TM 1-1510-218-10 2-72 CAUTION Propeller reversing on unimproved surfaces should be accomplished carefully to prevent propeller erosion from reversed airflow and, in dusty conditions, to prevent obscuring the operator's vision. The  propeller  blade  angle  may  be  reversed  to shorten landing roll.  To reverse, propeller levers must be  positioned  at    HIGH  RPM  (full  forward),  and  the power levers are lifted up to pass over an IDLE detent, then  pulled  aft  into   REVERSE  setting.    Power  levers must be pulled back through normal idle speed range before   being   positioned   in   REVERSE.    One  yellow caution   light,   placarded   REV   NOT   READY,  on  the caution/advisory annunciator panel, Figure 2-17, alerts the   pilot   not   to   reverse   the   propellers.      This   light illuminates only when the landing gear handle is down, and   if   propeller   levers   are   not   at   HIGH   RPM  (full forward).      This   circuit   is   protected   by   a   5-ampere circuit breaker, placarded LANDING GEAR RELAY  C   D1 and  LANDING GEAR CONTROL   D2  T , located on the overhead circuit breaker panel. 2-58.  PROPELLER TACHOMETERS. a.   Propeller  Speed  Indicator  (N₂)  C  D  .    Two tachometers on the instrument panel register propeller speed    in    hundreds    of    RPM,    Figure 2-26.        Each indicator   is   slaved   to   a   tachometer   generator   unit attached to the corresponding engine. b.  Propeller   Speed   Indicator   (N₂)  T   .      Two digital/bar graph propeller tachometers located on the center  of  the  instrument  panel  display  the  measured N₂    concurrently    in    a    1    alphabetic,    3½    numeric character  digital  display  and  a  38-segment  bar  graph display. The  indicator  has  a  digital  range  of  0  to  2500 RPM  while  displaying  current  N₂ and 0 to 4000 while displaying exceedence N₂.  The bar graph display has a range of 0 to 2500 RPM. The digital display will contain dashes (----) when the  measured  N₂  is  greater  than  2500  RPM.    When displaying exceedence N₂ values the actual measured value, not "----", will be displayed up to 4000 RPM. The  dashes  will  be  prefixed  with  an  "F"  if  an EEPROM failure has been detected and with and "E" if an event has been stored and played back. The indicator  performs  BIT  on  the  LCD  display, ROM (program memory), RAM (volatile data memory), and  EEPROM  (Non-volatile  data  memory),  and  input signal.    The  display  BIT  shall  consist  of  displaying  all active LCD segments for three seconds on power up.   If  the  ROM  or  RAM fails BIT the indicator display will be blank.  If the input signal fails BIT the indicator will prefix the digital display with "F". BIT failures are not stored in EEPROM.  Failures detected   shall   therefore   not   affect   the   display   after power   is   cycled   unless   the   failure   is   encountered again. Each exceedence event will consist of two 16-bit words.        One    word    will    contain    the    peak    RPM measured    during    the    event    and    the    other    word contains the event duration in seconds. A  maximum  of  50  events  can  be  recorded.    If more  than  50  events  are  recorded,  only  50  will  be stored.  As new events are recorded, the oldest events will be discarded. The indicator will prefix the digital display with an "E" when an event is being or has been recorded and not played back. The    indicator    will    start    recording    an    event immediately  when  the  indicator  has  measured  RPM greater than 2200 or when the indicator has measured RPM   exceeds   2081.      The   flashing   will   cease   and digital  display  will  be  prefixed  with  an  "E"  when  the indicator    begins    recording    an    exceedence.        The indicator  will  end  exceedence  recording  and  store  the event   when   the   measured   RPM   drops   to   2080   or lower. The even display takes precedence over normal digital  display.    The  bar  graph  will  continue  to  display current  N₂  data.    To  activate  the  event  display  press the  EXCEEDENCE  DISPLAY  switch  for  0.2  seconds or  longer.    The  most  recent  event  will  be  displayed first.  Each event display contains the following cycle: (1)   The  event  number  will  be  displayed  for 2 seconds. (2)   The event peak RPM will be displayed for 2 seconds. (3)   The   event   duration   in   seconds   will   be displayed for 2 seconds. All  three  data  items  displayed  during  playback will  be  prefixed  with  a  "P"  unless  an  EEPROM  BIT failure has been detected in which case the display will be prefixed with an "F". The  event  display  will  be  terminated  after  all  of the  recorded  events  have  been  displayed.    Once  the playback    sequence    has    been    performed    the    "E"