One of the most frustrating things for folks creating or updating airports is when their changes do not appear.  There can be a number of reasons for this but one of the most common is the loading order of files in FS.  The same order applies for both FS9 and FSX (and thus P3D).  

The first thing to understand is how FS removes the old when you create the new.  It actually does not remove anything but effectively hides (or excludes it).  There are a set of exclusion flags that are added when an add on airport is made.  These flags tell FS not to display airport. features from files containing the airport that are already loaded.  The screenshot below shows these flags.

So the last file loaded that contains an airport will generally 'exclude' the information in any files containing the airport that are already loaded.  Loading order (or scenery priority) is therefore very important..

In many cases users are not aware of all the files that may be on their system containing the airport.  Thus if some file containing the airport loads after their file then any changes they made are excluded.

The primary order is set by Scenery Library Settings.  This screenshot shows FS9 but the principle is the same for FSX/P3D.

1. In Settings select
2. Scenery Library

This list is based on the data found in the active scenery,cfg file.  The area with Priority 1 loads last (that is has the highest priority and gets the chance to exclude anything already loaded).

1. The top of the list is 'Airport Wizard scenery layer
2. Any airport files loaded in this layer will generally exclude any versions of the airport loaded in areas below it.

Stock area are those with the lowest priority and files in these areas load first.  This is how FS generally allows us to modify stock airports.  Because users should put their modified files at a higher priority then their work will replace the stock work.

Many of us assume that putting a modified airport file into Addon Scenery will ensure that our changes are recognized.  However the screenshot below shows that this may not be the case

1. The Addon Scenery Layer is not at the top
2. MyStuff is above it.  So If I update an airport and save the file to Addon Scenery my changes will show provided there is not a file in MyStuff that also contains this airport.  In that case it is the airport version that is in MyStuff that gets displayed.  If I am not aware of this then I will be left wondering where my changes went.
3. Note that if an area is not checked then it does not count.

Well we are not quite there.  The opportunity for unexpected consequences still remains.  The primary priority is set by scenery areas.  

1. Scenery areas are just file folders and they contain a collection of Bgl files.  
2. The next stage is how these are loaded and what effect that might have on our airport.
3. My airport is in 12FA_ADE9_JM.BGL.  However when I load the airport into FS my changes do not show up - what has happened?  Stage 2 of the file loading order is alpha numeric by file name within scenery area
4. AFD_12FA.Bgl also contains a copy of the same airport but it is different and does not contain my changes.  Because this loads after my file my changes are lost and the airport as defined in this file gets displayed.

Simple Airport Scanner is one tool that will find all addon copies of an airport and display them in the priority order

1. Two airports appear in more than one addon file  KHUT is selected and shows three versions
2. The primary priority is Layer and we can see that there is one version in Layer 6 and two in Layer 38
3. The file name list is displayed in the order that files are loaded with the last loaded file at the top
4. Since there are two files in layer 38 then the second stage priority is called on and the file that is lowest in the alpha numeric order of file name is the last to load

We can be pretty sure that KHUT will be displayed based on the contents of D:\...\KHUT_ADE9_JM.BGL.  If we are expecting the contents of one of the others then we will be surpised and will probably not see what we are expecting

Priority Matters.  

• Never assume the file you have saved is the one that FS is using.  
• Always make sure that you do not leave multiple copies of your airport in files with slightly different names
• If things do not appear as you expect then do check that you do not have a file loading after yours.  
• Prime suspects for this sort of problem are AI packages.  The almost always contain modified airports and almost always load at a higher priority than Addon Scenery.
• If in doubt use a scanner to check for duplicates.  BEWARE though that duplicates are not always a bad thing!