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Wolfgang,
The ‘failure to retract’ issue, to which you refer, sounds like a pump configuration issue.
For example, the current pump sold by Lancair for the 320/360/Legacy contains spool return springs. This configuration has the potential to lock up due to thermal heating. This configuration is simply not compatible with the design of the Lancair two-seat hydraulic systems.
With personnel turn-over at both companies, neither side knows why the return springs were added. However, both Parker and Lancair agree that a lock up on the ground is possible with the springs installed. This lock-up has been experienced by some, including the factory Legacy. It just has not been reported enough to drive an engineering change at Lancair.
Parker has graciously supplied parts to revert currently sold “CZZ” pumps back to the original 320/360 configuration with respect to spool operation. I have been forwarding these parts free of charge to anyone who needs them.
I applaud your ingenuity in devising your circuit. Unfortunately your energy seems to be a little misplaced. I would prefer to investigate and fix the root cause of any out of the ordinary behavior-especially on an airplane! If my engine was burning a quart of oil per hour, the fix would not to design a gadget that would pump oil from an onboard supply into the sump and replenish the lost oil. The solution, albeit painful in the short term, would be to open up the engine, find and fix the problem and then fly like other engines.
I understand that removing a pump, in particular those mounted behind the baggage bulkhead is no fun. Poor access is a real deterrent for any maintenance activity. That is simply human nature. In the grand scheme of things, it is a small price to pay.
My offer still stands to examine your (Lorn’s) pump or any other that is able to lock-up both circuits. I have not yet come across one that did not have an obvious cause-like the springs mentioned above, so any such reported behavior of great interest to me and is worth investigating.
Chris Zavatson
N91CZ
360std
www.N91CZ.net
From: Wolfgang <Wolfgang@MiCom.net>
To: lml@lancaironline.net
Sent: Wednesday, September 5, 2012 9:28 AM
Subject: [LML] Re: Thermal influence on Hydraulic Pressure
Let's not forget this thermal influence causing non-function can also happen in reverse. It has been reported that a thermal a soak on the ground after a cold night can and has caused a failure to retract after take off.
A while back I offered a solution of bumping the pump automatically whenever both pressure switches, HI and LO, are open due to high pressure. This would put the system back in normal operation mode without the need for pilot intervention
Wolfgang
----- Original Message -----
Sent: Tuesday, September 04, 2012 7:53 PM
Subject: Re: [LML] Re: Thermal influence on Hydraulic Pressure
Fred,
I wish we had an easy way of bleeding just a wee bit of pressure without momentarily dumping the gear in cruise.
The 320/360 pumps use a non-adjustable thermal relief valve rated at 2,500 psi. The actual cracking pressure is 2,500 to 2,900 psi. This is the lowest set-point Parker has available. 2,900 psi is a lot of pressure for a system that nominally operates at 1,200 psi. Hopefully the design margin for the cylinders etc. was chosen large enough.
The first time I saw a 2,000+ excursion was on the test bench. I was leak checking a cylinder. The test stand was near the hangar door. The sun swung around and hit the pressurized clyinder. I dont' know what the max pressure was since that gauge only went to 2,000 psi.
Chris
Chris Zavatson
N91CZ
360std
http://www.n91cz.net/
From: Frederick Moreno <frederickmoreno@bigpond.com>
To: lml@lancaironline.net
Sent: Monday, September 3, 2012 11:54 AM
Subject: [LML] Re: Thermal influence on Hydraulic Pressure
I have an MPV-50 engine monitor that includes continuous reading of the hydraulic system pressure on my Lancair IV/ I have seen the same thing, particularly on a flight that started on a cool morning departure from my home drome and took us to the desert for a 110+F day. On descent, pressure rose from its customary 1250 to 1500 psi at which point Bitching Betty started to complain so I bumped my speed brakes which are hydraulic, and lowered pressure back down to my customary range. Same thing sometimes happens in reverse, a hot soak on the ground, then a climb to 10-11,000 feet for a cool soak, and at some point the hydraulic pump kicks on to restore pressure to the desired level. My TR-182 used to the the same thing routinely when departing Phoenix in the summertime.
It is a real effect, and makes one wonder about very high pressure events arising from FL 270 (say) cold soak terminating in Phoenix on a July afternoon. Hmmmm....
Fred
-------Original Message-------
-took this photo after a descent from 16.5k to 1,500 ft.
OAT went from 42 to 94 degrees.
High Pressure gauge is pegged at 2,000+ psi
-Didn't get a photo, but the opposite happened on the ramp in Spokane. After sitting for an hour, the low pressure gauge was pegged at 1,200 psi+. Spokane might not be the warmest spot in the country, but at 85 deg F, it was 55 degrees warmer than the cold soak on the previous leg.
Temperature changes have a big impact on system pressure.
Chris Zavatson
N91CZ
360std
http://www.n91cz.net/
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