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Piper Seminole Landing Gear Questions

Old 02-10-2014, 08:03 AM
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Couple questions about the Seminole's landing gear that I have not been able to find answers to in the POH or through internet research.

What are the purposes of the pressure switch vs the electric uplimit/downlimit switches? How do they differ?

My understanding:

Pressure Switch - to turn pump off during gear retraction, once PSI reaches certain point. Also, turns pump on when retracted but pressure is lost.

Uplimit Switch (on each gear) - turns off red gear unsafe light when all 3 are closed.

Downlimit Switch (on each gear) - illuminates 3 green lights when extended. Turns pump off when all 3 are fully extended.

Is my understanding correct so far? Does the uplimit switch have any affect on the pump?

I find it very difficult to comprehend the electrical schematic in the POH, but I'm trying.
Does anyone have any other good resources for information about the landing gear system (diagrams, etc.)?

Thanks.
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Old 02-10-2014, 08:57 AM
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Originally Posted by TheStranger View Post
Couple questions about the Seminole's landing gear that I have not been able to find answers to in the POH or through internet research.

What are the purposes of the pressure switch vs the electric uplimit/downlimit switches? How do they differ?

My understanding:

Pressure Switch - to turn pump off during gear retraction, once PSI reaches certain point. Also, turns pump on when retracted but pressure is lost.

Uplimit Switch (on each gear) - turns off red gear unsafe light when all 3 are closed.

Downlimit Switch (on each gear) - illuminates 3 green lights when extended. Turns pump off when all 3 are fully extended.

Is my understanding correct so far? Does the uplimit switch have any affect on the pump?

I find it very difficult to comprehend the electrical schematic in the POH, but I'm trying.
Does anyone have any other good resources for information about the landing gear system (diagrams, etc.)?

Thanks.

what do you mean by "pressure switch", by any chance is that the "squat switch"? (only on the left main I believe) that when on the ground, is not pressed. But upon lift off the gear extends a bit due to gravity and the weight of the aircraft being lifted off the wheels. This switch when working properly completes the circuit for the gear retraction. It serves to protect the pilot or any occupants from accidentally raising the gear while on the ground.

there is a switch on all gears for the lights (uplimit? individually activates the green lights just as or slightly after down locks are enganged) and then one more switch in the nose gear as that is the switch the turns off the pump. (nose gear is slowest to deploy down since it goes into the wind)
I dont remember for certain, but the red light is only just for when the gear is in transition or as a gear warning since your speed is too slow and or the switch in the throttle quadrant (roughly 15" mp or less on either throttle, you can hear the click sometimes) has been pressed. This may be linked to the 3 green circuitry, but im not sure. As for what turns the pump off, its either the uplimit switches or another switch up in the nose. Im not entirely sure, but ill look into it. I have a seminole POH somewhere.

Also, instead of looking at the schematic, have you read the systems description? Much easier to follow along.

Last edited by Aviator89; 02-10-2014 at 09:09 AM.
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Old 02-10-2014, 01:40 PM
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TheStranger,

You are exactly correct so far. The only thing that keeps the gear retracted and in the wheel wells is hydraulic pressure. There is a pressure switch that upon reaching a certain psi (about 1800 I think) during retraction turns off the pump. It will also turn the pump on if the pressure falls. The up-limit switch just turns off the red gear unsafe light. The up-limit switch has no affect on the hydraulic pump at all.

During extension when the down lock engages it activates the down limit switch which gives you a green light. When all are down and locked they disable the gear warning horn.

Aviator89 also brings up a good point. There is a squat switch on the left main gear that prevents the gear being retracted while on the ground. There is also a squat switch on the right main gear that keeps the stall warning horn from activating while on the ground.
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Old 02-10-2014, 07:18 PM
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Originally Posted by TheStranger View Post
Couple questions about the Seminole's landing gear that I have not been able to find answers to in the POH or through internet research.

What are the purposes of the pressure switch vs the electric uplimit/downlimit switches? How do they differ?

My understanding:

Pressure Switch - to turn pump off during gear retraction, once PSI reaches certain point. Also, turns pump on when retracted but pressure is lost.

Uplimit Switch (on each gear) - turns off red gear unsafe light when all 3 are closed.

Downlimit Switch (on each gear) - illuminates 3 green lights when extended. Turns pump off when all 3 are fully extended.

Is my understanding correct so far? Does the uplimit switch have any affect on the pump?

I find it very difficult to comprehend the electrical schematic in the POH, but I'm trying.
Does anyone have any other good resources for information about the landing gear system (diagrams, etc.)?

Thanks.
Spot on.

It's also not uncommon in older aircraft to sometimes have that moto occasionally kick on at seemingly random times to repressurize the gear, although one could argue that maintenance is needed.
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Old 02-11-2014, 10:26 AM
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Originally Posted by JamesNoBrakes View Post
Spot on.

