View Full Version : How do you calculate DOI Dry Operating Index?

Founder

8th Mar 2006, 10:25

How is the DOI Dry Operating Index calculated when used in Mass & Balance calculations?

Is the DOI the same as the no of inches from the LEMAC to the CG at DOM? and can it then be calculated as the % Mac - the distanse of Datum to LEMAC?

/Tim

PRNAV1

8th Mar 2006, 10:38

Nice question. i would like to know too.

Can anyone help out on this one and some expanded info on on the load sheet calculations would be good if anyone has the patients to answer fully...

Cheers before hand.

Founder

8th Mar 2006, 21:09

Doesn't anyone know an answer to the question? I really need to know!! =)

john_tullamarine

8th Mar 2006, 23:41

Tim,

How is the DOI Dry Operating Index calculated when used in Mass & Balance calculations?

(a) start with the empty weight, CG, and IU (or moment) which follow from

EWIU = (EW * EWCG) / convenient moment value

(b) problem is that the EW configuration is not much use for routine operations as it doesn't include all that routine stuff we put in aircraft and don't want to have to recalculate every time we do a loadsheet calculation.

(c) trick is to define operating configurations which include useful stuff. This is done just the same as a normal longhand loading calculation starting with the EW data and ending up with the data relevant to whatever configuration you are considering. There is nothing sacred about what you call any particular operating configuration or what is included in it ... dry operating is just one of many and is meaningless unless you include a configuration definition with it ....

(d) the calculation is stock standard ..

EW.................EWCG................EWIU

extra bit.....at this location.....gives you this IU

add up............ignore...............add up

(replace the dots with spaces to make it look like a sum ... ) and you get ..

operating CG = (Total IU * the convenient moment) / Total weight (mass if you prefer).

If you have decided to call the operating CG the dry operating weight CG then you would call the total IU the dry operating index.

Is the DOI the same as the no of inches from the LEMAC to the CG at DOM?

No. DOI is an index unit value related to the DOM and DOCG as above.

and can it then be calculated as the % Mac - the distance of Datum to LEMAC?

%MAC is just another way of writing the CG. Note that %MAC is not the distance from the datum to the LEMAC .. rather the distance from the LEMAC to the CG expressed in the following way

%MAC = (distance from LEMAC to CG) * 100 / (length of the MAC)

As a bit of background .. why %MAC ? The OEM design aerodynamicists like to work with CG in terms of %MAC for sensible engineering reasons. They generally get to write the flight manual and run up the OEM's loading stuff so do they do it in the easiest way for the pilots ? .. of course not, they do it in the easiest way for the aerodynamicists. Sensible operators then rehash the information to simple CG values to make life easy for operations.

Founder

9th Mar 2006, 07:33

Tim,

How is the DOI Dry Operating Index calculated when used in Mass & Balance calculations?

(a) start with the empty weight, CG, and IU (or moment) which follow from

EWIU = (EW * EWCG) / convenient moment value

(b) problem is that the EW configuration is not much use for routine operations as it doesn't include all that routine stuff we put in aircraft and don't want to have to recalculate every time we do a loadsheet calculation.

(c) trick is to define operating configurations which include useful stuff. This is done just the same as a normal longhand loading calculation starting with the EW data and ending up with the data relevant to whatever configuration you are considering. There is nothing sacred about what you call any particular operating configuration or what is included in it ... dry operating is just one of many and is meaningless unless you include a configuration definition with it ....

(d) the calculation is stock standard ..

EW.................EWCG................EWIU

extra bit.....at this location.....gives you this IU

add up............ignore...............add up

(replace the dots with spaces to make it look like a sum ... ) and you get ..

operating CG = (Total IU * the convenient moment) / Total weight (mass if you prefer).

If you have decided to call the operating CG the dry operating weight CG then you would call the total IU the dry operating index.

Is the DOI the same as the no of inches from the LEMAC to the CG at DOM?

No. DOI is an index unit value related to the DOM and DOCG as above.

and can it then be calculated as the % Mac - the distance of Datum to LEMAC?

