Cirrus SR22 Pilot Operating Handbook page 361

Cirrus Design Section 9
SR22 Supplements
HDG (Heading) Mode – When HDG is selected, the autopilot will
engage the HDG mode, fly the airplane to, and hold the heading set on
the HSI. Subsequent heading changes are made using the HDG knob
on the HSI. For smoothest transition to HDG mode, it is recommended
that the airplane be aligned to within 10° of the selected heading
before engaging HDG. The HDG mode is also used in combination
with the NAV mode to set up a pilot selected intercept angle to a
course.
GPSS (GPS Steering) – Pressing NAV twice will cause the autopilot
to go to GPSS for smoother tracking and transitions. When GPSS is
selected, the autopilot can be switched between heading and GPSS
modes of operation. In the heading mode, the converter receives a
heading error signal from the heading bug on the Horizontal Situation
Indicator. GPSS converts this information and sends this heading error
directly to the autopilot.
In the GPSS mode, the converter receives ground speed and bank
angle digital signals that are calculated and converted to a
commanded turn rate. The turn rate is then scaled and converted to a
DC heading error signal that is compatible with the autopilot. The end
result is an autopilot that can be directly coupled to the roll steering
commands produced by the GPS Navigator, eliminating the need for
the pilot to make any further adjustments to the HSI course arrow.
REV (Reverse Course) – When REV is selected, the autopilot will
automatically execute high sensitivity gain for an approach where
tracking the front course outbound or tracking the back course inbound
is required. The APR and REV annunciators will illuminate when REV
is selected.
APR (Approach) – When APR is selected, the autopilot provides
increased sensitivity for VOR or GPS approaches. APR may also be
used to provide increased sensitivity for enroute course tracking.
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Revision 06: 05-09-11