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Low MACTOW on TO. Below 27% on 320.

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Low MACTOW on TO. Below 27% on 320.

Old 7th Aug 2013, 23:48
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Low MACTOW on TO. Below 27% on 320.

Basically have to trim backwards quite a lot sometimes, meaning C of G is mostly forward(unpreffered). Is this correct?

Also. Can somebody please explain to me why we need MAC ZFW?
Little confused with this stuff. Appreciate the help.

Thanks guys.
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Old 8th Aug 2013, 02:10
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Less than 25% CG is considered forward CG. It is forward CG so there is speed increment in Vapp to create higher aerodynamic forces for better handling. ZFW CG as the name signifies is CG without fuel load. MAC TOW is with fuel. This will keep changing towards ZFWCG as fuel is consumed. So to get ideal CG position for landing or for the flight ZFWCG must be in favourable position. This is obtained by proper distribution of cargo and passengers. Incidently the relationship of trim wheel with CG is only for takeoff. Inflight CG is given on Fuel prediction page.

Last edited by vilas; 8th Aug 2013 at 02:35.
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Old 8th Aug 2013, 08:07
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I believe the box calculates current CG from the ZFW CG and the current Fuel On Board, that's why you have to enter it into the box.
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Old 8th Aug 2013, 08:54
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Tail-heavy is nice for fuel burn, and should be what all load planners strive for, but other operational requirements of ground handling may (depending on carrier) carry more weight than chasing an optimum ZFW CG. This will be particularly true on shorter flights, where the fuel burn benefits of an aft CG are not as prominent as on a long-haul flight.

Some operational requirements that may run contrary to optimum CG are: Priority baggage loading (F, J and short connection bags loaded in the door position), separating mail, freight and bags in different holds, late connecting freight, mail or bags, carrying return catering or fly away kits. etc.

Of course, if you're flying LCC or charter it's relatively easy: Fill up the rear and stuff the rest in the front.

As mentioned above, the box needs to know ZFW CG so it can add the fuel quantity at any given time to obtain actual CG and thus trim setting.
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Old 8th Aug 2013, 09:45
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Heave!!!

W S T. 26 years of flying airline 'frames, I have NEVER had one which is anywhere near stick free between unstick and above V2 with ALL engines operating, let alone 1 engine out. Admittedly these craft have been conventional FBW, the steel wire type!

Always to trim I have had to trim nose up, never found neutral nor the slightest need for nose-down trim.

Hopefully some test-pilot type will spot this thread and put us right, in the meantime make sure you've had a good breakfast before rotation and it's a good idea to eat the cereal boxes as well, allegedly it builds up the biceps (ripping the boxes into flakes) making the physical task easier?
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Old 8th Aug 2013, 09:51
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The A320 family don't care where you load them for Specific Air Range (SAR) considerations.

Originally Posted by Getting to grips with fuel economy, page 10, para 3
The A320 family does not show the same SR variation with CG as the other aircraft. The aft CG produces worst SR at FL290, crossing over to show an improvement at higher flight levels. The SAR variation is much smaller also. This is due to a complex interaction of several aerodynamic effects. The SAR can be considered effectively constant with CG position. Loading is therefore not critical for fuel economy for the A320 family.
http://www.google.co.uk/url?sa=t&rct...,d.d2k&cad=rja
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Old 8th Aug 2013, 20:23
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Can't speak specificially to the Airbus but general comments -

It is forward CG so there is speed increment in Vapp to create higher aerodynamic forces for better handling

While the higher speed may well be associated with better control responses, I suggest this is not the consideration behind the speed increment.

One consequence of CG is that a more forward CG generally is associated with a higher stall speed than is the case at more aft CG - routine fact of life in the design and certification world.

A lot of performance numbers are based on stall speed - and, in this situation, V1 and Vref/Vapp are important to field length, for instance.

It follows that an OEM has the choice of a simpler AFM using the most critical (highest) stall speed or, at the OEM's choice, a more complex certification with multiple stall speeds based on CG.

If the vast bulk of operations are going to be aft of x% MAC, then that MAC is probably a better option for run of the mill stall speeds. For the less usual circumstance of a more forward CG, the appropriate higher stall speed is used and this is seen by the pilot as an AFM requirement to add a knot or two to the speed schedules.

why we need MAC ZFW?

Standard weight control requirement. Correct load is figured for BRW/TO and ZFW to cover the possible range of CG in flight associated with FBO.
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Old 9th Aug 2013, 00:59
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JT
You are right stall speed is higher in forward positon. Increased speed also increases elevator authority.

Last edited by vilas; 9th Aug 2013 at 01:09.
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Old 9th Aug 2013, 01:29
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.. but the stall speed consideration is the driving force behind the procedure, not a consideration of elevator loads and effectiveness ..
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Old 13th Aug 2013, 23:10
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Always to trim I have had to trim nose up, never found neutral nor the slightest need for nose-down trim.
as a load planner I regularly see nose down trim calculations on the narrowbody Airbii. As you approach the aft CG limit you will pass neutral and into ND trim.
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Old 14th Aug 2013, 00:00
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Tail-heavy is nice for fuel burn, and should be what all load planners strive for.............................re: SMTmember.
I flew many years in a sailplane and its performance was much more enhanced when I arranged for an aft CG. As a matter of fact, my elevator position was neutral in the 60-70 kn speed region, which helps to reduce drag. The L/D is positively enhanced and that is what it is all about in cross country flying.

