Hydraulic Problem

[ghancock]

So we sealed up the hydraulics (skydrol) and replaced all the o-rings and fittings.  Yesterday we were flying the airplane and the main hydraulics stuck in at 1000psi and the pressurization stayed on 3000psi.  We were trying to get the pressurization system to work and never did get it to function but at some point the main 3000psi system started to spike to 4000-4500psi and then back to 3000.  It only did this a few times and then settled back down to 3000psi but it was troubling.

Upon landing we didn't discover any leaks or skydrol anywhere.  The last o-ring I replaced seemed to be leak free and everything was fine, except that the pressure system was not working.

I get a call this morning from the mechanic saying they arrived at the hanger this morning to a floor full of skydrol appearing to be coming from the engine.  They didn't tell me which engine and I haven't been able to speak with them yet.  I'm a bit confused though as to how it developed a leak when in reality it should have been under a lot less pressure than when we were flying it.  Does this make any sense to anyone and anyone have any ideas what might have gone wrong?

thanks,

Glenn

[Willis]

Thanks Glenn,

As you work through these issues, please take the time to record the PN of the failed components.   Lots of good reasons:
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It comes handy when you are buying replacement parts
some makers are better than others (Kelly aerospace has some issues).
We may have some experience with the component (either designing them, making them or procuring them).
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Spikes in the indicated hydraulic pressure is usually due to air in the system.  It eventually gets purged when all the bubbles go to the reservoir and then vented.  The intermittent High pressure is when the pump is trying to catch up to bubbles being purged near an accumulator.  {bubbles shrink down stream of a pump and expand downstream of a control, orifice or actuator, when they pass through something, spikes ocurr}
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This comment may not address your issues but Ive seen this a lot.
Ive worked with fuel systems, hydraulic systems, Hydraulic test benches and have designed a lot of this equipment.  Most of the trouble I've seen is by inexperienced techs who think "  Tighter is Better" and if it doesnt break when you tighten it, then it's okay.  Virtually every failed hydraulic component Ive seen was over torqued, bent or struck by an object.
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There is a torque specification in the aircraft maintenance manual,  ususally the section up front that lists standards, parts, maintenance intervals, etc.  Technicians should read, recite and report the torques used on each aircraft.

1.  Hydraulic fittings, if they are clean and serviceable, require little torque to hold thier seal.  A 1/4 turn, is often enough to hold 3-5000 psig.  anything higher and you are crushing threads, smashing glands and o-rings.  you may bend an angle fitting.
2.  If there is a gland associated with a fitting, it is usually to accommodate an unusual take-off angle or bend.  The gland is a backing and often comes with an associated o-ring.  Neither of which are expensive or require a lot of torque to seal.
3.  Older hose fittings are just as good as modern ones.  Perhaps better, since they were designed to be reused, and have better seals.  If you remove one, clean and inspect it.  good threads, good seat, good nipple....keep it.  otherwise, get a new one.
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A lot of this is 20/20 hindsight for you now, but in the future, when you are delving into unknown territory, use only specified parts; not "higher-spec" or upgraded parts.  You need to first find, what works; then upgrade it.

Im not familiar with Aero Commander hydraulics (I worked for Pratt Whitney and Hamilton- Sundstrand). 

It may be some time before I get my hands on an Aero Commander hydraulic Schematic.  Im guessing they are similar across a lot of models.  If you have one, please post it or show me how to get a copy.

Thanks.

Bud

[ghancock]

Will do, in this case it was people problems not parts problems and while I'd like to post their serial numbers I probably shouldn't. :-)

Turned out most of the problems were o-rings that didn't actually get replaced and fittings that were either over tightened or not tightened enough.  Also have had hose installation problems that caused hoses to break in half.

It's been awesome but as you can see here, its at least home now. :-)

https://youtu.be/apdL9-0sMRs

Glenn

[Willis]

Well, last night I decided that if I really wanted to contribute to this forum, it could be in Hydraulics.

