TM 1-1500-204-23-4 NOTE Lubricate only if called for by the manufacturer's instructions covering the motor.  Most motors used in aircraft require no lubrication between overhauls. (3) Replacement criteria.  Motor replacement criteria are as follows: · Replace   if   the   insulation   on   leads   or   windings   is burned, cracked, or brittle. · Replace  when  any  unusual  bearing  or  gear  noises are detected during operation. · Replace  when  commutator  bars  are  rough,  scored, pitted, loose, high, or worn to the depth of insulating mica between bars. NOTE Remove motor from aircraft when replacing brushes. · Replace brushes that are cracked, broken, chipped, unevenly  worn,  or  worn  beyond  maximum  wearing depth slot. i.      Thermocouples.      Thermocouples   are   used throughout the aircraft to detect and measure temperature   changes.      A   thermocouple,   as   shown   in figure 3-13, is the junction of two dissimilar metals which generate a small electric current according to the rate of temperature  rise.    Thermocouples  are  commonly  used in   fire   warning   systems   and   temperature   indicating instruments on Army aircraft. 18 j.    Ignition  Systems.    Ignition  systems  on  Army aircraft   are   used   for   engine   starting   and   operation. Turbine   engine   ignition   systems   are   usually   operated only  for  a  brief  period  during  the  starting  cycle,  during takeoff  and  landing,  and  during  flight  in  bad  weather (i.e., icing conditions).  Turbine engines use capacitor or electronic ignition systems. (1) Troubleshooting.  Use applicable aircraft maintenance   manuals   to   troubleshoot   aircraft   ignition systems.        Table    3-3    is    an    example    of    a    typical troubleshooting    chart    for    an    Army    aircraft    ignition system. (2) Replacement criteria. Refer to applicable aircraft maintenance manual for replacement criteria. k.      Lighting    Systems.        Army    aircraft    lighting systems   include   cockpit   lights,   instrument   panel   and console    lights,    caution    and    warning    light    systems, position    lights,    anti-collision    light,    searchlight,    and transmission   oil   level   lights.      Instrument   panel   and cockpit  lighting  is  provided  to  enhance  night  vision  of the flight crew. (1)  Inspection.    Inspect  lights  for  corroded lamp socket terminals, shorted or broken wires, cracked lens, or burned out lamp bulbs. (2) Troubleshooting.  Use applicable aircraft maintenance   manuals   to   troubleshoot   aircraft   lighting systems.        Table    3-4    is    an    example    of    a    typical troubleshooting    chart    for    an    Army    aircraft    lighting system. (3)   Replacement   criteria.      Light   assembly may   be   repaired   by   replacing   damaged   or   defective component   parts.      If   light   case   is   damaged   beyond repair,    complete    unit    must    be    replaced.        Replace missing or burned out fuses, lamps, and discolored lens. 3-19