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a dual-axis AP
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.

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  • $\begingroup$ If you have it in altitude hold and you manually roll the aircraft would the a/p not give elevator inputs that could be inconsistent with the pilot's intent? $\endgroup$
    – TomMcW
    Commented Sep 23, 2016 at 17:42
  • $\begingroup$ I'm talking about it was single axis only. If it was controlling pitch only and you turned the wheel what would happen? Seems like the A/P would be confused and do unexpected things with the elevator $\endgroup$
    – TomMcW
    Commented Sep 23, 2016 at 19:17

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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).

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    $\begingroup$ That's the combo we had: lateral on, vert off. Makes it very strange when plane starts rolling left unexpectedly (autopilot) and now nose is pointing down and you have to pull up to keep altitude. Feels a bit like wrestling against the autopilot more than flying and controling a plane $\endgroup$
    – Radu094
    Commented Sep 24, 2016 at 17:05
  • $\begingroup$ How\why is it counterintuitive that you need to increase the aircraft's vertical load factor (i.e., pitch up) to hold altitude in a banked turn? $\endgroup$
    – Vikki
    Commented Oct 28, 2020 at 20:55
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    $\begingroup$ since the turn was initiated (and stoped) by the autopilot you never knew when to start or stop pulling. This delay results in unconfortable oscilations. But now the ap is attempting to compensate the new heading (you are pitching while in a bank so heading changed as far as ap is concerned) by rolling even more (or less). The result is a combination of coupled pitching down and up with ap banking left and right in a series of oscilations that made everyone sick $\endgroup$
    – Radu094
    Commented Oct 29, 2020 at 7:04
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The ROL steering mode in the image above does allow for lateral steering if the ALT mode is engaged to hold altitude. Control Wheel steering is a different mode that temporarily suspends AFCS control while CWS is engaged.

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  • $\begingroup$ ROL is what is displayed for wing leveler mode, per the manual. $\endgroup$
    – user14897
    Commented Sep 23, 2016 at 1:25

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