TM 1-1520-238-10 Change 4 3-55 a. CDU or DNS Memory Zeroize. The DNS memory zeroize function is used for maintenance pur- poses only after the CDU or signal data converter has been replaced. The CDU memory zeroize resets all flight plan dictionary page waypoints to bogus default values. This is done from the NAV top level present position page as follows: 1. ADMIN – Press. Observe that the ADMIN page is displayed. 2. Enter CDU on the scratchpad. 3. ZERO – Press. 4. FPLN – Press. Observe that all locations on the FPLN dictionary page displayed contain the similar default values: 31T BV 6355 8711 E 000.0 sph CL6 5. Using the up and down arrow keys, scroll through all FPLN dictionary pages, observing that all location contain the default value. 6. NAV – Press. Observe that the NAV top level present position page is displayed with default data next to the captions. b. BACKUP Mode. In the BACKUP mode, the last remembered radar velocities become the source for navi- gating. This mode provides the capability to the operator of manually entering GS and TKA via the NAV top level present position page. The BACKUP mode must be manually accessed. To op- erate the BACKUP mode do the following: 1. ADMIN – Press. Observe that the ADMIN page is displayed. 2. BACKUP OFF – Press. Observe that BACKUP OFF changes to BACKUP ON. 3. Enter GS data into the scratchpad and press VAB 7. Observe the data is now displayed on line 5 next to GS. 4. Enter TKA data into the scratchpad and press VAB 3. Observe that the data is now displayed on line 5 next to TKA. c. HBCM. In HBCM the DNS determines the hover bias velocities in both the longitudinal and lateral axis of the DNS. The computed bias velocity corrections are ap- plied to all future DNS hover velocity computations. HBCM is accessed via the NAV top level present position and ADMIN pages. NOTE Hover bias calibration only needs to be per- formed when the signal data converter or RTA has been replaced. 1. ADMIN – Press. Observe that the ADMIN page is displayed. 2. HBCM – Press. Observe that the HBCM page (fig 3-17) is displayed. NOTE If line 2 of the HBCM page does not dis- play HBCM READY, the DNS has failed. The time indicated on line 5 of the HBCM page increments only when DNS is per- forming hover bias computations. if the DNS goes into memory mode, the time display halts until the signal from the RTA is sufficient for DNS computations. 3. START – Press. Observe that on line 2 HBCM READY changes to HBCM ACTIVE and on line 5 the time displayed begins incrementing. NOTE The calibration continues until the CPG presses STOP to stop it. When the calibra- tion is stopped, the mode status will be dis- played. Allow the calibration to run for at least two minutes but not more than 8 min- utes. 4. STOP – Press. Observe that the time display freezes and a mode status message is dis- played on line 2. If applicable, DNS error mes- sages will be displayed on lines 3 and 4. The DNS indicates the HBCM update was accepted when an HBCM GO is displayed or not accepted when an HBCM NO-GO message is displayed along with a description of the exceeded limit(s) on other lines. Possible DNS error messages are: TIME BELOW 2 MIN TIME EXCEEDED 8.2 MIN BIAS EXCEEDS 0.3 KTS EXCESSIVE MEM CONDITION