TM 1-1510-225-10 3C-31 (6)   AP ENGAGE Push Button.  The engage switch is used to engage the autopilot.  Engaging the autopilot automatically engages the yaw damper.  The autopilot   may   be   engaged   with   the   airplane  in  any reasonable    attitude.        The    autopilot    is    normally disengaged by momentarily pressing the control wheel AP    DISC    switch.        The    autopilot    may    also    be disengaged by any of the following actions. 1.    Actuation   of   the   control   wheel   AP  DISC    button.        Disengagement    is confirmed  by  five  flashes  of  the  AP ENG annunciator. 2.    Pressing  the  respective  vertical  gyro FAST ERECT button. 3.    Actuation     of     respective     compass INCREASE / DECREASE switch. 4.    Selection of Go-Around mode.   Disengagement  is  confirmed  by  the AP   ENG    annunciator   flashing   five times    and    illumination    of    the    GA annunciators. 5.    Pulling  the  autopilot   AP   CONTROL circuit breaker. 6.    Pressing  the  autopilot   AP  ENGAGE push button. 7.    When  transferring  between  pilot  and copilot flight directors. Any  of  the  following  malfunctions  will cause the autopilot to automatically disengage. 1.    Vertical gyro failure. 2.    Directional gyro failure. 3.    Autopilot power or circuit failure. (7)   Elevator Trim Annunciator.    The  elevator trim annunciator indicates  UP or  DN when a sustained signal   is   being   applied   to   the  elevator  servo.    The annunciator   should   not   be   illuminated   before   the autopilot is engaged. 3C-21.  VHF NAVIGATION RECEIVERS. a.    Description. The VIR-32 Navigation Receiver    provides    200    50-kHz    spaced    VOR/LOC channels from 108.00 through 117.95 MHz and 40 GS channels automatically paired with localizer channels.   It also provides a marker beacon receiver. The  digital  VIR-32  provide  VOR,  LOC,  and  GS deviation   outputs,   high   and   low   level   flag   signals, magnetic  bearing  to  the  station,  to/from  information, marker  beacon  lamp  signals,  and  VOR  and  marker beacon audio outputs. CAUTION When monitoring for marker beacon passage,  always  listen  for  marker  beacon audio.    Do  not  rely  solely  on  the  marker beacon lights.  b.    Operating  Controls.     All  operation  controls for the VIR-32 are located on the CTL -32 NAV control.   Refer to Figure 3C-15. (1)   Compare    Annunciator    (ACT).          ACT momentarily  illuminates  when  frequencies  are  being changed.   ACT flashes if the actual radio frequency is not   identical   to   the   frequency   shown   in   the   active frequency display. (2)   XFR  /  MEM   Switch.    This  switch  is  a  3- position,  spring-loaded  toggle  switch.    When  held  to the XFR position, the preset frequency is transferred to the   active   display   and   the   receiver   re-tunes.      The previously  active  frequency  becomes  the  new  preset frequency and is displayed in the lower window.  When this switch is held to the  MEM position, one of the four stacked  memory  frequencies  is  loaded  into  the  preset display    Successive  pushes  cycle  the  four  memory frequencies through the display (…2, 3, 4, 1, 2 ,3…). (3)   Frequency Select Knobs.  Two concentric knobs control the preset or active frequency displays.   The larger knob changes the three digits to the left of the  decimal  point  in  1-MHz  steps.    The  smaller  knob changes the two digits to the right of the decimal point 0.05-MHz  steps.    Frequencies  roll  over  at  the  upper and  lower  limits.    The  two  frequency  select  switches are independent of each other such that the upper and lower  limit  rollover  of  the  0.1-MHz  digit  will  not  cause the 1.0-MHz digit to change. (4)   ACT  Button.    Push  the    ACT  button  for approximately   2   seconds   to   enable   the   frequency select   knobs   to   directly   retune   the   receiver.      The button   window   will   display   dashes   and   the   upper window  will  continue  to  display  the  active  frequency.   Push   the   ACT  button  a  second  time  to  return  the control to the normal two-display mode. (5)   TEST Button.  Press the  TEST button to initiate the radio self-test diagnostic routine.  (Self-test is active only when the TEST button is pressed.)