Plane: added Q_APPROACH_DIST

[tridge]

this sets a minimum distance to use the fixed wing approach logic. It is an alternative to just disabling the approach with Q_OPTIONS which some users do to avoid some short distance problems. This allows the approach to still be used for longer distances in QRTL but have it disabled for shorter distances The approach code is really needed for longer distances to allow the full fixed wing terrain following logic to be used

[timtuxworth]

This seems like a good idea, but the interaction with the Q_OPTIONS is very confusing. If I understand this correctly, DISABLE_APPROACH actually means disable fixed wing approach, is that right? and if so the new behavior seems to be:

16: Disable Approach - always disable use of fixed wing approach and airbrake stages in VTOL landing, overriding Q_APPROACH_DIST if set

Do I have that right?

[Hwurzburg]

@tridge does this impact the code that prevents switch to fw approach if in vtol closer than 1.5x maxrad? I assume not..