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The wiring diagram below is for the Landing gear circuit in a homebuilt airplane using Automation Direct pressure switches. These pressure switches turn the hydraulic pump off when a pre-set pressure is obtained. As pressure bleeds off over time the pump will re-energize when a pre-set lower pressure is reached.

I want to add TWO momentary push-button switches that can override/bypass the pressure switches. Why? Well, for example, if the pressure switch in the gear up part of the circuit should fail, I can still get the gear to retract by activating the override push button.

Another example would be on a lengthy cross country at high altitude where the colder temps cause the pressure that holds the gear up to fall enough to slow me down (because of drooping gear doors), but not enough to activate the pump automatically. In this example activating the gear up override/bypass switch manually will bring the pressure and the drooping doors back up.

Is it even possible to do this? I want the pressure switches to do their thing most of the time, but as I indicated there may be times when I want or need to operate the pump manually. In order to do that, however, the pressure switches will need to be completely bypassed.

In the diagram I posted, the red and green marks represent momentary push button switches. I think wiring those switches in as shown should be all that is needed, but I'd like to have an expert's opinion before I start running wires through the airplane. The alternative spot for the push button switches would be in the signal lines (white wires coming off the two pressure switches).

Thanks in advance for any insight you are able to provide.

Automation Direct Pressure switch wiring

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  • \$\begingroup\$ My cousin built his own full-scale amateur-built (no kit) airplane (flying usually out of Pearson.) It sounds like you are talking about a genuine homebuilt aircraft that you pilot. I just want to be sure, because if so folks here will want to be rather more cautious about the implications of their advice. \$\endgroup\$ – jonk Feb 11 at 2:50
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    \$\begingroup\$ I'm a pilot... Your back up gear extension should be a manual pump or gravity drop, not a secondary button. I have to be honest though, this wall of text was difficult to read and understand... \$\endgroup\$ – Ron Beyer Feb 11 at 3:22
  • \$\begingroup\$ that breaker should probably be 0.5A as that's all those pressure switches can handle. \$\endgroup\$ – Jasen Feb 11 at 3:45
  • \$\begingroup\$ just add the push buttons in parallel with the pressure sensors... you could even use a single two-pole push-button, as you still have to set the switch to select the pump direction. \$\endgroup\$ – Jasen Feb 11 at 3:48
  • \$\begingroup\$ Thanks Jasen. I was thinking the same thing and I will try that as soon as the weather warms just a bit. \$\endgroup\$ – DougW Feb 11 at 5:26
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Let me address a couple things as a pilot...

Why? Well, for example, if the pressure switch in the gear up part of the circuit should fail, I can still get the gear to retract by activating the override push button.

This is a big no-no as a pilot. The last thing you want to do is to bypass something that is failing. If your gear doesn't retract, leave it down, enter the pattern and land. Fixing issues on the ground is infinitely better than trying to fix/diagnose them in the air. Eastern Air Flight 401 was caused by the pilots fixation on a faulty gear light, losing situational awareness and crashing.

Another example would be on a lengthy cross country at high altitude where the colder temps cause the pressure that holds the gear up to fall enough to slow me down (because of drooping gear doors), but not enough to activate the pump automatically. In this example activating the gear up override/bypass switch manually will bring the pressure and the drooping doors back up.

You need to design your aircraft such that these things don't happen. Typically aircraft use pressure switches not analog sensors, and once they are up they are in a "locked" position. The gear system hydraulics rotate cam locks (or other devices) such that a loss of hydraulic pressure doesn't cause inadvertent gear extension.

As I said in the comments, you need to have a back-up system that is either a manual release (rotates the cam shafts and allows the gear to gravity drop) or a manual pump in a secondary hydraulic system. Do not use bypasses.

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    \$\begingroup\$ This. And bypassing a failing switch won't help a failing pump. \$\endgroup\$ – Brian Drummond Feb 11 at 12:23

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