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<<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> << Lancair Builders' Mail List >> <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> I have learned a great deal more about the LNC2 hydraulic system since relating my myriad of problems. A recap is in order: 1. Frequently, when the airplane was cold soaked, the pump would fail to shut off after gear retraction, requiring the circuit breaker be pulled. After about twenty minutes of flight and numerous breaker engagements, the pump would finally stop. This caused seasonal adjustment of the hi pressure switch to balance between enough pressure to raise the gear and stop the pump, but not too much to overstress the system with hi pressure. 2. There was enough loss of hi pressure to cause the pump to cycle every 45 seconds or so – an irritant on a long flight, even if only a minor pump blip. 3. Finally, this spring, there began a loss of lo pressure (down), causing the pump to cycle every 30 seconds during taxi. To me, this was an indication of a continuing system degradation that could lead to future gear failure rather than just a nuisance. Where to start? I first ordered re-build kits for all the cylinders and a new 3rd generation hi pressure switch. I also like to change as little as possible to understand just what it is I fixed (or broke). So, here it is: 1. The newer hi pressure switch is set up differently than the old one and I made a 180 degree tubing loop with an AN 90 degree pipe thread to flare and a 90 degree female pipe thread fitting to finally hook up the switch and substantially lower it so it would fit it the same space, located inside the loop. Note that when I examined an old flare fitting for the prior switch, I saw that it was slowly being worked back out of the sleeve and would have failed sooner or later. Be very careful with tube flares, but especially so with ones under hi pressure. This switch, although marked for 1000 psi, had to be adjusted (screw under wax plug between switch fast-ons) to a higher pressure to overcome the nose gear gas strut and spring-loaded gear doors. This switch exhibited a new problem – it oozed closed and the arcing created radio noise for 1.5 seconds before actually activating the familiar pump blip. Try that on a long cross country with radio static occurring every 30 seconds. The temporary solution was to pull the circuit breaker and start a 5-minute countdown to re-activate the circuit. Of course, by that time the switch had fully closed resulting in only the momentary blip from the pump bringing the pressure back up. 2. Next, I decided it was hopelessly messy to try and test every component so I might as well rebuild everything until the system worked. Don’t try this yourself, as you will later see the error of my ways. Since the door hydraulics were circa 1989, I rebuilt them first – no apparent change. Then I rebuilt all the gear actuators (nose gear original Aero-Tek, mains newer Wolstenholms) – no apparent change. This is not a big deal if you keep everything clean and liberally lubricate with hydraulic fluid during reassembly so as not to nick the o-rings or cylinders. 3. Frustration building, I ordered a replacement dump valve since there was anecdotal evidence of leaks in the dump valves. Note that my original Italian 3/8-inch bore valve was physically 3/16 inch longer than the Thai ¼-inch bore replacement valve. This required a little more work since I had to remove the header tank forward deck to get at its location behind the radio stack. Luckily, the tubing stretched enough to make up for the lost 3/16 inch. Result, no apparent change. 4. OK, I never liked the circa 1989 Oildyne pump anyway. I ordered a new one, just like the one going in my Legacy. I was reluctant to send my old one to Oildyne because I had heard less than favorable reports about their service and some older pumps had been assembled with the spool valve backwards, etc. Anyway, I had lost 25 pounds on my new diet and I was sure I could get in the back and work behind the baggage bulkhead without expiring. Of course, the new pump came with the larger reservoir and the infamous black ground wire. I did not want to change the way my hard lines were located so I removed the reservoir, tapped the hole in the bottom center post and drilled out and tapped the filler port in the cast pump housing, all like the old one. I mounted the old, smaller reservoir and the same T fittings and positioned the pump back on the aluminum mounting plate – Drats, the pump was taller than the old one. I opened a recess hole in the hat shelf but left the carpet covering in place with only a small bulge showing. After re-wiring, testing began – Hey, no more cycling on the lo pressure side! Indeed, after days between visits to the hangar, still no cycling of the pump. Unfortunately, the up side was a downer and the pressure switch was still causing noise. As a matter of fact, the hi pressure side was getting worse – more frequent cycling. 5. About this time, I was requesting help, any help. A visit to Ed De Chazal showed that pressures ran about 500 psi during gear retraction, but rose to 1200 psi to get everything buttoned up. Since he uses panel mounted analog pressure gauges, it could be that the spike to get the nose gear started doesn’t show up --- more on this later. Unlike mine, the leak back was about 100 psi in 25 minutes. Mine were beginning to show the doors dropping an inch in about 1 minute, but only 4 inches in 24 hours, the system probably holding at about 600 psi. This was definitely a hi pressure problem only. I finally talked to Oildyne, where they had seen problems when the internal relief valves were not set 200 psi higher than controlling pressure switches. They sent me a fax explaining how to adjust the valve pressure by opening the reservoir, loosening the obvious locknut on the same side as the port and that one full turn in on the hi valve would raise the relief pressure 600 psi. I had also gotten a promise from Lancair for a new switch, but it wouldn’t be available for a month. 