Cirrus SR22 Pilot Operating Handbook page 363

Cirrus Design Section 9
SR22 Supplements
Altitude Selector / Alerter
The altitude selector / alerter provides the autopilot with an altitude
preselect function, a programmable vertical speed function, as well as
provides altitude alert, decision height alert, and baro corrected
altitude display. The altitude selector reads and decodes altitude
information from the same altitude encoder that provides altitude
information to the transponder. The decoded altitude is baro corrected
and then compared to the selected altitude setting. When the decoded
and baro corrected altitude matches the selected altitude, the altitude
selector signals the autopilot to engage the ALT hold mode. The
altitude select (ALT SEL) function is operable only when the
transponder and encoder are operating and then both the autopilot
ALT and VS modes are selected.
The altitude selector also provides a vertical speed signal to the
autopilot pitch computer that is proportional to the amplitude and
direction of the selected or computed vertical speed. This signal is not
used by the autopilot until the autopilot VS mode is engaged. When
VS is engaged, the autopilot compares the selected vertical speed
signal with the existing vertical speed derived from the autopilot's
altitude transducer and maneuvers the airplane to attain the selected
vertical speed. The Vertical Speed (VS) select portion of the altitude
selector / alerter is showing a selected vertical speed (VS annunciator
on) and the autopilot Vertical speed (VS) mode is engaged.
The altitude selector / alerter also provides Decision Height (DH) and
Altitude Alert (ALR) selection. All selector function selection is made
through the altitude selector/alerter. Available functions are as follows:
P/N 13772-108 21 of 24
Revision 06: 05-09-11