%MAC is just another way of writing the CG. Note that %MAC is not the distance from the datum to the LEMAC .. rather the distance from the LEMAC to the CG expressed in the following way

%MAC = (distance from LEMAC to CG) * 100 / (length of the MAC)

As a bit of background .. why %MAC ? The OEM design aerodynamicists like to work with CG in terms of %MAC for sensible engineering reasons. They generally get to write the flight manual and run up the OEM's loading stuff so do they do it in the easiest way for the pilots ? .. of course not, they do it in the easiest way for the aerodynamicists. Sensible operators then rehash the information to simple CG values to make life easy for operations.

"If you have decided to call the operating CG the dry operating weight CG then you would call the total IU the dry operating index."

So just to check that I got this right, the moment of the CG at DOM is the DOI?

Do you divide it by something? In my studybooks we've got dry operating indexes as three digit sums for example 45,0

Perhaps something like this?

CG = X, DOM = 34000kg, DOI = 45,0 (45000?)

X = DOI/DOM (x = 45000/34000 = 1,31 which leads to CG at 31% MAC?)

Best Regards

Tim

hoey5o

9th Mar 2006, 08:54

Founder, you mention study books so the following real world answer may or may not help if it just exams your trying to pass.

Taking the B757/767 as an example.

An index unit ( IU ) is just another way of expressing a moment.

So remember a moment is the mass X distance of mass from some point.

The manufacturer specifies the "some point" as a datum point usually somewhere forward of the fuselage nose. The distance from it is usually in inches.

The DOW ( aircraft ready for service without fuel) is simply a sum of all the moments for catering, crew etc.

However with airliners the numbers can start to get very big and the C of G calculations can therefore become very user unfriendly.

So operators use index numbers (IU,s) with their own trim sheets to simplify the procedure.

On the B767 an IU of zero equals 20%MAC ( which sits about in the middle of the loadsheet). If it helps 20 %MAC is actually 972.6 inches back from the B767 datum point.

So any positive IU numbers make the aircraft more tail heavy and negatives more nose heavy. A typical DOI on the B757 would be +6

so IU equals : mass X (arm - 972.6)

They then divide the whole thing by 50000 to get the IU down to single numbers.

Founder

9th Mar 2006, 10:28

I'm starting to get this now =)

But lets say that you've got all the info you need to do a mass&balance calculation except the DOI. How do I get hold of it? is it published in a manual?

hoey5o

9th Mar 2006, 12:09

In the airline world it really is very simple.

The figures are all in the manuals. We actually have a laminated sheet in the cockpit with the weights and IU's.

It gives a Basic weight (with crew) with its index

A choice of 3 different catering weights with each index

Add the two together and you get a DOI

In addition there are published IU's for baggage/passengers and take off fuel.

Add it all together and you get a take off IU and hence a trim figure.

Founder

9th Mar 2006, 13:12

Well that sounds easy enough =) Too bad it isnt this easy at the atpl tests... I failed last time bacause I didn't know how to get the DOI. It isn't published in any manuals that we are allowed to bring or anywhere else. So the only solution that I see is to calculate it... =(

But I'm pretty sure that the DOI is = to the CG Position at dry operating mass in our case... I've done a few calculations and they turn out correct =)

If you're interested and have the time I'd apriciate you haveing a look at our manual and tell me if it's anything like in real life?

http://www.caa.co.uk/docs/33/CAP696.PDF

The Medium Range Jet starts on page 19...

Best Regards

/Tim

steve_oc

9th Mar 2006, 14:25

I guess the principle is the same as for helicopters:

Index = [Mass(Arm - RRA)]/x where arm=CG position, RRA=the CG position of the main rotor reference axis (equivalent to say 20%MAC in FW terms) and x=a convenient number

For DOI calculation, take the dry op mass of the aircraft (ie with all role equipment and crew) and the calculated dry op CG position (the arm). Correct the arm for the RRA (to get the + or - value). Divide the whole sum by 100 (in metric) or 10000 (in inches) to get a basic index value. For convenience we then add a random number (30 for most types) purely to have a positive index value.

