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Chris,
I am on my third (and last, I don't like the word "final" associated with
aviation) set of pressure switches because the first two did exhibit peculiar
characteristics such as the one you related. It was most disturbing to
climb out and retract the gear only to hear long pauses because, under
certain temperature conditions, the sw would pause until the pressure changed
enough to finish the retraction. As you earlier mentioned, I adjusted both
pressure switches to a slightly higher pressure than that set
when they were delivered. I don't have gauges, but intermittent
pauses were eliminated. I also believe the pump relief valves were factory
set at about 400 psi above the operating pressure range of the switches - no
problem there.
There is yet another failure mode: If an intermittent relay was
selected (either up or down) and power remained on to that relay for an extended
period, the relay can fail because the coil winding insulation may melt and foul
the solenoid - sometimes locking it open or closed, depending. I
have seen this at least twice. This can be caused by extensive
ground testing with the master sw on, power to a relay and the pump
breaker pulled. Another way is in flight where the pump breaker was pulled
to eliminate gear recycling because of a small reduction in system
pressure but not enough to open the gear doors.
Luckily, even these type of failures will not defeat the "emergency" gear
drop process. Even weak main gear over center locking springs can be
corrected by a skid. A very weak nose gear gas spring is more attention
getting.
Scott
I have great respect for hydro-electric systems in that I once owned a
SkyMaster (337). Its hydraulics are described in two notebooks
full of page after page printed with different colored lines
representing pressures in the many lines and states of valves, doors and
wheels.
In a message dated 1/29/2013 9:55:47 A.M. Central Standard Time,
chris_zavatson@yahoo.com writes:
Scott,
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.
Chris Zavatson
N91CZ
360std 1,400hrs
From: "Sky2high@aol.com"
<Sky2high@aol.com>
To:
lml@lancaironline.net
Sent:
Tuesday, January 29, 2013 5:27 AM
Subject: [LML] Re: Thermal influence on
Hydraulic Pressure
Chris,
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
In a message dated 1/29/2013 6:54:27 A.M. Central Standard Time,
chris_zavatson@yahoo.com writes:
Wolfgang,
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).
...................................
Wolfgang>>
<<..........Automatic action by my module can prevent
these problems for both up and down gear operation.
Wolfgang>>
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.
Chris Zavatson
N91CZ
360std, 1,400 hrs
http://www.n91cz.net/
From: Wolfgang
<Wolfgang@MiCom.net>
To: lml@lancaironline.net
Sent: Saturday, January 26, 2013 11:24
AM
Subject: [LML] Re:
Thermal influence on Hydraulic Pressure
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.
Wolfgang
----- 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.
Charley Brown
Legacy #299 200 hr
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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