6. I raised the hi relief pressure by 300 psi – no apparent change. During this time I also acquired the Monroy Air Traffic Detection System (more in another missive) and placed the antenna on the hat shelf. Every time the pump ran (now every 15 seconds), I was warned that a collision was imminent with aircraft within a half mile. Egad, gag me with a spoon! A discussion with the Monroy people indicated how I might suppress motor brush noise with a .01 Mfd 1000 Volt Disc Capacitor between the power lead and ground. 7. I finally came to realize that I had to test individual components. How could I do this easily? After pulling the seats, I noticed that the crosses were buried under the center console – another “never again”. Instead, using flare end caps and capped flare ends, I isolated each door actuator up side, one at a time, until all were out of the system – no apparent change. Now I know that the problem is not with the door actuators and I can easily isolate each gear system since, if the gear isn’t in retraction, there is no door action. Or, “if the glove doesn’t fit, you must acquit” – same logic. Now I could open the hi pressure line as it goes thru the cockpit close out rib bulkhead fitting and isolate the gear. Bingo! The right main actuator was the culprit. I ordered an overnight delivery of a new main actuator and disassembled the diseased one. Lookee here! A very faint, but observable 1 inch long diagonal cylinder wall mark (scratch?) across the area where the piston o-ring would rest in the “up” position. Did this leak since I got it a few years back? Did I exacerbate the problem when I re-built the actuator? I will never know. 8. The new actuator was amply pre-lubricated with hydraulic fluid before moving it and then installed. The new pressure switch was installed (one marked for 1100 psi) and the pump, pressure switches and relays were rewired to include capacitors for the motor and pressure switches and diodes for the relay coils. Testing began. At Last! After adjusting the new pressure switch to a higher pressure (at least 1200 psi), the gear not only retracted, but also stayed that way. No more door droop. Hour long flights have not required the pump to cycle at all after a crisp retraction. No noise, no false warnings, no trouble. The conclusion: The gear down leak-back condition was fixed by replacing the pump although the cause may have merely been a weakening lo pressure relief valve spring. I think I had a very bad hi pressure switch but, since I can’t get the pump to cycle in the air, I can’t tell if the new one or the noise suppression circuitry have resolved that problem. Even though I am happy I rebuilt the actuators, it may have been unnecessary. A new main actuator solved the hi pressure leak-back condition. Observations: 1. Everything will eventually require maintenance, so don’t bury components, leave wiring loops and, even high current carrying wire can be connected with ring terminals bolted together so that they could be separated at some future date. In the Legacy, I will install a capped Tee in the hi and lo pressure hydraulic lines for pressure testing or the future installation of digital strain sensors. Each gear segment (left, right and nose) will have one of the new mini-valves installed to isolate the segment for quick, non-messy resolution of hydraulic problems. 2. The Legacy hydraulic system will be wired similarly to my 320 – the relay breaker will be cascaded off the pump breaker so that pulling the pump breaker will remove electricity from the system. The breaker will be located within easy reach – perhaps as now, just forward of the armrest on the center console. The “pump powered” indicator light will be wired with isolation diodes and a fuse since the wire for that circuit is not capable of carrying the 50 amps it is tied onto. 3. The pressure required to lift (or lower) the gear is about 500 psi, but the higher pressure to break the gas strut nose gear lock may be dependent on the strength of the gas strut and the pressure to completely close the gear doors is about 1200 psi. On the down side, a fresh and powerful gas strut (100 lbs+) may cause the nose gear extension to create a negative pressure on the down side and delay the nose gear door opening after the sequence valve extends, thereby causing the nose gear to bang into the door to force it open -- this may be the primary cause of the nose gear door scratches. Next time you have your LNC2 on jacks, have someone use the dump valve to slowly let the system leak down and watch the nose gear. 4. There are adequate filters on the hi and lo intakes in the pump but, in the LNC2’s system, some fluid remains in the cylinders at either end. Because they are not completely flushed during cycles, there is o-ring wear residue (blackened fluid) and other particles which can remain in the cylinder until the least desirable moment – then chip the o-ring or scratch the cylinder. There is no real need for an additional hydraulic filter in this system. 5. Although some people recommended that an accumulator system could be used to solve my problem, usually these are reserved for emergency gear down. In any event, with a leaky system, even accumulators won’t work since they would have to be continually charged. Internal leaks must be fixed. 6. The pressure switches available thru Lancair have a sufficient dead band of several hundred psi if the rest of the system has integrity. I hope my experience will help others better understand their own hydraulic system. Scott Krueger LNC2 N92EX _ LML website: http://www.olsusa.com/mkaye/maillist.html LML Builders' Bookstore: http://www.buildersbooks.com/lancair Please send your photos and drawings to marvkaye@olsusa.com.