Cruise C of G

[Checkmate]

I have been flying Boeings for quite a few years. Every now and then someone bright spark decides its time to over type the default value. How do they calculate this value. I do sometimes fear they just pick a number. I have and still am happy with the default, but can be converted. Your thoughts gentlemen?

[SMOC]

CX have a procedure in the FCOM with a Fuel Qty vs. MACZFW correction chart.

You take the MACZFW from the loadsheet then subtract the figure from the correction chart, the resulting figure is input to the FMC.

The procedure is used only when max alt is a factor.

[huckleberry58]

Your cruise C of G is related to the MAC (Mean Aerodynamic Chord) and essentially governs your max cruising level and hence your optimum cruising level. Recall the old 172 days with your lift/drag/thrust/weight couples. Well, when you enter a higher cruise C of G value (up to a point), it effectively shifts the lift vector closer to the weight vector. This in turn requires less downward force on the rudder and hence reduces the effective 'weight' of the aircraft (in reality the rudder produces less trim drag) and therefore you are able to cruise at a higher FL.

[barit1]

For "rudder" please read "horizontal stabilizer".

AHA! NOW it makes sense!

[john_tullamarine]

Minor pendantry only ...

Your cruise C of G is related to the MAC (Mean Aerodynamic Chord)

Keep in mind that the practical significance of the MAC to the line pilot is nil. The above statement is a bit like saying "a CG of 100.5 inches is related to a scale from zero to 300 inches .." ie, perfectly true but not illuminating.

requires less downward force on the rudder ... the rudder produces less trim drag

Hopefully, you mean tailplane and elevator ?

[framer]

Rudder??? must be a fast jet guy flying knife-edge....wicked

[alatriste]

CG position on cruise calculation.

The easiest way to know your CG position at any time, is to correct the LIZFW that shows your Load &Trim sheet by the fuel index corresponding to the fuel currently onboard. Treat this index as a virtual new LITOW and enter into the CG envelope, considering your current weight, to find out the new CG in terms of MAC.
This method works as far as you have manual load sheets onboard, an these are made according IATA standards AHM 516,519. Does not matter aircraft type.

REGARDS

[Checkboard]

when you enter a higher cruise C of G value (up to a point), it effectively shifts the lift vector closer to the weight vector. This in turn requires less downward force on the rudder and hence reduces the effective 'weight' of the aircraft

The CofG value in the FMC is in %MAC. Higher values indicate a rearward CofG, lower values a forward CofG. The weight vector moves with the load, not the lift vector. Entering a higher CofG value into the FMC tells the FMC that the load is distributed rearward.

The horizontal stabiliser and elevator normally produce a balancing "down" force in the cruise. A rear CofG requires less force (the weight being closer to the tail) and a forward CofG requires a higher force (weight away from the tail.) If the CofG is forward, the higher down force being produced by the tail must be countered by a higher angle of attack (and consequent induced drag) from the wing - so the performance is better with a rear CofG and worse with a forward one. Some airlines have load policies to take advantage of this and load the aircraft holds from back to front to move the CofG as far back (within the limits) as possible.

The default value on the FMC is simply the most limiting (forward) CofG case. While the methods above are accurate, the differences are so small that (on the 737) I usually enter a figure half-way between the take-off CofG and landing CofG shown on the load sheet.

[huckleberry58]

Hi. I had just came back from an 11hr flight when i wrote my msg so yes I meant 'horizontal stabilizer'.

[Checkmate]

Thanks to you all for taking the time to answer. BRGDS