Example for a Super Puma L2:

DOM say 6000 kg

CG say 4.66 metres aft of datum

RRA is 4.67 metres aft of datum

DOI is {[6000(4.66-4.67)]/100}+30 = 29.4

Alex Whittingham

9th Mar 2006, 15:39

If you're talking about this question:

Q Use the given Annex to determine the Dry Operating Index for a DOM of 35,000kg and a %MAC of 14%.

(a) 35

(b) 40

(c) 45

(d) 50

You go to page 31 of the manual, enter the graph with DOM from the left, move across until you cross the 14%MAC line, read down to an index of 40.

Founder

9th Mar 2006, 16:29

If you're talking about this question:

Q Use the given Annex to determine the Dry Operating Index for a DOM of 35,000kg and a %MAC of 14%.

(a) 35

(b) 40

(c) 45

(d) 50

You go to page 31 of the manual, enter the graph with DOM from the left, move across until you cross the 14%MAC line, read down to an index of 40.

Didn't know about that... Thanx =)

john_tullamarine

14th Mar 2006, 00:38

Now that I have had a chance to run my eye over the CAA loading manual,

(a) I see that Tim's original question appears to relate specifically to the manual rather than the general question it appeared to be. See below for some suggestions.

(b) There may be some use for a few comments on the manual for those who are using it for their exams and will then have to rationalise some aspects when they get out into the real world. There appear to be a couple of strange things in the manual ...

Please note that exams are an artificial animal and my comments are intended to draw your attention to things which relate to the real world .. the exam world (with which I used to be involved years ago) is a somewhat different creature .... My comments are not exhaustive and only relate to those things which jumped out at me during a quick run through the booklet. For the exam you need to consider what the examiner wants to see as well as trying to learn some correct techniques for later life. So far as the CAA exam system is concerned ... I have no familiarity with it so I will leave it to Alex to offer comment on my comments in respect of what the CAA examiner might be looking for ...

(a) p3 a better definition for CG is that point about which (if taken as the datum) the sum of all moments is zero .. ie there is no residual turning tendency. Has the same effect but is often a bit easier to understand for the new kids on the block.

(b) p4 DOI. Tim may have misunderstood this definition .. an index is not the same as a CG or arm. If reworded one might describe the DOI as being the loading index which relates the DOM and DOCG. Specifically, the terms are related by

DOI = (DOM x DOCG)/reference moment used to convert moments to loading indices.

Note that, in the real world of trimsheets, the IU formula can get a bit more involved than this depending on what the designer is trying to achieve .. but the basic equation will be tucked away in there somewhere ...

(c) p4 conversion units .. I am staggered at the use of such inappropriate levels of significance for exam loading calculations even if it is relevant to ICAO Annex guidance. Be very aware that the conversion factors given ought not be used to infer that loading calculation are made to such levels of accuracy. There are so many sources of error in real world loading calculations that one is doing very well if the loaded CG calculation is within 5-10 mm of the actual.

(d) p5 for a light aircraft weighed on platform scales, the use of standard axle (wheel) arms is inappropriate as this will vary with oleo extension. However, for exam use, it is a convenient simplification. For weighing, the arms should be determined by measurement for each occasion. Same comment applies for p12.

(e) p5 BEM, BECG, BEM/100 (BEI, if you like) are individual to a given aircraft and will change any time that the aircraft configuration changes outside prescribed tolerances or is reweighed. Same comment applies to p12. It is quite possible that candidates could get the idea from the CAA manual that these numbers are fixed .. they are very variable and change regularly throughout the operating life of a typical aircraft.

(f) p6 baggage arms. Typically the arms quoted for any loading position will be to the centroid (think middle) of the load (considering the shape of the loading volume for baggage). Be very wary of just throwing things in the back of the aircraft if you are dealing with loads of moderate to high density .. your effective centroid might end up being a tad different to the one with which you presume to do your sums. More than a few aircraft have come to grief due to a combination of misloading, miscalculating, and overloading in the boot ...

