shows aircraft position and heading with respect to a
radio course and rotating heading dial.
(13) Aft Lubber Line Fixed heading mark at
the aft position.
(14) Reciprocal Course Pointer Positioned
on the heading dial in a direction 180 from the course
(15) Heading Knob Positions the heading
bug on the rotating heading dial and displays pre-
selected compass heading.
(16) Vertical Warning Flag When in view,
indicates the vertical information is unreliable.
(17) Vertical Deviation Pointer Displays
glideslope deviation. Pointer is in view only when
tuned to a localizer frequency. Aircraft is below glide
path if pointer is displaced upward and each dot
represents approximately 0.4 displacement.
(18) To-From Pointer Two flags, 180 apart.
One always points in the direction of the station along
the selected VOR radial.
(19) Course Pointer Like the heading bug,
the course pointer also rotates with the heading dial to
provide a continuous readout of course error to the
computer. When one of the radio modes is selected,
the vertical command bar in the ADI will display bank
commands to intercept and maintain the selected radio
(20) Heading Bug And Heading Knobs The
notched orange heading bug rotates with the heading
dial so the difference between the bug and the fore
lubber line index is the amount of heading error
applied to the flight director computer. In the heading
mode, the ADI will display the proper bank commands
to turn to and maintain this selected heading.
c. HSI Failure Warning System. The HSI
failure warning system monitors and compares lateral
steering signals for VOR, ILS, localizer, TACAN. The
system also monitors and compares vertical steering
signals for ILS glideslope. Lateral steering failures are
displayed by the HSI NAV flag, and vertical steering
failures are displayed by the HSI VERT (glideslope)
warning capabilities and failure warning test switches
for both pilot and copilot. Failure warning priority is
provided for failures within the active NAV receiver.
The pilot and copilot test switches are on the
autopilot/flight director transfer panel located on the
The circuit receives 28 Vdc power from the #1
and #2 avionics buses routed through each respective
navigation receiver, i.e., NAV 1 / NAV 2, TACAN.
This design provides a built-in power redundancy
since only one navigation receiver is required to be
operating at any given time to provide sufficient power
for effective failure warning to exist. Once active, the
failure warning system automatically monitors steering
signals for the navigation system selected by the
operator and displayed on the HSI.
The EFWS monitors the failure prone circuits of
the Course Deviation Indicator (CDI) system. The CDI
system receives input from the VOR/LOC, TACAN,
and GLS (Glideslope systems) through a common
printed circuit (PC) board. The EFWS compares these
input signals to an expected output signal. If the
signals do not match, this indicates failure of the
switching relays, connector pins, or the PC board
traces for the LT / RT or UP / DN deviation steering
displayed. The EFWS cannot determine if the input
signals are unreliable; therefore, the EFWS will not
detect a failure of the VOR/LOC, TACAN, or
ANTENNA systems. It will not detect a failure of their
associated wiring, connectors, or switching relays. In
addition, the EFWS will not detect a failure inside the
The failure warning system includes an individual
and separate self-test circuit for the pilot and copilot.
Once activated, the self-test circuit will bias both VERT
and NAV flags in view when the deviation exceeds
+7.5 millivolts. The flags will bias out of view when the
deviation is less than +7.5 millivolts, or when the self-
test circuit is de-energized (provided a valid signal is
A failure flag is displayed when the NAV signals
monitored by the HSI failure warning system differ by
more than 7.5 millivolts for both lateral and vertical
deviation. Filtering is provided to eliminate false
warnings due to excessive noise and/or spurious
7.5 millivolts correspond to approximately
0.1 dot displacement of the CDI and GS
3C-18. AUTOMATIC FLIGHT CONTROL SYSTEM.
a. Description. The Automatic Flight Control
system is a completely integrated autopilot/flight