TM 55-1510-221-10
(fig.
NOTE
The TACAN ground speed reading will be
accurate only when the aircraft is on a
course directly to or from the TACAN
station.
g.
Shutdown procedure: TACAN power switch
3-20) - OFF.
3-29. AUTOMATIC FLIGHT CONTROL SYSTEM.
The RC-12H aircraft is certified with
wingtip pods installed. Should the pods
be removed, the autopilot system must be
replaced with a standard C-12D autopilot.
Effected wiring must also be changed.
a. Description. The Automatic Flight Control
System is a completely integrated autopilot/flight
director/air data system which has a full comple-
ment of horizontal and vertical flight guidance
modes. These include all radio guidance modes, and
air data oriented vertical modes.
When engaged and coupled to the flight director
(FD) commands, the system will control the aircraft
using the same commands displayed on the attitude
director indicator. When engaged and uncoupled
from the flight director commands, manual pitch
and roll commands may be inserted using the pitch
wheel and turn knob.
When the autopilot is coupled, the flight director
instruments act as a means to monitor the perfor-
mance of the autopilot. When the autopilot is not
engaged, the same modes of operation are available
for flight director only. The pilot maneuvers the air-
craft to satisfy the Flight Director commands, as
does the autopilot when it is engaged.
b.
Air Data Computer. A digital air data com-
puter located in the forward avionics compartment
provides the altitude information for the pilot’s
altimeter indicator, altitude alerter, and transpon-
der. The computer also provides altitude and air-
speed hold function data to the flight control com-
puters. The air data computer receives 28 VDC
power through, and is protected by, a 2 amp circuit
breaker placarded AIR DATA - ENCDR located in
the AVIONICS section of the overhead circuit
breaker panel. All air data computer functions are
automatic in nature and require no flight crew
action.
c. Flight Director Mode Selector. The Flight
Director Mode Selector (fig.3-22), located on the
pedestal, provides for selection of all modes (except
go-around which is initiated by a remote switch
located on the left power lever for the flight director.
The top row of split light annunciated pushbuttons
contains the lateral modes and the bottom row con-
tains the vertical modes. The mode buttons will illu-
minate when manually selected, or automatically
selected through other modes.
The split light pushbutton annunciators, illumi-
nate amber for armed conditions and green for cap-
tured. When more than one lateral or vertical mode
is selected, the flight director system automatically
arms and captures the submode. Mode annuncia-
tions are also presented on remote annunciator
blocks, located above the pilot’s Attitude Director
Indicator (ADI), and on the pilot’s ADI. Operating
modes and annunciation events of the Flight Direc-
tor system are detailed in figure 3-21.
d. Controls/Indicators and Functions (FD
Mode Selector, jig. 3-22):
(1.) Heading Mode Switch (HDG).
Engages heading mode. Commands aircraft to
acquire the heading indicated by a heading marker
on the pilot’s HSI.
(2.) Navigation Mode Switch (NAV).
Engages the navigation mode selected.
(3.) VOR Approach Mode Switch (APR).
Engages approach mode. Commands aircraft to
intercept and track ILS inbound course.
(4.) Back Course Mode Switch (BC).
Engages backcourse mode. Commands aircraft to
intercept back course ILS.
(5.)
VNAV Mode Switch. Not used.
(6.) Standby Mode Switch (SBY). Engages
standby mode.
(7.)
Indicated Airspeed Hold Mode Switch
(IAS). Engages indicated airspeed hold mode.
(8.) Vertical Speed Hold Mode Switch
(VS). Engages vertical speed hold mode.
(9.) Altitude Preselect Mode Switch (ALT-
SEL). Engages altitude preselect mode.
(10.) Altitude Hold Mode Switch (ALT).
Engages altitude hold mode. Commands aircraft to
maintain pressure altitude.
e.
Autopilot Modes of Operation.
(1.) Heading Select Mode (HDG). In the
HDG mode the flight director computer provides
3-38
