Alternate Forward CG
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In my company we often have to use either 20% or 22% mac to get out.
We have figures published for both of these for the different flap settings on all our limiting rnw's. 737-800/900 by the way. Standard loading nearly always has us around 22-23% mac. Makes all the diff some days.
MTOW is a figure calculated to ensure the aircraft can safely operate out of a given rnw on a given day with give conditions ie wet/dry and temp. Normally it is calculated for the worst case situation ie. the actual CofG being on the forward limit.
Now if you have your load sheet and on that day it tells you that you have an actual CofG on that day of say 24%mac you now know that you are safe useing figures for MTOW calculated for 24%mac. Airlines are unlikely to have figures derived and tabluated for you for every given CofG possiton. For this reason in my company we have tables/(BLT) giveing you MTOW figures for 20%mac and 22%mac.
If you know you have a CofG aft of the CofG on the tables you know you are safe.
In simple it is a method of degrading the safety margin in order to operate the aircraft from limiting airports safely.------
Granted not a great statement the same safety margin exists just the amount the actual aircraft performance on the day exceeds the required performance is reduced.
We have figures published for both of these for the different flap settings on all our limiting rnw's. 737-800/900 by the way. Standard loading nearly always has us around 22-23% mac. Makes all the diff some days.
MTOW is a figure calculated to ensure the aircraft can safely operate out of a given rnw on a given day with give conditions ie wet/dry and temp. Normally it is calculated for the worst case situation ie. the actual CofG being on the forward limit.
Now if you have your load sheet and on that day it tells you that you have an actual CofG on that day of say 24%mac you now know that you are safe useing figures for MTOW calculated for 24%mac. Airlines are unlikely to have figures derived and tabluated for you for every given CofG possiton. For this reason in my company we have tables/(BLT) giveing you MTOW figures for 20%mac and 22%mac.
If you know you have a CofG aft of the CofG on the tables you know you are safe.
In simple it is a method of degrading the safety margin in order to operate the aircraft from limiting airports safely.------
Granted not a great statement the same safety margin exists just the amount the actual aircraft performance on the day exceeds the required performance is reduced.
Last edited by Todders; 27th Jun 2008 at 21:33.
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A lot of folks getting too involved in the weeds level, here ... considerations are all to do with certification ..
(a) takeoff performance starts with the AFM stall speed for the weight.
(b) AFM stall speed usually is determined at the forward limit for reasons tied up with the tail download consideration mentioned in several posts above .. ie the forward limit normally gives the highest flight test stall speed within the envelope CG range at a given weight.
(c) the consideration is a bit like derate in that the OEM is perfectly entitled to do several mini-certifications for the aircraft. In the same way derate lets the operator take advantage of lower Vmcg/Vmca, restricting the AFM forward cg limit permits the operator to take advantage of a lower stall speed for a given weight . .this lower stall speed then flows into the other speeds pertinent to takeoff speed schedule and comes up with a few extra kilos RTOW especially for runway limited circumstances .. this may not apply at lower weights if the basic performance is minV1-limited.
(d) in all cases, to take advantage of the permission, the operator has to have the supplement in the AFM (generally at humungous cost .. it has always ever been .. and will always ever be ... the same)
(e) In simple it is a method of degrading the safety margin in order to operate the aircraft from limiting airports safely.
Not quite .. the minimum "safety margins" remain the same for certification .. what the OEM is doing is a multiple certification .. a bit like having two slightly different aircraft models within the same Type
(f) technically speaking, you should have the tool to calculate takeoff-data for each CG position within the env
Those of us with the tech background could come up with some plausible data .. however, at the end of the day, the operator can only operate within the boundaries of the AFM .. so, if secondary CG limits are not published then this argument remains academic.
(a) takeoff performance starts with the AFM stall speed for the weight.
(b) AFM stall speed usually is determined at the forward limit for reasons tied up with the tail download consideration mentioned in several posts above .. ie the forward limit normally gives the highest flight test stall speed within the envelope CG range at a given weight.
