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× Welcome to the CPL Performance question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.

• Posts: 119

Just wondering... working through these leading charts...sharp objects are out of my reach But to work out the moment... the equation is moment index= weight, in kilos times by Arm in millimeters divided by 10000... so then if the weight is given in pounds, and my Arm in inches... do I have to convert them to kg and mm?
Or do I just remember to convert them should the question ask for the answer in kilos?

• Posts: 2213

To find the moment index.

If you check the examples in the book and in the supplement, you will see that in every case the first step is to simply multiply the weight in kilograms or pounds by the arm in mm or inches (depending on the system) and then divide by the constant given in the example.

For Alpha. there is no need to calculate a moment at all - you simply follow the arrows on the chart after you enter the weight in each location.

For Bravo, you multiply pounds by inches and divide by 1000 as indicated in the supplement.

For Charlie, you multiply kilograms by mm and divide by 100 as indicated in the supplement.

For Echo, you multiply kilograms by mm and divide by 10000 as indicated in the supplement.
The following user(s) said Thank You: JC

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 592

Some thoughts -

Any given loading system is designed by the WCO to suit the aircraft data. The system can be set up based on metric or imperial units (or any other units one might prefer - doesn't matter at all providing things are done consistently). The only problem using any system if the starting data is in different units to the system's design, is that the numbers will be incorrect and it won't work. Hence the need to convert to the system's units prior to entering the data.

On this point, there is one problem you all need to keep in mind for your flying if you are using trimsheets (along the lines of the Alpha system). It is very common for the trimsheet designer to use a datum position which is different to the manufacturer's.

The manufacturer of most light aircraft will choose a datum convenient for the design organisation, the design process, and general spares convenience. The loading system, usually designed by whichever of the aerodynamics group folks drew the short straw, follows from that datum.

The Industry WCOs, however, are not constrained by the OEM data and can choose whatever datum they may consider appropriate. Generally, for accuracy reasons with graphical systems, the best datum position is one which is somewhere in the aft half of the CG envelope. Most of us have our own little preferences - I tend to use the aft-most CG limit which gives a nice boxy envelope in the trimsheet.

The problem arises should another WCO issue data to be used with the trimsheet and, for whatever reason, didn't realise that the datum used was non-standard (ie not the same as that chosen by the OEM). In this case the data issued will be incompatible with the trimsheet.

For instance, years ago, a good friend (LAME and WCO) reissued the LDS for a cabin class light twin (for all the right reasons after some minor modifications) but didn't know enough about such systems to recognise that my design used a non-standard datum. The POH data he issued was totally incompatible with the existing trimsheet. One of the considerations I take, when designing trimsheets with a non-standard datum, is to endeavour to make the sheet's design incompatible with entry data based on the OEM datum so that, where the problem described above occurs, the pilot should be alerted to the problem because he/she can't get a sensible solution from the trimsheet. In the case cited, the CP for the operator, who was a mate of mine, caught up to me the next day (prior to a charter) with the complaint that the trimsheet didn't work. We fixed up his problem quickly (ie calculated the correct data) and he was able to head off for his charter. When I gingerly raised the subject with the LAME, the latter was horrified. An hour or so of briefing and he had a good understanding of the problem which was fine for any work he had to do in the future.

As a side note, pretty well all heavy aircraft OEMs provide for two datum positions to get around the accuracy problem The design datum is similar to what you see in light aircraft, while the second datum is chosen for loading system work only and, usually, is referred to as the "trim datum".

So, if you are flying an aircraft with a trimsheet loading system, you need to be able to recognise whether the entry data for the sheet are compatible with the sheet. It is probably worthwhile reading through the trimsheet thread on this site to get an idea of some of these concerns - see bobtait.com.au/forum/rpl-ppl/5236-aircra...loading?limitstart=0

Some additional comments to Bob's post, for the Alpha, to amplify his observations.

When you enter the IU at the top of the page, you are doing exactly the same thing as when you fill in the first line entries for a conventional tabular load calculation. In particular, you are entering the empty/basic/(whatever term is appropriate) weight IU for the loading system. As you come down to each trimline, in turn,

(a) the distance you move (left/right) is the distance of the calculated IU (ie the IU change for the load at that load position) measured against the background IU grid (for which the scale is the IU entry line at the top of the sheet)

(b) the direction of the arrow (if to the right, you are adding the IU calculated for that load position, if the left, you are subtracting it) indicates the numerical effect on the progressive IU total exactly as you would do in a longhand calculation by adding/subtracting IU values for each load position.