(g) p6 while this aircraft has a constant fuel tank centroid, be aware that many light aircraft (and just about all larger aircraft) do not and the use of a constant fuel loading arm can prove embarrassing. With respect to the comment at p7 para 2.6, if the fuel arm is not constant it may not be adequate to check the correctness of loading only for TO, landing, and ZFM as there are aircraft where the fuel usage can take the CG outside and then back inside the envelope ... trap for young players ... Similar comment applies for p13 para 2. Generally a well designed trimsheet will allow for this sort of problem and protect against inappropriate loading.

(h) p9 the upper forward CG limit is a parabola not a straight line when you convert from mass plotted against CG to mass plotted against moment (or IU). Same applies to p15. One often sees the parabola approximated by a straight line which is OK provided that the slope of the mass by CG line is towards the middle of the envelope .. if it is the other way (as it is for some aircraft), the use of a straight line approximation is inappropriate and incorrect as the parabola curves inwards toward the envelope. Whether this is or is not significant depends on the individual geometry of the envelope but it needs to be kept in mind if you are playing with loading systems .. and some pilots do get co-opted to do some with smaller operators ...

(i) p13 standard masses. It is inappropriate to use standard masses for small numbers of occupants due to the variability of body mass within a given population. As the number of occupants increases the variability evens out a bit and the use of an appropriate standard mass becomes acceptable.

(j) p23 Fig 4.7 When using zone distributions, the loading is presumed to be evenly distributed throughout the zone so that the presumed centroidal loading arm is valid. It is very easy to generate a very incorrect loading by adopting an asymmetric loading distribution but doing the calculation using the zonal centroid loading arm. A well designed operator's trimsheet will consider the operator's loading practices and provide protection against this sort of misloading.

(j) p25 Item (h) I fail to see a schedule of fuel arm against volume (or mass) of fuel. I do hope that you good folk are not using the data p22 at Fig 4.5 for fuel loading arms ... that would be just a little bit naughty as this aircraft will not have a constant fuel arm. However, if that is the case, what explanation do your instructors give you when the longhand and trimsheet answers disagree to some extent ? Alternatively, have I overlooked a fuel arm table (intended for longhand calculations) tucked away somewhere in the booklet ?

An alternative is to use the trimsheet fuel data and reverse engineer the equations but I would expect that exam candidates are not required to have such a level of understanding of trimsheet design.

Perhaps someone out there might be so kind as to remove the confusion from my poor old brain and tell me what loading arm fuel data you use for the longhand calculation ... ? Alex ?

(k) For the manual calculation of loading on the twin jet, there is no indication of the starting data although there is for the trimsheet. I suspect that Tim's original question was driven by his frustration in not being able to find these data.

The general answer to the question is given in my previous post. For the specific case, there are several alternatives likely ..

(i) the data will be given in the exam

(ii) one can use the complement of Alex' suggestion ... starting with the trimsheet entry data read 34300 kg and 45 IU to give a CG of approximately 16%MAC (I did prefer life when I could read tiny numbers with the naked eye ..) and then calculate the CG to be 647.1 in. ... then do the longhand sum .. this would be my guess as to what the examiners are looking for.

(iii) alternatively, you could reverse engineer the trimsheet to get the numbers a bit more accurately. My quick back-of-a-fag-packet-looksee suggests that the trimsheet equation for DOI is something like

DOIU = 50 + (CG - 650) x mass [in kg] /20000

Juggling the algebra a bit gives

CG = ((DOIU - 50) x 20000 / mass) + 650

.. for the same entry data as used above, this would give

CG = 647.08 which is pretty close to the figure obtained directly from the trimsheet of 647.1.

