备用放系统的工作原理和它的液压源从哪里来？

747-400

  各位大神，我想问问777备用放组件，就是安装在左轮舱后板的一个角落里的一个小组件。它的工作原理和它的液压源从哪里来？

headfirst1

For your reference:

The alternate extension system operates independently of the
normal extension and retraction system.

The hot battery bus supplies power to control and operate the
alternate extension system. The ELMS contains relays that
control the power to the alternate extend power pack.

The alternate extend power pack has a hydraulic pump
operated by an electric motor. The alternate extend power
pack pressurizes fluid from the center hydraulic system. This
fluid then goes to door release/safety valve modules for each
landing gear and unlocks the landing gear doors.

Fluid then goes to alternate uplock release actuators for each
landing gear and unlocks the uplocks for the landing gear.

The landing gear doors open and the landing gear extend by
airloads and their own weight. The landing gear tires may
contact the landing gear doors during an alternate extension.

The alternate extend pressure switch stops operation of the
power pack after the extend cycle is complete.