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Rob,
Most likely the original VEP switches will be internally corroided and not
usuable if they have never been put into service. I went thru that
situation years ago (1990's) during the extended build time. It cost a
fortune to have the VEP company even just test them them, more than a new set
from Lancair would cost for the new style. And the new style is install
and fly, whereas the VEP's require adjustments during new flight hours, a big
inconvenience.
Gary Edwards
LNC2
Sent: Wednesday, January 30, 2013 5:28
AM
Subject: [LML] Re: Thermal influence on
Hydraulic Pressure
Rob,
Lancair carries a new type that is much more reliable. I
forget the brand name.
Chris Zavatson
N91CZ
360std 1,400 hrs
Chris,
I
am interested in your comment about the “Old VEP pressure switches”. The 360
kit I am building comes with those pressure switches. What do you recommend we
should use in place of them?
Thanks
again.
Rob
Stevens
Perth,
Western
Australia.
From:
Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Chris
Zavatson
Sent: Tuesday, 29 January 2013 11:56 PM
To:
lml@lancaironline.net
Subject: [LML] Re: Thermal influence on
Hydraulic Pressure
Yep, the failure
modes are completely unrelated to each other.
There is a second
failure-to-retract mode. In fact, it is the only one I experienced in my
plane. It was with the old VEP pressure switches. They would stick
open on occassion. Then the problem is not too much pressure, it is not
having any.
Failure to retract
can be a bit more serious if in IMC or taking off at high density
altitude.
The failure to extend
can be fixed by simply raising the operating pressure of the low side
circuit.
I have looked at
pumps spanning the 95 to 2011 manufacturing time frame. The only one
that would lock up on the ground had spool return springs. Since the 70g
rattle-your-spool controversy appeared, I have also been measuring the spool
resistance in every pump I get my hands on. -haven't found an
outliers yet, but I will keep looking.
While
waiting for Wolfgang's answer, some 200/300 series pilots are becoming
concerned that they can't get the gear down when needed. First, Lorn's
secondary problem is that he didn't look for the "gear down and locked" lights
before landing. While the gear down pressure switch saw enough
pressure to not allow the pump to start, the failure to recognize that
the gear was not down contributed to the belly in landing.
To
allow people to be more comfortable about the hydro-electric system they
should be reminded of this
1.
The failure to RETRACT the gear because of high pressure on both sides is
a problem because of the safe design. Opening the dump valve
doesn't resolve the problem because the system is in a static state with
the gear down and locked. There are no physical forces able to
change the system condition - no G-maneuvers, no slips or skids,
nada.
2.
The failure to EXTEND the gear because of high pressure on both sides is
different. Opening the dump valve (allowing down side fluid to flow to
the upside) will allow the gear to "emergency" extend because of gravity on
the mains and the nose gear gas spring. OK, maybe the nose gear
might not extend all the way at 120 KIAS (max gear extension speed), but the
pressure lock has been broken and the pump will start if the switch is down
and the pump breaker is closed. Close the dump valve to complete the
process. As a matter of fact, assuming everything is normal and the gear
down switch is selected but nothing happens, it may merely require momentarily
cracking the dump valve to relieve the hydraulic
constipation.
Failing
to retract is a nuisance. Failing to extend is, uh, more serious, but
resolvable.
Scott
Krueger AKA Grayhawk
I seem to recall
you dismissed the idea of a three-way valve previously.
<<That's a sound and positive fix.
- - However it requires changing the dump valve,
running a return line and pump modification (replace the
reservoir).
...................................
<<..........Automatic action by
my module can prevent these problems for both up and down gear
operation.
Could you please
explain how your module will prevent the gear extension lock-up
failures. Recall Lorn's photo after his extension failure and gear up
landing. Both pressure gauges showed around 500 psi. The system
is hydraulically locked with one switch open and one
closed.
This
has been covered at great length before. Some take the position that "real
pilots" don't need anything but the dump valve.
I
have proposed two answers,
1)
replace the dump valve that only opens a port between the HI and LO sides
with a dump valve that dumps back to the pump reservoir.
2)
install a small electric module I make across the pressure switches
that runs the pump to relieve the over pressure in the "wrong"
line automatically.
-----
Original Message -----
Sent:
Saturday, January 26, 2013 2:07 AM
Subject:
Re: [LML] Re: Thermal influence on Hydraulic Pressure
This has happened to me even at the beginning of a
flight, if the temps in the hangar have gone from cold on a previous day,
to warm on flight day. I guess cold temps shrink the fluid and suck
more from the reservoir into the lines, and when it warms, the pressure
builds on both sides of the system, so that the "up" and "down" pressure
switches are both open and the gear won't move until I pop the bleed
valve.
Nasty when the gear won't retract just after blasting
off into a low overcast.
On Jan 25, 2013, at 7:23 AM, randy snarr
wrote:
. I dont like
having to open the dump valve but it is necessary once in a while
especially when it is cold. The pressures on both sides of the system
climb due to the engine heat and the gear sometimes will not move without
dumping the pressure.
--
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