Most hydraulic pumps used on modern aircraft are of the constant pressure variable delivery type. Although they are constant pressures this is somewhat of a misnomer. If the pump is in a 3000-PSI system it will put out 3000-PSI when there is no demand. When a demand is placed on the pump the pressure will drop to the level it takes to move the control surface or the landing gear. If the loads are light and it only takes 1100-PSI to effect the work then the system gage will show that level. When the surface or the gear reaches the desired position the demand will reduce to zero and the pressure will rise to 3000-PSI.
During the windmilling condition the pump will show a 3000-PSI level but the pump may not be able to react to a demand on the system (due to low flow). Some servo systems have internal bypass valves to allow manual reversion or, to reduce resistance to movement when the pressure in the system is less than a preset level (Approximately 700-1000-PSI. In this condition when there is minimal or no demand and the system pressure is at the 3000-PSI level the bypass valves will be closed. When a demand is placed on the system the pressure will drop and the bypass valves will open. To counter this designers incorporate redundant hydraulic systems ,power transfer systems , ram air turbines and electric pumps
If all hydraulic systems are lost then the pilots revert to thrust management for steering.
