Is there a minimum speed for the ALT mode to operate or will it hold the
altitude even in a hover
The machine we fly (a 4 axis AW139), ALT will work all the way down to the hover if you wish...just like the RHT (or RADALT hold). As an example, if both radalts failure during an Automatic Hover, ALT will automatically engage after a very brief period....does a reasonable job although just not as "crisp" as RHT hold.
Cheers