So, Ive started buying up all the AeroCommander Maintenance Manuals, for all the 500 and 600 series aircraft.  I'll be going through them and figuring out how thier pneumatic and hydraulic systems were designed to operate.  Hopefully, I can post some schematics and spreadsheets in the hydraulics forum for reference.

It should be interesting and give me something to do thats constructive.

Bud

[SKYFLYER]

Well Bud you are about to become one of the super guru's of the Commander world... and in the area most have little to no knowledge of... AWESOME!!!
Let me know any of the manuals you cannot find as I may still have a bunch boxed up somewhere in the hangar...

[Willis]

We'll see.

I've ordered the MM for 500 B (and others) and the 680 series MM.

I need any Service Manuals and any Illustrated Parts manuals  if they are available.

It may get expensive, but if I can get them piecemeal, I can get them copied (I usually make 3 copies of rare stuff like this, JIC)

The first ones wont be here for 2 weeks.....and wouldn't you know it, I have to go down to Sandy Eggo for a week.  (I know... tough duty, but those brew-pubs dont visit themselves.)
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Upside is, I get another bite at the machine tool apple.  SD is "Candy Land" for gear-heads like myself.  I just sold my Southbend Lathe and am looking for its replacement    Its like christmas!

[ghancock]

Bud,  I'll take a look at my manuals and see what I have on hydraulic system and scan them for you. I appreciate the explanations and it sounds awesome that you're going to get involved.  As a perfect example:

The front nose wheel steering cylinder is leaking.  It is the 90 degree fitting that the hose attaches to that is leaking.  The shop installed it but here is the rub.  Did they tighten it too much?  Did they not tighten enough?  How do you know when tight is enough and what is the trick when the end result is important to make sure you're not too tight or too loose when you reach that point?

Do you torque these fittings?  If so how?

Or is it all about luck?

Glenn

[Willis]

Bud,  I'll take a look at my manuals and see what I have on hydraulic system and scan them for you. I appreciate the explanations and it sounds awesome that you're going to get involved.  As a perfect example:

The front nose wheel steering cylinder is leaking.  It is the 90 degree fitting that the hose attaches to that is leaking.  The shop installed it but here is the rub.  Did they tighten it too much?  Did they not tighten enough?  How do you know when tight is enough and what is the trick when the end result is important to make sure you're not too tight or too loose when you reach that point?

Do you torque these fittings?  If so how?

Or is it all about luck?

Glenn

Thanks for any documents you scan for me.  I'll PM you an e-mail where you can send them.  Alternately, you can post them to a web site like Google photos and share the link.  I use "Picasa" for my projects.

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 Angle fittings that fit into an actuator or some other Cast/Machined body are used to provide for a correct take-off angle.  This is so you dont put strain on the fluid hose or tube.   Since this angle is variable the nipple end cannot seat properly because you dont know how deep it will go when you "Clock" the fitting. 

An angle fitting will usually have a backing nut to hold the fitting at the proper angle once that's established.  Underneath that backing nut should be a gland or o-ring that is providing the hydraulic seal.

The Backing Nut will be torqued anywhere between 10- 30 ft-lbs.    There are two sources for torque values. (a)  the maker of the body receiving the fitting and (b) a Mil-Spec.  Almost universally, fittings have a mil-spec equivalent, so a place like Parker-Hannifin will issue a spec for their fittings.    Use two wrenches, one for the angle fitting (it should have a flat) and one for the backing nut.

It usually takes two attempts to get the angle/fitting/torque done properly.  Once to get the alignment correct, and once to actually torque it down.  The MM, and IPC should be consulted to see if the hose/tubing routing is correct.  Mine was done improperly for 10 years until someone pointed it out to me on the IPC.  The routing with the least strain in both the UP and DOWN positions is ususally the correct one.

Look at the Illustrated parts catalog for your AC (if you have one, if not, then thats a goal to get posted on this website.).