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Given the following facts for airliners:

  • Fuel can be transferred in flight from one tank to another
  • Hydraulic energy can be transferred without fluid transfer from one circuit to another in flight using a PTU
  • Many hydraulic components are fed by several hydraulic circuits and thus may use fluid from several hydraulic circuits simultaneously

Can hydraulic fluid go from one circuit to another while in flight?

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    $\begingroup$ I'm pretty sure the answer is "no" since it would remove the redundancy you get with separate systems. If you could leak between systems this would give you a single point of failure for all the systems. $\endgroup$ – Ron Beyer Jan 12 '17 at 18:09
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    $\begingroup$ Those components fed by several hydraulic circuits use several hydraulic cylinders in parallel, each fed independently of the others by its own circuit. $\endgroup$ – Peter Kämpf Jan 12 '17 at 20:44
  • $\begingroup$ @PeterKämpf I also susect that but I cannot find any evidence. If you have any, please share. $\endgroup$ – Manu H Jan 12 '17 at 23:40
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This depends on how the systems are plumbed, and while this is likely an exception, for some airplanes like the EMB-145, the answer is yes.

The EMB-145 has 2 hydraulic systems. System #1 serviced the outboard brakes and system #2 serviced the inboard brakes and the parking brake. The parking brake actuated all of the brakes and this let you transfer fluid between the systems through the outboard brakes. This isn't a normal procedure, but one maintenance would walk you through if you had low fluid in one system and more than you needed in the other to balance the systems if you were out somewhere without the ability to service the hydraulic fluid.

The procedure was either

  1. Depress and hold toe brakes
  2. Set parking brake
  3. Release toe brakes
  4. Release parking brake

or the opposite, depending on which way you wanted to transfer fluid.

This works in flight or on the ground (don't do this in flight lest you get distracted and end up landing with the parking brake set)

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  • $\begingroup$ That had to be an interesting ride! $\endgroup$ – FreeMan Jan 13 '17 at 21:06
  • $\begingroup$ Reporter: "I imagine it must be very difficult to steer via differential braking, when the parking brake is set." / Jose Jimenez: "You know, a lot of people think that." / Reporter: "...Is it true?" / Jose Jimenez: "Yes!" $\endgroup$ – A. I. Breveleri Jan 15 '17 at 15:03
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I suspect @RonBeyer's comment is right and the fluids cannot mix.

According to the diagram in this answer (explaining the 3 hydraulic systems in an A320) the rudder (for ex.) happens to be fed by the Green, Blue, and Yellow system.

I found this description of the hydraulic system and control of a B737's rudder. They say

The rudder PCU [Power Control Unit] consists of an input shaft / crank mechanism, a dual concentric servo valve to control porting of the fluid to the rudder actuator, […].

The rudder actuator is a tandem actuator, having two internal piston areas for each hydraulic source (A & B). The actuator is capable of positioning the rudder panel with either one or both main hydraulic sources available, […]

emphasis mine

Given that I doubt the fluids can mix.

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