(c) when you get down to the envelope, the final line which drops into the envelope is the total IU for the load arrangement, again, exactly the same as what you calculate in the manual add-up-the-values system

All the trimsheet does is make the calculation quicker and easier, while achieving functional accuracy similar to the longhand calculation. (This does presume that the trimsheet design is sensible - as you can see in the thread cited above, that sometimes isn't the case, unfortunately. Hopefully, CASA will address that issue at some stage as we still see the occasional dreadful attempt at trimsheet design, as can be seen in the other thread's examples of how not to do it).

Engineering specialist in aircraft performance and weight control.

• Posts: 7

I am not follow two points. I have been taught that you use the datum in the handbook. How can you change that. How can a graph system have more accuracy than the calculator answer.

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 592

Pete, that's a very common concern. Let me endeavour to answer your questions without going too deep into the mathematics.

(a) the datum in the handbook (POH) comes from the OEM's workup for Type Certification. For just about all light aircraft, the OEM chooses a datum somewhere towards the front of the aircraft for design convenience - the numbers for locations are positive (plus, rather than minus) with the result that the numbers increase steadily as one progresses towards the rear of the aircraft. Makes it easier for all concerned, including pilots.

(b) there is absolutely no sanctity about what location the OEM chooses for the design datum - one is as good as another, by and large.

(c) the only time datum selection is important is where one is using graphical loading systems. In this case, the weight and balance calculations made with the graphical system will have the highest accuracy if the datum selected is in the back half of the CG envelope. Why ? For such a sheet, as drawn, the loading system designer can opt for the maximum length between the forward and aft limit which makes it easy to plot and read data accurately on the sheet. There is some further discussion on this point in the other thread at bobtait.com.au/forum/rpl-ppl/5236-aircra...loading?limitstart=0

(d) so, if the datum's position is a matter of choice, why don't we all just pick datum positions willy-nilly. Answer - there is no purpose to be served by doing so and, in any case, any given loading system must be consistent in its use of whatever specific datum is chosen for the particular system. So, unless there is a good reason to do so, we stick with the OEM datum for everyone's convenience. The ONLY sensible reason for choosing a different datum position is if the loading system is going to be figured graphically, such as with a trimsheet.

For instance, the Alpha loading system we use in the CASA exams is for the Piper Turbo Lance II (as can be determined, quite easily, by a quick back-of-a-fag-packet bit of reverse engineering). There is a discrepancy between the sheet and the FAA Type Sheet in that the Alpha forward limit is truncated at lower weights. While the only way to check this is to refer to an old DCA Civil Mk 1 AFM for the Model (which I don't have) the difference is entirely consistent with DCA practice back in the late 70s, prior to the introduction of the GAMA Spec No 1 POH format. Sometimes, early format AFM/POH appeared to be a bit optimistic in respect of forward CG limits. If the OEM/importer chose not to provide additional substantiating documentation (or run some additional flight tests to prove the envelope claims) the usual response was for DCA to restrict the forward limit. In this case, it appears that the limit has been constrained on the basis of the Lance II. For the purposes of the discussion, it is a minor point.

The original Alpha trimsheet was an Industry document designed by Norm Overmeyer (a Sydney engineer) for whatever registration aircraft. At some stage the then Examiner co-opted the sheet as a suitable exercise for the exams. Subsequently, he arranged for Bruce Clissold (then based in Adelaide) to redraw the sheet to tidy up some bits and pieces. The two sheets look very similar but are slightly different so they can't be used interchangeably. There is a bit more of the story in this thread bobtait.com.au/forum/performance/5279-we...0-may-2015-book#8645

Now, when Norm designed the original sheet, he chose (as was his routine practice) to use a datum different to that chosen by the OEM. In his case, he chose to use the fuel arm (FS 93.6) which is just a little ahead of the aft CG limit (FS 95). The resulting trimsheet has the usual boxy envelope appearance when this sort of approach is adopted. Were you to compare the Alpha sheet to one drawn using the OEM datum, they would look dramatically different. So, you are using a trimsheet with a non-standard datum each and every time you run a weight and balance exercise using the Alpha aircraft.

