Convert %MAC to INDEX UNITS

[piratepete]

When an airplane is re-weighed typically after a heavy check, a new MAC is calculated.For the TRIM SHEET, this MAC needs to be converted to INDEX UNITS by reference to (BOEING) the "Customized Loading Schedule Substantiation Document".Typically this has a Catalog Number of D043T620xxxyC where xxx is the airline code and y is the serial number.Anyone have one of these for a 767-200? If yes please PM me and I will give my email, beers on me.

[FE Hoppy]

A new MAC is calculated?

No it isn't!

[piratepete]

Thanks for the useful answer.So how to find the INDEX conversion? Any useful answer appreciated.

[Trackdiamond]

Why not contact Boeing Performace Engineering for guidance?

[XPMorten]

He probably means the location of the empty CG has been adjusted.

[hawk37]

Pirate Pete

As XP pointed out, it sounds like the new weighing is used to calculate a new % MAC, ie a new location for the balance point, which for simplicity is often determined as a percentage along the designated mean aerodynamic chord. As for converting to "index units", there is probably a formula specific to the manufacturer.

[dusk2dawn]

CHAPTER 4 - INDEX AND GRAPHIC LIMITS (http://avstop.com/ac/weightbalance/ch4.html)

[Dadanawa]

Hi Pete,

For the 737-400 in pounds.

Arm = 648.5 + (65000 x (DOI - 40)/ Wt).

%MAC = (Arm - 625.6) /1.345

DOI = (Wt x (((%MAC x 1.345)+625.6) - 648.5) / 65000) + 40.

If you have the formulas for Arm and %MAC for the 767, you can substitute their numbers.

Hope this helps.

[piratepete]

Hoppy, just got a new re-weigh report and guess what....yes a new %MAC.Please engage brain before spouting rubbish.

Other guys, thanks for the feedback.

[Wizofoz]

Well, you did write "New MAC", not "New %MAC". Hoppy might have been a touch pedantic, but he was right.

In terms of a new index, what datum does the manufacturer specify?

[Trackdiamond]

A common pprune disease...if someone offers a comment that you don't like or approve..rather commend them for at least taking their time to read and digest your request..even if it leads to their indigestion...no need to spit back insults.Not professional.This is PROFESSIONAL pilot Rumours forum...not merely Pilot tongue wags and spit fires ...etiquettes!

[piratepete]

TD, Merry Christmas! I have been a PROFESSIONAL pilot since 1975.I think you are proving your own point by writing this statement.I rest my case.

Wizzy....being too pedantic is not in the proper spirit of co-operation, a CRM basic isnt it?....cant someone just help me instead of perpetuating all this pilot-bashing.Come on.Help one another.

This might be a PROFESSIONAL pilot Forum but I think the RUMOUR aspect always seems to over-ride the P angle......happy new year.

[keith williams]

I do not have access to the formula that you have asked for but if you are using paper load & trim sheets and they are anything like the 737 ones, it should be possible to convert %MAC to Index units using this.

Just plot the new Dry operating C of G position using the weight and the % MAC. Then drop a line vertically down or vertically up to the Index Scale. Read off the new DOI where this vertical line crosses the Index Scale.

If you know the Dry Operating weight, % MAC and DOI prior to the recent reweighing, you can test the validity method using the data for the previous figures.

[john_tullamarine]

When an airplane is re-weighed typically after a heavy check, a new MAC is calculated.

[As you have done, I use MAC loosely to mean %MAC.]

Not a common approach but no reason why not and, depending on the trimsheet design, it may be necessary for the entry argument.

This latter consideration is for those trimsheets where the IU is not specified explicitly but is determined implicity by use of a graphical entry using empty weight and MAC (or CG) to generate IU. In general, there is no advantage in doing things this way but some trimsheet designers like it. One such which comes to mind is the original OEM Nomad trimsheet.

When calculating the IU for a more conventional trimsheet system, the general equation for the entry argument is

IU = C1 + W * (CG - trim datum)/C2

where

C1 is a simple shift to get rid of those pesky negative IU numbers.

If C1 is not used then the zero IU is at the trim datum CG position. Trimsheet datum can be picked out quite easily if the sheet has an MAC grid and is the MAC whose line is vertical.

If a trim datum is not used the standard datum usually is way out front with the result that the envelope slopes towards the right with an adverse effect on trimsheet accuracy.

W is the empty weight

CG is the calculated empty weight CG

trim datum is a reference CG position chosen by the trimsheet designer to improve usability and accuracy in service.

Typically, the trim datum will be somewhere in the CG envelope with the specific position chosen to suit the designer's preferences. The trim datum changes the effective datum for calculations; the end result is to change the appearance of the CG envelope as drawn in the trimsheet by making it more vertical. Each designer, naturally enough, has the view that his/her trim datum philosophy is superior to all others ;)

C2 is the usual moment to IU factor

If you have the MAC, rather than the CG, it is just a matter of swapping out the CG for CG expressed in terms of MAC in the equation. Just complicates things for no real benefit unless you set it up on a PC.

He probably means the location of the empty CG has been adjusted

That is a near guarantee with a reweigh. However, one checks that the running CG position is sensibly close to the new weigh figure.

Addendum: This old thread (http://www.pprune.org/tech-log/177477-load-sheet-construction.html) has some details on the trimsheet side of things.

[barit1]

Not to be redundant nor excessively pedantic: Does CRM include mind-reading? When piratepete specifies MAC but intends %MAC, do we infer the error, or take him literally?

:{

[keith williams]

Probably not, but effective CRM does require effective communication.

If your fellow pilot says something that appears to be nonsense it is better to ask for clarification, rather than to simple say "no it isn't".

[john_tullamarine]

.. and, I would hope, we might be a little more relaxed on PPRuNe than in the cockpit.

However, you are quite correct .. we both ought to have talked about %MAC as the incorrect terminology is a source of confusion.

.. in our defence .. it is a very common usage ..

Mea culpa maxima.