Hydraulics - Centre System 
General
The centre system powers the:
The system consists of a reservoir, two electric motor-driven pump and a Ram Air Turbine (RAT) pump.
Fluid Supply
Hydraulic fluid is supplied to each hydraulic pump from an independent reservoir. The reservoir is pressurised by the Bleed Air system to ensure positive fluid flow. A quantity measuring system provides information to the EICAS status display. RF displays when a reservoir requires refilling prior to dispatch (quantity < 0.75).
The RSVR light illuminates and the EICAS advisory message C HYD QTY displays when the reservoir quantity is low (quantity < 0.5). The RSVR also illuminates and the EICAS advisory message C HYD RSVR PRESS displays when the reservoir air pressure is low.
The reservoir has a stand pipe to maintain reserve hydraulic fluid for use by the Reserve Brakes and Steering in the event of a centre system hydraulic Leak. With fluid level at the top of the standpipe, quantity is approximately 0.15.
Electric Motor Driven Primary Pumps
The two centre electric motor-driven primary pumps are similar to the left and right system electric motor-driven pumps. The C2 pump is load shed automatically to reduce electrical loads if only one electrical generator is operating and both Electric Pump Switches are ON.
The pump PRESS light illuminates and the EICAS advisory message C HYD PRIM 1 or C HYD PRIM 2 displays when the pump output pressure is low. The pump OVHT light illuminates and the EICAS advisory message C HYD 1 OVHT or C HYD 2 OVHT displays when the pump temperature is high.
Air-Driven Demand Pump
A bleed-air driven demand pump provides additional hydraulic power either on demand or continuously for periods of high system demand. The demand pump also provides a backup hydraulic power source for the electric motor-driven primary pumps.
The Air Demand Pump Selector determines the pump mode of operation. In the AUTO position, the air pump operates whenever system pressure is Low or when large demand systems such as landing gear, flaps/slats and ground spoilers are operated. In the ON position, the pump runs continuously.
The pump PRESS light illuminates and the EICAS advisory message C HYD DEM PUMP displays when the pump output pressure is low. The pump OVHT light illuminates and the EICAS advisory message C DEM HYD OVHT displays when the pump temperature is high.
The Hydraulic Driven Generator is automatically powered by the centre hydraulic system when electrical power is lost from both main AC busses. The centre Air Demand Pump will then operate continuously to ensure sufficient hydraulic pressure to drive the generator.
System Pressure Indications
The SYS PRESS light illuminates and the EICAS caution message C HYD SYS PRESS displays when the hydraulic system pressure is low.
Hydraulic system pressure displays on the EICAS status page. When both electric motor-driven pumps are inoperative, the EICAS status page displays the Ram Air Turbine output pressure.
Reserve Brakes and Steering Isolation
Inflight, the centre number one electric primary hydraulic pump is automatically isolated by two reserve brakes and steering control valves, if system quantity is sensed low, to provide hydraulic pressure exclusively to the Reserve Brakes and Steering system.
The Reserve Brakes and Steering ISLN Light illuminates to indicate the centre hydraulic system is isolated.
Pushing the Reserve Brakes and Steering Switch signals the number one electric pump to operate regardless of the pump switch position. The switch also signals the control valves to move to the isolated position.
When the Reserve Brakes and Steering switch is OFF, the VALVE light illuminates to indicate the control valves) disagree with the signalled position from the automatic isolation feature. When the switch is ON, the VALVE light indicates the valve(s) disagree with the manually selected switch position.
Ram Air Turbine
- See: Ram Air Turbine
Hydraulic Driven Generator
The Hydraulic Driven Generator is automatically powered by the Centre hydraulic system engine driven pump when electrical power is lost from both main AC buses.
- See also: Electrics - HDG
