropriate CG using the trim wheel indication once in the cruise exactly as you suggest.

The 4% correction is almost always conservative as you observe which got me thinking....

I hazard a guess most people are flying around with an artificially "more forward" CG location than they need to be...

[CaptainSandL]

Maybe it is the traditional answer because it is the correct answer :D

If you have no aux tank then you don’t need a full 4% correction. If you are flying a standard 3 tank classic then the greatest adverse fuel trim change is from full wings (9,200kg) to 6,500kg. This gives a change of -2% or a reduction in manoeuvre altitude capability of 100ft.

Remember that if you are scratching about for that last 100ft you probably shouldn’t be up there - especially if there is any turbulence.

S&L

PS Say hello to JP for me

[SR71]

S&L,

Not meaning to offend...but if you don't have an aux tank, the 4% correction is erroneous like you say. That was my concern...

It doesn't bear any resemblance to what the CG is doing and on a Classic you've got an indication of exactly where it is yet no-one seems to use it!

:confused:

[Flight Detent]

Hi all,

Not only does the NG not have the said MAC scale on the stab trim, there is also quite a large range of auxiliary tank configurations available, so be careful with these rules of thumb!

Cheers, FD :bored:

[eckhard]

Take care if using the CG indicated on the stab trim wheel, as this is only valid for a takeoff flaps setting. In other words, with the flaps up, the trim pointer merely indicates units of trim and not CG. To prove this, note the indicated CG in the cruise and then work it out from MACTOW and fuel quantity; the two figures will be different.