Hello,

I have a question about the blue hydraulic pump/system of an Airbus. Maybe it’s the same principle in an Boeing airliner.
In the blue Airbus hydraulic system, you have an electrical driven hydraulic pump. The motor is running and due to an axial piston pump, pressure will be generated. You can adjust the flow by adjusting the angle of the plate.



You have for example 10 Psi in the inflow line [No.1] and when the elec. engine is turning, the plate [No.3] is also turning. This slides the piston out and the chamber is filled with fluid. At 180° later the piston is pressed to the front and we increase the pressure to 3.000 Psi in our outflow line [No.2]. Here a picture of the blue hydraulic pump:



My Question is:
How do you ensure that you have no pressure compensation when the plate [No.3] is adjusted at 90° and the electrical engine is still running?
I mean when the plate is in the 90° position, all chambers have the same volume of fluid, but the pressurised fluid (red) has 3.000 Psi and the non-pressurised fluid (blue) has (maybe) 10 Psi. The function to adjust the plate into an 90° angle is to maintain hydraulic pressure so you have no fluid flow when the system is in idle state. But when the electric engine still turns when the plate is in the 90° position, the chamber (red) with the pressurised fluid would get connection with the non- pressurised line.
This would continuously pressurise the (red) pressurised line and this make no sense. Also it would damage the Boost Pump.

My first idea was you have a check valve but this is not the case in the blue hydraulic pump, other than in the green engine driven pump. So can someone explain me how does the blue hydraulic system work?

I don't think the question is Airbus specific or even axial piston pump specific, but rather a question of the basic principles of hydraulic power.

If I understand your question correctly you're concerned about the fluid trapped in the cylinder on the pressurized (red) side being rotated around to the unpressurized (blue) side creating a "backflow" of pressure.

First of all we have to understand how pressure is created in a hydraulic system. Pressure is a flow of fluid that is constricted by something. It's both working together. Without constriction you produce lots of flow, but no pressure. And vice versa without flow you can't create pressure (or rather use that pressure to do work).

In the pump itself you change the amount of space the fluid has (constriction) by moving the piston in and out, creating suction and pressure.
Considering the whole hydraulic system you change the amount of fluid (flow) you put in the confined space of the system to pressurize it.
When there is demand in the system (an actuator moving for example, basically increasing the volume of the hydraulic system), the pumps need to mach the flow rate of that demand to keep the system pressurized.

Unlike gases, fluids are incompressible, so the volume is set. The trapped fluid in the "red" piston simply has not enough volume to create flow once the restriction of the cylinder is removed (opened to the "blue" side).
That's the reason why you need a gas pre-charge in hydraulic accumulators. Fluids can't store energy in form of pressure. Once the constriction is removed (you open the accumulator to the rest of the (unpressurized) system) the gas pre-charge is doing the work, creating flow by forcing the fluid out. The fluid is the medium to transmit the force, not to create or store it (in a useful way like doing work or pressurizing more volume).

Additionally the pistons in the axial piston pump have small diameter holes drilled through their center to lubricate the swashplate, so the pressure can be released through these bores. That's the case drain fluid.

I think that plate is never adjusted to 90 deg, there is always some minimal flow to lubricate pump and ot is called case drain line which returns fluid back to the reservoir. There is also filter on this line on B737, A320/330. All emdp on A320 are same PN so You can swap them, when no spare one is available. Yellow one can be inop for 10 days, but You have to open cargo doors by hand pump.

Thank you very much for the explaination, it was a big help!