MLG actuation ALTN brakes NLG steering LG ALTN ext system ACMP C1 Filter MOD Filter MOD Filter MOD Filter MOD RAT check out module ACMP C2 ADP C1 ADP C2 RAT NLG Actuation MLG steering Trailing edge flaps PFCS Leading edge slats Sample valve GND SVC disk RTN Return filter MOD Drain valve RSV ISLN valve NG ISLN valve GND SVC disk press SYS  pressurized XDCR RSVR pressurized switch From RSVR servicing station Center SYS RTN Pressurized air Supply Pressure Return Figure 12-68.  Center hydraulic system. Filter Center Hydraulic Isolation System (CHIS) Filter modules clean the pressure and case drain output of  the hydraulic pumps. A return filter module cleans the return  flow of hydraulic fluid from the user systems. The module  can be bypassed. The heat exchanger cools the hydraulic  fluid from the ACMP case drains before the fluid goes back  to the reservoir. ADP case drain fluid does not go through  the heat exchangers. The ACMP and ADP filter modules each have a pressure  transducer to measure pump output pressure. A temperature  transducer in each filter module measures the pump case  drain temperature. A system pressure transducer measures  hydraulic system pressure. Pressure relief valves in each ADP filter module prevent  system overpressurization. A pressure relief valve near  ACMP C1 supplies overpressure protection for the center  hydraulic isolation system (CHIS). The CHIS supplies engine burst protection and a reserve  brakes and steering function. CHIS operation is fully  automatic. Relays control the electric motors in the reserve  and nose gear isolation valves. When the CHIS system is  operational, it prevents hydraulic operation of the leading- edge slats. If there is a leak in the trailing edge flaps, the MLG actuation  and steering, or the flight control lines, the reservoir loses  fluid down to the standpipe level (0.00 indication). This  causes a loss of these systems, but the alternate brake system  The output of the other center hydraulic system pumps goes  to the trailing edge flaps, the MLG actuation and steering, and  the flight controls. If there is a leak in the NLG actuation and  steering or LE slat lines, there is no further loss of hydraulic  fluid. The alternate brakes, the trailing edge flaps, the MLG  actuation and steering, and the PFCS continue to operate  normally. The reserve and nose gear isolation valves are normally  open. Both valves close if the quantity in the center system  reservoir is low (less than 0.40) and the airspeed is more than  60 knots for more than one second. When CHIS is active,  this divides the center hydraulic system into different parts.  The NLG actuation and steering and the leading-edge slat  hydraulic lines are isolated from center system pressure. The  output of ACMP C1 goes only to the alternate brake system.  ACMP C1 gets hydraulic fluid from the bottom of the center  system reservoir. All other hydraulic pumps in the center  system get fluid through a standpipe in the reservoir. This  gives ACMP C1 a 1.2-gallon (4.5 liter) reserve supply of  hydraulic fluid. 12-46