Cirrus DESIGN SR20 Information Manual page 330

Section 9 Cirrus Design
Supplements SR20
steering is accomplished by autopilot steering commands to the
aileron trim motor and spring cartridge.
The pitch computer receives altitude data from the altitude encoder
pressure transducer plumbed into the static system, an accelerometer,
and glideslope information from the HSI and #1 NAV radio. Pitch axis
command for altitude hold, vertical speed hold, and glideslope tracking
is accomplished by pitch computer commands to the elevator trim
motor and trim cartridge.
28 VDC for autopilot and altitude selector/alerter is supplied through
the 5-amp AUTOPILOT circuit breaker on the MAIN BUS #1.
All Autopilot mode selection is performed by using the mode select
buttons and VS knob on the autopilot programmer/computer in the
center console. Annunciators in the programmer/computer display
window annunciate modes. Refer to Figure 1 for an illustration of the
programmer/computer.
RDY (Ready)– Illuminates when autopilot is ready for engagement.
When the airplane's Battery Master switch is turned on and the rate
gyro RPM is correct, the RDY annunciator will come on indicating the
autopilot is ready for the functional check and operation. The autopilot
cannot be engaged unless the RDY light is illuminated.
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.
NAV (Navigation) Mode - When NAV is selected, the autopilot will
provide intercept and tracking of GPS, VOR, and Localizer courses.
For course intercept with full-scale deviation, the autopilot
automatically sets up a 45° intercept angle at maximum gain and
sensitivity (turn is limited to 90% of standard rate). The point at which
the turn to capture the course begins is dependent upon closure rate
and airplane position. When the course is intercepted and the HSI
course deviation needle centered (indicating course capture), the
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Revision 04: 08-15-07