Why does a dual-axis autopilot not allow operation in single-axis? Image source: KFC 225 Pilot's Guide (PDF)



In a dual-axis AP, when no lateral mode is selected, a wing leveler (ROL) mode is activated and can't be disengaged unless another lateral mode is selected. Similarly for pitch attitude hold (PIT). (Those two modes are shown above.)

But why, for example, can't the AP hold only the altitude, and the pilot steers laterally. As far as I know any AP with more than one axis doesn't allow single-axis operation. Control wheel steering (CWS / attitude hold) is not what I mean.

Why's that? Regulation? Design limitation?

One benefit of allowing single-axis – I think – is for climbs and descents when there is no speed hold mode. The pilot lets the plane steer left and right, and they control the climb/descent by manipulating the power/trim. Instead of guessing the proper vertical speed – which keeps changing if it's a long climb/descent, unlike trimming for airspeed.
It is neither a design nor regulation limitation. Best explanation I have is that manual control on one axis with the autopilot on another is a bit impractical and goes against normal piloting training techniques. For example banking left/right involves using both ailerons and rudder, but also adding a little bit of pitch up to increase the wing load in the turn.

I used to fly like this a while ago when the pitch axis of one of the autopilots was inop (failed auto-test). It was actually allowed to dispatch (and use the autopilot) but only lateral (HDG/NAV) control was available, with manual pitch. It was uncomfortable to use, counter-intuitive, and we usually just disconnected and flew the thing manually (well, maybe except cruise).