A320 C.G Being Less Than 25% ...Question

[kuwait340]

Hello All....


in the A320 if the MAC T.O CG is less than 25% been indicated on the load sheet...

if max RTOW is required for that day...we must decrease the weight by 1000 kgs and increase V1,Vr and V2 by 1 knot.

if lower than MRTOW and Flex power is used ....we must decrease the flex by 2 degrees and increase V1,Vr and v2 by 1 knot.

in the QRH chapter 4.00 " in flight performance" page 4.01 "speeds"
on the bottom of the page it says "For C.G less than 25% add 2 knots to Vls and Vref"

so the question is ... for landing ...when the C.G is been known that it is less than 25% for T.O..do you have add 2 knots on Vapp ? ...that's what i understand when they say add 2 knots on Vls (by mental calculation not on mcdu because you can't change Vls) ..when you add vls + 2 that mean you have to increase the Vapp by 2 knots to stay 5 knots more than the Vls.Or do the FMGC take care of that and increase the Vls and Vrefs and Vapp?...one pilot told me that you add 2 knots on Vref incase of total Fac's loss so you won't have O,F,S speeds . ?

by the way..always when fully configured for landing ...i always notice that the C.G wheel is less than 25 % even if the load sheet shows that the Ldg c.g is more than 25 %.

thanks guys

[sebxl]

Very interesting question for which I only have part of the answer.
Concerning the pitch trim wheel, it is a take off wheel only available for take off, used for take off setting and if you have a look during cruise to both this wheel and fuel pred page (where cg pos is) you ll see quite a difference. Because there is a difference between the position of THS and true CG.

For the rest I'll wait for the other to comment on it
Try to email Airbus via your ops office

[kuwait340]

thanks for the reply..

i'll be waiting for the comments .

[Dani]

A wealth of questions... - I try:


Different operators handle the 25% increment differently. Obviously that's approved by AI. We e.g. used the imcrement only from 27% on.

The "wrong" trimwheel setting certainly also has to do with the "FMGC vs. FAC" mistery: Both do not show the same weights (FMGC derived from pilots input, FAC from AOA).
I cannot completly follow your "mental calculation" but I presume that the imcrement is a safety margin in tailheavy configuration. As we all know most aircraft are more stable when they are nose heavy.
I doubt the story about FAC loss. If you loose FACs you fall very fast into alternate law, thus having a 10 kts increment. That's more of a realistic speed adjustment.


Any opinion on that?
Cheers,
Dani

[BSD]

Kuwait340,

Sadly, I know nothing of Airbus types, so I could be talking out of my hat! Please excuse me therefore if what I say next is total tosh.

Could it be that V2 is based on 1.2 x Vs whereas approach speeds are based on 1.3 x Vs? Thus raising the speed in the take-off case ensures a greater margin to provide sufficient control response/effectiveness, which is a given in the landing case as a result of the greater margin above Vs.


BSD.

[kuwait340]

thanks guys...

i will do my home work...and come back soon.

cheers

[airbus757]

In the t/o you describe a nose heavy condition. Because of this the tail has to exert more downward force to counteract the condition which has a net effect of decreasing the performance. On the 340 we can use this to our advantage if the a/c is tail heavy. We get a net increase in performance if cg is greater than 29.

As to this affecting the landing performance you can bet your bottom dollar it has been considered and if it made enough of a difference it would be published in the landing performance data.

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[Dani]

But we are talking about A320...

25% is considered to be a rather tailheavy configuration. Normal T/O trim range is between 25 and 35, while mostly it is around 30.

I assume this is about the same on the A340?

Dani

[kuwait340]

refrence

Fcom 2 chapter 02.10 page 3 top of the page note.

[Bullethead]

Take off CoG has no real bearing on the landing CoG as you have burnt off a large amount of fuel and the CoG will changed accordingly by the time you are on approach, also you will be using a much increased flap setting for landing than taking off and and flying at a slower speed so the trim setting will also be affected.

