Does Max and min cg limits change?

[yaverona]

Dear Sirs,

I fly with A320 and I have a cg envelope on FCOM vol3 which shows minimum and maximum cg limits. Same cg envelope is presented in the Weight and Balance Manual.

A couple of days ago, I received a new load and trim sheet from performance department which has more limited cg envelope than the others.

When I ask them, why we have different envelope. They told me that because of the aircraft has 180 seats.

It is suprized me that max and min cg limits changing by configuration. CG position changes by configuration and passengers, catrings so on. But how the cd limits change?

As far as I know cg limit is a structural limit and we should configure the aircraft according that limit.

Any body can explaine to me that are cg limits change by configuration?

Best Regards

[ft]

To account for in flight movements and uncertainties in the pax distribution, both regarding seating and passengers who refuse to conform to the standard weights, the CG envelope is curtailed. Chances are this is what you are seeing. A different configuration could mean different sectioning of the cabin which would in turn mean different curtailments of the envelope.

Edit: BTW, the the aerodynamics and flying qualities have a larger impact on the CG envelope than the limits set by the structure.

Regards,
Fred

[Genghis the Engineer]

The fwd CG limit is generally a structural limit, but it's not unusual to find an overriding handling limit nonetheless.

The aft CG limit is, I think, invariably a handling limit, not structural.

G

[mutt]

Genghis,

Not always the case with airliners, we can reduce the CG envelope to increase the aircrafts takeoff performance. This is generally applied to the forward CG.

Yaverona,

There is a master CG envelope for your aircraft, there is nothing stopping the airline from being more restrictive than the master.

Mutt.

[john_tullamarine]

As one who has designed more trimsheets than I care to remember, I am always surprised to find a trimsheet which DOESN'T have "limits" different to the TC.

The designer should (I would prefer to say "must", but too many either don't bother or don't understand the need) run an error analysis on the sheet design and contain the errors so determined by whatever means is appropriate for the particular sheet. Most times, this is conveniently done by constraining the sheet "limits" so that, if the trim is calculated to be on the sheet limit then, with the error included, it is still not outside the TC limit.

Why an error analysis ?

While a well-designed trimsheet gives an answer as accurate as a longhand calculation for practical loading purposes, there are various errors involved in loading calculations which ought not to be ignored and which will bite people on the tail during the court case .....

Some examples of these errors ..

(a) weighing data ... poor aircraft preparation, non-axial cell loading during weighing, mis-measurement of geometry during the weighing etc. He who believes the fanciful empty weight numbers often generated to the tenth of a kilo and millimetre .. is fooling himself ... the scheduled empty weight data can only ever be approximate due to unavoidable errors in the weighing .. we do, however, do our best to keep them under reasonable control.

(b) use of standard figures - fuel SG, pax weight, baggage weight, etc

(c) use of presumed seating occupancy if the trim is based on seat zones rather than rows. Similar concerns with presumed centroidal baggage and freight loading.

(d) CG movement with inflight fuel burn .. this one sometimes is done very poorly by the designer fraternity.

(e) movement of crew and pax during flight.

JT

[yaverona]

hi mutt,

I respect your comment;

Yaverona,

There is a master CG envelope for your aircraft, there is nothing stopping the airline from being more restrictive than the master.


But my question is that Do we need to restrict CG envelope when we add more seats to aircraft? Couldn' t we use master CG envelope?

Basically, when and why wee need to more restiriction than master envelope?

I can understand to limit CG envelope for performance reason. But same time instead of limit cg envelope we can ask ground handling agent for some certain cg point when they will use during loading.

I hope I could explaine what I mean

[ft]

"Basically, when and why wee need to more restiriction than master envelope?"

When there are uncertainties in the method you use for calculating your CoG.

E g if your method of calculating the index generated by the pax distribution can possibly give you an index which is x units higher than the true index, you need to reduce the index limit in the envelope used by x units. This curtailment applied for pax, cargo, in flight movements and what do you have will change the envelope as seen "on the sharp end" - even though the master envelope remains the same.

The point is to use an artificial envelope which has enough margin to the actual envelope to to make sure that the possible errors in the calculations used can never put you outside the actual envelope.

[john_tullamarine]

yaverona,

Two points,

(a) restricting the envelope in the trimsheet is only one way to get around the error problems, although often the easiest.

(b) the problem associated with increasing numbers of seats is related to zone load planning.

A .. generating a NEW trimsheet ...

(i) if the trim is calculated on individual rows, then we just increase the size of the sheet by increasing the number of trim lines to account for additional rows

(ii) if we prefer to use zones, then there will be a tendency to expand the zone to add a row or two .. this increases the error as the range of loading distributions increases for each zone as constrasted with that presumed in the IU calculations for the zone.

B .. using the same trimsheet.

If the row distribution changes, ie individual row arms change, the same sheet can be used but with a different error analysis and different adjusted limits. However, unless the designer is very clever, this approach generally is too restrictive to be of much practical use

To answer your basic question of why we cannot (or rather, should not) use the TC envelope (the "master" envelope, if you will) is that the calculated CG in practical trimsheets is invariably wrong to some extent. At the court case after the prang, you need to be able to show that you met the legal requirement, viz., the aircraft was not outside the TC envelope limits. If you use the TC limits, then you are going to have a major problem here ...

One needs to keep in mind that the trimsheet is just a clever and convenient way to simplify doing the CG sums .. the requirement is to do whatever is necessary to stay within the TC envelope limits. Except for simple, small aircraft, almost all trimsheets will have constrained envelope limits to get around the error analysis problems.

[yaverona]

hi john_tullamarine,

thanks for your good explanation. As far as I understand, we have more limited envelope than TC to protect us from manual load sheet error due to zone distrubitioni etc..

Is it mean that we can use TC envelope, if we use computer with seat row index.

[Mad (Flt) Scientist]

Well, manual calculation isn't the only source of error. So in addition to computerised calcs and seat ID, you'd in theory need to weigh the passengers, their baggage, the aircraft, etc., etc.

Only if your load calcs, and the data they use, are of the accuracy commensurate with using the type certificate CG should you do so.

[john_tullamarine]

.. may I concur with my good colleague's observations, above.

The main test is "can I demonstrate to the judge (or the audit team from the Regulator) that the cg was within the TC limits" ? The method you adopt to achieve that goal is less important than achieving the goal ...

The answer to yaverona's question must always come down to a sensible error analysis and this can involve consideration of quite a few sources of error.