Unable to align vertices along a custom orientation Why when I scale vertices by zero with custom orientation, instead of getting a straight line, I get a little bit curved line.




You can see that vertices aren't perfectly aligned. 

What I expected is to get perfectly straight line aligned along the custom orientation as shown in the picture below:
I don't see a problem in your file?

Just in case.. to flatten to a plane .. in 2.79..


Select 3 points which define the plane
CtrlAltSpace create the Custom Orientation
With pivot set to 'Active' as you have done, SZZ0
To check the result, F make a face out of your arc, select it, and set an orthographic view to look straight down its side by hitting ShiftNumpad 3