TM  55-2840-241-23 the two drain ports located on the aft face of the filter cover. Pressure taps are provided before and after (labeled BF and AF) the filter element to permit mea- surement of filter pressure drop if desired. 1-15.  Double  Check  Valve. The  double  check  valve  assembly  is  a  mechanical device  placed  in  the  governing  pressure  (Pg)  circuit between the power turbine governor and the main fuel control  to  dampen  surges  in  the  Pg  pressure  signal. (See  figure  1-4.)  It  incorporates  two  individual, diaphragm operated valves within one unit. The valves are located in parallel with each other and open in opposite directions. Movement of air through the check valve assembly in either direction is permitted by one of the valves. However, when a stable signal is pro- vided, a very small flow of air is experienced with the valve toward its closed position. Any pressure surges in the Pg signal will be dampened by spring loading on the  valve. 1-16. Pc Air Filter. The Pc air filter is a 10 micron filter located in the Pc air supply line leading from the diffuser scroll to the power turbine governor. It incorporates a permanent type,  wire  mesh,  cleanable  element.  It  prevents  the governor and fuel control pneumatic components from being  contaminated  with  foreign  particles  by  filtering Pc air flow to the governor and control. 1-17. Fuel Check Valve. The  fuel  check  valve  is  located  in  the  fuel  line between the fuel control and the fuel nozzle. It has a cracking pressure of 22-28 psi and serves to prevent leakage of fuel through the fuel nozzle into the combus- tion chamber if the fuel cutoff valve is inadvertently opened or leaks while the engine is not operating. 1-18. Fuel Nozzle. The  fuel  nozzle  is  a  single-entry,  dual-orifice  type unit. It contains an integral valve for dividing primary and secondary flow. This fuel nozzle relies on the fuel check valve to provide fuel cutoff when the fuel mani- fold pressure falls below a pre-determined pressure, to keep fuel out of the combustion chamber at shutdown. 1-19.  Lubrication  System. The lubrication system is a dry sump type with an external  reservoir  and  heat  exchanger.  A  gear  type pressure and scavenge pump assembly is mounted within the power and accessory gearbox. (See figures 1-6 and 1-7.) The oil filter, filter bypass valve, and pres- sure regulating valve are in a unit which is located in the upper right-hand side of the power and accessory gearbox housing and are accessible from the top of the engine. A check valve is located between the housing and the filter unit. Indicating type magnetic chip detectors are installed at the bottom of the power and accessory gearbox, and at the engine oil outlet connec- tion. All engine oil system lines and connections are internal with the exception of pressure and scavenge lines to the compressor front support, the gas producer turbine  support,  and  the  power  turbine  support. 1-20. Ignition System. The engine ignition system consists of a low tension capacitor  discharge  ignition  exciter,  a  spark  igniter lead, and a shunted-surface gap spark igniter. The system receives its input power from a 14 to 29-volt, d-c power  source. 1-21.  Temperature  Measurement  System. The temperature measurement system has four chromel-alumel single junction thermocouples in the gas  producer  turbine  outlet  (TOT)  and  a  associated integral  harness.  The  voltages  of  the  four  thermocou- ples are electrically averaged in the assembly and delivered by the assembly lead to an engine terminal block for attachment to the airframe temperature indi- cating  system. 1-22.  Anti-king  System. a. Anti-icing is provided for the compressor inlet guide vanes and front support hub by the use of com- pressor discharge air. The air is taken from a port at the twelve o’clock position on the front face of the dif- fuser scroll. An anti-icing air shutoff valve is installed in the port and is manually operated from the flight deck to control anti-icing airflow. Anti-icing air tubes direct the flow of air from the valve to fittings on each side of the compressor front support. The air is then routed through an annulus around the OD of the front support  and  through  the  inlet  guide  vanes  and  is  dis- charged into the inlet air stream. b. The approximate effects of anti-icing air flow on performance available to pilot at power levels above 40,000 N1 (gas producer) speed are immediate and definite. 1-10 Change 14