TM 1-1520-238-10 3-48 Change 3 Table 3-14. AN/ASN–137 Control and Display Functions – continued Control/Display Function FPLN Displays the first (H-1-2) of 7 waypoint dictionary pages. Dictionary pages may then be scrolled using the UP/DOWN ARROW keys. NAV 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. FDLS Used to access the FDLS Test page. DNS and CDU FDLS tests are initiated from this page. COM Not active. IFF Not active. ATHS Not active. TGT Not active. CODE Not active. DATA Not active. 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 time. NOTE 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. c. HBCM. 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- tion computations.