Cirrus SR22 Pilot Operating Handbook page 360

Section 9 Cirrus Design
Supplements SR22
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 autopilot pitch
servo.
The altitude selector provides altitude and vertical speed pre-select
capability for the autopilot. A pre-programmed altitude and vertical
speed can be input into the altitude selector/alerter and then coupled
to the autopilot. The autopilot will then follow the selected vertical
speed until the selected altitude is reached. Then the altitude selector
will signal the autopilot to hold the selected altitude. The altitude
selector/alerter receives uncorrected altitude data from the same
altitude encoder used by the transponder. In addition to the preselect
functions, the altitude selector provides altitude alert, decision height,
and altitude readout.
28 VDC for autopilot and altitude selector/alerter is supplied through
the 5-amp AUTOPILOT circuit breaker on the Essential Bus.
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.
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Revision 06: 05-09-11