continues to get hydraulic power from ACMP C1. If there is  a leak in the lines between ACMP C1 and the alternate brake  system, all center hydraulic system fluid is lost. The nose gear isolation valve opens for any of these  conditions: The first condition permits the flight crew to operate the  NLG steering when airspeed is less than 60 knots (decreased  rudder control authority during taxi). The second condition  permits operation of the NLG actuation and steering if the  hydraulic leak is in the part of the center hydraulic system  isolated by the reserve isolation valve. The third condition  Some aircraft manufacturers have equipped their aircraft with  a high pressure pneumatic system (3,000 psi) in the past. The  last aircraft to utilize this type of system was the Fokker F27.  Both valves open again automatically when the center system  quantity is more than 0.70 and airspeed is less than 60  knots for 5 seconds. Both valves also reset when the center  system quantity is more than 0.70 and both engines and  both engine-driven pumps operate normally for 30 seconds.  [Figure 12-69] permits operation of the NLG actuation and steering if there  has not been an engine burst and the other hydraulic systems  are pressurized. The nose gear isolation valve opens when  pressure is necessary at the NLG. If the NLG is not fully  retracted or the NLG doors are not closed, the nose gear  isolation valve opens to let the NLG complete the retraction.  When the landing gear lever is moved to the down position,  the nose gear isolation valve opens to let the NLG extend  with center system pressure. Central Hydraulic System Reset Nose Gear Isolation Valve Aircraft Pneumatic Systems •  Airspeed is less than 60 knots. •  Pump pressures for ACMP C2, ADP C1, ADP C2,  and the RAT is less than 1,200 psi for 30 seconds. •  Left and right engine rpm is above idle, left and right  EDP pressure is more than 2,400 psi, and the NLG is  not up, the NLG doors are not closed, or the landing  gear lever is not up for 30 seconds.  Figure 12-69.  Center hydraulic isolation system. 12-47 RSV and NG ISLN valves close • Airspeed > 60 kts and C SYS qty < 0.4 (1 second delay) Nose gear ISLN valve open • Airspeed < 60 kts ACMP C2, ADP C1, ADP C2, and RAT press < 1,200 psi (30 second delay) • L & R eng rpm > idle, L & R EDP press > 2,400 psi, and (NLG is not  up or NLG doors are not closed,  or the LG CTRL lever is DN) (30  seconds delay) CHIS reset • Qty > 0.7 and airspeed < 60 kts  (5 second delay) • Qty > 0.7 and L & R eng rpm >  idle and L & R EDP press >  2,400 psi (30 second delay) ALTN Brakes TE Flaps PFCS MLG Actuation and steering NLG Actuation and steering ACMP C1 CTR HYD SYS RSVR ACMP C2 ADP C1 ADP C2 RAT M M LE Slats PCV Lines to slat hydraulic motor Nose gear ISLN valve RSV ISLN valve Fseu (2) P310 stby PWR management panel P110 left PWR management panel Supply Pressure LE slat lines Main hydraulic systems - center hydraulic isolation system - functional description HYDIM CL