(e) Now, let's talk a little about accuracy and precision. These two terms are related but not quite the same. Generally, accuracy relates to the consideration of how close to the real (or true) answer some calculation might get, while precision relates to repeatability between multiple calculations.

When we run a calculation on an electronic calculator, we can obtain an "answer" (of sorts) apparently correct to a squillion decimal places. In reality, that is nonsense - the arithmetic calculation is just an artefact of the calculator's design and has nothing to do with either the real precision or accuracy (other than in respect of the internal floating point operations in the calculator). It is expected that the user will be a bit sensible in how the calculator's arithmetic capabilities might be applied to real world use.

Without going into a lot of engineering stuff, you can presume that the starting empty weight data you get in a light aircraft POH will be to an accuracy of no more than a few kilos. For the empty weight CG, you might get to an accuracy of 5 mm. These sorts of figures presume that the data has been obtained with a high level of care and good quality equipment. Often, the accuracy will be far worse.

There is absolutely no point running calculations on a calculator to umpteen decimals - it is meaningless in respect of real world things. The calculator's internal accuracy is not in question, only its sensible application to the real world out there.

If you run a calculation on a well-designed trimsheet, with a modicum of care, you will get accuracy well within the accuracy of the empty weight and CG data and the loads you put into the aircraft. Functionally, the calculator gives you nothing superior to the trimsheet.

So it's not so much that the trimsheet is more accurate in its calculations than the electronic calculator. Rather, both are capable of providing accuracies well within the reasonable accuracy achievable for a weight and balance calculation.

Engineering specialist in aircraft performance and weight control.

• Posts: 167

For loading system Alpha, you have to use the graph because everything is on graph which would require two points method to be used. it is unlike bravo,chalie and echo.

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 592

While that's true enough at a superficial level, a common principle still prevails - all these different loading system styles (and the CASA exam cohort is but a sample of the range available out there) are both the same while being a little different.

They are the same in that they achieve the same goal by, essentially, the same means - add up weights, multiply weights by arms to calculate moments (or IU, you prefer), add up moments, divide total moment by total weight to arrive at a loaded CG.

They are superficially different in that their appearance is different - some use a variety of graphical means to calculate, some manual arithmetic processes, while others are totally computer based.

Any aircraft can be loaded using any style or version of loading system you may choose. It would be an exercise of elementary difficulty, for example, to recast the Alpha trimsheet into the style used in the Bravo, Charlie, Echo, or whichever other approach one might prefer.

The main goals for the student, whether at PPL/CPL/ATPL level, should be to have

(a) an understanding of how these systems are developed (albeit at a very basic level). This goal, unfortunately, generally is missed although, to the extent reasonably practicable, we endeavour to fill in some of the gaps in forum discussions, such as this one.

(b) a practical competence in the use of the usual range of typical systems used in the Industry

Engineering specialist in aircraft performance and weight control.

• Posts: 7

If you have Alpha how can you have different Alpha like bravo or charlie.

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 592

Pete - fair question.

The answer is to do with the fact that, in reality, there is no significant difference, other than style, between the systems. They do the same task in much the same way, even if they do look a little bit different.

How about I rework the Alpha sheet to fit the style of, say, the Echo ? You can then run a few calculations to show that both systems give you the same answer (within practical limits of reading and plotting accuracy).

It would be just as easy to do the same sort of exercise for the Bravo and/or Charlie style of loading systems. If we did all three, you would see that you still get the same answers (to within practical limits of reading and plotting) regardless which of the system styles you chose to use.

There is a host of different loading system styles which have been used over the years. Really, it doesn't matter at all which style is used. Some are a little more accurate but, if they are designed carefully and used accurately, the answers you get will be acceptable regardless of which system style you might prefer. Often the style used just reflects the personal preference of an aircraft owner or operator Chief Pilot. The responsibility of the loading system designer is to keep the customer happy while, at the same time, making sure that the system is of adequate flexibility and accuracy, ie fit for purpose.

Engineering specialist in aircraft performance and weight control.