Piper Arrow: gear down and locked

E_Dawg

Moderator
2 Qs, thanks in advance:

#1) What is the hydraulic pressure in the system when the gear is down and locked and everything is working normally? The POH, of course, doesn't say.

#2) The gear 'locks' mechanically and automatically when the gear is down, which is why you can get the gear down without ANY hydraulic pressure if you need to. What actually causes it to lock? And how does it unlock when the gear is raised?
 

Tired

New Member
There is no specific pressure in the system when the gear is down. The pump helps bring the gear down, however, the pump cuts out when all three downlock squat switches are triggered. There doesn't need to be any pressure in the system because downlocks, which are basically a hook held over-center by a spring, hold the gear down. There is a published pressure to hold the gear up since there are no uplocks. If you need to extend to gear manually you pull the emer. gear extension, which basically connects the two sides of the gear system together, allowing gravity and slipstream to bring the gear into place. Once the gear is extended the downlocks, driven by a spring engage, holding the gear down and giving you three greens. The downlocks are unlocked by hydraulic pressure when the pump runs in the retract direction.
 

sixpack

New Member
[ QUOTE ]
There is no specific pressure in the system when the gear is down. The pump helps bring the gear down, however, the pump cuts out when all three downlock squat switches are triggered...

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Are your sure about this?
I thought (on the Seminole at least) that the pressure was maintained by a pressure switch.
 

Tired

New Member
Pressure is only maintained when the gear is in the up position. This is not typical, in many aircraft there is no pressure when the gear in not in transit. However, the Arrow and the Seminole, along with other piper aircraft have no uplocks, so the gear is held by system pressure alone, with a pressure swtich that cuts the pump off, and will turn it back on if pressure is lost with the gear handle in the up position.

If pressure was needed to keep the gear down you'd notice two things. First, because pressure typically leaks down over time when you turn on the master the pump would kick on. Also, if the system failed the gear could collpase. Hence the need for downlocks.

Pressure in the system after shutdown can even be harmful. After shutdown in the LearJet we bring the hydro pressure down to zero. Reason is that somehow fluid can leak past seals or something casusing the nosewheel to collapse.
 

Tired

New Member
Just like to add that the Seminole/Arrow are great trainers because you can do real emer. gear extensions all day long without inspections or going to mx. It's also simple, unlike the procedure to emer. extend the gear in a Cessna single.
 

PhotoPilot

New Member
I second Tired's answer. Retracted position is maintained by hydro pressure and regulated by pressure switches and travel limit switches. Extended position is maintained by overcenter locks, hooks, and springs and is regulated by travel limit switches.

That said, I don't know that the pressure is dumped after extension. Though there's no pressure switch to kick the pump back on after a slight loss over time, I would think that there's still slight residual pressure remaining after extension. I'm sure it's not much, as there's a much higher pressure required to retract then extend (the hydro system is serving only to assist a system that is essentially gravity/spring actuated), but I'd think there would still be some there.

Can anyone confirm or disprove that? I checked the POH and it's not mentioned in there . . .
 

ananoman

New Member
This is from the Piper maintenance manual for the Arrow.

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DESCRIPTION. Hydraulic fluid is supplied to the landing gear actuating cylinders by an electrically powered reversible pump located aft of the baggage compartment at right side station 156.00. The fluid reservoir is an integral part of the pump. The pump is controlled by a selector handle located on the instrument panel to the left of the control quadrant. As the handle is selected to either the up or down position, the pump directs fluid through a single line to a manifold and from that manifold to each individual actuating cylinder. As fluid pressure increases at one side of a cylinder piston, fluid at the opposite side is directed back through another manifold to the pump reservoir. The two manifolds and their connecting lines serve either as pressure or return passages depending on the rotation of the pump to retract or extend the gear.
On the manifold through which pressured fluid passes during gear retraction, there is located a pressure switch which opens the electrical circuit to the pump solenoid when the gear fully retracts and pressure in the system increases to 1800 plus or minus 100 psi. The switch will continue to hold the circuit open until pressure in the system drops to 1500 plus or minus 100 psi, which at this point the pump will again operate to build up the pressure as long as the gear selector handle is in the up position. The down position of the gear handle has no effect on the pressure switch.

