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P/N 11934-S19
Section 9
Cirrus Design
Supplements
SR20
Section 7 - Systems Description
The airplane is equipped with an S-Tec System Thirty two-axis
Automatic Flight Control System (Autopilot). The autopilot roll axis
uses an inclined gyro in the turn coordinator case as the primary turn
and roll rate sensor. In addition to the turn coordinator instrument and
rate gyro, the case encloses an autopilot pick-off, a gyro RPM detector,
an instrument power monitor, and the autopilot roll computer/amplifier.
An altitude transducer plumbed into the pitot-static system provides
altitude information to a separate pitch computer installed within the
console. Roll axis steering is accomplished by autopilot steering
commands to the aileron trim motor and spring cartridge. Pitch axis
command for altitude hold is accomplished by pitch computer
commands to the elevator servo. Low electrical power will cause the
Turn Coordinator to flag and low RPM will cause the autopilot to
disconnect. 28 VDC for autopilot operation is supplied through the 5-
amp AUTOPILOT circuit breaker on the Essential Bus. Turn
Coordinator instrument dimming is controlled through the INST light
dimmer on the bolster switch panel.
The system also incorporates a GPSS (GPS Steering) converter and
switch to apply roll steering commands from GPS1 to the autopilot.
The GPSS function consists of a GPSS/HDG switch, a GPSS
converter box, and associated wiring. During normal flight operations,
the system can be switched from heading to GPSS modes of
operation. In heading mode, the converter receives a heading error
signal from the Horizontal Situation Indicator (HSI). The converter
processes the signal and sends the heading error to the autopilot. In
the GPSS mode, the converter uses digital groundspeed and bank
angle signals from GPS1 that are converted to a commanded turn
rate. The turn rate is scaled and converted to a heading error signal for
the autopilot. This allows the autopilot to be directly coupled to the
GPS1 roll steering output, without the need for the pilot to make
adjustments using the HSI course (OBS) control or the HSI heading
(HDG) bug.
All Autopilot mode selection and annunciation is performed with the
Turn Coordinator Control Knob and the A/P ALT HOLD buttons on the
pilot’s and copilot’s control yokes.
Refer to Figure 1
for an illustration of
the following listed modes and controls:
Revision 1: 12-07-04
Summary of Contents for SR20
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