REFLECTOR: Gear Hydraulic System 'Nuggets' I learned

[Dugas Bellsouth]

Glad it was a discoverable but frustrating challenge (education is a challege). Thanks for sharing your discoveries. 
If I were planning repeated activation of the gear and jacking was a huge issue I would disconnect the Bolt to the  mains cables at the mains cylinder shaft. 
You did not mention that the swat switch must be bypassed for those who did not build there craft. 
Happy you made it back. I was stranded in Billings Montana with delaminated rt main gear and the reflector, CSA and the factory were awesome getting me a gear over the weekend. 
Gotta love it. 

Rene' Dugas
Sent from my iPhone4

On Aug 13, 2011, at 11:40 AM, "Bob Jackson" <bobj at computer.org> wrote:

> Thank you Burrall, Kerry, Rene, Larry – and most especially, Scott Swing and John Abraham at the factory (who continue to provide such great support, and so much detailed information and great troubleshooting suggestions) for help with my Unsafe Gear UP indication, and how to fix my landing gear hydraulic problems while ‘on the road’ out West.  What a great resource the Velocity community is blessed with to have so many builders, and the factory, so willing to contribute their own experiences and suggestions as we each run into our own little challenges and issues along the way.
>  
> Here’s a summary of all that I learned – maybe it will help someone else sometime when you’re stuck!
>  
> Short Answer
> After I pulled the ‘map case’ out of the top of the keel, just behind the stick, and the keel access panel by the co-pilot’s knee, I noticed a ‘hiss’ and a mist of hydraulic fluid each time I pushed in the gear pump circuit breaker.  There was a ‘pin hole’ leak in the rear of the nose gear hydraulic cylinder near where it anchors to the floor inside the keel – either the ‘T’ fitting itself is cracked, or the I think more likely, the back of the cylinder barrel is cracked out and      allows cylinder to leak as pressure builds up.  I’m happy I only have to replace the NG cylinder (and not remove the canard to replace the gear pump!) while I’m “on the road.”
> To complete my final leg to Oregon (where I will have more time to make repairs), I took off from Vernal UT (KVEL), raised the gear (still with an Unsafe UP indication and a cycling gear pump light, so I also pulled out the gear pump circuit breaker), and made a low pass down the runway so the Dinaland FBO people could take a look.  They came up on Unicom and said the nose gear doors were partly open, but the mains appeared to be all the way up.
> I climbed to 15,000 ft to clear the mountains and headed directly for Oregon (KHIO), pushing the circuit breaker momentarily occasionally to hold the gear at least up in this state as I continued along.  I stayed a little above gear speed, and on average made about 150 kt GS, with 15 kt of headwind, on 9.5 gph LOP, covered the 664 NM in less than 5 hrs, and landed with about 50 gals of gas, which is ~18 mpg – the Velocity is a heck of an airplane!
> At KHIO the tower said the mains were still up and the nose gear doors still partially open.  They came down normally and locked overcenter with the pump and I was safely on the ground.  Oregon even provided perfect weather as I came across the coast range between Mt Jefferson and Mt Hood into Portland.
>  
> Hydraulic System ‘Nuggets’ Learned
> Hydraulic Tank Fluid Fill Level – Tank fluid level will be higher with the gear up than down (and on the ground) because all the cylinders are retracted when the gear is up, and the cylinder shafts displace fluid (that ends up in the reservoir) when they are retracted.  Scott Swing says you should set the hydraulic fluid level in      the tank with the gear retracted, whenever possible, but about two thirds full with the gear down should work.  There shouldn’t be any other ways for the pump or the tank to vent or leak fluid other than it being filled too full, or possibly being cracked.
> Gear Switch Temporarily Inoperative – It is possible for the gear switch to become inoperative because both the UP and the DOWN pressure switches can both be above their cut-off pressures.  This can occasionally be caused by large changes in environmental temperature/pressure (particularly like sitting on a hot ramp in the sun?).  It is easily fixed by first making sure the Gear Switch is DOWN, then pushing the RESET button momentarily to force the pump to first run in the DOWN direction.  After that, the gear should come down or go back up normally.  Never push the RESET button with the gear switch UP, because then the pump may get both UP and DOWN current at the same time.  This condition can also be caused by a hydraulic leak anywhere in the system (any hydraulic cylinder with a bad seal, a poorly sealing shuttle valve inside the hydraulic pump, a poorly sealing gear down dump valve, etc) that allows the pressure on both side of seals to equalize.
