but i don't understand it
I had to get out pencil and paper to work it through as well, so don't feel bad at all. The Airbus explanations are a bit (and unnecessarily) convoluted in my view .. but, nonetheless, correct. The aim, now, is to work out just where you are having problems and then fix those problems. Keep firmly in mind that this stuff is pretty straightforward but it requires meticulous housekeeping to avoid error.
the same equation is able to compute %Mac correctly for one aircraft and isn't able to compute this %mac for an other.
If the equation is correct, then there must be something different about the aircraft. That is what we need to find out.
The best way would be to meet pilot.
I suggest not .. the average pilot has a practical competence with the completion of trimsheets .. but generally a limited understanding of their design and construction.
The only equation I don't have is equation %Mac,
You will have to show us what data you are using .. ie trimsheet, software output, etc. Otherwise we cannot determine where the error occurs. My guess is that you either have
(a) an incorrect implementation of the equation, or
(b) different and incompatible trimsheet designs.
Until we can see and test we probably can't go much further.
If you don't want to post such documents by all means email a scan direct to me and I can sanitise the original for an explanatory post.
So i search on Web, generic equation or not,which is able to compute %Mac for any aircraft,
This is possibly a cause for your problem. The %MAC equations discussed in previous posts work just fine but only for the correct and applicable datum position. Because the IU equation for trimsheets can vary according to the preference of the trimsheet designer one requires a fairly complex equation to cover all the possibilities and then there remains the problem of reverse engineering an individual trimsheet to find equation constants.