TM 1-1510-225-10 3C-47 (5)   TEST    Button    –    TEST    button    checks indicator R/T unit and flag operation. c.     Normal Operation. (1)   Setting   Decision   Height   Altitude.      The radio   altimeter   also   provides   altitude   and   decision height   information   to   the   GCAS.      Set   the   decision height   altitude   using   the   decision   height   set   knob.   Setting the radio altimeter decision height marker to a decision    height    altitude    will    cause    the    GCAS    to announce  the  "Minimums,  Minimums"  aural  warning when  the  aircraft  transitions  through  the  selected   DH altitude. (2)   Testing.      Operating   the    TEST    button causes  the  flag  to  come  into  view  and  altitude  pointer to indicate approximately 100 feet.  Release of button causes pointer to return to existing altitude and flag to retract. 3C-29.  VERTICAL SPEED INDICATORS. The   vertical   speed   indicators   are   mounted   on both   sides   of   the   instrument   panel.      The   face   is placarded  VERTICAL SPEED,  UP, and  DOWN.  The range  of  the  instrument  is  from  0  to  4000  feet  per minute,  with  the  first  2000  feet  graduated  in  100-foot per  minute  increments  and  the  remainder  in  250-foot per minute increments. 3C-30.  TURN AND SLIP INDICATOR. Refer  to  Chapter  2,  Paragraph  2-77 for detailed information. 3C-31.  GYRO MAGNETIC COMPASS SYSTEM. a.     Description. Two identical compass systems   provide   accurate   directional   information  for the  aircraft  at  all  latitudes  of  the  earth.    For  heading reference, two modes of operation are used: directional   gyro   (FREE)   mode,   or   slaved   (SLAVE) mode. b.    Operation. In     areas     where     magnetic references  are  reliable,  the  system  is  operated  in  the SLAVE  mode.    In  this  mode,  the  directional  gyro  is slaved    to    the    magnetic    flux    valve    that    supplies magnetic  reference  for  correction  of  the  apparent  drift of the gyro.  In FREE mode, the system is operated as a   free   gyro.      In   this   mode,   latitude   corrections  are manually introduced using the INCREASE / DECREASE  switches  located  outboard  of  each  HSI.   The  slave/free  mode  is  selected  as  desired  using  the SLAVE   /   FREE   switches   also   located   outboard   of each  HSI.    Both  compass  systems  (No.  1  and  No.  2) are   ac   power   dependent   and   are   powered   by   the selected inverter. Gyro  compass  1  provides  heading  information for the pilot's HSI and copilot's RMI.  Gyro compass 2 serves the copilot’s HSI and the pilot's RMI. c.     Compass    Synchronization    Annunciator.   Consists of the symbol  2 or  X  (dot or cross) displayed in  a  window.    When  the  compass  system  is  in  the slaved  mode,  the  display  will  oscillate  between  the  2 and X, indicating the heading dial is synchronized with gyro stabilized magnetic heading. 3C-32.  VERTICAL GYRO SYSTEM. The pilot's and copilot's vertical gyro systems are independent and powered by the ac power bus located in the nose avionics compartment.  The purpose of the vertical gyro systems is to provide the pilots with visual indications   of   aircraft   pitch   and   roll   attitudes  on  the flight   director   indicators.      The   gyroscope   develops through synchros, pitch, and roll signals representative of the aircraft attitude. Vertical reference is established  by  two  gravity  sensitive  switches,  which control a torque motor for each gyro axis.  High or low erection  rate  of  the  gyro  is  accomplished  by  applying high  or  low  voltage  to  the  respective  torque  motor.   Panel mounted switches, placarded   NO.  1 (pilot) and NO.  2 (copilot)  V  GYRO  FAST  ERECT,  provide  the means  for  fast  erection  of  the  gyros.    Pressing  the FAST  ERECT  switch  will  erect  the  gyro  to  within  1.0 of    pitch    and    roll    within    60    seconds    of    power application, and erect to within 0.5 within 2 minutes. 3C-33.  TOTAL AIRSPEED / TEMPERATURE INDICATOR. a.     Description.    The  combined  Total  Airspeed, Static   Air   Temperature   indicator   provides   a   digital display  of  both  TAS  and  SAT  or  TAT  as  generated  by the air data computer. b.    Display.  The input signals are displayed as follows.  Refer to Figure 3C-21. (1)   TAS – Displays TAS. (2)   SAT – Displays SAT in two digits and the ± sign from -99° to + 50° C.