It's also not uncommon in older aircraft to sometimes have that moto occasionally kick on at seemingly random times to repressurize the gear, although one could argue that maintenance is needed.
true, if you see a gear unsafe light mid flight, then it goes away a few seconds later, you have a hydraulic leak and the pump is trying to keep the gear retracted. This is due to the gear handle position selected and the fact the gear is held up by pressure, no "uplocks", that would be stupid for this kind of airplane. Do you know where the hydraulic reservoir is in your aircraft? If i remember correctly from my brief time in the PA44, its rear of the baggage door.

on another note, 1800 psi? Seriously? that is insane, you sure its not 180, or 18? 1800 is one hell of a explosion if something goes wrong. Not to mention requires one heck of stong compressor. Ever been near a scuba tanks or paintball hpa tank when they fail? lethal, and at the very least deafening.
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Old 02-11-2014, 11:46 AM
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Most aircraft hydraulic systems operate above 3,000 psi so 1800 is actually kind of low for a hydraulic system.
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Old 02-11-2014, 11:52 AM
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Originally Posted by Aviator89 View Post
true, if you see a gear unsafe light mid flight, then it goes away a few seconds later, you have a hydraulic leak and the pump is trying to keep the gear retracted. This is due to the gear handle position selected and the fact the gear is held up by pressure, no "uplocks", that would be stupid for this kind of airplane. Do you know where the hydraulic reservoir is in your aircraft? If i remember correctly from my brief time in the PA44, its rear of the baggage door.

on another note, 1800 psi? Seriously? that is insane, you sure its not 180, or 18? 1800 is one hell of a explosion if something goes wrong. Not to mention requires one heck of stong compressor. Ever been near a scuba tanks or paintball hpa tank when they fail? lethal, and at the very least deafening.
Its been years since ive touched a seminole but I think I remember the pressure being constant right around 17 or 1800. Pump comes on at 1400 to re-pressurize.

This could possibly be completely inaccurate=)
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Old 02-12-2014, 10:52 AM
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Thanks for the replies guys. After spending (too) much time studying the electrical schematic, I finally figured it out! (big deal for me at the moment )
My initial thoughts were correct but here are the definitive answers based on the electrical schematic:

Pressure switch: part of circuit to close/open switch to turn pump on/off based on pressure

Up limit switches: ONLY part of circuit to close/open switches for red "gear unsafe" light (all gear must be retracted in place to extinguish light); is NOT part of any circuit involved with pump operation

Down limit switches: part of BOTH green gear extended lights circuit AND pump operation circuit. They are two sides of the same coin, or switch in this case. The switches can either be closed for the green lights to come on (by being fully extended), or closed for the pump to continue to operate (by not being fully extended). Even if just one gear is not fully extended, it will allow current to continue to flow to operate pump. ALL gear must be locked in place for pump to shut off.
Additionally, current will continue to flow to the red "gear unsafe" light until ALL gear is locked down into place and switches closed to the green light circuit.

Hope that explanation makes sense.

Aviator89,
Thanks.
The pressure switch is part of the circuit that is needed to retract the gear. Once fully retracted and certain PSI is reached, the switch opens, shutting off the pump. If, during flight, the pressure drops below set PSI (due to leak), the switch closes circuit, activating the pump to attempt to obtain set PSI. The up limit switches are NOT connected to pump circuitry (see above), only the pressure switch turns pump off when being retracted.

You are correct about the squat switch operation.

The switches involved with the green lights are the down limit switches (see above). ALL three gear must be fully extended to open the gear pump circuit (not just nose gear). Even if just one gear is not fully extended, the gear pump will continue to receive current and operate. Individually, once extended, they close green light circuits.

The red light will be on if:
1. The gear selector is "up" but the airplane is on the ground (squat switch).
2. Gear selector is selected "down" but not fully extended (all green lights are not on) - in transit
3. Gear selector is "up" (or gear not fully extended - in transit) and manifold pressure falls below 14" or flaps are extended past 1 notch.

So, yes, the red light is linked to the 3 green circuitry.

GEFT10,

Thanks. And you're correct, the hydraulic schematic confirms that it's 1800 PSI. You succinctly summed up the gear system in a lot fewer words than I just did.
I was not aware about the stall horn squat switch, I'll have to look into that. Thanks.
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Old 02-13-2014, 06:38 AM
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Dont ask too much about that one.. some of us experienced seminole drivers and educators get in heated arguments about why they installed it. Lol when in doubt I call piper or recall old emails from them.
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Old 02-16-2014, 03:02 PM
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yimke,

At the risk of opening a can of worms, I need to inquire about the stall warning horn squat switch. I have been unable to find much info about it. The explanation I found that made the most sense (and there weren't many) was that its purpose is to prevent stall warning horn activation due to prop slipstream while taxiing. What light can you or others shed on this situation?

On a related note, the Seminole has two stall warning tabs. The best explanation I found for this is that they provide adequate stall warnings at varied AOAs, since flaps will lower the wings AOA. Any thoughts on the 2 tabs?

Thanks.
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