Sometimes the trimsheet IU equation can get a bit complicated depending on what the designer is doing or trying to achieve but the basics of mass x arm will be in there somewhere ... For the equation above,

(iv) 50 is a convenient number to shift the entry argument line to the left to get rid of the minus numbers .. quite arbitrary but it is normal to use a number which puts the zero point at the left of the entry line.

(v) 650 inches is the trim sheet datum. This is quite arbitrary (within a range of sensible options) and is intended to ensure that the envelope is fairly boxy or upright so that is can be stretched laterally on the sheet to maximise execution accuracy. In addition, with a complicated fuel line such as one sees with swept wing jets, it is convenient to put the trimsheet datum somewhere in the middle of the fuel grid to minimise the fuel index movement .. and this is what the designer appears to have done with this sheet ... The trimsheet datum is no different to any other loading datum .. it is just an imaginary nail in the side of the aircraft on which to hang the tape measure ...

(vi) 20000 is the moment chosen by the designer to convert moments to IU. Why 20000 ? Who knows .. and it doesn't really matter. My guess is that this sheet came out of Boeing ..

(vi) sometimes you will comes across mm-in and kg-lb conversion factors and sometimes, as in this sheet, you will see mixed imperial and metric units. Just a consequence of engineers being so comfortable with converting from one system of units to another (we have to work in all sorts of strange units ..) that often we forget it can confuse the living daylights of non-engineering folk ..

Note that one needs to be a bit careful extracting data from a trimsheet. When we design these, we often tweak the numbers for error control and it can be a bit difficult to work out just what is going on. However, I cannot think of any reason why the CG overlay would not be drawn "correctly" so extracting a CG in the manner described should be pretty safe.

Hopefully that sorts out Tim's initial question ?

Normally, I wouldn't bother correcting misconceptions with loading systems as they are just too endemic in the Industry. However, seeing Tim is a new chum and we ought not to add too much to his confusion, the following comments are relevant to the thread .. (no criticism of other posters intended at all)

(a) [i]the moment of the CG at DOM is the DOI

Not quite.

Think in terms of the DOM, DOCG and DO moment being related by

DO moment = DOM x DOCG

and the DOIU coming in with

DOIU = DO moment / reference moment

(b) The DOW ( aircraft ready for service without fuel) is simply a sum of all the moments for catering, crew etc.

We know what the poster means but it is more accurate to say something along the lines of

The DO configuration index (DOI) is ...

(c) We actually have a laminated sheet in the cockpit with the weights and IU's.

Keep in mind that the entry data changes regularly and are just the numbers which represent the particular aircraft's configuration. The referenced laminated sheets would disappear and be replaced with new ones every time that the data changed.

(d) But I'm pretty sure that the DOI is = to the CG Position at dry operating mass in our case

This follows from a simple misreading of the CAA manual and a bit of bad luck. DOI can NEVER be equal to a CG position .. apples and oranges. It is feasible to generate systems where the numbers can come out the same but that is an artefact of the system only and not an equality.

RHLMcG

14th Mar 2006, 04:50

JT,

That explains a few things.

I was interested in your fuel CG comments. As I figure it, you leave out the 50 IU number to get the formula for the fuel line, otherwise you don't get the right numbers.

It is interesting to see that the calculated fuel CGs are fairly close to those stated in CAP696.

Main tanks 650.7 as against 650.66

Centre tank 600.4 as against 600.14

Full tanks 628.8 as against 628.56

I assume that the differences are due to rounding off of the IU values in the trim sheet fuel table ?

Your comment about not using the full tank CG values for the long hand calculations becomes a little clearer when the CGs are calculated for the fuel table. I calculate CGs varying from 610 (empty) inches moving back to 650.7 inches (full main tanks) and then moving forward to 628.8 (full tanks).

john_tullamarine

14th Mar 2006, 05:32

As I figure it, you leave out the 50 IU number to get the formula

Correct. The scale shift number (50) is a design trick to get rid of the minus numbers in the entry IU argument and has no other application. For the individual trimlines, use the equation less this number.

