Alt CG on B777..

[Yani Yani]

Is there some way of figuring out the actual cost savings using the alternative aft CG system on the B777 ? I know some B777 operators are using it and others not. Loading an aircraft using the system and applying it to performance calculations results in among other things lower Vspeeds for takeoff.

Cheers

[CONCOMBRE]

Hi Yani!

Could you tell me more about that "alternative CG" ? :)
I wasn't aware about such a procedure! Maybe I forgot to attend the right course!!! :}
My airline currently fly 777, mainly balanced around 24%

bst rgds

[Pugilistic Animus]

Perhaps the performance figures are recalculated on a reduced stall speed at aft CoG? ---as this would cause the tail to carry a greater percentage of the load

[Mad (Flt) Scientist]

I *think* this has been discussed before, but "cg" and "aft" are both under the four letter search threshold, so not easy to find the thread(s).

From what I recall, it's actually the use of a Forward CG limit which is moved aft of the 'normal' one. That than allows the stall speeds etc to be calculated for that "less forward" limit, which will give slightly lower stall speeds (due exactly to the tail lift/downforce issue mentioned above).

If my recollection is correct, it should really be called an alternative forward cg option, because AFAIK the aft limit itself isn't moved, and is in amy case pretty much irrelevant* for most performance purposes.

* unless you start mucking about with Vmca etc., but I can't see anyone going down that route ....

[CR2]

On the 744F alternate fwd cg enables a higher payload to be carried from a restrictive runway. You get a performance calculation for the full envelope, for 20ish % MACTOW and 24ish % MACTOW. Can give you a couple of tons more.

[bigmountain]

When the CG is moved aft, the aircraft pitching moment is reduced .This results in a reduced downward tail load required to trim the aircraft.Since the required wing lift is the sum of the aircraft weight and the tail-download for trim, this aft CG reduces the required wing lift.This reduction of required wing lift manifest itselfs as reduction in speed for the same pitch attitude and weight that reduces the stall-limited speed.The wing lift reduction also reduces the takeoff attitude -limited speed ( the parameter used to set the margin to tail strike rotation). These reduction in speeds reduces the field length required for given takeoff weight;the reduction in tail down load reduces the trim drag which improves climb capability
Using the alt cg option on the B777-300 B777-300 ER allows the possibility of are Higher Assummed temp or increasing the performance TOW at the particular airfield when the Takeoff MAC is 26% or greater.
This ALT CG selection is usually available when the Boeing OPT ( optmized Performace Tool) is used via the EFB or the Laptop.

BM

[Yani Yani]

Thanks for input guys.

I aware of the benefits of using the Alt CG principle, eg reduced V speeds and increased assumed temp, which reduces the general wear and tear on the aircraft components and therefore leads to cost savings. My actual query is how to calculate the actual cost savings using this method. It is perhaps a question for the Boeing Company.....

[GMDS]

Take the TO weight calculation with and then without alt cg. With the difference go into the flightplan and check what +/- burn you get.
Its not accurate, but the computer curve should consider the +/- downforce in kg/lb needed to trim the aircraft by giving us the resp. max TOW.

GMDS

[halas]

Alt CofG is used for takeoff. As mentioned above it offers slightly lower V figures and a degree or two in assumed temp TO thrust.

Cruise CofG is different - and dependant, if l am correct, on your company and configuration of the type being flown. Unless you are operating the 77W where 7.5 is currently the default setting. Otherwise it would be closer to 30.

Therefore cost saving means not off loading freight when taking off from WSSS at 29 degrees at MTW (and using TOGA to do it) for a six hour flight.

halas