I think we asked last year with no response.
If you look in your joystickbrand.jsm file, the axis settings shows this from the file before you configure anything in the game.
X Axis
0,32767,65535,0.050000,0.100000,0.250000
NOTUSED,NOTUSED,0,0 <--Mode1
NOTUSED,NOTUSED,0,0 <--Mode2
NOTUSED,NOTUSED,0,0 <--Mode3
NOTUSED,NOTUSED,0,0 <--Mode4
Looks like you can set a different scaling of an axis in each of the 4 possible modes. So you can set 3 custom scaling's, since I suspect Mode 2 for vehicles doesn't matter too much. That means turning off auto mode select when you spawn your plane or vehicle. You would have to remember to pick your mode or set one button to pick the next mode.
Here is an example I could set:
From Mode 1
Button [15] ---> Toggle Mode 2
Button [15] ---> Toggle Mode 3
Button [15] ---> Toggle Mode 4
Button [15] ---> Toggle Mode 1
So every time I hit my button 15 it toggles to the next Mode of key mappings and axis scaling's. I think when you manually toggle modes you get a Mode number message in the text buffer.