TM 1-1510-225-10
3C-26
When passing over the station, an overstation
sensor detects station passage removing the VOR
deviation signal from the command until it is no longer
erratic. While over the station, course changes may
be made by selecting a new course on the HSI. If the
NAV receiver is not valid prior to the capture point, the
lateral beam sensor will not trip and the system will
remain in the HDG mode. After capture, if the NAV
receiver, compass data or vertical gyro go invalid, the
command cue will bias out of view. The NAV CAP
annunciator on the mode selector will extinguish if the
NAV receiver becomes invalid.
(b) VOR Approach Mode. The VOR
approach mode is selected by pressing the NAV
button on the mode selector with the navigation
receiver tuned to a VOR frequency and less than
20 DME miles from the station. The mode operates
identically to the VOR mode with the gains optimized
for a VOR approach.
(c) Localizer Mode. The localizer mode
is selected by pressing the NAV button on the mode
selector with the navigation receiver tuned to a LOC
frequency. Mode selection and annunciation in the
LOC mode is similar to the VOR mode. The localizer
deviation signal is gain programmed as a function of
radio altitude, time and airspeed. If the radio altimeter
is invalid, gain programming is a function of glideslope
capture, time and airspeed. Other valid logic is the
same as the VOR mode.
(3) Approach Mode Selector. The Approach
(APR) mode is used to make VOR, TACAN, localizer,
and ILS approaches. This mode is more sensitive
than the NAV mode.
(a) Localizer Approach Mode. The
approach mode is used to make an ILS approach.
Pressing the APR button, with a LOC frequency tuned,
arms both the localizer and glideslope modes. No
alternate NAV source can be selected and the NAV
receiver must be tuned to an ILS frequency. When the
APR button is pressed and the above conditions are
met, both the NAV and APR modes are armed to
capture the localizer and glideslope, respectively.
Operating LOC mode is the same as described above
except, if the radio altimeter is invalid in APR mode,
gain programming is a function of glideslope capture,
time, and airspeed.
With the APR mode armed, the pitch axis can be
in any one of the other pitch modes except go-around.
When reaching the vertical beam sensor trip point, the
system automatically switches to the glideslope mode.
The
pitch
mode
and
APR
ARM
annunciators
extinguish and APR CAP annunciator illuminates on
the controller. At capture, a command is generated to
asymptotically
approach
the
glideslope
beam.
Capture can be made from above or below the beam.
The glideslope gain is programmed as a function of
radio altitude, time and airspeed. The APR CAP
annunciator on the mode selector will extinguish if the
GS receiver becomes invalid after capture.
Glideslope capture is interlocked so that the
localizer must be captured prior to glideslope capture.
If the glideslope receiver is not valid prior to capture,
the vertical beam sensor will not trip and the system
will remain in the pitch mode. After capture, if the NAV
receiver, GS receiver, compass data or vertical gyro
becomes invalid, command cue will bias out of view. If
the radio altimeter is not valid, the glideslope gain
programming is a function of GS capture, time and
airspeed.
(4) Back Course Mode Selector. The Back
Course (BC) mode is selected by pressing the BC
button on the mode selector. The normal front course
for the localizer beam is set for the selected course.
Back course operates the same as the LOC mode with
the deviation and course signals reversed to make a
back course approach on the localizer. When BC is
selected, and outside the lateral beam sensor trip
point, BC ARM and HDG will be annunciated on the
mode selector. At the capture point, BC CAP will be
annunciated with BC ARM and HDG extinguished.
When BC is selected, the glideslope circuits are
locked out.
(5) Standby Mode Selector. Standby (SBY)
Mode is selected by pressing the SBY button on the
mode selector. These resets aid the other flight
director modes and biases the command cue from
view. While pressed, SBY acts as a lamp test causing
all mode annunciators to illuminate and the flight
director warning flag on the ADI to come in view.
When the button is released, the mode annunciator
lights extinguish and the flight director warning flag
retracts from view.
(6) Indicated Airspeed Hold Mode Selector.
The Indicated Airspeed (IAS) hold mode is selected by
pressing the IAS button on the mode selector. When
IAS is selected, it overrides the APR CAP, GA, ALT,
VS, ALTSEL CAP, or PITCH HOLD modes. In the IAS
mode, the pitch command is proportional to airspeed
error provided by the air data computer. Pressing and
holding the TCS button allows the pilot to maneuver
the aircraft to a new airspeed hold reference without
disengaging the mode. Once engaged in the IAS
mode, the mode will be reset if the air data computer is
not valid and the command cue will bias out of view if
the VS is not valid.