TM 1-1510-223-10 Selecting go-around mode Selecting standby mode EFIS 1/2 reversionary selection NAV, ATT, or HDG reversionary selections (4)   VOR (NAV) mode.   The VOR mode provides for    automatic    intercept,    capture,    and    tracking    of    a selected VOR radial, using the selected navigation source displayed on the EHSI.  The navigation source displayed on   the   EHSI   is   a   function   of   the   VOR/localizer   (V/L) pushbutton     selector     switch     located     on     the     display controller  (fig.    3-8).    Prior  to  engaging  the  VOR  (NAV) mode, perform the following: 1. VOR navigation receiver control panel - Set desired VOR frequency. 2. V/L pushbutton selector switch (display controller) - Depress to select VOR1 or VOR2. 3. Course pointer (EHSI) - Set desired course on heading dial. 4. Heading marker (EHSI) - Set desired intercept heading for selected course. With the aircraft outside the normal capture range of the  VOR  signal  (typically,  course  deviation  indicator  on EHSI  off  course  by  more  than  2  dots),  engage  the  NAV switch-indicator on the flight director mode selector.  HDG switch-indicator on the flight director mode selector panel will   illuminate   ON,   and   the   NAV   switch-indicator   will annunciate  ARM.    HDG  (green)  and  VOR  ARM  (white) will  also  be  annunciated  on  the  EADI.    The  flight  control computer is now armed to capture the VOR signal, and is generating   a   roll   command   to   fly   the   heading   select mode. When reaching the lateral beam sensor trip point, the system  will  automatically  drop  the  heading  select  mode and will switch to the VOR capture phase.  The EADI will display the following indications: VOR annunciator (white) will extinguish HDG annunciator (green) will extinguish NAV (green) will be annunciated and will be enclosed in a white box for 5 seconds to indicate the capture phase The    heading    pushbutton    switch-indicator    on    the autopilot/flight  director  mode  selector  will  extinguish  and the NAV pushbutton switch-indicator will switch from ARM to CAP The flight control computer now generates the proper roll   command   to   capture   and   track   the   selected   VOR radial. Course    select    error    signal    is    established    by    the heading selected by the course pointer on the EHSI.  This signal    represents    the    difference    between    the    actual aircraft heading and the desired course.  The course error signal is then sent from the symbol generator to the flight control computer. The    radio    deviation    signal    is    routed    from    the navigation   receiver   to   the   symbol   generator   and   flight control computer.  The signal is lateral gain programmed in the flight control computer. The   lateral   gain   programming   is   performed   as   a function of DME distance to the station (if available), true airspeed, and barometric altitude.  This gain programming adjusts  for  the  aircraft  going  toward  or  away  from  the VOR station. NOTE If possible, avoid using DME hold or having N/A as  a  distance  display  annunciation  during  VOR capture and tracking.  When in this condition the flight control computer cannot use DME distance for gain programming. When  flying  a  VOR  intercept,  the  optimum  intercept angle is less than 45 degrees.  When the aircraft satisfies VOR track conditions, the course error signal is removed from  the  lateral  steering  command.    This  leaves  radio deviation, roll attitude, and lateral acceleration to track the VOR signal, and to compensate for beam standoff in the presence  of  a  crosswind.    The  system  will  automatically compensate  for  a  crosswind  of  up  to  45  degrees  course error. As  the  aircraft  approaches  the  VOR  station,  it  will enter a zone of unstable radio signals (zone of confusion). This zone radiates upward from the station in the shape of a truncated cone.  In this area, the radio signal becomes highly erratic and it is desirable to remove it from the roll command.  The overstation sensor monitors entry into the zone  of  confusion  and  removes  radio  deviation  from  the roll command. The  system  also  uses  the  collocated  DME  signal  (if available) to adjust tracking gains. When  over  the  VOR  station,  the  system  will  accept and follow a course change of up to 30 degrees. (5)   VOR  approach  mode.   The  set-up  procedure for  the  VOR  approach  mode  is  the  same  as  for  VOR tracking, except the VORAPR pushbutton switch-indicator on  the  mode  selector  panel  is  depressed  instead  of  the NAV  switch-indicator.    ARM  will  be  annunciated  by  the VORAPR switch-indicator on the mode selector panel and APR will be displayed in white on the EADI. 3-51