Fuel Savings with aft C of G
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Fuel Burn vs C of G.
On a dc10-30
the difference in fuel burn between full forward CofG and max Aft Cof G is a a whisker over 1%.
Aft is 1% less.
Does not sound like much but an 11 hr flight @ 10 tonne /hr
= over a tonne saved.
It has nothing what so ever to do with weight off the runway, or VMCg or even VMCA
BBG
the difference in fuel burn between full forward CofG and max Aft Cof G is a a whisker over 1%.
Aft is 1% less.
Does not sound like much but an 11 hr flight @ 10 tonne /hr
= over a tonne saved.
It has nothing what so ever to do with weight off the runway, or VMCg or even VMCA
BBG
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Current MD-11 info is that an optimum aft CG, about 32% aft of the mean aerodynamic chord saves about 2.7% of total burn. VMCG and VMCA are always figured for max aft CG.
CG position is constantly displayed in the cockpit and the fuel system uses the tail tank to control the CG to the best aft position in flight.
CG position is constantly displayed in the cockpit and the fuel system uses the tail tank to control the CG to the best aft position in flight.
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Quote kijangnim
That's what I thought you might mean. 
My company are currently pushing us for 23-28%MAC of the 747 for fuel efficiency. They don't give us any numbers to back up their request though. Maybe that should be a project for me!
Sorry badly written 
, a forward C of G will enable you to increase your takeoff weight since the arm C of G tail (rudder) will be longer thus provides better control in case of an engine failure, we tried it on the B767, and the difference can be up to 3 tonnes.
, a forward C of G will enable you to increase your takeoff weight since the arm C of G tail (rudder) will be longer thus provides better control in case of an engine failure, we tried it on the B767, and the difference can be up to 3 tonnes.
My company are currently pushing us for 23-28%MAC of the 747 for fuel efficiency. They don't give us any numbers to back up their request though. Maybe that should be a project for me!
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Greetings MIKE 773
What I did was to run different FRWD C of G settings using the takeoff performance software provided by Boeing, cause at that time the airline wanted to certify the B767 at a higher take off weight ( which is a paper certification, nothing to change in the airplane structure
)
the delta takeoff weight they wanted to obtain was far beyong what we needed, and the 3 tonnes was sufficient
get in touch we flight ops engineers (in your airline) I am sure they will be delighted to help you.
What I did was to run different FRWD C of G settings using the takeoff performance software provided by Boeing, cause at that time the airline wanted to certify the B767 at a higher take off weight ( which is a paper certification, nothing to change in the airplane structure
)the delta takeoff weight they wanted to obtain was far beyong what we needed, and the 3 tonnes was sufficient
get in touch we flight ops engineers (in your airline) I am sure they will be delighted to help you.
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Thanks for that, kijangnim. I'm sure our Flight Ops Engineering guys have the data. I might see what I can find out. Much better than blindly following "the rule", although that's what they expect sometimes!
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"...a forward C of G will enable you to increase your takeoff weight since the arm C of G tail (rudder) will be longer thus provides better control in case of an engine failure..."
There is no "takeoff weight" adjustment based on C of G variations.
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Let's see if I get this right:
A more aft CG gives an increase in Vmcg (minimum control ground), which increases the lower V1 limit (Decision speed, go or no-go). And if the ASDR (accelerate-stop distance required) exceeds the ASDA (available), TOM (actual take-off mass) would have to be reduced as a consequence of a more aft CG.
If you meant a more forward CG, this would result in a reduction the Vmcg and a possible reduction in V1. This does not necessarily mean an increase in TOM is available, but if the ASDA was limiting (unbalanced take-off), you would be able to do so (a reduction in V1 would decrease ASDR, but increase TODR).
And as far as my understanding goes, an increase in TOM can be "substituted" with a FLX take-off.
But I must say my "knowledge" are pure theoretical, so a confirmation from someone with practical experience would be excellent.
A more aft CG gives an increase in Vmcg (minimum control ground), which increases the lower V1 limit (Decision speed, go or no-go). And if the ASDR (accelerate-stop distance required) exceeds the ASDA (available), TOM (actual take-off mass) would have to be reduced as a consequence of a more aft CG.
If you meant a more forward CG, this would result in a reduction the Vmcg and a possible reduction in V1. This does not necessarily mean an increase in TOM is available, but if the ASDA was limiting (unbalanced take-off), you would be able to do so (a reduction in V1 would decrease ASDR, but increase TODR).
And as far as my understanding goes, an increase in TOM can be "substituted" with a FLX take-off.
