AN/ASN137 Control and Display Functions continued
Displays the first (H-1-2) of 7 waypoint dictionary pages. Dictionary pages may then be
scrolled using the UP/DOWN ARROW keys.
Displays NAV top level page. Pressing the NAV FAB will override all other display pages and
return the CDU display to the NAV top level page.
Used to access the FDLS Test page. DNS and CDU FDLS tests are initiated from this page.
3.15.1 Modes of Operation.
There are four modes of
operation: navigation, backup, hover bias calibration
(HBCM), and test (On Command and Continuous). When
electrical power is first applied to the helicopter, the CDU
is powered and the NAV top level page is displayed as
shown in paragraph a. The DNS is not powered at this
All data and messages shown in displays
are typical representations.
a. Navigation Mode.
This is the initial power-up
mode of the DNS. In navigation mode, power is
applied to all DNS components. Computed pres-
ent position data is derived by the DNS comput-
er from doppler radar data. This is the most ac-
curate navigation mode.
b. Backup Mode.
The backup mode is manually
selected by the CPG. Backup mode is usually
selected for only one of two reasons: DNS FDLS
has detected a failure of the doppler RTA or a
heading input failure. In this mode the DNS com-
puter uses the last valid doppler radar ground
speed for navigation calculations. Dynamic dop-
pler radar velocities will not be available. The
(HARS) will go into free inertial when the DNS is
in backup mode. The CPG can change the
ground speed used by entering his computed or
best estimation of ground speed on the NAV top
level page. Ground speed can only be entered
by the CPG when the DNS is in backup mode.
The HBCM calibrates the DNS system
for small velocity errors that may be present in
the doppler receiver/transmitter subsystem. The
velocity bias corrections computed by the DNS
computer and are applied to all subsequent dop-
pler radar velocities. The velocity bias (error),
when present, will be most noticeable when the
helicopter is at a hover or slow speeds. The rea-
son is that at a hover the velocity error, although
small is about the same amount as the actual
helicopter velocity. When HBCM is selected, the
CPG can manually start and stop the calibration.
If a calibration NO-GO status is displayed at the
conclusion, a recalibration may be restarted by
the CPG. If a GO calibration is computed, the
bias velocities will automatically be stored and
applied continuously to all subsequent naviga-