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The POH of a Cessna T210N (which is a retractable-gear aircraft) states that the hydraulic pump will run 1 - 2 times per hour to restore system pressure bleed down, and whenever the gear up / down lever is repositioned for the whole gear cycle and 1 - 2 seconds more, to restore system pressure. Also, the emergency hand pump can only extend the landing gear, but not retract it. I think it's the same for other RG's (such as the C177RG, C172RG and C182RG).

Now to the question: First, why does the pump need to run all the time while the gear is moving ? Since there should be no pressure loss. Second, why can the emergency hand pump only extend the gear but not retract it - I thought all the emergency hand pump does is building pressure as the electrical hydraulic pump does, and gear position is selected by the gear lever.

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Since there should be no pressure loss.

There is no fluid loss, but there most definitely is pressure loss.

Pressure is energy. More precisely, energy is pressure times volume. A hydraulic actuator works by letting a fluid flow from a high pressure reservoir to a low pressure one¹, extracting the energy from the high pressure to do useful work. So yes, there is a pressure loss², and since the energy required is more than what is stored in the high pressure reservoir, the pump must run during the gear motion.

I thought all the emergency hand pump does is building pressure as the electrical hydraulic pump does, and gear position is selected by the gear lever.

When extending the gear, gravity is doing most of the work, but when retracting it, you need to work against the gravity. Therefore you need more pressure to retract the gear than to extend it.

I don't know whether the hand pump is only connected to the extension piston as user3528438 mentions, but even if it is connected to both, it may not be able to reach the higher pressure needed for retraction.


¹ The low pressure reservoir is often the actuator itself—a piston that extends, increasing the volume of fluid it contains.

² Even if the pressure in the extending piston isn't much lower, pushing the fluid into it takes energy from the reservoir that the pump has to replenish by pumping more fluid there.

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    $\begingroup$ Additionally, if you've lost hydraulic pressure, you can fly with the gear down to either return to the airport you just took off from, or, if fuel allows, to your destination. Additionally, if you've got a hydraulic failure, you may have other issues to deal with than hand pumping your gear up. You can't land with the gear up. Well, not without generating a very large repair bill. $\endgroup$
    – FreeMan
    Apr 29 at 16:47
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It comes down to the basics of hydraulics.

The pressure of the system is maintained by a pressure reservoir when the pump is not running, but the valve would leak a bit so the pressure "bleeds down" slowly.

However the pressure reservoir can in no way power the entire stroke of the actuator, hence the pump needs provide a continuous supply of pressure during the entire motion of the actuator.

To your last question, I think it's because of the failure mode of the system. If the hand pump is necessary, then that means either the mechanical pump has lost power or somewhere along the hydraulic lines is leaking. To defend against the latter you would want to minimize the length of hydraulic lines that the hand pump connects to, so the reasonable choice is to connect the hand pump directly to, and only to, the extending side of the actuator.

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  • $\begingroup$ You mean an accumulator no? $\endgroup$
    – John K
    Apr 28 at 19:54
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    $\begingroup$ This is the fundamental difference between pneumatic and hydraulic systems - hydraulic fluid is supposed to be incompressible, so for work to be done, there must be transfer of fluid - even a small increase in volume, due to a piston moving, will result in a big drop in pressure. $\endgroup$ Apr 29 at 11:48
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As user3528438 pointed out, the reservoir seldom has enough capacity to run the systems it is feeding. It's purpose is more like being a dampener, as the pump would otherwise produce quite spikey pressure waves across the system. Also maintaining the pressure in the system will help keep air (or impurities) in the system.

A reservoir large enough to cycle gears would be quite heavy, as required pressure is not exactly very small.

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