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Wanted clever PA32 engineer's thoughts - Gear extention problem on Piper Lance



 
 
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  #1  
Old July 17th 03, 05:56 PM
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Default Wanted clever PA32 engineer's thoughts - Gear extention problem on Piper Lance

Others may be able to give more knowledgeable advise, but I am somewhat
familiar with the gear system on the PA28R, and I believe the system on the
PA32R is similar.

As I see it, you have expressed two problems. The first is that the gear
won't extend reliably with the normal hydraulic activation (accompanied by
the squeal from the pump). The second, potentially more dangerous problem
is alluded to at the end of your post when you state, "however once released
with the emergency extension lever the hydraulic pump would push them to the
locked position if required." The whole idea of the emergency extension
system, which is essentially a valve that releases the hydraulic pressure
holding the gear up and thus allowing it to free-fall into the extended (and
locked!) position, is that it works independently of both the pump and the
electrical system. Your statement seems to suggest that the gear will
free-fall, but may not lock unless given a "boost" from the pump. Bad
situation if you ever loose your pump!

This second problem may in fact be related to the first problem. If you can
figure out why the gear does not reliably free-fall to a down and locked
position, you may also find the reason why the hydraulics won't reliably
extend the gear down. Once place it start looking would be the pressure
switch, which is common to both the main hydraulic circuit and the emergency
bypass circuit.

One more thing: I do not believe it is normal for the pump to squeal, even
momentarily, at the end of the gear cycle, since the pressure switch should
cut it off immediately when pressure spikes upon full extension, but I could
be wrong. Anyway, this would also point to the pressure switch as the
culprit.

I presume your mechanic has checked to make sure the system is purged of air
and is not leaking.

Hope this might be of help. Good luck.

-Elliott Drucker
  #2  
Old July 17th 03, 06:06 PM
Montblack
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("Richard Kelly" wrote)
snip
Gear allways retracts correctly and without problems. On occasions when
extending the landing gear it does not come down at all, all you can hear

is
a loud squeal as the hydraulic pump appears to pumping against a brick

wall
(similar noise to when holding a car's power steering against the stops)

It
allways extends with the emergency extention without problems.


Whether it's computers, boats, cars, plumbing, etc, it seems when sh%*
happens to me, mystery problems invariably have 2 (unrelated) things going
on - at the same time. Trips me up every time - until I figure it out ....or
stumble upon the (hidden) second problem.

Good luck solving your mystery.

Ass-u-me nothing!!!

--
Montblack



  #3  
Old July 18th 03, 12:47 AM
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On Thu, 17 Jul 2003 13:43:23 +0100, "Richard Kelly"
wrote:

Will you settle for another Usenet junkie's thoughts? Worth exactly
what you paid for 'em...

On a retract PA28 or 32, the retraction cycle starts by picking up the
gear handle. The pump will run in "up" mode, high (compared to
extension) pressure is applied to the "up" side of the retract
cylinders picking up the gear. For the pump to stop running, the
required pressure must be seen at the pressure switch. Am reasonably
sure that the "up" switches are for position indication only, the
"high" pressure switch is what stops the pump.

If the pressure bleeds off in flight, the pressure switch logic
changes, the pump runs, pressure builds back up, switch shuts pump
off. Pressure in the system is all that holds the gear "up".
Emergency extension is accomplished by manually opening a valve in
between the "up" side and the "down" side of the hydraulic system and
also by actuating a switch turning on the pump in "down" mode. It is a
semi-crappy valve, and you will not find a parts breakdown on it from
Piper (unless things have changed in the last couple of years). I
could tell you that it is likely similiar to a park brake valve on an
Aztec (sharing the same seals), but then I would have to kill you.

The gear should indeed extend and lock properly when this valve opens,
with- or with-out assistance from the pump. If your aircraft has not
been modified to remove auto gear extension, "too slow" results in
only the mechanical valve opening, the fluid passes from the "up" side
of the pistons in the retract cylinders, through the open valve, and
back into the cylinders on the "down" side of the pistons and the gear
extends via gravity, and with some spring assist.

Before I forget, it is typical for the powerpack to squeal slightly at
the end of the normal retraction cycle. The gear is mechanically
rigged to be in the proper position with the pistons in the retract
cylinders at full travel-when the pistons smack the ends of the
cylinders, the pressure builds rapidly, the switch makes, pump shuts
off. Absolute max retract pump pressure is regulated by a "high"
pressure relief valve (part of the pack)..

