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Date: Thu, 23 Feb 2006 00:37:36 -0500
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From: "Robert R Pastusek" <rpastusek@htii.com>
Subject: RE: [LML] Re: Pump chatter (was Hydraulic pack - spool valve)
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Craig Blitzer wrote:

  My hydraulic pump cycled on and off all the time in my L-IV-P when
operating
either the gear or flaps....Is this normal or should the pump run
continuously during all hydraulic operations without having to "bump"?

Craig,
The IV/IV-P series aircraft have an accumulator in the hydraulic system.
This is a round steel container with a single hydraulic line plumbed into
it. The plans call for it to be located on the right side of the fuselage,
approximately adjacent to the right main when retracted, but lots of
builders put them in other places. In any case, you might want to run a
simple test to see if it's functioning as designed. First, pressurize the
hydraulic system by turning on the master power. The hydraulic pump should
operate for a short time if the system is not fully pressurized (to
approximately 1000 PSI). When the pump stops running, turn off the master
switch, and activate the flap handle. The accumulator should provide enough
pressure to cycle the flaps up and down 2-5 times. Mine will do four full
cycles, but if it will cycle the flaps twice, the accumulator is probably
OK? If the flaps won't cycle with the hydraulic pump turned off, the
accumulator is "flat" and should be replaced.

With this confirmed, you can check other parts of the operation. The system
is a "pressure demand" system. That is, it's normally pressurized at
approximately 1000 PSI any time the master switch is on. Any time the
pressure drops below the pressure switch pre-set value, it turns the pump on
to bring the system pressure back up to the switch off value. For my system,
the start pressure (low) is about 750 PSI, and the turn off pressure (high)
is about 950 PSI. Operating the flaps on my airplane may cause the pump to
run for a short period, usually near the end of the flap movement, but not
always. A second activation will always cause the pump to run if it didn't
do so on the first activation.

The hydraulic fluid demand of the gear is significantly more than the flaps,
so any gear activation fully depletes the accumulator before the gear have
moved to the commanded position. This should cause the hydraulic pump to
turn on and stay on until a short period after the gear are in the commanded
position. The reason is that it takes only about 300-500 PSI to operate the
gear cylinders, so they initially move by pressure stored in the
accumulator, then by the hydraulic pump running until they reach the
commanded position. Once the gear is in position, the pump continues to run
to rebuild system pressure, then turns off.

The pump motor cycling you describe is not normal. It should "bump" no more
than once during flap operation, and should come on during gear operation
and stay on until a few seconds after the gear has reached it's commanded
position. The intermittent operation you describe could be caused by a
defective pressure switch, but I'd bet on one of two more likely causes. If
the flap test above works as outlined, check the gear and flap operating
levers on the center console to be sure they are moving to their correct
up/down position. If you've done any work in this area that realigns the
parts, or allowed wiring to get in the way, this is the most likely culprit.
The gear lever is particularly sensitive to being just a fraction out of
position, and will allow hydraulic fluid to "bypass" from the high to the
low side if it is. This normally causes the pump motor to cycle periodically
when the gear is up (or down), and could cause the motor to cycle during
transit. Basically, when the valve is out of position just a bit, it causes
an internal leak in the system. This could also cause a restriction in the
system that doesn't allow unrestricted flow of fluid through the system.
This could cause the pump to restore normal system pressure, then cycle when
the pressure "leaks" down a bit. This could also be caused by a crimped or
bent hydraulic line, and I only suggest you check this last because it could
be difficult to isolate. So, if the above checks don't identify the problem,
start looking for a restriction in the system plumbing--it could be in the
high side or the low side.

I'd be interested in what you find.

Bob Pastusek