The hydraulic pump is a gear type unit driven by a 14 volt reversible motor. To prevent excessive
pressure on the hydraulic system due to fluid expansion, there is a thermal relief valve incorporated in the pump that will open at 3000 plus 300 or minus 200 psi, thus allowing fluid to flow to the pump reservoir. Other valves in the pump system channel fluid to the proper outlets during gear retraction or extension. In the base of the pump is a shuttle valve that allows fluid displaced by the cylinder piston rods to return to the reservoir without back pressure. This shuttle valve has a delivery pressure of 400 to 800 psi during the extension cycle. Also in the system is a bypass or free-fall valve that allows the gear to drop should a malfunction in the pump circuit occur. To prevent the gear from extending too fast, there is a special restrictor fitting on the
side of the valve.

The emergency gear lever, used for emergency extension of the gear, manually releases hydraulic
pressure to permit the gear to ?free-fall? with spring assistance on the nose gear. The lever must be held in the downward position for emergency extension. For a description of the landing gear and electrical switches, refer to Section VII, Landing Gear and Brake System.

Hydraulic
High Pressure 2400 ± 200 psi
Low Pressure 600 ± 200 psi
Flow Rate @ 1000 psi 60cu. in. per min.
High Pressure Control 2400 ± 200 psi
Thermal Relief 3000 + 300, - 200 psi
Hydraulic Fluid MIL-H-5606
Pressure Switch
Open (OFF) Pressure 1800 ± 100 psi
Close (ON) Pressure 300 ± psi below opening pressure




[/ QUOTE ]

It looks like 400-800 psi is used for extension, but as others have said, there is no mechanism to increase this as it leaks down. It also looks like there is a .020" bleed hole that connects the low pressure 'down' side of the system to the hydraulic reservoir. If this is so, it would quickly return the pressure in the low pressure side of the system to zero after the gear is down. This is not a problem as once the gear is extended, the 'J' hooks and springs will keep the gear in the down and locked positon without the aid of hydraulic pressure.
 

stuckingfk

Well-Known Member
Here is something that shows the Arrow's Hyraulic System during an emergency gear extension.



Pressure relief valve & gravity drop the gear, with the large assisting. Once the pressure is relieved, the hydraulic oil is then dumped into the reservoir in the back.
 

ananoman

New Member
[ QUOTE ]
Once the pressure is relieved, the hydraulic oil is then dumped into the reservoir in the back.


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When the emergency gear extension valve is opened, the fluid will flow from the bottom of the hydraulic cylinders into the top of the cylinders as the gear falls. Since the volume of the 'top' of the cylinder is greater than the 'bottom' (since there is no piston rod taking up space in the 'top' side), some fluid will probably be sucked out of the reservoir to fill the extra volume as the gear reaches full extension.
 

stuckingfk

Well-Known Member
[ QUOTE ]
When the emergency gear extension valve is opened, the fluid will flow from the bottom of the hydraulic cylinders into the top of the cylinders as the gear falls. Since the volume of the 'top' of the cylinder is greater than the 'bottom' (since there is no piston rod taking up space in the 'top' side), some fluid will probably be sucked out of the reservoir to fill the extra volume as the gear reaches full extension.

[/ QUOTE ]

That makes sense.

Our teacher just gave us the basics when it does that, but it seems to me that some fluid has to flow on the relief side to get the gear to come down. But you are right.
 

Tired

New Member
[ QUOTE ]
That said, I don't know that the pressure is dumped after extension.

[/ QUOTE ]

Yes, there can be pressure in the system after the gera is down, there is nothing to relieve the pressure (other then the overpressure relief ofcourse).
 
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