power pack repair

SteveG · #1 10-05-20, 04:28 PM

Hello all,

My 337A with the all hydraulic power pack has developed a fluid drip from the priority valve which vents to the exterior. Guidance from anyone with experience in remedying this problem would be greatly appreciated. Rick Cox of Cox Airparts has quoted me $2450 for an overhaul and is unwilling to repair as necessary. This seems like an awfully expensive o-ring replacement but considering the work required to r&r this unit I certainly do not wish to do it more than once. That would seem to preclude a salvage yard part of unknown condition. Anyone else attempted a field repair and how did that go? Any and all insight gratefully received.

Regards, Steve G.

cessnadriver · #2 10-05-20, 06:56 PM

Steve G. IF YOU ARE SURE IT'S LEAKING AT THE PRIORITY VALVE! I personally have NOT disassembled the power pack, but looking at the parts manual, FIG 113, Items 44-49 is the priority valve. ALTHOUGH not much room to work, you'd have to remove all the hydraulic fluid, remove the reservoir top and remove the priority valve, from the inside of the reservoir, with a socket, replace the two o-rings Items 45 and 46. Re-install the valve, fill reservoir with H-5606. Then check for leaks. Again I have NOT preformed this type maintenance but looks fairly simple.
Regards, Bill Story
'65 C337 owner/pilot/A&P

SteveG · #3 10-05-20, 07:24 PM

Hi Bill,

Thanks for the reply. But my analysis of this is less optimistic. Removing the cover and working from the top down into the bottom of the canister does not seem feasible. The service manual says to remove the unit from the aircraft, a job I conservatively estimate as being 5 hrs. of backbreaking work. And then there's the matter of setting the pop-off pressure. Apparently requiring capping outlet ports and connecting a lever pump & gauge. Both the manual and the overhaul shop suggest that once opened it is indeed a Pandora's box of cascading problems and that an overhauled unit may be the more cost effective solution in the long run. That said, I'm still considering the field repair option if anyone can report an actual success story.

JAG · #4 10-06-20, 10:53 AM

Steve,
I have not tried the field repair - but my thoughts are like Bill's; the maintenance manual has the steps to do this. Removing it sure seems like a nightmare, but once you take the seats out, drain the fluid and get up under there with some good lighting, it is not that bad.

I have built myself a hydraulic power cart with a regulator and pressure gauge that I use for swinging the gear - if you have something like that it would be fairly straight forward to use something like that to do the testing (may need a few extra hoses, etc.

If that seems like too big a job to pull off - my next step would be to take Rick up on his overhauled unit. The PowerPack is the brains of the landing gear.
Jeff

SteveG · #5 10-06-20, 08:12 PM

Hi Jeff,

Thanks for the reply. Unless I completely misunderstood Bill, I think he was suggesting pulling the valve in situ rather than removing the entire assembly but trying to work from the avionics access plate downwards doesn't seem feasible and won't allow for portable power to be connected. I have a power cart, in fact posted a picture awhile back in a different thread topic, it just seemed like using it was like using a chainsaw when a scalpel was the right tool, no finesse in measuring the pop-off pressure. The service center rebuild would be convenient but I'm usually disappointed every time I try just throwing money at a problem and 25 c-notes for some o-rings just seems excessive. When I suggested same I just got the usual blather about "well, you know, liability and all". The only reason I'm considering it is I don't much want to revisit this project site next year when the other shoe falls and I end up wishing all the seals had been replaced while it was out and open.

JAG · #6 10-07-20, 10:44 AM

Sorry Steve - when I reference Bill, I was just referring to doing it yourself - but if I was doing it, I would likely take the PowerPack out, and replace some of the other seals as well. I recently had a leak on mine and had to replace the o-ring and the backup ring on the "landing gear down" fitting on the bottom of the unit - was just old and installed improperly and finally started to leak.

Bill's suggestion of going through the top is not a bad idea if you don't have too many avionics in the way - mine has good access since I installed G5s in the panel.

I like the chainsaw analogy - ha ha. I suppose even a handpump set-up could be used. I have used this handpump in the past for engine pre-oiling. May work for this? I think you can find higher pressure ones...

https://www.amazon.com/VEVOR-Hydrost...89684056&psc=1

JAG · #7 01-28-21, 03:56 PM

All this talk of Power Packs has jinxed me, and thought I would update the group. The leak I had in my power pack on a fitting progressively got worse - even after putting in a new o-ring and backup ring. I found the culprit over the weekend...the manifold at that fitting has a small crack in it - hence the source of the leak that was showing up on the downline fitting.

