Bolt  Elbow  Body assembly  Fitting for left  engine installation  Fitting for right  engine installation  Mounting bracket nut  Nut  Arm assembly  Bolt  Glass bowl Lower gasket Cap Stiffener Drain valve Lid assembly Retainer spring Filter Upper gasket Flat screen Standpipe Filter ring Elbow 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 22 21 Drain valve Inlet Outlet Figure 14-60.  A filter assembly on a light twin reciprocating-engine  aircraft. Figure 14-61.  A large-area double-screen filter passes fuel through  the outer cylindrical mesh and the inner conical mesh. Turbine engine fuel control units are extremely close  tolerance devices. It is imperative that fuel delivered to them  is clean and contaminant free. The used of micronic filters  makes this possible. The changeable cellulose filter mesh type  shown in  Figure 14-62  can block particles 10–200 microns  in size and absorbs water if it is present. The small size of  the mesh raises the possibility of the filter being blocked by  debris or water. Therefore, a relief valve is included in the  filter assembly that bypasses fuel through the unit should  pressure build up from blockage. Fuel filters are often used between the engine-driven fuel  pump and the fuel metering device on reciprocating, as well as  turbine-engine aircraft. While these are technically part of the  engine fuel system, a common type used on turbine engines is  discussed here. It is also a micronic filter. It uses finely meshed  disks or wafers stacked on a central core. These filters are  able to withstand the higher pressure found in the engine fuel  system downstream of the engine-driven pump.  [Figure 14-63] Indication of a filter blockage may also appear in the cockpit  through the use of a bypass-activated switch or a pressure  differential switch. The bypass valve physically activates a  switch that closes the circuit to the annunciator in the first  type. The differential pressure type indicator compares the  input pressure of the fuel filter to the output pressure. A  circuit is completed when a preset difference occurs. Thus,  14-35