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pablo
25th May 2008, 12:18
Hi!

I've been asked this question, and I don't find the reply on the manuals (maybe it's on the schematics and I don't see it).
I'm not sure if it's a latch that holds it up, or it's hydraulic pressure.

Another fellow instructor told me that it's held up by hyd press, and in the event of loosing hyd press the gear comes down, but... so far I couldn't find this in manuals, and it's just his word.
At least I'd like to hear more opinions.

Your help is appreciated!

Kind regards / Pablo

Halifax
25th May 2008, 12:57
It has been a while since I have flown the PA28R, PA34 & PA44 series of Piper aircraft, but from memory on all these aircraft the undercarriage was held up by hydraulic pressure only (no up locks).
If you lose hydraulic pressure the undercarriage will extend!

The best place to confirm this is a schemetic of the electro hydraulic power pack in the Information/Flight manual from Piper.

Emergency extension was achieved by dumping the hydraulic pressure. On the Arrow there is a three position lever between the seats which allows selection of Auto (normal retraction/extension and auto extension) Emergency extension (Pushed down) and Override (Locked up, Auto system is disabled). One other feature of this system is that the undercarriage will automatically extend if you get too slow and if hydraulic pressure is lost.

I can't recall if all the Piper Cherokee series had the Auto Extension function but the PA28R 180/200 definately did.

I also believe that on some aircraft the Auto extend function was locked out.
Cheers Halifax

pablo
25th May 2008, 16:30
Hi Halifax,

I'll keep on looking at the schematics!
Thanks a lot for your help!

cheers/ Pablo

NutLoose
26th May 2008, 15:00
OK this is the Description lifted straight from an old copy of the Saratoga Maintenance manual ( similar system )

3C7
GENERAL
This chapter contains instructions for the overhaul, inspection, and adjustment of the landing gear, extension
and retraction, and brake systems; and, adjustments for the electrical limit, safety and warning systems.
Information on the hydraulic pump and distribution system is found in Chapter 29, Hydraulics.
A. DESCRIPTION AND OPERATION
The SARATOGA IIHP and SARATOGA II TC are equipped with a retractable tricycle air-oil strut
type landing gear which is hydraulically operated by an electrically powered reversible pump. A
selector handle on the instrument panel to the left of the control quadrant is used to select gear up or
down positions.
Gear positions are indicated by three green lights located above the selector lever for gear down and
locked position. A red gear warning light located on the annunciator panel indicates gear unsafe
condition. There are no indications that the landing gear has fully retracted other than all lights are out.
When the landing gear moves to the down position and each down lock hook moves into its locked
position, a limit switch is actuated to its normally closed (NC) condition. This will illuminate a
green light indicating that the individual gear is safely down and locked. The activation of all three
down lock switches will also shut off the hydraulic pump. When the Day/Night switch is in the night
position, the green lights will dim. When the gear begins to retract and the down lock hook disengages,
the down limit switch actuates to the (NC) position and is in series with the (NC) circuit of the up limit
switch allowing the gear unsafe light to come on. The gear unsafe light will remain on until the gear is
up and all up limit switches are actuated to their normally open (NO) position. When the gear begins to
extend, the up limit switches will again move to their NC position. Since the down limit switches are
already in their NC position, the unsafe light will illuminate, and the gear horn will sound, until the
down lock hooks engage, moving the down limit switches to their NOposition.
The red gear unsafe light also operates simultaneously with the warning horn. Both serve a dual purpose.
Their primary purpose is to give warning when power is reduced below approximately 14 inches of
manifold pressure and the landing gear has not reached the down and locked position. This circuit is
controlled by the three paralleling down limit switches connected in series with the throttle switch. The
secondary function of the warning light and horn is to give warning when the gear selector handle is up
while the airplane is on the ground. The warning light will also illuminate, and the warning horn will

sound, when the flaps are extended more than 10° and the landing gear is
not down and locked. A
micro switch installed on a cam located on the flap torque tube will, when closed, give an indication
that the gear is not down and locked regardless of the gear lever up or down position. When the
airplane is setting on the ground, the warning circuit is controlled through the (NO) side of the safety
switch (squat switch) located on the left gear and on the up position of the gear selector lever. When the
airplane is raised from the ground, such as in flight, far enough to move the safety switch to its (NC)
position, current is directed in series through the hydraulic pressure switch, the pump switch, and the
up positioned selector lever. The up limit, safety, pressure and selector switch, and pump solenoids are
all protected by the landing gear control and warning circuit protector. (Refer to Chapter 91 for electrical
schematic.)
Each landing gear is retracted and extended by a single hydraulic cylinder attached to the drag link
assembly of the nose gear, and the side brace link assemblies of each main gear. As the gear retracts,
doors partially enclose each gear through mechanical linkage. Each main gear is held in its up position
by hydraulic pressure on each cylinder. There are no up locks, and loss of hydraulic pressure will allow
the gears to drop. The landing gears are normally extended and retracted by the operation of the gear selector switch

