TM 55-1510-222-10
Figure 3-20. Flight Director Modes and Annunciators
(5) Localizer Mode. The Localizer Mode is
selected by depressing 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.
(6) Back Course Mode (BC). The back course
mode is selected by pressing the BC button on the
mode selector. 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
radial sensor trip point, BC ARM and HDG annunciators
will illuminate. At the capture point, BC CAP will be
annunciated with BC ARM and HDG annunciators
extinguished. When BC is selected, the glideslope
circuits are locked out.
(7) Localizer Approach Mode (APR). The
approach mode is used to make an ILS approach.
Pressing the APR button with a ILS frequency tuned,
arms both the NAV and APR modes to capture the
localizer and glideslope respectively. No alternate NAV
source can be selected. 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 make a gradual
interception of 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 in 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 sensor will not trip and the system will remain in
the pitch mode. After capture, if the NAV
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