In a single channel ILS approach, the autopilot must be disengaged at or below 140 FT ARTE.
In an approach using V/S mode, the autopilot must be disengaged at or above 300 FT ARTE.
The ILS is a precision approach. The precision approach has the benefit of a glide slope to aide the crew with the descent. This company has a policy for the crew to disconnect the automation at a specific height.
The V/S (Vertical Speed) mode would be used in a non-precision approach (there is no vertical guidance); Localizer, NDB/ADF or VOR. The same company has a policy for the crew to disconnect the automation at a different, and more restrictive, height when flying a non precision approach (V/S mode) than a precision approach (single channel ILS).
Having the pilots disconnect the automation on a non precision approach at a more restrictive height gives them more time to manually take over, establish “feel” and be stabilized. I am just guessing here...........
I do not know the company, the specific reasons or even the type of equipment.
We hardly fly any approach other than the ILS or RNP/RNAV. Occasionally there is the stand alone GPS and rarer still the non-precision approach using something other than LNAV/VNAV.
Hope it helps.
Northbeach