TM 1-1510-225-10 3C-39 (3)   Light  Sensor   –   The   built-in  light  sensor automatically controls the display brightness. (4)   Power    Switch     –    The    latching    push- on/push-off  PWR  switch  controls  the  power  applied  to the IND-42A indicator. (5)   Mode Selector Switch  – The non-latching push-button  SEL  switch  selects  the  information  to  be displayed   in   the   display.      (When   power   is   initially applied, NM  (distance) is shown in the numeric display and   ID    (DME    station    identifier)    is    shown    in    the alphanumeric  display).    Pressing  the   SEL   switch  will sequentially select  KT (velocity), MIN (time-to-station), and  I D  (2,  3,  or  4  letter  station  identifier).    KT,  MIN, and I D are shown in the alphanumeric display and  NM  is continuously shown in the numeric display. (6)   Channel  Switch  –  The  momentary  push- button  CH  switch  sequentially  selects  the  information from the next DME channel and lights the appropriate channel annunciator 1, 2 (or 3 if enabled). (7)   Annunciators  – The annunciators provide an   indication   of   which   DME   channel   is   selected, system operational information, and units of measure.   The following list describes the annunciators. (a)  1,  2,  3  –  Sequentially  controlled  by the   CH   button   to   indicate   which   DME   channel   is providing    the    information    being    displayed    in    the numeric and alphanumeric displays. (b)  WPT – Not used. (c)  NM   –  Automatically  illuminates  after power on when valid DME data is available.  Indicates that the numbers displayed in the numeric display are slant range DME distance in nautical miles. (d)  HLD  –  Indicates  that  DME  hold  has been selected on the CTL-32 NAV Control. (e)  KT  –  Sequentially  controlled  by  the SEL button and indicates that the numbers displayed in     the     alphanumeric     display     are     the     computed groundspeed in knots.  Groundspeed is based on rate of   change   of  DME  distance  and  accurately  reflects slant-range   speed   only   when   the   aircraft   is   flying directly to or from the station. (f)   MIN  –  Sequentially  controlled  by  the SEL button and indicates that the numbers displayed in the alphanumeric display are the computed time-to- station in minutes.  Like the groundspeed (KT) display, time-to-go  is  also  based  on  rate  of  change  of  DME distance. (g)  ID   –   Automatically   illuminates   after power  on.    The  DME  ident  is  transmitted  once  every 30  seconds  and  it  is  possible  that  2  minutes  could elapse   before   the   station   ident   is   displayed   in   the alphanumeric display.  The station identifier is usually 3 letters, but can be 2, 3, or 4 letters, depending on the facility being used. CAUTION It is not practical to provide monitoring for all conceivable system failures and, however unlikely, it is possible that erroneous operation could occur without a fault  indication.    It  is  the  responsibility  of the  pilot  to  detect  such  an  occurrence  by means  of  crosschecks  with  redundant  or correlated     information    available    in    the cockpit. c.     Operating Procedures.  (1)   Equipment   Turn   On   –   Apply   power   by pressing the   PWR  button  on  the  IND  42A.    The   NM  and    ID    annunciators    and    the    channel    1    or    2 annunciator will be illuminated.  If the DME-42 is within range  of  the  selected  station,  the  numeric  display  will show  the  slant  range  distance,  and  after  two  DME idents    have    been    received    and    processed,    the alphanumeric  display  will  show  the  2,  3,  or  4  letter station identifier. CAUTION The  DME-42  may  not  decode  the  identity transmissions correctly, it is the responsibility of the pilot to verify reception  of  the  intended  ground  station through the audio identifier as necessary. (2)   Frequency Selection – The NAV frequency  for  DME  channel  1  is  the  frequency  shown in  the  active  frequency  display  of  the  pilot's  CTL -32.   The  frequency  can  be  changed  using  the  ACT button or the   XFR  /  MEM  switch.    DME  channel  2  is  the frequency shown in the active frequency display of the copilot’s CTL-32, and DME channel 3 is the frequency shown  in  the  preset  frequency  display  of  the  pilot's CTL -32. Using   the   pilot's   and   copilot's   active   and   the pilot's  preset  displays,  select  the  desired  VORTAC  or ILS  frequency  for  paired  DME  channels,  or  the  VHF NAV   frequency   for   unpaired   channels.      The   DME channel  that  is  associated  with  the  VORTAC,  ILS,  or unpaired VHF NAV frequency is automatically selected.