TM 1-1510-218-10 3C-38 (b)   When the aircraft reaches an altitude 1000 feet from selected altitude, the  ALT annunciator on upper right corner of pilot's altimeter and the   ALT ALERT  annunciator   in  the  annunciator  block  above the  copilot's  airspeed  indicator  will  illuminate  and  a warning  horn  will  sound  for  1  second.    The  lights  will remain illuminated until the aircraft reaches an altitude 250  feet  from  the  selected  altitude.    If  the  aircraft  now deviates   from   the   selected   altitude   by   more   than 250  feet, the ALT lights will again be illuminated.  The lights  will  remain  illuminated  until  the  aircraft  is  once again within 250 feet of the selected altitude. 3C-20.  GYROMAGNETIC COMPASS SYSTEMS (C14A). a.   Description. Two identical compass systems  are  installed  to  provide  accurate  directional information for the aircraft.  For heading reference, two modes of operation are used, directional gyro (FREE) mode  or  slaved  compass  (SLAVE)  mode.    In  areas where magnetic references are reliable, the system is operated in the slaved compass mode.  In this mode, the directional gyro is slaved to the magnetic flux valve that  supplies  a  magnetic  reference  for  correction  of gyro precession.  In the free gyro mode, the system is operated   as   a   free   gyro.      In   this   mode,   heading corrections are manually introduced using the pilot's or copilot's   INCREASE   /   DECREASE   switches.      The slave/free   mode   is   selected   as   desired,   using   the pilot's or copilot's SLAVE / FREE switches. b.  Gyromagnetic Compass System Controls, Indicators, and Functions. (1)   Compass   System   Selector   Switch.      A three-position     switch,     placarded HEADING ALL SYSTEMS     ON     COMPASS     1     /     NORM     /     ALL SYSTEMS   COMPASS   2,  located  left  of  the  pilot’s EHSI,    controls    the    selection    of    compass    system.   When  the  switch  is  set  to  the   ALL  SYSTEMS  ON COMPASS 1 position, both EHSI’s, both flight director computers, and the flight management system receive heading information from compass system number 1.   When the switch is set to the NORM position, the flight director computer number 1, EHSI number 1, and flight management system receive heading information from compass  system  number  1,  while  the  flight  director computer   number   2   and   EHSI   number   2   receive heading  information  from  compass  system  number 2.   When  the  switch  is  set  to  the   ALL  SYSTEMS  ON COMPASS 2 position, both EHSI’s, both flight director computers, and the flight management system receive heading information from compass system number 2. (2)   Heading Comparator Warning Annunciator    Compass.        The   EADI   compares   the heading  inputs  from  system  number  1  and  system number  2  and  displays  a  yellow  flag  placarded   HDG on the top of the EADI to show a miscompare between the   two   heading   inputs   to   the   pilot’s   and   copilot's EHSI. (3)   Gyro  Slave  /  Free  Switches.     The  pilot and copilot are each provided with a switch placarded GYRO  SLAVE  /  FREE,  located  on  the  extreme  left and  right  sides  of  the  instrument  panel.    When  the switch   is   in   the   SLAVE   position,   the   pilot's   flight instruments  are  receiving  heading  information  based on  magnetic  north.    When  the  switch  is  in  the   FREE position,  the  respective  pilot's  or  copilot's  EHSI  and FMS are receiving heading information referenced to a free gyro. (4)   Gyro Increase / Decrease Switches.   The pilot   and   copilot   are   each   provided   with   a   three- position    switch    spring-loaded    to    the    center    (off) position, placarded  GYRO  INCREASE /  DECREASE, and located on the extreme left and right sides of the instrument   panel.      When   the   respective   pilot's   or copilot's gyro slave/free switch is in the FREE position, these  switches  may  be  used  to  increase  or  decrease the   magnetic   heading   indication   of   the   respective pilot's or copilot's EHSI. (5)   Heading   Source.      The   heading   source field  annunciates  the  source  for  the  aircraft  heading input  on  the  EHSI’s.    Magnetic  north  heading  can  be referenced  with  or  without  the  directional  gyro  slaved to a magnetic flux detector.  Table 3C-11 indicates the heading source annunciations.