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