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The isolation valve is an integral part of the landing gear extension/retraction systems on most larger aircraft.

What is its funtion?

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    $\begingroup$ Which aircraft are you talking about? $\endgroup$
    – Bianfable
    Jan 15 '20 at 12:31
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    $\begingroup$ Why is this question closed? Someone who knows what is asked, cannot post the answer now. $\endgroup$
    – Koyovis
    Jan 15 '20 at 22:22
  • $\begingroup$ How can I reopen the question? Can someone help with this. $\endgroup$
    – nakamura
    Jan 16 '20 at 3:05
  • $\begingroup$ The question was closed by 5 users. It is now in the re-open stack, 3 votes for re-opening have been registered, and 1 to keep closed, 2 more re-open votes would be required. Only high reputation users can vote to close or re-open. I personally can see enough detail in the question to provide an answer $\endgroup$
    – Koyovis
    Jan 16 '20 at 4:27
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The function of an isolation valve in a landing gear hydraulic system, is to isolate a critical part of the hydraulic system upon pressure loss in all redundant supplies.

For instance for the B737, from this site:

If both system A and B lose pressure, the accumulator isolation valve closes at 1900psi and you are just left with residual hydraulic pressure and the pre-charge. The gauge will indicate approx 3000psi and should provide 6 full applications of brake power through the normal brake lines (so full antiskid is available) As the brakes are applied the residual pressure reduces until it reaches 1000psi at which point you will have no more braking available.

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As a general answer: the purpose of an isolation valve is to separate two (or more) parts of a hydraulic system from each other. The separation is necessary to prevent a failure of one part of the system affecting the rest of the system. For example, a leak in landing gear actuator system might bleed the entire system - rendering the whole hydraulic system inoperable.

An isolation valve may also be used to prevent system operation if certain conditions are not fulfilled. Say, if airspeed is above a predetermined limit, the isolation valve would remain closed preventing the operation of the gear.

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