TM 1-1510-223-10 The   lightning   sensor   system   (LSS)   is   used   to detect and locate areas of lightning activity within a 100 nautical   mile   radius   around   the   aircraft   The   system provides   the   operator   with   a   visual   display   of   the position  and  rate  of  occurrence  of  the  lightning  activity on the EHSL.  The lightning sensor system detects both visible  and  high  energy  invisible  electromagnetic  and electrostatic   discharges   (lightning)   indicating   areas   of turbulent activity.  After evaluating the LSS display, and its  relation  to  precipitation  as  indicated  by  the  weather radar   display,   the   operator   can   effectively   plan   the proper    course    to    avoid    hazardous    weather.        The lightning sensor system consists of a receiver/processor, an   antenna,   a   mode   selector   switch   (located   on   the radar   control   panel,   fig.      3-29),   and   the   pilot's   and copilot's EHSL.  The system is powered by a 2-ampere circuit-breaker placarded LSS, located on the overhead circuit breaker panel (fig.  2-7).  Because the system is a passive device (it does not transmit), it can be operated safely  on  the  ground.    Weather  in  all  directions  around the   aircraft   may   be   monitored,   even   before   starting engines. a. Lightning    Sensor    System    Mode    Selector Switch (fig 3-28). (1) OFF. Disables lightning sensor system. (2) SBY.      In    the   standby   (SBY)   mode, lightning data is not displayed.  However, the receiving and processing equipment is active and lightning strikes are being counted and accumulated. (3) LX.        The    LX    mode    is    the    normal working mode of the lightning sensor system.  Lightning strikes are collected, processed, and displayed. (4) CLR/TST.  When clear/test is selected, all   memory   of   past   strikes   and   symbols   are   erased. After 3 seconds the system enters the test mode. In  the  test  mode,  simulated  lightning  signals  are fed  into  the  test  loop  in  the  antenna  and  a  lightning strike  is  simulated  at  a  bearing  of  45  degrees  at  25 nautical miles.  A lightning alert is also generated along the  outermost  range  ring  at  a  bearing  of  45  degrees. This  simulated  strike  will  progress  in  severity  through level three within 15 seconds of entry into the test mode. If  the  mode  selector  is  left  in  the  CLR/TST  mode,  the alert   and   strike   symbols   will   reduce   in   severity   and disappear.  The lightning rate symbol will disappear after approximately 2 minutes. NOTE The lightning sensor system antenna is used in    this    test,    and    as    a    result,    any    real lightning activity that occurs while the test is in operation may also be displayed. b. Lightning  Sensor  System  Display.   The  LSS shows   areas   of   lightning   activity   on   the   EHSI   with symbols.    Each  lightning  symbol  represents  the  center of  a  circular  area  with  a  radius  of  nine  nautical  miles. Three  different  lightning  symbols  are  used  to  represent three different rates of occurrence of the lightning within each 18 nautical-mile diameter circle.  The lightning rate symbols    represent    the    lightning    rates-of-occurrence during  the  last  two  minutes.    The  symbol's  location  on the   display   represents   the   average   position   of   the lightning  that  has  occurred  during  the  last  two  minutes inside  each  18  nautical-mile  diameter  area.    Lightning may   not   be   occurring   at   the   center   of   the   symbol. Because  it  is  easier  for  the  LSS  to  detect  lightning  at close  distances  than  at  far  distances,  the  number  of lightning    strokes    required    for    each    rate    symbol    is adjusted  for  distance  to  the  storm.    Figure  3-32  shows the number of strokes required for each symbol versus range to the storm.  Figure 3-33 shows the three rate-of- occurrence symbols. c. Lightning Alert System.   It is possible to see a   lightning   flash   visually,   and   see   no   change   in   the display.  This is normal, because, unless one of the rate thresholds  are  crossed,  there  will  be  no  change  in  the symbol,    other    than    a    slight    position    update.        The lightning alert function shows the operator that the unit is operational, and shows real-time lightning activity.  The lightning    alert    function    places    a    magenta    rate-one symbol near the outer range mark of the display, at the proper bearing, for each lightning flash (fig.  3-33).  The lightning alert symbol is displayed for five seconds, and gives    the    operator    confidence    that    the    system    is functional  even  though  there  may  be  no  change  in  the white  lightning  rate  symbols.    It  is  also  an  indicator  of real-time activity. d. Lightning Sensor System Mode Annunciations.   The  following  mode  annunciations  may appear on the EHSI display. (1) LX/F  (amber).    Indicates  that  self-test has detected a fault. (2) LXIS (green).  Indicates that the system is in the standby (SBY) mode. (3) LX/CL    (green).       Indicates    that    the system is in the clear (CLR) mode.  This annunciator will appear for approximately 3 seconds after the CLR/TST mode   has   been   selected.      After   this   time   the   mode annunciator will switch to LX/T. (4) LX/T (green).  Indicates that the system is  in  the  test  (CST)  mode.    This  annunciator  may  be replaced with LXMN. (5) LXmn  (amber).  In the CLR/TST mode an amber LXn  will be displayed, where mn is a two digit failure code.  Table 3-8 explains these codes. 3-83