REFLECTOR: Gear Hydraulic System 'Nuggets' I learned

Sat Aug 13 11:40:44 CDT 2011

Thank you Burrall, Kerry, Rene, Larry – and most especially, Scott Swing and
John Abraham at the factory (who continue to provide such great support, and
so much detailed information and great troubleshooting suggestions) for help
with my Unsafe Gear UP indication, and how to fix my landing gear hydraulic
problems while ‘on the road’ out West.  What a great resource the Velocity
community is blessed with to have so many builders, and the factory, so
willing to contribute their own experiences and suggestions as we each run
into our own little challenges and issues along the way.

Here’s a summary of all that I learned – maybe it will help someone else
sometime when you’re stuck!

Short Answer

*	After I pulled the ‘map case’ out of the top of the keel, just
behind the stick, and the keel access panel by the co-pilot’s knee, I
noticed a ‘hiss’ and a mist of hydraulic fluid each time I pushed in the
gear pump circuit breaker.  There was a ‘pin hole’ leak in the rear of the
nose gear hydraulic cylinder near where it anchors to the floor inside the
keel – either the ‘T’ fitting itself is cracked, or the I think more likely,
the back of the cylinder barrel is cracked out and allows cylinder to leak
as pressure builds up.  I’m happy I only have to replace the NG cylinder
(and not remove the canard to replace the gear pump!) while I’m “on the
road.”
*	To complete my final leg to Oregon (where I will have more time to
make repairs), I took off from Vernal UT (KVEL), raised the gear (still with
an Unsafe UP indication and a cycling gear pump light, so I also pulled out
the gear pump circuit breaker), and made a low pass down the runway so the
Dinaland FBO people could take a look.  They came up on Unicom and said the
nose gear doors were partly open, but the mains appeared to be all the way
up.
*	I climbed to 15,000 ft to clear the mountains and headed directly
for Oregon (KHIO), pushing the circuit breaker momentarily occasionally to
hold the gear at least up in this state as I continued along.  I stayed a
little above gear speed, and on average made about 150 kt GS, with 15 kt of
headwind, on 9.5 gph LOP, covered the 664 NM in less than 5 hrs, and landed
with about 50 gals of gas, which is ~18 mpg – the Velocity is a heck of an
airplane!
*	At KHIO the tower said the mains were still up and the nose gear
doors still partially open.  They came down normally and locked overcenter
with the pump and I was safely on the ground.  Oregon even provided perfect
weather as I came across the coast range between Mt Jefferson and Mt Hood
into Portland.

Hydraulic System ‘Nuggets’ Learned

*	Hydraulic Tank Fluid Fill Level – Tank fluid level will be higher
with the gear up than down (and on the ground) because all the cylinders are
retracted when the gear is up, and the cylinder shafts displace fluid (that
ends up in the reservoir) when they are retracted.  Scott Swing says you
should set the hydraulic fluid level in the tank with the gear retracted,
whenever possible, but about two thirds full with the gear down should work.
There shouldn’t be any other ways for the pump or the tank to vent or leak
fluid other than it being filled too full, or possibly being cracked.
*	Gear Switch Temporarily Inoperative – It is possible for the gear
switch to become inoperative because both the UP and the DOWN pressure
switches can both be above their cut-off pressures.  This can occasionally
be caused by large changes in environmental temperature/pressure
(particularly like sitting on a hot ramp in the sun?).  It is easily fixed
by first making sure the Gear Switch is DOWN, then pushing the RESET button
momentarily to force the pump to first run in the DOWN direction.  After
that, the gear should come down or go back up normally.  Never push the
RESET button with the gear switch UP, because then the pump may get both UP
and DOWN current at the same time.  This condition can also be caused by a
hydraulic leak anywhere in the system (any hydraulic cylinder with a bad
seal, a poorly sealing shuttle valve inside the hydraulic pump, a poorly
sealing gear down dump valve, etc) that allows the pressure on both side of
seals to equalize.
*	Cycling Gear Pump and Pressure Switch Adjustment – It is possible
for the gear pump to cycle on frequently for many reasons.  With my
hydraulic leak, mine was cycling about once per second.  Other causes can be
internal leaks, (like inside any of the hydraulic cylinders, the shuttle
valve in the pump, or the dump valve), or possibly even because the
microswitch pressure is set too high and one of the pressure relief valves
inside the pump is relieving it.  This can be fixed by either lowering your
microswitch pressure, or raising the pumps internal relief pressure (you
must remove the reservoir tank to gain access to the internal pressure
relief valves).  The Figure on page 20 of the attached Mercruiser Hydraulic
Pump manual shows these pressure relief valves, as well as the shuttle
valve, poppet valves, and all the rest of the internals very well, if you
are interested.
*	Gear and Hydraulic Troubleshooting ‘Tricks’ – You should be able to
safely jack up only the nose (to allow working on the nose gear linkage in
the keel on the ground) and leave the main gear down (and un-jacked) if you
are sure the main gear are down and locked (remove rear access panel to
verify), keep the gear switch in the Down position, and don’t turn on the
batteries or power up.  Then you can support only the nose of the plane,
open the emergency gear down pressure dump valve and manually push the nose
gear up to gain better working access.  To verify that you don’t have any
leaks inside the hydraulic pump, temporarily plug both the Up and the Down
hydraulic lines right where they come out of the pump, and run the pump to
ensure it comes up to and holds pressure, both in the Up and the Down
directions.

Again, thanks to everyone for your help!

Bob Jackson

N2XF

Original problem post:

  _____  

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Bob Jackson
Sent: Tuesday, August 09, 2011 9:38 PM
To: 'Velocity Aircraft Owners and Builders list'
Subject: REFLECTOR: Low Hydraulic Pressure or Pump Check Valve Leaking?

I’m stuck in Vernal UT after flying from Orlando, heading for Oregon.  After
take-off from Vernal on the third and last leg to Portland, I got an unsafe
gear up light.  After returning and investigating (no problem getting the
gear back down), I noticed the hydraulic reservoir was down more than a
third of the way from the top (we normally keep our fluid level (with the
gear down) about ½” from the top of the reservoir.  I refilled the reservoir
and tried again.  This time I noticed the pump (light) was cycling even on
the ground, and got the same ‘Unsafe UP’ light indication after take-off.

Does anyone have an ideas or suggestions?  I assume that the pump can’t
create enough UP pressure because the up-side pressure relief valve won’t
let it.

Attached is a PDF copy of the Mercruiser Outdrive Maintenance manual – pg 20
has a good diagram of the internal design of (our) gear pump, including
shuttle valve, pressure relief valves, etc.  Can anyone explain:

1.	why the pump’s tank level is different for the Gear UP and Gear DOWN
directions (isn’t there the same amount of fluid on both sides of the
pistons?)
2.	if you’ve ever seen a situation where the both the UP and the DOWN
sides get pressurized so that neither pressure switch will any longer allow
you to change the gear’s position (different problem from mine in Vernal,
but just asking since I heard of it once)?
3.	What could cause my hydraulic reservoir to go down by 1/3 during
just the first two legs from Orlando, when it’s never gone down rapidly like
that before.  We don’t seem to have any big leaks in the overall hydraulic
plumbing that I’ve been able to find yet.
4.	Is there any way for the hydraulic pump or reservoir to ‘vent’
fluid, other than from bad hydraulic connections that are external to it?

Thanks for the help,

Bob Jackson

N2XF

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