Main tanks 650.7 as against 650.66

Centre tank 600.4 as against 600.14

Full tanks 628.8 as against 628.56

I'm surprised to see such close alignment. I would not have been surprised to see a bigger delta given the apparent round off for the IU .. surprises, surprises ....

I calculate CGs varying from 610 (empty) inches moving back to 650.7 inches (full main tanks) and then moving forward to 628.8 (full tanks)

Not hard to see some potentially big errors creeping in if one used a constant CG for the fuel calculation. This certainly can be done (and is quite common in trimsheets for smaller aircraft with non-prismatic tanks) but its use must go hand in hand with some appropriate error containment method .. typically an adjustment to the envelope limits as plotted on the trimsheet.

FLEXJET

14th Mar 2006, 08:48

You will find in any company's "Ground Operations Manual" the BASIC INDEX AND MAC FORMULA :

see correct formulas on my next post, thanks to john_tullamarine

Index = [W - (Sta - Ref. Sta.) +K] / C

% MAC = [(C x (I - K) + Ref. Sta. – LEMAC] / (MAC / 100)

With :

W = Weight actual.

Sta. = Station horizontal distance in inches or metres from station zero to the location.

Ref. Sta. = Reference station/axis. Selected station around which all index values are calculated.

K = Constant used as a plus value to avoid negative index figures.

C = Constant used as a denominator to convert moment values into index values.

I = Index value corresponding to respective weight

MAC = Length of Mean Aerodynamic Chord in inches or metres.

LEMAC = Horizontal distance in inches or metres from the station zero to location of the Leading Edge of the MAC.

john_tullamarine

14th Mar 2006, 10:07

FLEXJET,

I am a tad bewildered by your post ... and, after thirty-odd years as a consulting engineer in this Industry with particular expertise in this field, I thought I had seen pretty well all the variations on the theme ...

Could I invite you to fax me a copy of ' .. any company's "ground operations manual" ' and that might ease my bewilderment ... ?

Index = [W - (Sta - Ref. Sta.) +K] / C

You might need to revisit this equation as it cannot possibly be correct. IU must derive from moment which, in turn, must derive from a product of weight (or mass) and arm. While there may be a range of sideline bits and pieces in it as well, you just cannot avoid having these basics. Perhaps you might offer some additional commentary in support of your thesis ? Alternatively, is it just a typo and you meant to post ..

Index = K + [W * (Sta - Ref. Sta.)] / C

.. which then would be fine for the entry IU argument ?

% MAC = [(C x (I - K) + Ref. Sta. – LEMAC] / (MAC / 100)

I think that even I have to admit defeat with this one... I can't make any sense of it at all in terms of working out where you might have taken an incorrect turning along the road. At the end of the day, the lefthand term is non-dimensional while the righthand term has a mass unit left over languishing all by itself .. this expression just cannot represent an equality.

Could I politely request a bit more information regarding just how you came up with this "equation" ?

FLEXJET

14th Mar 2006, 11:03

You are (of course) correct John.

I should have written :

Index = [W - (Sta - Ref. Sta.)] / C + K

% MAC = [{(C x (I - K)}/W + Ref. Sta. – LEMAC] / (MAC / 100)

For a better view of the formulas, see Sheet 3 of this document :

At the request of the company to whose manual FLEXJET has referred, the URL link is removed. The manual is available on their website but they prefer not to have it on a site over which they do not exercise any control. For the remainder of discussion on this thread, if anyone refers to the manual, can we make the references so that the company concerned is not identified explicity. Thanks, John

john_tullamarine

14th Mar 2006, 11:18

Thanks for that .. I'll go through the document in the next few days when I have a couple of hours spare and come back to you with my thoughts.

Hopefully your boss doesn't get upset at having this document in the public domain ... ?

However, looking at your revised, posted expressions, there are still some problem areas to sort out as, for both expressions, the two sides of the "equation" remain dimensionally incompatible and, as a result, cannot be equations.

However, I can probably work out where things have gone astray by reviewing the company manual and, perhaps, the AFM if needs be ....

