"Dan Luke" wrote:

The heading bug plays no part in NAV tracking on S-Tec 30 or 50
autopilots. Why is it necessary on tha KAP-140?

If I recall correctly, those S-Tec autopilots require the pilot to establish
the aircraft on course before engaging the NAV course tracking mode. Once
on course, the error signal from the CDI is probably sufficient to remain
there.

The KAP 140 has the capability to compute an automatic intercept heading and
will take you to the desired radial or DTK then turn on course and track.
You can be anywhere, pointing in the wrong direction altogether, with a
full-scale deflection of the CDI, and the autopilot will turn you around and
get you on course. This requires a reference heading for orientation.