TM 1-1510-225-10 3C-41 station.  Ground range from the station can be  computed  by  using  altitude  and  slant range.  The difference between slant range and  ground  range  is  usually  insignificant except     when     flying     over     the     selected ground  station.    For  example,  an  aircraft that is 100 nm ground range from a station and flying at 35,000 feet altitude is approximately  100.17  nm  slant  range from the  ground  station.    When  flying  directly over  the  ground  station,  the  DME  distance information     is     the     aircraft     altitude     in nautical miles above the station. (b)  Press the SEL button on the IND-42A until  KT  is  annunciated.    The  alphanumeric  display now  shows  computed  groundspeed  in  knots  if  the computed groundspeed is between 50 and 999 knots.   Accuracy of the display is +1 knot or +1%, whichever is greater, and will be true only after 30 to 60 seconds.   Dashes  will  be  shown  in  the  alphanumeric  display  if the computed groundspeed is less than 50 knots. (c)  Press the SEL button on the IND-42A until  MIN  is  annunciated.    The  alphanumeric  display now shows the time in minutes to or from the station if the  computed  groundspeed  is  greater  than  50  knots.   Dashes  will  be  shown  in  the  alphanumeric  display  if the computed groundspeed is less than 50 knots.  The MIN display ranges from 0 to 120 minutes.  Accuracy of the display is +1 minute. NOTE The  KT  and  MIN  indications  are  accurate only    if    the    aircraft    is    tracking    directly toward   or   away   from   the   ground   station and  the  distance  is  sufficient  for  the  slant range and ground range to be nearly equal.   For  example,  at  an  altitude  of  12,000  feet above  the  VORTAC  station  and  at  a  slant range    of    10    nm    from    the    station,    the groundspeed indication is lower than actual aircraft groundspeed by approximately 2%.  The MIN indication will have a corresponding error. If  the  DME  is  put  into  hold,  the  ID  display will   automatically   be   selected   when   the HLD   annunciator   illuminates.      The   pilot may still select other modes (KT, MIN), but when he stops pushing the SEL button, the display   will   revert   to   the   ID   mode   after approximately  5  seconds.  This  is  a  safety feature to remind the pilot not only that the DME  is  in  hold,  but  also  to  show  which station is being held. DME   hold   is   not   functional   for   the   third DME channel (#1 NAV pre-select). Therefore, this safety feature is not applicable. When the #1 NAV is in the hold mode, the pre-selected    frequency    (channeling    DME #3) remains active although not displayed. (5)   Self-Test. An extensive self-test diagnostic  routine  can  be  initiated  in  the  DME-42  by pushing the TEST button on the CTL-32 NAV Control.   The  self-test  routine  takes  approximately  10  seconds to   complete.      After   initiating   self-test,   all   display segments and annunciators on the IND-42A illuminate for  a  lamp  test.    If   NM and   ID  were  being  displayed, the   numeric   display   will   show   a   test   distance   of 100  NM.  The alphanumeric display will show AOIC at the completion of the test routine if no faults have been detected.    The  DME  aural  output  will  be  the  Morse characters   AOK.      If   KT   was   being   displayed,   the alphanumeric display will show a test groundspeed of 100  KT.  If MIN was being displayed, the alphanumeric display will show a test time of 60 MIN.  Completion of self-test is indicated when either the IND-42A displays return  to  normal  or  the  word  DIAG  along  with  a  self- test  fault  code  is  displayed  on  the  IND  42A.    Tables 3C-4  and  3C-5 provide a partial list of diagnostic and fault codes. NOTE Due to the approximate 10-second self-test cycle  time,  the  test  should  be  made  as  a preflight check and not during critical flight times.  Table 3C-4.  DME-42 Diagnostics CODE DESCRIPTION 01 Power supply 02 Synthesizer 03 Transmitter 04 Video processor 05 Receiver 06 Distance processor 07 Microprocessor ROM 08 Microprocessor RAM Table 3C-5.  IND-42 Diagnostics    CODE 90 Bus failure (no data) 91 TEST mode failure (no response) 92 Distance word missing 93 VEL, TTS word missing