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Wolfgang,
I know that this was discussed a while back, but it occurs to me
that this module could induce catastrophic failure if it activates a
few times in flight, e.g. if the pressure for some ungodly reason is
NOT relieved the pump could build up so much pressure that it blows
a seal or worse.
Bypassing the pressure switches AUTOMATICALLY is not, IMHO, an
actual solution but rather a work-around that could unintentionally
cause unexpected consequences. While fixing the pump with the
latest mods and installing the three-way dump valve is probably the
best actual solution (fix the problem, not the symptoms), if you
want to do a work-around it's probably safer to use the momentary
push-button solution (e.g. manually bypassing the pressure switch by
intentional pilot command) than to have an automatic system
repeatedly exercising a workaround without the pilot's knowledge.
Everyone,
By now some of you know that I experienced engine loss a few months
ago and ended up attempting to land on a highway. Unfortunately an
SUV pulled onto the highway at the last moment, and the energy spent
waiting to get past the SUV was simply too much to leave enough for
a flare. The result was a hard touchdown, breaking off the left
main and slamming the left wingtip onto the highway. The plane
pulled hard left, leaving me concerned that the plane would cross
the grass median into oncoming traffic. There was just enough
energy left to momentarily re-raise the left wing, apply the right
brake and rudder to straighten out, and make a split second decision
as to whether I could land on two wheels safely or whether
attempting that would result in either resuming course towards
oncoming traffic or result in flipping in the median. Given the
choice, I chose to raise the remaining gear and slide rather than
risk flipping and being trapped in a scenario that might possibly
have involved gasoline leaking on a hot engine with us trapped
inside. Google N351E for photos of the result.
Why is that pertinent to this discussion?
Prior to that I had had a gear-up landing from pump failure and
other factors involving a potential partner. I spent 3 yrs of
weekends rebuilding and a year flying before this incident. In the
mean time, I became VERY familiar with my systems. And, I just
don't want to see any more of our community end up busting their ass
if it is avoidable or spending your time fixing what might have
remained a perfectly good airplane.
SOooooooo....
Chris has made an EXTREMELY generous offer to all of you. If you
don't KNOW that your pump has the latest mods, take him up on it.
He is sharp and an obsessively careful individual.
Install the 3-way valve if you have the time and inclination.
Install the push-button override, too.
But do NOT install automated work-arounds that may be disguising or
exacerbating real problems.
Please.
Blue skies,
Bill Reister
From:
Wolfgang <Wolfgang@MiCom.net>
To:
lml@lancaironline.net
Sent: Sun,
April 17, 2011 10:13:18 AM
Subject:
[LML] Gear operation fix for the LNC2
Here are some details on
the Gear Fix Module - - - - -
There is a potential for
the landing gear to fail to operate in the LNC2.
The spool in the hydraulic
pump can come off it's end position and seal the fluid
volume in the lines. When the system incurs a large
temperature rise, the trapped fluid expands raising the
pressure in the lines, tripping both pressure switches
open and making the hydraulic pump inoperable.
A module to fix this gear
operation failure in the LNC2 using the standard
hydraulic system is now available. It is installed
across the pressure switches and monitors their
operation. If both switches are tripped open by high
pressure, the pump is made to run in the direction set
by the gear switch re-seating the spool in it's end
position restoring normal pump operation and relieving
pressure in the offending side. This happens
automatically without pilot intervention.
The module is 2.5" x 1.5" x
.75", weighs 2.3 oz. and has two pairs of leads that
connect directly to each pressure switch with 1/4" spade
terminals. No other wiring is necessary. Price is
$250.00 each. Simple installation instructions included.
----- Original Message
-----
Sent: Saturday,
April 16, 2011 8:24 PM
Subject: Re: [LML]
Re: LNC2 Gear Hydraulics
Wolfgang,
What is your gear fix module?
Bryan
On Fri, Apr 15, 2011 at 10:11
AM, Wolfgang <Wolfgang@micom.net>
wrote:
From a design
point of view, I still have a big concern
about relying on the friction from an O-ring
to keep the landing gear functioning properly.
Vibration levels can exceed 70 G's. I would be
much happier with a spring loaded ball detent
or something similar.
That's why I came
up with the gear fix module.
Wolfgang
----- Original
Message -----
Sent:
Wednesday, April 13, 2011 8:25 AM
Subject:
Re: [LML] Re: LNC2 Gear Hydraulics
A symmetric back-pressure circuit and
spool will eliminate the possibility of
installing the spool backwards as
happened with pumps manufactured in
about the '96 through '98 time frame.
The spool must still remain in place
after the pump shuts down to prevent
both high and low circuits from locking
up simultaneously.
Using o-ring 013-70 requires 30 g's
to move the spool.
Using o-ring 013-90 requires 70 g's
to move the spool.
This includes the return force generated
by the poppet springs (410462).
The difference between the two
o-rings is durometer. I have been using
the softer one without any problems.
The stiffer o-ring (intended for pumps
with back pressure circuits) should
leave no doubt. The first thing to do
with a pump that has a history of
locking up both sides is to measure the
return force for the spool and make sure
it has an o-ring installed.
Chris Zavatson
N91CZ
360std
From:
Wolfgang <Wolfgang@MiCom.net>
To:
lml@lancaironline.net
Sent:
Mon, April 11, 2011 4:56:57 AM
Subject:
[LML] Re: LNC2 Gear Hydraulics
When
I first got into this problem with
the hydraulic pumps last year, I
had a few conversations with the
product manager of the Oildyne 108
pump. There have been many changes
in it's production life and one of
them was to make the spool
symmetrical and have the relief
circuit the same both ways.
Wolfgang
Wolfgang,
Yes, I have considered this.
The spool with O-ring is a very
tight fight. While I have not
tried to measure the force to
move it, I would estimate
something over 50 g's to move it
(I'll measure this the next time
I have a pump opened up) Could
it be that a pump somehow ended
up mounted such that it was
subjected to some resonant
frequency? Seems like a remote
possibilty given all the
variables and the mass of the
pump. How about wear and
tear? Perhaps
sitting dry and idle for ten
years would do it. A missing
O-ring or a spool that doesn't
utilize an O-ring would allow
the spool to move very easily.
Parker does make spools that
don't even have O-rings. Did
these end up getting installed
by chance?
Based on reports from
the LML, the behavior seems to
be rather binary. On the one
hand we have planes with over
1000 hours and more than ten
years of service that have
never once had the spool move
off the poppet. On the other
hand, we have planes with
circuits that lock up right
out of the box (put into
service).
This leads me to
believe there is a difference
in configuration. The easiest
way to verify this is to
simply open up a pump that is
known to lock up hi and low
circuits simultaneously.
While I have opened up
several pumps (primarily to flip
around spool valves), I have not
yet worked on one that had this
lock up issue. I am optimistic
the difference will be quite
obvious upon inspection.
Chris Zavatson
N91CZ
360std
Sent from my iPad
Chris, have you
considered what effect
airframe vibration has on
the position of the spool
in the pump and the
problem in general ?
Wolfgang
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I would like to
extend the offer
to the whole
group. I think
the entire
community would
benefit from
understanding why
some pumps are
allowing the
system to lock up.
If anyone has a
pump that exhibits
this behavior and
could part with it
for a week, I am
willing to check
it out on the test
stand and examine
the internal
configuration.
Chris Zavatson
N91CZ
360std
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