Incidentally; the spin tendency did not change due to the built in docility of the wing's design. But...............when it did spin (on my input) it became a scary machine with a nose down attitude almost vertical ! Recovery always was standard and within 400 ft (even with half water ballast). All the more reason to keep the speed up below 1000 ft circuit height !
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Old 14th Aug 2013, 07:32
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Originally Posted by john_tullamarine
It follows that an OEM has the choice of a simpler AFM using the most critical (highest) stall speed or, at the OEM's choice, a more complex certification with multiple stall speeds based on CG.
Under FAR25 the OEM doesn't have that option:
Originally Posted by FAR25.103
(b) VCLMAX is determined with:
(...)
(5) The center of gravity position that results in the highest value of reference stall speed
The AFM stall speed information should tell.

EDIT:
Then there is also the manoeuvre margin of FAR 25.143(g) to consider.

Do Vref and landing distance increase with forward c.g.?

Last edited by HazelNuts39; 14th Aug 2013 at 08:31.
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Old 14th Aug 2013, 11:12
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HN39,

I stand corrected with regard to your far more extensive background in these matters than mine.

Then what might be the significance of the speed additive ?
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Old 14th Aug 2013, 12:23
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JT,

The official source for stall speed, Vref and landing distance is the AFM.

If the AFM doesn't schedule these as a function of cg, then Vilas' speed additive to Vapp is in the realm of a few knots for grandma and the kids, I should think.

Perhaps the manoeuvre margins to alpha-floor and alpha-prot are a bit marginal at Vref and forward cg?

Is there a reference for it?

Last edited by HazelNuts39; 14th Aug 2013 at 12:32.
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Old 16th Aug 2013, 18:10
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at least for the A320 airbus differentiates between "basic forward C.G." (25%CG in take off, approach and landing configuration) and "alternate forward C.G" (forward CG limit) in their stalling speed tables (Vs1g) found in the FCOM. moreover if you consult the A320 QRH for in-flight-landing performance a note tells you:
For CG <25% add 2kts to VLS CONF FULL and VLS CONF 3
which corresponds pretty much to the average difference you'll read out form the Vs1g-tables mentioned above.

Do Vref and landing distance increase with forward c.g.?
so VREF increases with a forward CG since VREF=1.23*Vs1g for the bus.

Perhaps the manoeuvre margins to alpha-floor and alpha-prot are a bit marginal at Vref and forward cg?
in my eyes there is still plenty of margin to alpha-floor, etc even if you don't account for a forward CG, since a forward cg alters the VS not significantly.

interesting: there is no such differentiation for the 319 and 321.

first post complete
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Old 20th Aug 2013, 06:30
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From Airbus Weight and Balance document
4.5.2. Maneuverability and elevator efficiency
This limitation concerns all flight phases (ref. B.1.2.3.2) but it is important just before landing, at

low speed, when the elevator must enable the pilot to flare out.

Thus, elevator maximum deflection and aircraft maneuverability limit the CG position forward.




Elevator maximum deflection + Maneuverability margin : FWD limit (landing)

Last edited by vilas; 20th Aug 2013 at 06:31.
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Old 20th Aug 2013, 07:54
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Originally Posted by sierra_mike
at least for the A320 airbus differentiates between "basic forward C.G." (25%CG in take off, approach and landing configuration) and "alternate forward C.G" (forward CG limit) in their stalling speed tables (Vs1g) found in the FCOM.
Thank you for that information - very interesting. Can you clarify if this is to cover different aircraft S/N's having different C.G. limits, or can a given airframe be operated to the basic limit in one flight and the alternate limit on the next flight?
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Old 20th Aug 2013, 09:28
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HazleNuts39
From Airbus weight and balence document.
when determining the aircraft takeoff performance (TOW, TOD), the calculation is always performed at the most forward-certified CG position. Therefore, for aircraft having a naturally aft CG position (tail heavy aircraft), everyday operations might be penalized using this very forward value.
The A320 and the A340 usually have a DOW CG of around 30%MAC and their most forward certified CG limit is around 17%MAC.These


“tail-heavy” aircraft usually have an aft takeoff CG position. This means that their everyday performance is better than the certified performance for the most forward CG position.
So a second CG position forward limit has been certified for these two aircraft.

at 25%MAC for the A320 at 26%MAC for the A340.

All takeoff performances in the certified Airplane Flight Manual are given for both these CG positions.

Last edited by vilas; 20th Aug 2013 at 09:29.
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Old 20th Aug 2013, 10:34
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It is considered FWD CG past 27% when using an electronic performance module and 25% when using paper charts for performance
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