TM 55-1510-221-10
Figure 3-22. Flight Director Mode Selector (MS-
500)
inputs to the command cue to command a turn to
the heading indicated by the heading bug on the
HSI. The heading select signal is gain programmed
as a function of airspeed. When HDG is selected, it
overrides the NAV, BC APR and VOR APR modes.
In the event of a loss of valid signal from the gyro
or compass, the command cue on the AD1 is biased
out of view. automatically select
(2.)
Navigation Mode (NAV). The Naviga-
tion Mode represents a family of modes for various
navigation systems including VOR, Localizer,
TACAN and INS.
(a.) VOR Mode. The VOR Mode is
selected by selecting either VOR 1 or VOR 2 on the
Course Indicator select switch on the pilot’s instru-
ment panel, and then depressing the NAV button on
mode selector with the navigation receiver tuned to
a VOR frequency. (A VOR indicator, NV1 or NV2
on the pilot’s HSI, will illuminate. VOR NAV infor-
mation will display on the pilot’s HSI and RMI).
Prior to VOR capture, the command cue receives a
heading select command as described above and the
HDG mode switch is illuminated along with the
NAV ARM annunciators. Upon VOR capture the
system automatically: switches to the VOR mode;
HDG and NAV ARM annunciators extinguish;
NAV capture (NAV CAP) annunciators will illumi-
nate. At capture, a command is generated to capture
and track the VOR beam. The course error signal is
gain programmed as a function of airspeed. Cross-
wind washout is included which maintains the air-
craft on beam center in the presence of crosswind.
The intercept angle is used in determining the cap-
ture point to ensure smooth and comfortable perfor-
mance during bracketing.
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 cap-
ture point, the lateral beam sensor will not trip and
the system will remain in the HDG mode. After cap-
ture, if the NAV receiver, compass data or vertical
gyro go invalid, the ADI command cue will bias out
of view. Also, the NAV CAP annunciators will
extinguish if the NAV receiver becomes invalid.
(3.) 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 glide slope capture, time and airspeed.
Other valid logic is the same as the VOR mode.
(4.) VOR Approach Mode. The VOR
Approach Mode is selected by depressing the APR
button on the mode selector with the navigation
receiver tuned to a VOR frequency and less than 10
DME miles from the station. The mode operates
identically to the VOR mode with the gains opti-
mized for a VOR approach.
(5.) Back Course Mode. 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 lat-
eral beam 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.
(6.)
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 cap-
ture the localizer and glide slope respectively. No
alternate NAV source can be selected. Operating
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