TM 55-1510-220-10 2. Low battery charge.  A low battery unit charge will cause INS to shut down when INS is operating on battery  unit  power.    Both  WARN  annunciator  on  CDU  and BAT annunciator on MSU will illuminate and not extinguish until the mode selector is set to OFF.  The battery unit should be replaced when this failure occurs. 3. Interpretation of failure indications.  It is    important    to    be    able    to    correctly    interpret    failure indications    in    order    to    take    effective    action.    Failure indications   are   listed   below   under   two   main   categories: WARN   annunciator   illuminated,   and   WARN   annunciator extinguished.  Under each of these categories are listed other indications which will give the operator sufficient information to take action. 1. WARN  annunciator illuminated.  Take the following action: a. If action codes 01, 02, 03, 04 or 05 are displayed See table 3-2. b. No   action   or   malfunction   codes displayed Indicates NAV unit computer failure. c. Improper  displays  Indicates  NAV unit computer failure. 2. WARN   annunciator   extinguished.      If CDU  displays  are  blank,  incorrect  or frozen CDU failure is indicated. NOTE It  is  not  possible  to  load  displays  from  the keyboard.        A    temporary    failure    of    a numerical  key  may  prevent  loading.    If  a number  cannot  be  loaded  into  latitude  or longitude  displays,  after  pressing/wiggling the key several times, the cause may be the momentary   hang-up   of   another   key.   To identify   the   faulty   key,   rotate   the   data selector to DSRTK/STS.  The right digit on right    display    will    indicate    suspect    key. Press and release suspect key several times. To  test  whether  the  keyboard  problem  is corrected, try pressing any other numerical   key.      Its   number   should   now appear  as  the  right  digit.    If  this  test  is successful,    press    the    CLEAR    key    and return     data     selector     to     original     data position.    Otherwise,    a    CDU    failure    is indicated. 4. CDU    BAT    indicator    annunciator    is illuminated. CAUTION Operation  on  battery  is  an  indication  that there may not be aircraft power to blower motor  with  resultant  loss  of  cooling.    The INS    can    operate    only    a    limited    time (normally 15 minutes) on battery power be Table 3-2.  Malfunction Code Check Step Indicator or Indication Control Operation Display Indication 1 WARN light Lights 2 Data selector Rotate to DSRTK/STS RH data display Action code second and third digits 3 TEST switch Press and release RH data display Lowest number malfunction code which has occurred since this procedure was performed replaces action code. 4 Repeat step 3 repeatedly, recording all malfunction codes until second and third digits again indicate an action code or go blank.  Refer to Table 34 for action codes and recommended action and to Table 3-5 for malfunction code definition. 5 If WARN light extinguishes and two digits go blank, failure was intermittent and has been cleared.  If digits do not go blank, perform action according to displayed recommended action code. 3-72        Change 2