Go to
C:\Program Files\Eagle Dynamics\DCS A-10C\Config\Input\Aircrafts\A-10C\headtracker\
open default with notepad++
replace line ....HEADTRACKER_Z....
with this:
{combos = {{key = 'HEADTRACKER_Z'}}, action = iCommandViewZoomAbs, name = 'Zoom View'},
and the zoom works fine.