The CoG scale on the trim wheel is not indicative of the actual aircraft CoG it is only a scale by which you set the stab for take off.

Anyhow who can fly to within two knots? :}

Regards,
BH.

[kuwait340]

it is not a matter of flying within 2 knots, as you know the A/THR will do a great effort to maintain the Vapp .

i did some studies on the C.G below 25%...and with reference to the FCOM 2 mentioned above, and the QRH page mentioned earlier ....you can connect the dots.

cheers

[SIDSTAR]

The question of FMGS vs FAC derived speeds has raised its head again in this thread. The FAC weights can be in error by a very large amount depending on the signal from the AOA probes from which they are estimated. 0.1 degree error in the AOA probe can give errors of up to a couple of tons!

There is absolutely no need to use anything other than the FMGS weights and speeds unless a "Check gross weight" message is received. If in doubt, you have the loadsheet to rely on for your ZFW plus the fuel remaining to calculate the current weight. Anyone attempting to use the FAC weights in flight is asking for trouble and it is specifically not recommended by AI.

The trim wheel is indeed almost useless in flight, except in Alternate Law. It is not even very accurate for takeoff but the autotrim system will look after you once airborne, provided all is in working order.

[Jetset320]

Take off CoG has no real bearing on the landing CoG

So when due to incorrect loading 10 pax are asked to be re-seated from the front to the back for example. Can they retake their original seats after takeoff and remain there for landing? Lets exagerate; what about 30 pax?

[Bullethead]

Jetset320

You'd have to examine the landing CoG, without a specific example I'd say that once in the cruise the pax could move around but they would more than likely have to resume their seats for landing. The pax may have been moved to satisfy a landing trim problem not a take off problem.

The zero fuel trim, of course will not change, but the actual CoG will change as fuel is burnt off. It usually moves forwards.

Regards,
BH.

[Jetset320]

Hope I'm not hijacking the thread, but how exactly is the landing CG checked then? In a trim sheet we check that the ZFW CG and the T/O CG are within the envelope.

[EMIT]

DANI

A CG of 25 % is a FORWARD CG, which is called nose heavy.

If you support the aircraft at its' CG, it will be in balance.
If the balance point is forward, the nose must be heavy in relation to the tail.
If the balance point lies far aft, the tail is heavy compared to the nose.

As far as landing CG is concerned - all computor loadsheets will show Zero Fuel CG, TakeOff CG and Landing CG.

If you make the loadsheet yourself with LPC laptop, you will only get a solution if all three mentioned CG's are within limits.

If you reposition large numbers of people, you, or your dispatcher will have to recalculate the loadsheet to check whether CG stays within limits for all conditions.

Note - reposition of normal quantities, e.g. some people to the bathroom, cabin attendents walking around with trollies, etc, are accounted for by setting the limits sufficiently safe.

[Slasher]

Jetset320 if you have a CG range TO thru to ZFW then the LW CG lies in between. If both are in range then LW is in range. The extreme would be a dead-stick dry tanks landing, ie landing at ZFW.

[airbus757]

EMIT you are correct. So how does all that come into play when we are discussing t/o performance?

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[Dani]

Emit, I stand corrected, thanks.
I was confused because on a manual loadsheet (or on the graphs of LPC) you wander forward the bigger the number is. But this is of course to get a higher nose down trim, not a more nose heavy condition.

But still we don't know why we have to add 2 kts at 25 or lower.

It must be that at the lower CG numbers, we come closer to a limitation, so they invented this 2 kts trick.

Dani

[FlightDetent]

Hope I'm not hijacking the thread, but how exactly is the landing CG checked then? In a trim sheet we check that the ZFW CG and the T/O CG are within the envelope.
Good question, wrong place. Airbus has no limits on ZFW CG (A320) but provides landing CG envelope - see AFM. If your company decides to use those to create an ZFW envelope, it is them you need to ask.

FD.

[airbus757]

Dani

But still we don't know why we have to add 2 kts at 25 or lower.



I have explained why in my previous post. Perhaps I wasn't clear?

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