TM 1-1520-238-10 Change 8 3-63 NOTE Waypoints stored via the Data Transfer Unit (DTU) will not be accessible if the BBC is the bus controller. The CPG can store up to 20 waypoint coordinates in the CDU with the MUX switch in SEC (Emer- gency return routes to be used as a back- up in the event of a FCC failure). If HARS is free inertial for 20 or more sec- onds, the HARSVEL? prompt is dis- played on the CDU. If HARS is free iner- tial for 20 or more seconds and groundspeed is less than 10 knots, the electronic trim ball will flash. Care should be taken when engaging HAS as uncom- manded aircraft drift may occur. Aircraft position should be carefully monitored using visual cues and symbology. If the GPS system is not keyed or is track- ing less than 4 satellites, HAS perfor- mance may degrade over time. Hover Position Box and Velocity Vector accura- cy will be degraded. In normal operation, the Navigation System redundantly integrates data from the sensors. The EGI is the primary sensor for inertial data with the HARS as the backup. The GPS is the primary sensor for helicopter position. The Doppler provides helicopter velocity data. The GPS, if keyed and verified, will also provide velocity data. The fire control system and flight symbology use the redundant data. There are three instruments and two subsystems that receive dedicated inputs from the sensor array. The HSI, RMI, and RAI receive data directly from the HARS for heading and attitude. The DNS provides data directly to the distance–to–go and No.1 needle displays on the HSI. The DASE and DNS receive data directly from the HARS. For those instruments, including the DASE and DNS, that receive data directly from a sensor, there is no redundan- cy. If the sensor providing the data has failed, then the af- fected instrument or subsystem will not function. The dis- tance–to–go and steering information is always displayed on the CDU and HMD symbology. The velocity vector, ac- celeration cue, vertical speed indicator, and heading tape are driven by the navigation system and not by any single sensor. This allows redundancy in the displays in the event of invalid data or failure of any sensor. No one ve- locity sensor can cause the error. However, because the Hover Augmentation System (HAS) of the DASE receives velocity data directly from the HARS, it is possible for a ve- locity error in the HARS to affect HAS and not affect the Navigation System. In the event that the HARS has been in a free inertial condition for longer than 20 seconds the HARSVEL? message is displayed in the CDU, NAV sta- tus page. If the HARS has been in a free inertial condition for longer than 20 seconds and the groundspeed of the helicopter is less than 10 knots,the pilot HMD trim ball will flash. The purpose of the flashing trim ball is to alert the pilot that if the HAS is engaged there may be an initial drift of the helicopter. NOTE Repeated HARS moving alignments (sea start or airborne start) will eventually fail HARS internal calibration test and have to be zeroed out or replaced. The FCC has been modified to automatically mode the HARS to an operate condition, via the HARS squat switch control interface. This occurs whenever the navigation mode is water, total engine torque (sum of both engines/ symbolic torque) value exceeds 40% or the aircraft squat switch indicates an airborne condition. Once the HARS is moded to operate, either via the HARS mode switch or the squat switch control as described above, the HARS is never allowed to return to an align mode, even if the HARS mode switch is returned to fast and norm. The FCC uses the INS, DNS or Zero Source velocity to aid in the HARS. If the aircraft is on the ground, the NAV mode is land and the total engine torque has been less than 40% for 90 seconds or more, the FCC supplies the HARS a Zero Source velocity. This allows for extended ground op- erations without the HARS velocity incurring any drift. If the HARS is not allowed to perform regular norm align- ments, it will eventually fail internal calibration test and have to be zeroed out or replaced. Once the HARS is in an operate condition, the FCC provides automatic magvar corrections to keep the HARS heading effectively caged to the INS heading. There is no requirement to place the HARS mode switch to norm after landing if ground opera- tions will exceed one minute. With the integrated navigation system, the Hover Position Box drift varies according to whether the EGI or HARS is in use as the inertial sensor, and if using the EGI, whether or not the GPS is operating in PPS (keyed) or SPS (not keyed) mode or operating at all. Using the EGI with the GPS keyed produces optimal HAS/Hover Position Box drift performance. The amount of drift may be up to 6 feet the first minute, and as much as 23 feet after 5 minutes. All other modes of operation