(c) the consideration is a bit like derate in that the OEM is perfectly entitled to do several mini-certifications for the aircraft. In the same way derate lets the operator take advantage of lower Vmcg/Vmca, restricting the AFM forward cg limit permits the operator to take advantage of a lower stall speed for a given weight . .this lower stall speed then flows into the other speeds pertinent to takeoff speed schedule and comes up with a few extra kilos RTOW especially for runway limited circumstances .. this may not apply at lower weights if the basic performance is minV1-limited.
(d) in all cases, to take advantage of the permission, the operator has to have the supplement in the AFM (generally at humungous cost .. it has always ever been .. and will always ever be ... the same)
(e) In simple it is a method of degrading the safety margin in order to operate the aircraft from limiting airports safely.
Not quite .. the minimum "safety margins" remain the same for certification .. what the OEM is doing is a multiple certification .. a bit like having two slightly different aircraft models within the same Type
(f) technically speaking, you should have the tool to calculate takeoff-data for each CG position within the env
Those of us with the tech background could come up with some plausible data .. however, at the end of the day, the operator can only operate within the boundaries of the AFM .. so, if secondary CG limits are not published then this argument remains academic.
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From the Airbus Instructor Support A-320 Family:
Some considerations about the CG
The location of the CG has significant influence on Performance, on Loading flexibility, on structure and on handling characteristics when in Direct Law.
All those factors contribute to define the CG envelope.
- Performance considerations
The weight and lift forces do create a pitching moment which is counteracted by the THS setting.
When the CG is located forward, the resulting pitching down moment is counteracted by a large THS nose down
setting which induces a lift decrease and a drag increase.
As a general rule, FWD CG penalizes the Performance.
• At Take-off and landing, it affects:
* The Stall speeds: Typically on A330/A340, the stall speed increases by 1.5 kts when CG varies from 26% to full forward CG. This affects take-off and landing speeds thus associated distances.
* The rotation maneuver: It is "heavier", thus longer at forward CG.
This affects the take-off distance. For example, on an A340 at 250 t, the TOD increases from 3165 m to
3241 m, when CG varies from 26% to full forward CG, which represents a 2.42% TOD increase (T/O, FLAP3, PACK: OFF, ISA, ALT 0).
* The climb performance itself: For example, if a climb gradient of 5% is required (e.g. due to obstacles) in the previous take-off conditions, the MTOW is reduced from 257.6 t down to 256.2 t when CG varies from 26% to
full forward CG.
This is why on A320 and A340 take-off performance charts are provided at forward CG (which in most cases is penalizing) and at 26%; these last charts may be used provided the actual aircraft CG is at least 28%
ANP
Some considerations about the CG
The location of the CG has significant influence on Performance, on Loading flexibility, on structure and on handling characteristics when in Direct Law.
All those factors contribute to define the CG envelope.
- Performance considerations
The weight and lift forces do create a pitching moment which is counteracted by the THS setting.
When the CG is located forward, the resulting pitching down moment is counteracted by a large THS nose down
setting which induces a lift decrease and a drag increase.
As a general rule, FWD CG penalizes the Performance.
• At Take-off and landing, it affects:
* The Stall speeds: Typically on A330/A340, the stall speed increases by 1.5 kts when CG varies from 26% to full forward CG. This affects take-off and landing speeds thus associated distances.
* The rotation maneuver: It is "heavier", thus longer at forward CG.
This affects the take-off distance. For example, on an A340 at 250 t, the TOD increases from 3165 m to
3241 m, when CG varies from 26% to full forward CG, which represents a 2.42% TOD increase (T/O, FLAP3, PACK: OFF, ISA, ALT 0).
* The climb performance itself: For example, if a climb gradient of 5% is required (e.g. due to obstacles) in the previous take-off conditions, the MTOW is reduced from 257.6 t down to 256.2 t when CG varies from 26% to
full forward CG.
This is why on A320 and A340 take-off performance charts are provided at forward CG (which in most cases is penalizing) and at 26%; these last charts may be used provided the actual aircraft CG is at least 28%
ANP
