It is now possible to have one or more local apt.dat files each containing data for one or more airfields. The airfields in the local apt.dat files take precedence over the contents of the apt.dat.gz file that is distributed as part of fgdata. - in much the same way as local custom scenery takes preference over terragear scenery.
If you are using WED, the apt.dat output by that program is suitable.
To use it add a folder NavData/apt in your fg-scenery and place your private files in that folder.
This is an extract from the author's (Florent Rougon) post in the devel list.
No need for any command-line option. As I wrote in
<https://sourceforge.net/p/flightgear/mailman/message/35424973/>, you
have to put your apt.dat files (any name ending with .dat or .dat.gz is
OK) in a structure like the following:
Scenery path
| -- Airports
| -- Models
| -- NavData
| -- | -- apt
| -- | -- | -- vrmg.dat
| -- | -- | -- vrmh.dat
| -- | -- | -- vrmk.dat
| -- | -- | -- vrmm.dat
| -- | -- | -- vrmo.dat.gz
| -- | -- | -- vrmt.dat
| -- Objects
| -- Terrain
where “Scenery path” can be any of you scenery paths (e.g., any
component of --fg-scenery/$FG_SCENERY; the path given to --terrasync-dir
is also one). The files read are all NavData/apt/*.dat[.gz] files inside
each scenery path, followed by the default $FG_ROOT/Airports/apt.dat.gz
(last in precedence order, thus allowing your custom sceneries to
override airports already present in $FG_ROOT/Airports/apt.dat.gz).
In the typical case, just put your apt.dat files(*) in the NavData/apt
subdirectory of your custom scenery and they will be picked up
automatically. In each of these files, you may include as many airport
definitions as you want.
(*) Actually, any name ending with .dat or .dat.gz---but the syntax
inside, of course, must be valid "apt.dat syntax". And if the file
name ends in .gz, of course the contents must be gzipped-compressed.
Of course your apt.dat changes should continue to be submitted to the Xplane gateway so that they will eventually be included in the main distribution.
Alan