But I must say my "knowledge" are pure theoretical, so a confirmation from someone with practical experience would be excellent.
Moderator
I've been watching this thread develop and, I must confess, I am getting a little confused along the way ... time to fill some holes in my knowledge bank, I guess ....
(a) conventionally, certification stalls are done at forward CG as this gives the highest stall values.
(b) takeoff speed schedules then are based on this conservative stall paradigm.
(c) Fwd C of G will increase your take off weight (gives better control in case of engine failure)
this seems a bit avant garde .. conventionally, the (forward CG determined) stall determines the speed schedule independent of CG variation in operations ? Perhaps someone can throw some light on Kijangnim's comment ?
(d) we tried it on the B767, and the difference can be up to 3 tonnes
I'm not a 767 man but have played in the AFM in the past with no recollection of such a certification-driven variation. Perhaps Kijangnim can amplify the comment as I'm missing something along the way here ?
(e) If you meant a more forward CG, this would result in a reduction the Vmcg and a possible reduction in V1
while there is nothing to prevent multiple Vmcg demonstrations, it is not routinely addressed .. ie, the usual deal is Vmcg is fixed and determines min V1
(f) but if the ASDA was limiting (unbalanced take-off), you would be able to do so
in general, no ... min V1 usually is fixed for the conditions
(g) an increase in TOM can be "substituted" with a FLX take-off
I think you might be confusing flex with derate .. two generally similar considerations for the line pilot ... but very different animals when it comes to Vmcg/min V1 considerations.
Would be nice if someone could give us a bit of hard data on where the variation of RTOW with CG originates ? Certainly could be arranged .. but not generally seen.
yours in confusion ....
(a) conventionally, certification stalls are done at forward CG as this gives the highest stall values.
(b) takeoff speed schedules then are based on this conservative stall paradigm.
(c) Fwd C of G will increase your take off weight (gives better control in case of engine failure)
this seems a bit avant garde .. conventionally, the (forward CG determined) stall determines the speed schedule independent of CG variation in operations ? Perhaps someone can throw some light on Kijangnim's comment ?
(d) we tried it on the B767, and the difference can be up to 3 tonnes
I'm not a 767 man but have played in the AFM in the past with no recollection of such a certification-driven variation. Perhaps Kijangnim can amplify the comment as I'm missing something along the way here ?
(e) If you meant a more forward CG, this would result in a reduction the Vmcg and a possible reduction in V1
while there is nothing to prevent multiple Vmcg demonstrations, it is not routinely addressed .. ie, the usual deal is Vmcg is fixed and determines min V1
(f) but if the ASDA was limiting (unbalanced take-off), you would be able to do so
in general, no ... min V1 usually is fixed for the conditions
(g) an increase in TOM can be "substituted" with a FLX take-off
I think you might be confusing flex with derate .. two generally similar considerations for the line pilot ... but very different animals when it comes to Vmcg/min V1 considerations.
Would be nice if someone could give us a bit of hard data on where the variation of RTOW with CG originates ? Certainly could be arranged .. but not generally seen.
yours in confusion ....
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In a normal (not ALT 1 or ALT2) dispatch, T/O performance charts assume the CG to be at the worst position (FWD limit). Every time your actual CG is behind the FWD limit, you will have excess performance both for accelerate stop as well as for accel Go, even if your TOW=MTOW. If you want to use this excess performance to improve your TOW limitations, you must use certified procedures ALT 1 or ALT 2, which no longer assume the CG is in the FWD limit, and the loading has to be done is such a way so not to have a CG forward of the new limits for ALT 1 or ALT 2 as applicable. Actually, taking your CG AFT of the FWD limit improves your TOW capacity. This is applicable to the 767.
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"while there is nothing to prevent multiple Vmcg demonstrations, it is not routinely addressed .. ie, the usual deal is Vmcg is fixed and determines min V1"
Ah, thank you for that info. While a fixed Vmcg assuming worst CG position might be used in diagrams in the POH, does anyone know if any FMS uses this kind of calculations?
"in general, no ... min V1 usually is fixed for the conditions"
Would you care to elaborate?
I understand that my example is pure theoretical, because in many cases Vmcg might be fixed, as you mentioned.
What about using fixed derate to decrease Vmcg, thus increasing TOM if takeoff is Vmcg limited? Wouldn't that be a similar scenario, except we are reducing Vmcg in different ways?
"I think you might be confusing flex with derate .. two generally similar considerations for the line pilot ... but very different animals when it comes to Vmcg/min V1 considerations."
