Does it already exist in FS?  Generally Airport designed for FS9 will work in FSX.  There are features in FSX airports that will not work in FS9.

There are several tools you may use if designing in FS9.  The most obvious for airport design is AFCAD 2.2.  This is freeware and allows the design of airports.  A more recent alternative is AFX which works for both FS9 and FSX.  It is similar to AFCAD.  It is payware and can be obtained from Flight1.  My own ADE9X is also freeware and is similar to AFX and AFCAD in airport design functionality.  It also allows for the placement of objects and so on. 

There are several other tools that will place objects in FS9 including Rwy12 Object Placer and SBuilder.  These are both freeware.  Instant Scenery from Flight1 can do this also.  It is payware with the advantage of in Sim placement.

You should find a plan or airport diagram of your airport if you wish to enhance it.  An image from Google Earth or similar can also be used as a background image in some of the tools mentioned above to make positioning of features easier and more accurate.

1 main thing that you might want to take into consideration, is the fact that FS9 and FSX GPS Coordinates *DO NOT* match. if you fly multiplayer with someone on FS9 and you are in X, you will probably see them land no where near the runway that you are landing on.   


What you would have to do, is basically build your airport in FS9 (or X) to start with, just to have all the files that you want to put in.   Then when you go to put it in your other Sim, you will have to put it in, and EXCLUDE all the default scenery that is showing through or poking holes in your newly updated addon.


(disclaimer) that is, unless you put it out in the middle of the desert like I did for my KDVI - Desert View International.

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