wrote in
link.net:

I make reference to the heading, but once coupled I have no autopilot
input to modify or adjust heading in any manner, so I certainly do not
fly heading while in NAV, APPROACH, or AUTOLAND modes. If I am hand
flying using the flight director while in NAV or APPROACH modes then,
indeed, I am using heading to track the course (course and vertical
path in case of ILS). But, the autopilot completely takes over
steering of heading when its doing the task, as its alway been with
any autopilot I've used since 1960 or so.

But not so with most of the Century autopilots out there, including ones
installed as late as 1980's). While the newer ones do as you describe,
all the older ones utilize a combination of data from the AI, DG, and
NAV input.

The AI provides the "wing leveler" primary input, while the DG and NAV
signals are summed to provide a heading. Amazingly effective, for such
a trivial analog approach, it will (if carefully adjusted) intercept and
track a NAV signal. However, in a strong crosswind, it will do so "off"
the actual courseline (parallel to it). The Century manual states that
"if this bothers you" you should simply adjust the heading bug on the DG
to an amount "offset" the other way.

The two annoying features of this system a
1. Every time the desired course changes, you have to adjust the heading
bug on the DG accordingly.

2. Like all such systems, there is no turn anticipation. So it is
always a compromise on each new acquisition between undershoot and
overshoot. These systems have a sensitivity adjustment that sets this.
Problem is: a) too little, and it will fly S-turns about the desired
course line, and b) too much, and it will wing rock itself until you
toss your cookies. Getting it set "just right" is an inflight specific
adjustment unique to each individual autopilot unit and N# combo.

jmk