Load sheet construction

[john_tullamarine]

The devil is in the detail but the basics are -

(a) review the TCDS, POH, Weight and Balance manual, etc., to get a feel for the aircraft. Often it is a good idea to draw a picture of the load arrangement on the trimsheet so that everyone knows what is what ...

(b) choose a datum.

The datum has a significant effect on sheet accuracy and is the single most important decision the designer makes in designing a trimsheet. The typical ICAO/GAMA datum for light aircraft up near the nose is very inappropriate for any graphical loading system as the completion errors can be quite significant. Large aircraft often use a similar datum for station identification convenience but then run with a second, more appropriate position for trim calculations .. for lighties, the trimsheet designer has to do something similar.

The datum should result in a boxy, squarish shaped envelope (plotting weight against IU - remember that IU is just a non-dimensional moment). Trick is to use a datum station somewhere inside the envelope ... for the typical lightie, often the aftmost envelope limit works quite well .. although one might equally choose a loading station in the same vicinity to remove one trim line from the calculation.

To get a feel for this, try several datum positions ranging from forward through aft of the envelope. Forward datum positions result in a thin trim envelope sloping from bottom left to upper right on the sheet (typical GAMA POH envelope picture) .. central gives a fairly vertical envelope (desirable as the scale can be expanded to give maximum completion accuracy) ... aft gives an envelope similar to the forward case but sloping the reverse way .. ie from bottom right to upper left (not much point doing this as there is no benefit compared to either of the other options).

A lot of folk are uncomfortable with this idea .. there is NOTHING sacred about the manufacturer's datum .. the datum is no more than a nail on which one hangs one's tape measure. HOWEVER, once you choose your datum, you stick with it .. there is no changing horses mid-race ..

(c) pick a divisor which gives sensible numbers ... bit of a judgement call here and is worked out in conjunction with the selection of trimline division weights.

(d) revised calculation arms become

arm = load FS - datum FS

and the general IU formula for each trimline is

delta IU = (load FS - datum FS) * delta weight / divisor

(e) if the general formula is used for the entry argument at the top of the sheet .. in the form

IU = (CG limit - datum FS) * envelope weight / divisor

the zero position is at the datum, ie we get plus and minus numbers. .. bit messy, so it is usual to adopt a simple fix and add a suitable number for the entry IU to get rid of the minus numbers .. general formula for the entry line (only) - and the envelope (including MAC grid if you want to put that in) - now becomes

IU = scale shift value + (CG - datum FS) * envelope weight / divisor

(e) draw in the entry argument IU scale as a background grid.

(f) draw the envelope at the bottom of the sheet, including the admin bits (scale identification and suitable titles.

Note that constant CG lines on the weight against CG envelope become straight lines on the IU diagram .. but sloping straight lines on the CG envelope become parabolae on the IU envelope .. so you may have to plot a few points to fair a nice line on the trimsheet. Beware of CG envelope lines with negative slope (forward limit) and positive slope (aft limit) as these may NOT be simplified to straight lines on the IU envelope .. try plotting the parabola to see the problem. If the CG envelope line is a curve, then you will need to plot enough points to figure out the faired line on the IU envelope.

(g) if you want, draw in the MAC (or CG value lines - same thing but different numbers and names only) as an envelope overlay. Use the general IU formula to work out where these lines are to be plotted

(h) draw in sufficient trimlines to account for each loading position to be assessed.

(i) for each loading position, pick a suitable delta weight so that the equivalent delta IU is around 5 - 10 mm physical dimension on the trimline .. this usually works out pretty well but is up to the assessment of the designer. Alternatively, one can use "standard" things like numbers of passengers (at so many kg per passenger), volume of fuel (at such and such an SG), etc.

(j) draw in the ticks along each loading trimline. It doesn't matter where you start as the trimline only gives a delta IU for the delta weight ... the important thing is the distance to be moved to account for the delta weight, not absolute position on the trimline. If you prefer to have sloping lines to simplify the interpolations when using the trimline, make a copy of the first trimline with ticks a short distance away but with the ticks slightly moved on the second line. Draw in the sloping lines so that there is an overlap .. by this I mean that a vertical line coming into the trimline must intersect one or two sloping lines.

(k) draw some vertical guide lines to make it easier to keep things aligned when completing the trimsheet.

(l) put in the admin things for each trimline (weight/division). Also add an arrow so that we know which way to move to account for the delta weight. If you choose to put slope lines instead of ticks, it is usual to slope them in the same direction as the arrow .. so that you "bounce" off the slope line in the direction of the arrow.

(m) add in the addition table to one side of the trimlines. Add any limits you might want to publish.

(n) add in any regulated information required by the jurisdiction ..

(o) some sheets may include the standard load message form but that is just a matter of adding it in as a second drawing on the one sheet.


Beyond that, it is really a matter of just making the document have a pleasing appearance, emphasising simplicity so that the poor sods who have to use it can do so quickly and accurately.


Three additional considerations ..

(a) if the fuel line has a CG varying with volume (ie a non-prismatic tank), you can address this by one of the following techniques ..

- if the variation is not too great, treat the fuel line as a constant arm and adjust the envelope lines to account for the error

- plot the trimline as a curve reflecting the fuel CG variation and then plot a series of parallel lines which form guide lines .. you will need a weight or volume scale at the side of this trimline

- if the curve is a bit complicated, draw it as a separate grid and then use straight lines in the trimline to be the guidelines

(b) for aircraft which check the basic trim at ZFW but still need to calculate a takeoff CG for stab trim (eg Gulfstreams and HS125 etc.) you can account for the resulting two envelopes by using a scaling grid for the takeoff envelope which moves and resizes the scaling for the IU line scale .. this allows you to plot the two envelopes one over the other which keeps the completion accuracy high. Just a drafting sleight of hand thing, really.

(c) keep in mind that the real world has lots of small (and not so small) errors in loading calculations. You need to do an error analysis to account for these ... the usual trick is to constrain the published envelope limits to account for the errors .. ie we acknowledge that the final plotted data is in error but constrain it so that the TC envelope is not compromised.. this often results in an error in the stab trim setting but that is not usually at all critical. A minor variation here is to use LMCs (last minute changes) envelope lines .. same philosophy but presented is a different way.

Ignoring (c) above, don't let anyone tell you that the basic trimsheet (ie not considering error analyses) is not accurate. .. reasonably designed and executed, a trimsheet loaded CG should be accurate to something in the order of several mm. This is a bit artificial, of course, as the empty weight and CG data for the aircraft usually is in error somewhat more than this.


Once you have designed your first couple of hundred sheets they become pretty routine .. but expect to have to rework the first few quite a few times until you get the hang of things ...

I have trained up work experience uni engineering students (who often knew not much about aircraft beyond the academic and nothing at all about loading) in a few days to the stage where they could design a pretty passable trimsheet .. no rocket science needed for these things .. just a bit of attention to detail.



As folk might offer any comment, I will add more words to clarify any points above which might not have been written as clearly as they might have been .... it is getting a bit late at night here, you see.