For large transport aircraft, there is a requirement for power assistance to help the pilot move the
control surfaces. These systems require a constant head of pressure and are termed closed
systems, as diagram 9.29 shows. These allow the operation of multiple services at one time,
give instant response to control inputs, and have a greater output force due to higher working
pressures. Further requirements for large aircraft hydraulic systems are the ability to store energy
for use in emergencies, produce power in the event of normal pump failure, and prioritise fluid
flow to the primary services. These are the primary flying controls and the wheel brakes