View Full Version : FMC CG entries

25th Oct 2005, 03:53
The loadsheet MACTOW or CG position is computed to 1 decimal place but the FMC accepts CG entries only in whole numbers. What do you guys do at your outfit? Round it upward/downward? Any company policy on this, or is it silent?

Rounding it upward, methinks, would ensure the stab position is ever so slightly nose down thereby requiring a positive elevator input for rotation. Reduces any likelihood of the nose rotating prematurely.

25th Oct 2005, 04:36
The MAC data from either trimsheet or computer calculation is only as good as the data going in and the design of the system doing the calculating.

The "accuracy" of the IU and/or MAC, likewise, is highly dependent on a variety of matters.

In general, if the nominated CG is within 5-10 mm for empty (depending on aircraft size, geometry, and scales being used) you are doing pretty well, considering all the avenues for error in establishing the empty weight data. For instance, I can recall investigating one Type's dreadful history for an operator many years ago .. turned out that inappropriate jacking procedures were causing significant unexpected scale indication errors.

As the load goes on, the error potential expands and one ought not to be expecting too great a loaded CG accuracy.

I see people, from time to time, plying CG numbers which are away with the fairies in that the implied accuracy is totally unsustainable.

In respect of the present question, and acknowledging that I don't have the documents etc to hand, I would suggest that it doesn't matter whether you round up or down for the FMS.

So far as stab trim is concerned, there is more than enough fat in the certification FT arena to ensure that one need not worry too much about round off effects ....

The Devil's Advocate
26th Oct 2005, 11:51
I'd agree with j_t on this in that it should not matter about rounding up or down, the figures are so little it will have un-noticible effect during rotation.

Also on the 737 NG that I fly, the FMC will accept CofG to one decimal place, plus after it calculates a stab trim, most people I fly with would only use that figure to check it's almost the same as the figure given on the loadsheet, but would in fact trim the aircraft to the loadsheet trim given.

If I have this correct in my head gengis, rounding it upwards, you would be moving the CofG towards the tail and therefor increasing the likelihood of the nose rotating.

Somebody correct on this if i'm wrong, i'm just picturing a trim sheet in my head :oh:

26th Oct 2005, 12:16
Depending on what update you have (U5.0 & above), you can enter a CRZ CG on the PERF INIT page.

But I always understood that all this does is spit out a MAX ALT and OPT ALT that are correlated to the actual loading of the aircraft on the day.

Not saying anyone would, but you can artificially increase your max Mach at altitude by entering a more rearward CG location in this position.

But it is purely a software issue and has no correlation with stab position whatsoever. U10 and above will spit out a stab trim setting for a particular CG location but you still have to manually set the trim. Its just a database lookup table behind it.

Assuming your loadsheet is reasonably accurate, the only way you can adjust the propensity of the aircraft to come unstuck at T/O is to set a trim setting slightly more nose-up than the computed one. The dangers of this are obvious.

For most efficient cruise, you do want a rearward CG location, so all the bags should be in the boot.

There is a trade-off in terms of controllability but there always is isn't there?


26th Oct 2005, 13:51

"If I have this correct in my head gengis, rounding it upwards, you would be moving the CofG towards the tail and therefor increasing the likelihood of the nose rotating."

Yes it would. The point i was trying to raise is that by deliberately entering a slightly aft C of G & then using the FMC computed trim setting, the stabiliser would then be set up for a more aft C of G than actually exists. In so doing, minimise any probability of the nose rotating prematurely.

But I am persuaded by JTs point, essentially that the difference is too minute to be noticed anyway.

26th Oct 2005, 14:07
I totally agree with j_t but just to share;

The airbus family FMGC accepts decimal values (for ex: 25.4).

And consider a trim sheet made by hand. Do you think your pencil's tip is so sharp to find a correct decimal value ?

26th Oct 2005, 23:13
.. and I wouldn't underrate manual trim sheets.

As an aside, one needs to keep in mind that the output of the trimsheet is IU and the numeric value of the IU is entirely an artefact (artifact - whichever you prefer) of the sheet's design. That is to say, a sheet designer can make the IU whatever sort of values he might choose for whatever strange reason motivates the design philosophy...

If the sheet is well conditioned in its design and executed with a modicum of care by the user, the calculated CG can easily be within 1-2 mm of the longhand calculation results. However, many trimsheets are designed with little consideration for error management and the results can be highly variable.

I can recall a sheet I designed many years ago for a corporate operator. The semi-graphical/longhand system which they had been using previously was a bit of a pain and they wanted to simplify the calculation exercise.

As the particular aircraft presented some design difficulties for the trimsheet (one of the reasons it hadn't been done before) I put a lot of effort into the job and we ended up with a very nice product. (The sheet design subsequently was pinched by all and sundry and I would not be surprised to see it surface on this Type in all sorts of strange places ..)

Outcome was that the first asy training session using the trimsheet with their previous standard training ballast configuration showed the CG to be out of limits. They were a little sheepish when I investigated and showed that the previous system, due to its poor design conditioning, was sufficiently error prone that they had been conducting such training routinely outside the envelope (not significantly, but it could have been embarrassing at the the Enquiry).

The Devil's Advocate
27th Oct 2005, 04:08
My appologies gengis, I see whare your coming from. What, if any, is the the effect on the 747 if you allow the aicraft to be out of trim by only a small degree. I have yet to try it on the 737 but I have noticed on the 146 that a required trim of 4 units set to 3 units will practically have no difference on the feel of the controls.

I'm mostly wondering after doing a few 747 loadsheets with freighters how noticible the trim would be if I was out by a few decimals of a degree.

28th Oct 2005, 04:56
Advocate, the answer is no discernable difference. As JT has pointed out, the loadsheet is rubbish in-rubbish out anyway and the margins are a lot wider than estimated CGs to within a decimal point. In any case, the 747-400 has a nose gear pressure switch which senses actual load on the nose & uses this to predict an approximate trim band (744 has 3 trim greenbands - mid/centre/aft). Whatever you enter into the FMC gets compared with this nose gear pressure switch predicted band & if they differ, you will get an FMC message "Check GG". So long as the stab is anywhere in the correct greenband, the airplane will fly. That is a way larger margin than 0.2% MACTOW.