TM  55-1510-221-10 4.   Realignment   -   INS   shut- down. Perform complete alignment   procedures. 5.   Position   update.   Recom- mended  if  time  is  not  avail- able  for  realignment. NOTE Perform   position   update   using   parking coordinates   in   accordance   with   proce- dure:   “Insertion   of   Geographic   Waypoint Coordinates.“   If   parking   coordinates   are not  available,  proceed  as  follows: Continue  operation  in  NAV,  if  INS  accu- racy  appears  acceptable. Perform   position   update   using   best   esti- mate  of  parking  coordinates. 6.   Downmoding   to   standby: Mode  selector  -  STBY. NOTE INS  can  be  downmoded  to  standby  opera- tion   which   will   maintain   navigation   unit at  operating  temperature  with  gyro  wheels running.  INS  is  downmoded  to  standby  as follows: Do  not  leave  INS  operating  unless  aircraft or  ground  power  and  cooling  air  are  avail- able  to  system. 7.  Shutdown:  Mode  selector  - OFF. N O TE INS  will  retain  inertial  present  position data  computed  at  time  INS  is  down- moded.  This  value  is  compared  with  pres- ent  position  inserted  for  next  alignment and   difference   is   used   to   determine   azi- muth  gyro  drift  rate. e.   Abnormal   Procedures: (1.) General.  INS  contains  self-testing  fea- tures  which  provide  one  or  more  warning  indica- tions  when  a  failure  occurs.  The  WARN  light  on  th CDU  provides  a  master  warning  for  most  malfunc- tions   occuring   in   the   navigation   unit.   Malfunctions in  the  MSU  or  CDU  will  normally  be  obvious because   of   abnormal   indications   of   displays   and lights.   A   battery   unit   malfunction   will   shut   down INS  when  battery  power  is  used. (2.)   Automatic   INS   shutdown. (a.)   Overtemperature.   An   overtemp- erature  in  navigation  unit  will  cause  INS  to  shut down  (indicated  by  blank  CDU  displays)  when mode  selector  is  at  STBY  or  ALIGN  during  ground operation.  The  WARN  light  on  CDU  will  illuminate and  will  not  extinguish  until  mode  selector  is  rotated to  OFF.  The  cooling  system  should  be  checked  and corrected  if  faulty.  If  cooling  system  is  satisfactory, navigation  unit  should  be  replaced. (b.)  Low  battery  charge.  A  low  bat- tery  unit  charge  will  cause  INS  to  shut  down  when INS  is  operating  on  battery  unit  power.  Both  WARN light  on  CDU  and  BAT  light  on  MSU  will  illumi- nate  and  not  extinguish  until  the  mode  selector  is  set to  OFF.  The  battery  unit  should  be  replaced  when this  failure  occurs. (c.)   Interpretation   of   failure   indica- tions.  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   light   illuminated,   and   WARN light   extinguished.   Under   each   of   these   categories are  listed  other  indications  which  will  give  the  oper- ator  sufficient  information  to  take  action. 1. WARN   light   illuminated. Take  the  following  action: a. If  action  codes  01,  02, 03, 04, 05 are displayed - See table 3-2. b.  No  action  or  malfunc- tion  codes  displayed  - Indicated computer failure. c. Improper   displays   - Indicates   NU   computer failure. 2.   WARN   light   extinguished.   If CDU   displays   are   blank, incorrect   or   frozen   -   CDU failure  is  indicated. 3-71