Originally Posted by FCS Explorer
what would be the use of "arming" auto pilot? either it's on or off.
You're right in that you wouldn't "arm" the autopilot as such.
But with the autopilot engaged, you can arm certain modes, which won't engage until the engage conditions are met.
As an example, to capture a VOR radial, you would arm VOR mode, then fly towards the radial in HDG mode. At the right moment, VOR mode will engage and start the turn towards the radial, and the HDG mode disengages.