- Replacement of OBJs used for modeling the airplane or cockpit.
- Alternate or modified ACFs*.
- Generally, the livery system shouldn't be used for changing the plane's behavior - it's just paint!
First, to be clear: an aircraft file is the .acf file that contains the X-Plane specific data needed to simulate the plane; the aircraft package is the folder containing that .acf file. A livery is a painting scheme for the 3-d model of the airplane, and a configuration is an instance of a plane with certain features, e.g. with or without G1000, with P&W vs. Rolls Royce engines, etc.
Configurations of an aircraft should be created by putting more than one aircraft file
(.acf) in a single aircraft package. Because the graphic and sound resources needed for the aircraft are accessed relative to the .acf file, you can build a family of aircraft with some common aspects and some unique aspects to each plane.
Files used by an aircraft fall into three broad categories:
- Files found by a fixed formula using the .acf name, e.g. be20_paint.png. Let's call these "file-specific".
- Files found by a fixed formula without using the .acf name, e.g. the contents of the aircraft plugins folder. Let's call these "package specific".
- Files that are explicitly named in the .acf file, like airfoils and OBJs. Let's call these "flexible" (since this naming scheme could be used in any way).
- The aircraft paint scheme is file specific.
- The aircraft panel background is package specific (but you can effectively have each file use a different panel background by setting the panel type differently for each plane).
- Sounds are actually either, which effectively makes them flexible. Non-generic instruments are package-specific.
- Objects, generic instruments and airfoils are flexible.
Now if there are aspects of a multi-configuration aircraft package that don't work right now, we can look at possible changes to the sim to make this work. But it appears to me that just about everything necessary to make multiple configurations is already available in the sim now.
As a final note, the question here (livery vs. multi-file aircraft pack for configurations) is one of file formats, and thus of data organization and contracts between authors and programmers. It is not a question of user interface. The user interface can be reshaped to make multi-aircraft packages look like liveries, or liveries look like multi-aircraft packages. But I suspect that most of the interest in extended liveries is on the file-format side.
* The one exception for liveries is the tail number -- given the strange state where the tail number, as painted into the livery, is also written into the ACF, it wouldn't be bad to be able to override this property. Some people are already doing this using plugins.