> Cycling Gear Pump and Pressure Switch Adjustment – It is possible for the gear pump to cycle on frequently for many reasons.  With my hydraulic leak, mine was cycling about once per second.  Other causes can be internal leaks, (like inside any of the hydraulic cylinders,      the shuttle valve in the pump, or the dump valve), or possibly even because the microswitch pressure is set too high and one of the pressure      relief valves inside the pump is relieving it.  This can be fixed by either lowering your microswitch pressure, or raising the pumps internal relief pressure (you must remove the reservoir tank to gain access to the internal pressure relief valves).  The Figure on page 20 of the attached Mercruiser Hydraulic Pump manual shows these pressure relief valves, as well as the shuttle valve, poppet valves, and all the rest of the internals very well, if you are interested.
> Gear and Hydraulic Troubleshooting ‘Tricks’ – You should be able to safely jack up only the nose (to allow working on the nose gear linkage in the keel on the ground) and leave the main gear down (and un-jacked) if you are sure the main gear are down and locked (remove rear access panel to verify), keep the gear switch in the Down position, and don’t turn on the batteries or power up.  Then you can      support only the nose of the plane, open the emergency gear down pressure dump valve and manually push the nose gear up to gain better working access.  To verify that you don’t have any leaks inside the hydraulic pump, temporarily plug both the Up and the Down hydraulic lines right where they come out of the pump, and run the pump to ensure it comes up to and holds pressure, both in the Up and the Down directions.
>  
> Again, thanks to everyone for your help!
>  
> Bob Jackson
> N2XF
>  
> Original problem post:
> From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of Bob Jackson
> Sent: Tuesday, August 09, 2011 9:38 PM
> To: 'Velocity Aircraft Owners and Builders list'
> Subject: REFLECTOR: Low Hydraulic Pressure or Pump Check Valve Leaking?
>  
> I’m stuck in Vernal UT after flying from Orlando, heading for Oregon.  After take-off from Vernal on the third and last leg to Portland, I got an unsafe gear up light.  After returning and investigating (no problem getting the gear back down), I noticed the hydraulic reservoir was down more than a third of the way from the top (we normally keep our fluid level (with the gear down) about ½” from the top of the reservoir.  I refilled the reservoir and tried again.  This time I noticed the pump (light) was cycling even on the ground, and got the same ‘Unsafe UP’ light indication after take-off.
>  
> Does anyone have an ideas or suggestions?  I assume that the pump can’t create enough UP pressure because the up-side pressure relief valve won’t let it.
>  
> Attached is a PDF copy of the Mercruiser Outdrive Maintenance manual – pg 20 has a good diagram of the internal design of (our) gear pump, including shuttle valve, pressure relief valves, etc.  Can anyone explain:
> why the pump’s tank level is different for the Gear UP and Gear DOWN directions (isn’t there the same amount of fluid on both sides of the pistons?)
> if you’ve ever seen a situation where the both the UP and the DOWN sides get pressurized so that neither pressure switch will any longer allow you to change the gear’s position (different problem from mine in Vernal, but just asking since I heard of it once)?
> What could cause my hydraulic reservoir to go down by 1/3 during just the first two legs from Orlando, when it’s never gone down rapidly like that before.  We don’t seem to have any big leaks in the overall hydraulic plumbing that I’ve been able to find yet.
> Is there any way for the hydraulic pump or reservoir to ‘vent’ fluid, other than from bad hydraulic connections that are external to it?
>  
> Thanks for the help,
> Bob Jackson
> N2XF
>  
>  
>  
> <GearPump_HydraulicSchematic_p20.pdf>
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