Roll Steering vs. Deviation Only Autopilots
The vast majority of the existing autopilot installations are deviation only autopilots.
That means they track the CDI needle and are reactive systems, essentially “needle
chasers” and do not have the ability to turn short or anticipate turning maneuvers.
Roll steering capable autopilots function in a very different fashion. They can accept roll
steering commands from the CNX80, which allows for smooth transitions from leg to leg
and allows the CNX80 to fly the procedure turns and holding for you. This is why the
CNX80 has two different symbols on the map for the active flight plan. The solid
magenta line being course information (driving the CDI deflection which in turn drives
the autopilot,) and dashed line flight plan showing where GPS roll steering is available
but not CDI deviation guidance.
This was done by design to accommodate the limitations of GA HSI’s/CDI’s which
unlike air transport systems, do not have the ability to slew the course automatically.
That means the pilot gets to twist the knob in order for the CNX80 to read the resolver
and the autopilot to track the guidance.
The magic of roll steering is that this knob twisting and needle chasing is completely
unnecessary with roll steering. This allows the CNX80 to drive the autopilot laterally to
track the course or perform the procedure turn or other complex turning maneuver
without the need to twist the course selector knob, since reading the CDI resolver is
unnecessary. You can literally forget about turning the course selector knob and watch
the airplane fly the flight plan or procedure (of course we don’t recommend this as good
pilot practice.)
The upshot of all this is that if you are limited by a deviation only autopilot, the roll
steering maneuvers must be complete by the pilot manually. For full instrument
approaches, make sure that the CNX80 transitions on the procedure turn and then set the
inbound course and capture the inbound course using NAV mode (or as applicable.)
Deviation only autopilots may utilize a “happy box” that accepts the roll steering output
commands from the CNX80 and turns them into voltage outputs that mimic the inputs
from the HSI/DG heading bug. This allows the CNX80 to drive the autopilot in heading
select mode as if you were twisting the heading bug to track the flight plan.
Currently there are a few on the market that work with various autopilots and your
installer should be able to help. UPS AT has an engineering unit in development that will
be interoperable with a number of different autopilots. Availability is not known today,
but we should have something definite shortly.
