Hydraulics in aviation - what do the different colours mean.

Following on from yesterday's story about the A330 losing hydraulics in flight, we look at the different colours of hydraulics in aviation.

There are three continuously operating hydraulic systems, designated green, yellow and blue.

Each hydraulic system has its own reservoir. The reservoirs are pressurised by bleed air to prevent cavitation. The normal source is engine 1, but if pressure becomes low bleed air is taken from the cross-bleed duct.  

The green and yellow systems are normally pressurised by engine driven hydraulic pumps. These pumps have a upstream shutoff valve that cuts off the flow of hydraulic fluid when the associated engine fire button is pushed. The yellow hydraulic system may also be pressurised by an electric pump. This pump may be powered by either AC2 or external power. It operates automatically to partially pressurise the yellow system when the cargo doors are moved. It may be selected on manually using a switch on the hydraulics panel, in which case it fully pressurises the yellow system. 

The blue system is normally pressurised by an electric pump powered by AC1. This pump operates whenever AC power is available unless the aircraft is on the ground with both engines shut down. In this case it may be operated using the BLUE PUMP OVRD pushbutton on the maintenance panel.

Hydraulic power source of systems (excluding flight controls)
     Green                       Yellow                                  Blue
Landing gear            Nose wheel steering wheel         Emergency generator
Normal brakes         Alternate and parking brakes
Eng 1 reverser                 Eng 2 Reverser
Yaw damper 1                  Yaw damper 2

                                           Cargo doors