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What will happen if hydraulic fluid starts leaking during a flight? What will happen to the overall performance of the flight, and how will the pilot compensate for the loss of the system?

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    $\begingroup$ It's more or less like leaking blood. If the leak is slow then you are OK until you really run out of fluid, which could take a while. If the leak is so wide that you can feel the loss of pressure, then you will be in trouble in a few seconds. $\endgroup$ – user3528438 Feb 15 '18 at 3:42
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    $\begingroup$ it also depends on whether or not the plane's hydraulic system has a redundant backup system which comes into play upon failure of the primary system. then the issue becomes whether or not the failure has affected both systems or only the primary. $\endgroup$ – niels nielsen Feb 15 '18 at 5:25
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    $\begingroup$ The pilots hear a ding, and an annunciator lights up. One pilot finishes the flight while another flips through a 900 page book, looking for a procedure, when they get to the gate they give up on the 900 page book and call maintenance. The passenger is 2B complains about the cabin being too cold. Someone in 41C never got their rum and coke. The next flight is delayed by 47 minutes. $\endgroup$ – ryan1618 Feb 16 '18 at 3:39
  • $\begingroup$ To quote an unnamed aircraft maintainer in response to a question about an apparent hydraulic leak, "If it ain't leaking, it's empty. We'll mop it up and top it off. You're good to go." $\endgroup$ – Gerry Feb 6 at 12:05
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That depends on the aeroplane. On the B737 for instance, there are three independent hydraulic systems: System A, System B, and Standby System. Normally Systems A and B are operating and Standy is off, only switched ON when necessary from a malfunction.

The flight systems are powered redundantly, with more than one hydraulic system powering the essential aircraft systems. The following systems are powered by the hydraulics:

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The hydraulic systems are each redundantly powered as well. For instance hydraulic system A is powered by an engine driven pump and an electrical AC pump. System A is protected from leaks in the engine driven pump lines by a stand-pipe: should a leak occur there, fluid leaks away until the fluid level reaches the stand-pipe, at which point the fluid level is approx. 20% and the engine driven pump circuitry is empty. System pressure is then maintained by the electric pump. Should a leak develop in the electric pump circuitry, the fluid quantity in system A reser- voir, and therefore system pressure, steadily drop to zero.

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System B has similar anti-leak features, with a stand-pipe that allows for the Power Transfer Unit (PTU) to operate on the remaining fluid. The PTU is a dual hydraulic pump/motor: one end is driven by System A, the other end can pressurise the fluid in System B.

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    $\begingroup$ Do we need gear up landing if the system A fails? Looks strange. $\endgroup$ – h22 Feb 6 at 10:12
  • $\begingroup$ @h22 agreed - that looks funny. Gear will probably lower in a hydraulic failure by gravity & wind pressure, though. $\endgroup$ – FreeMan Feb 6 at 20:39

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