No, I am thinking of the Assumed Temperature method, ASST or FLX takeoff. If you have a gross mass margin for the actual runway conditions, you can use this method. Is this not correct?
With fixed derate I understand derating the engine in the FMC N1 limit page, which is also allowed in contaminated runway conditions.
Thank you for your input, I appreciate learning from the pilots with practical experience. Please do correct me if I'm way off.
Ah, thank you for that info. While a fixed Vmcg assuming worst CG position might be used in diagrams in the POH, does anyone know if any FMS uses this kind of calculations?
"in general, no ... min V1 usually is fixed for the conditions"
Would you care to elaborate?
I understand that my example is pure theoretical, because in many cases Vmcg might be fixed, as you mentioned.
What about using fixed derate to decrease Vmcg, thus increasing TOM if takeoff is Vmcg limited? Wouldn't that be a similar scenario, except we are reducing Vmcg in different ways?
"I think you might be confusing flex with derate .. two generally similar considerations for the line pilot ... but very different animals when it comes to Vmcg/min V1 considerations."
No, I am thinking of the Assumed Temperature method, ASST or FLX takeoff. If you have a gross mass margin for the actual runway conditions, you can use this method. Is this not correct?
With fixed derate I understand derating the engine in the FMC N1 limit page, which is also allowed in contaminated runway conditions.
Thank you for your input, I appreciate learning from the pilots with practical experience. Please do correct me if I'm way off.
Moderator
First up the references to ALT-1 and -2 give me the answer to my concerns .. presumably the restricted forward limits are associated with lower speed schedules which, in turn, give an improved RTOW for a runway limited case. Now I can go back to sleep in comfort ...
"in general, no ... min V1 usually is fixed for the conditions" - Would you care to elaborate?
With a fixed certification basis then, given the ambient conditions, min V1 generally is a fixed number limited by Vmcg
What about using fixed derate to decrease Vmcg
.. that changes the certification basis (derate functionally is a "recertification" of the engine .. as opposed to flex which is just a reduced thrust setting) .... so all bets are off and we start anew ..
No, I am thinking of the Assumed Temperature method
If I am reading your earlier post correctly, it appears that you see flexing as providing an opportunity to increase RTOW .. it is this point which suggests to me that you are confusing flex and derate. On shorter runways, where a takeoff may well be min V1 limited, use of derate may give you a reduced min V1 (due to the reduction in certification Vmcg). As the weights will be low in any case, this reduction in speed schedule may then permit an increase in RTOW when compared to the higher thrust operation.
Important thing is to be aware that flex and derate are quite different .. although, functionally, the pilot will see much the same results for most operations. Both, of course, can be combined to give you a flex reduced derated takeoff.
If I have misunderstood your earlier post (and that may be the case) please do post some clarification and we will continue the discussion ...
"in general, no ... min V1 usually is fixed for the conditions" - Would you care to elaborate?
With a fixed certification basis then, given the ambient conditions, min V1 generally is a fixed number limited by Vmcg
What about using fixed derate to decrease Vmcg
.. that changes the certification basis (derate functionally is a "recertification" of the engine .. as opposed to flex which is just a reduced thrust setting) .... so all bets are off and we start anew ..
No, I am thinking of the Assumed Temperature method
If I am reading your earlier post correctly, it appears that you see flexing as providing an opportunity to increase RTOW .. it is this point which suggests to me that you are confusing flex and derate. On shorter runways, where a takeoff may well be min V1 limited, use of derate may give you a reduced min V1 (due to the reduction in certification Vmcg). As the weights will be low in any case, this reduction in speed schedule may then permit an increase in RTOW when compared to the higher thrust operation.
Important thing is to be aware that flex and derate are quite different .. although, functionally, the pilot will see much the same results for most operations. Both, of course, can be combined to give you a flex reduced derated takeoff.
If I have misunderstood your earlier post (and that may be the case) please do post some clarification and we will continue the discussion ...
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Thank you John, for that elaboration.
You may have misunderstood my initial statement. What I meant by "substituting" increase in TOM for FLX, is that if you have a gross takeoff mass margin, meaning you have enough runway to increase TOM, you could instead use flex to save engine life. And that is of course used in cases where you have no need to increase TOM.
Have I understood it correctly?
You may have misunderstood my initial statement. What I meant by "substituting" increase in TOM for FLX, is that if you have a gross takeoff mass margin, meaning you have enough runway to increase TOM, you could instead use flex to save engine life. And that is of course used in cases where you have no need to increase TOM.
Have I understood it correctly?