The gear's "up", the handle's "up", there is trapped "up" pressure
holding the gear "up" until you select gear "down". Pump supplies
reduced (compared to retraction) pressure to the "down" side of the
cylinders, as there is no up-lock system, the gear immediately begins
to extend.

The pump will run , generating volume of flow, but not a lot of
pressure, as the gear will pretty much extend and lock itself
(remember?). When all three gear down-and-locked switches are
actuated, the pump will shut off. There is no "down" pressure switch,
however extension pressure is limited by another relief valve.

The down-and-locked switches should actuate slightly before the
extension mechanism/cylinder pistons run out of "travel". For the
normal extension cycle, the pump should shut off electrically before
it bangs up against the relief valve. If the gear is sloppy
mechanically (common on the nose gear), and/or a d-&-l switch is
rigged on the edge of being actuated, the switch relaxing will cause
the pump to immediately run until the related cylinder/extend
mechanism moves enuff to actuate the switch.

There is no system pressure supplied to the properly down-and-locked
gear, it is secured by mechanical means only.

The book gives the range for proper extension pressure. This is
typically checked by plumbing a gage into the extend side of the
system, holding a main gear leg physically "up" and selecting gear
down. Once the other two gear legs are down and stopped, the relief
valve will be limiting the pressure, as indicated on the gauge. A
leaking emergency extension valve can also "bypass" extension
pressure, causing a lower indication.

The high points are pressure switch "up" limit (pump shut-off), d-&-l
switches "down" limit, emergency extension is a valve in the center
leg of the "H" connecting the extend and retract hydraulic lines.

This is only a general system overview, subject to subsequent
technical correction, as I haven't had access to a Piper manual (or
worked on one) for several years.

I'm sorry but I cannot recall if the extend relief valve is an
internal (to the pack) or external relief valve. I also am not sure
how the powerpack/system mechanically "swaps" from "extend" pressure
to "retract" pressure. My long distance usenet diagnosis is that
electrically the gear system logic is to extend, but it is hanging
hydraulically somewhere in between-the up pressure switch is
electrically out of the system, and the d-&-l switches aren't able to
shut the pump off.

Hope this helped;

TC


Help !!! Anyone ever heard of this one.

big snip


  #4  
Old July 18th 03, 10:33 AM
Richard Kelly
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I think my closing statement may have been slightly confusing, in that when
I said if the gear is released with the emergency extention the power pack
will push them into place. This gave the impression that the gear will not
free fall into the down and locked position - It will. However what was
missing from the statement is that if I prevent the gear from falling ie
hold the nose leg back (simulating too fast an airspeed) and then someone
resets the circuit breaker the pump will happilly power the leg into
position.

In other words I am not able to extend the gear using the pump because it
just sits and squeals but if this situation occurs and the gear is released
using the emergency extention the pump is then happy to continue push the
gear into its down position

Ric


"Richard Kelly" wrote in message
...
Help !!! Anyone ever heard of this one.

snip


  #5  
Old July 18th 03, 10:51 PM
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On 18-Jul-2003, "Richard Kelly" wrote:

This gave the impression that the gear will not
free fall into the down and locked position - It will. However what was
missing from the statement is that if I prevent the gear from falling ie
hold the nose leg back (simulating too fast an airspeed) and then someone
resets the circuit breaker the pump will happilly power the leg into
position.



Oh! Well, given that new info I would guess that the most likely culprit is
the shuttle valve in the power pack. This is the valve that directs the
pump's pressure to the retraction or extension sides of the hydraulic
circuit. The high pressure with the gear retracted may be somehow
preventing the valve from moving from the "retract" to the "extend"
position. However, when you reduce the pressure (with the emergency
extension valve) the shuttle valve is then freed up to move to the proper
position.

Actually, given all your symptoms the problem could be with the pressure
switch (external to the power pack) being set too high, so that the
hydraulic pressure with the gear retracted is excessive, which might be what
keeps the shuttle valve from moving properly. I am sure that Piper
specifies a correct pressure range in its service manual, and this can be
tested by a qualified A&P (err... "engineer").

All of the above are just the musings of an amatuer, of course, but
hopefully it might help.

-Elliott Drucker
  #6  
Old July 22nd 03, 12:35 AM
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On 20-Jul-2003, "Richard Kelly" wrote:

Elliot

Thankyou, I will go and investigate further for this one (too high
pressure)

Much appreciate everyones help



The advice is hopefully worth more than you paid for it. All we ask in
return is that you let us know what you find, so we can all have a good
laugh at how far off the mark we were.

-Elliott Drucker
 




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