This setting allows you to build small airports at anytime in the game.
Under normal circumstances, as the game progresses, small airports become grayed out and can no longer be built. This can be a pain if you wish to build an airport but don't have the space for a city airport. If this patch is enabled, you will always be able to build small airports.
This feature is disabled by default. You can enable this patch through the configure patches -> construction window or use the following syntax in Openttd.cfg:
[patches] always_small_airport = true