GENERAL (cont.)
There is an emergency system for extending the gear in the event the gear fails to extend after placing
the gear selector switch in the down position. A gear down emergency free fall valve, located under the
floorboard on the left side of the forward baggage compartment, allows the landing gear to extend
when opened by the operation of a push pull cable knob, located on the instrument panel just below the
primary gear selector switch.
To help the nose gear to extend, there are two springs, one inside the other, mounted on arms above the
gear links. The main gear requires no assist springs. Once the gear is down and the down lock hooks
engage, a spring maintains each hook in a locked position until hydraulic pressure releases it. A further
description of the hydraulic system will be found in Chapter 29, Hydraulic System.
The nose gear is steerable through a 45 degree arc by use of the rudder pedals. As the gear retracts,
however, the steering linkage becomes disengaged from the gear so that rudder pedal action with the
gear retracted is not impeded by the nose gear operation. A shimmy dampener is also incorporated in
the nose wheel steering mechanism.
The two main wheels are equipped with self-adjusting heavy duty double piston, single-disc hydraulic
brake assemblies. Toe brakes are standard on both pilot and copilot’s rudder pedals.
A parking brake is incorporated with a handle and is applied by pulling back on the handle and, while
holding the handle back, pushing forward on the button located left of the handle. To release the hand
brake, pull aft on the handle and allow it to swing forward. Hydraulic fluid for the cylinder is supplied
by a reservoir installed on the left forward side of the fire wall.

NutLoose
26th May 2008, 15:28
Again out of the Sararatoga manual, the Hydraulic system, but it will be more or less the same system throughout the Piper range. REMEMBER ON BOTH OF MY REPLIES, parts locations and types as well as systems can and will vary. This will just give you an over view on how it works and you can use that to understand your systems from the information you have, for more ask your engineers, they will gladly show you the manual for your aircraft............ well I would.



This chapter provides an overview of the main hydraulic system which is used for landing gear extension
and retraction. Detailed information on the combination hydraulic pump and reservoir and the hydraulic
lines are provided herein. See 32-30-00 for detailed information on landing gear extension and retraction
components and operation.
The brake system, although hydraulically operated, is not included in Chapter 29 as it is entirely
independent of the gear retraction system. The brake system and its components are covered in Chapter 32.



A. Description
Hydraulic fluid is supplied to the landing gear actuating cylinders by an electrically powered,
reversible, pump located below the raised floor in the forward baggage compartment at F.S. 45. A
reservoir is also an integral part of the pump. The pump is controlled by the landing gear selector
switch located on the instrument panel to the left of the throttle quadrant. As the switch is placed in
either the up or down position, the pump directs fluid through the particular pressure line to each
individual actuating cylinder. As fluid pressure increases at one side of a cylinder piston, fluid at the
other side is directed back through the other line to the pump. Both lines serve either as pressure or
return passages depending on the rotation of the pump to retract or extend the gear. (Refer to
Figure 29-1.)
A pressure switch is installed on a cross fitting connected to the pump mount assembly. During
landing gear retraction the pressure switch is the primary means to shut down the pump. This switch
opens the electrical circuit to the pump solenoid when the gear fully retracts and the pressure in the
system increases to 1800 ± 100 psi. As long as the gear selector handle is in the up position the switch
will continue to hold the circuit open until system pressure drops to 200 to 400 psi. At that time, the
pump will again operate to build up pressure to prevent the gear from free falling. The pressure switch
has no effect on the system when the gear selector is in the down position.


The hydraulic pump is a gear type unit, driven by a 28-volt reversible motor,
designed to operate in a pressure range of 2000 - 2500 psi.

To prevent excessivebuildup of pressure in the hydraulic system due to expansion, a thermal relief valve is incorporated in the pump.

The relief valve will open at 3000 ± 500 psi. Other
valves in the pump channel fluid to the proper outlet during retraction or extension of landing gear. A
shuttle valve located in the base of the pump allows fluid displaced by the cylinder pistons to return to
the reservoir without back-pressure. (Refer to Figure 29-1 and Chart 2901for specific pressures.)
A by-pass or free-fall valve allows the gear to drop should a malfunction in the pump system occur.
This valve is manually controlled by the Emergency Gear Extension Knob, located on the instrument
panel, below the gear selection switch. A special restrictor nipple on the main gear retraction line
prevents the gear from extending too fast.





Pulling the emergency gear extension knob out manually releases hydraulic pressure, permitting the

gear to free-fall, with spring assistance on the nose gear.





How's that for an answer :}

pablo
26th May 2008, 18:29
Hi NL,

thanks a lot for your reply. Sure it helps a lot!

cheers / Pablo