MCG PathConstraintAxis

Requirements: Max 2016 + Extension1/2 + SP3. Max 2017/18.

I will stress again, for Max 2016 you MUST have the extension (1 or 2) to have any spline functionalities.

This MCG gets rid of the flipping problem as with the default Path Constraint. It also allows the constraint to be based on an axis of a controlling object. This works with multi-spline paths, the default Max Path Constraint works with single spline paths only

Installation:

https://knowledge.autodesk.com/support/3ds-max/learn-explore/caas/CloudHelp/cloudhelp/2016/ENU/3DSMax/files/GUID-086875CC-4C32-4215-BFEC-2B356F9D2A53-htm.html

Then controller can be found in Motion Panel>Assign Controller

Ver 1.01: now includes Arc Length (Distance) mode

Ver 1.02: if you keyframe Up Vec, now will see proper red brackets and values

Ver 1.03: correct mirror (Y axis) bug

Ver 1.04: increased Arclen to 999999

Tip: for a loooooong complexed flightpath, it is far better to correct the internal 'gyroscope' by keying the Up Vec rather than keying the Roll. So if you see your plane slightly off axis, correct it by keyframing the Up Vec (usually the Y component, along with Z at 1.0). Because the sooner the 'gyroscope' is corrected the better it carries though the rest of the flight.