FLEXJET

14th Mar 2006, 11:32

John, the link to the AHM is from an airline I don't work for...

I googled for such a document to show that the formulas are used by operators.

Just google "BASIC INDEX AND MAC FORMULA" and you will find the document.

john_tullamarine

14th Mar 2006, 11:37

That's fine .. I'll check with them to see if they mind it being on PPRuNe ...

Founder

14th Mar 2006, 11:45

I must say that this discussion has taken an interesting turn, and it's what I intended it to do... There is a lot of discussion at my school about what's relevant to know and not to know and I think that how to calculate the DOI is one such thing that pilots should know? Am I right?

Best Regards

Tim

john_tullamarine

14th Mar 2006, 11:49

My view is a bit biased as I am a sort of pilot-engineer mix with a foot firmly in both camps. However, one of Tech Log's strengths is its ability to act as a training facilitation vehicle and the mods revel in this sort of thread.

Generally the pilot is not called on to calculate DOI but there is no reason why you should not be able to do so .. in any case, knowledge is generally a useful thing in its own right ...

Founder

14th Mar 2006, 12:34

in any case, knowledge is generally a useful thing in its own right ...

Well said...

john_tullamarine

15th Mar 2006, 06:02

Now that I have scratched my head for the requisite period of time, the errors in FLEXJET's URL link reference have become clear.

The manual linked to by FLEXJET contains expressions for index and %MAC both of which contain errors which appear to have been generated by a combination of (perhaps) carelessness and typographical mistakes.

Following the reference nomenclature, the correct equation for index is

I = (W x (Sta - Ref Sta))/C + K

This has been printed in the manual incorrectly by substituting a minus sign for the multiplication sign after W, probably a simple typo. In addition the divisor line has been omitted.

The manual's logic is then a bit unusual in that the IU equation is solved for Sta which results in

Sta = Ref Sta + ((I - K) x C)/W

which is then plugged into the standard %MAC equation of

%MAC = ((Sta - LEMAC)/MAC) x 100

which gives

%MAC = ((Ref Sta + ((I - K) x C)/W - LEMAC)/MAC) x 100

which can be adjusted to read

%MAC = (Ref Sta + ((I - K) x C)/W - LEMAC)/(MAC/100)

Unfortunately, somewheres along the way

(a) I was replaced by 1

(b) the W got lost in the complexity. Interestingly, this error is in the manual but in FLEXJET's transcription the W is resurrected .. I am not too sure how that came about .. ?

Two things come out of this ..

(a) be wary of indiscriminately pinching something from the net as it might be wrong ...

(b) I wouldn't choose to go down the algebraic path chosen by this particular operator ...

FLEXJET

15th Mar 2006, 16:05

(b) the W got lost in the complexity. Interestingly, this error is in the manual but in FLEXJET's transcription the W is resurrected .. I am not too sure how that came about .. ?

ATPL Mass & Balance recollection... :confused:

In fact I have a copy of this formula at home (taken from an A330 GOM/AHM), as I did some research about this subject earlier this month...

I ended up mixing the web linked and paper formulas as I was certain they were the same !

Two things come out of this ..

(a) be wary of indiscriminately pinching something from the net as it might be wrong ...

You are so right !

john_tullamarine

15th Mar 2006, 21:34

ATPL Mass & Balance recollection

.. you mean ... ground schools teach this nonsense ?

I have a copy of this formula at home (taken from an A330 GOM/AHM),

.. you mean ... other people do it this way as well ?

Absolutely mind blowing.

Now, I can see no reason for taking a simple equation (surely we agree that the standard %MAC equation is simple ?) and turning it into an unwieldy complicated thing when there is no apparent advantage gained for anyone.

The evidence of the errors which have come to light in this thread's discussions should be adequate indication of the undesirability of this needlessly complex way to do something which, inherently, is very simple ....

Perhaps someone can offer an explanation as to the philosophy behind this apparently strange phenomenon ? Or is it just a case that Aussies like to take the easy way ... ?