Figure 2-51.  Engine control system.  passes through an aircraft shutoff valve that is tied to the  fire detecting/extinguishing system. An aircraft furnished  in-line fuel filter may also be used. Fuel entering the fuel  control unit first passes through a 10-micron filter. If the filter  becomes contaminated, the resulting pressure drop opens the  filter bypass valve and unfiltered fuel then is supplied to the  APU. Shown in  Figure 2-52  is a pump with an inlet pressure  access plug so that a fuel pressure gauge might be installed  for troubleshooting purposes. Fuel then enters a positive  displacement, gear-type pump. Upon discharge from the  pump, the fuel passes through a 70-micron screen. The screen  is installed at this point to filter any wear debris that might  2-38 TT4.5 Bleed Valve P0 NL T13 PT0 TM [WF/P3] Engine Control System EFCU cross-link Fault Status IND Control Mode IND Engine monitoring system Flight data recorder Potentiometer IGN Power Cont Mode Sel ECU PWR NH Backup NH Mechanical Linkage P3 28V PWR Igniter IGN Cont PMG Fuel  Control Starter Gen Control Solenoids and Relays NL  Trim and  Engine ID EFCU Mode Sel SW TT0 400 Hz PLA SYMBOLS Core spool speed Fan spool speed Free stream total pressure Power lever angle Total temperature Interturbine temperature Fan exit air temperature Indicated airspeed NH NL PT0 PLA TT0 T4.5 T13 P