I have worked on many aircraft over the years, and in tight places (inside the fuel tanks of a Herc, under the dash of a Stinson 108....you get the picture? But I have to tell you, removing that damn power pack on my 1966 Skymaster was one of the worst jobs I have ever done! They built the damn airplane around that power pack!!!! Needless to say - I must correct earlier posts I made about thinking it was a straight forward job. I removed the seats, carpet and side panel on Sunday in short order, and decided last night after work, I would go out to the hangar and remove it, and be home by 9pm....wrong. I took my time draining and capping lines, etc, thinking that if I just have patience, things will go smoothly. Well, that is until...I started to take out the first mount bolt. Of course I am smart enough to start with the most difficult one, and let me tell you did that bolt learn some new cuss words, take some skin and try my patience. It took me about 30 minutes to finally figure out how to get one of wrenches on it (after trying 5 different types and styles. When I finally figured out the one that would do the job, it was another 30 minutes of backing it out, one flat at a time. If that wasn't enough, getting the damn overflow line disconnected was another joy - since new avionics occupy the space above the damn power pack.

Anyway - this job is not for the faint of heart. it took me all of 5 hours to remove that damn power pack, and I still have to go back and do some clean up in the hangar now having just dropped the tools to head home for a shower at 10pm.

Finally - I will be sending the unit up to Rick and Brian Cox to replace the manifold and overhaul this unit to 'like new' condition. I am a bit pleased with myself having purchased an auctioned power pack a few months ago from Mark (advertised on a thread here) - I am able to use the parts off of this power pack and save myself close to $1,000.

Steve - not sure if you have done the work on yours yet, but thought I would give you some feedback. I grossly underestimated the time and effort it would take to remove this pig of a part. I am guessing the install will take a bit longer, as I have to remove some avionics to make the install go a little smoother.
Jeff

SteveG · #8 01-28-21, 05:53 PM

Hello again fellow masochists,

I've been meaning to update my experience in this matter for quite some time but Jeff's latest post requires that I take the time to do it now. Sorry Jeff for the jinx. At the time of my initial inquiry I was amazed that after a few hundred post views only two persons proffered any comment and neither one had actually experienced that or a similar problem. So I now take some solace, at your expense, that I am not alone in the universe.

I elected to remove the power pack and send it to Cox for their overhaul/repair. My initial estimate of five hours to pull was, for me, optimistic at best. Like all mental estimates, what you can do in your minds eye actually takes twice as long to physically accomplish. And yes, removal of the aft, outboard mounting bolt is a one hour project in and of itself. You cannot see it or feel it, you can just barely engage it with an open end wrench and rotate it one flat at a go while flipping the offset each time. I rigged a flexible borescope so I could visualize the orientation. By the time you have the power pack out you will have used at least one of every type of socket, open end, offset & crow's foot wrench in your arsenal and still you'll have to grind down a couple of custom configurations. I had the exact same thought as Jeff in that the unit is factory installed early in the airframe build long before instruments, avionics or even the subpanel are installed with little concern for the poor SOB that has to try to service it later. The electrical connection and the fittings accessed through the avionics panel are additional sources of frustration.

My experience with Cox was of mixed results. They turned the work in a reasonable amount of time. The cost was expensive but as quoted. I reinstalled the unit, refilled the reservoir, bled the brake lines, placed the aircraft on jacks, plumbed the hydraulic cart and then tried to operate the gear doors with the e-pump. The doors would open as they should but would not close. After all the foregoing work and money the disappointment was off the chart. Trying to troubleshoot the fault was made more difficult because you just cannot believe that the newly overhauled power pack is at fault. The thought of having to pull it out and send it back is enough to drive a man to drink. But after some time with wiring and hydraulic plumbing schematics it becomes clear that yes, the power pack is inoperative, brand new & just out of the box. The fault proved to be the power pack microswitch which senses the position of the gear selector handle. It was frozen in the depressed state such that the solenoid was not being energized in sequence after all the downlocks are made. Cox sent a replacement which I swapped out, rigged to suit and all was well thereafter. I sent the defective switch back to them but never received a satisfactory explanation of the problem. It appeared to me that the switch was contaminated with a dielectric sealant that they use on the exposed solder joints.

On the reinstall, pro tip no. 1 is to loosen all the flared fittings. The reworked power pack will be slightly different in the position of all the AN fittings and will require a lot of tedious work to get everything back into alignment. Pro tip no. 2 is to ream the mounting holes out to at least 125% of their initial diameter so that you have some play to work with. Pro tip no. 3 is to forget about reinstalling that fourth bolt. Three bolts and all hard tubing make the finished product one with the airframe.

Best of luck to you Jeff. To anyone else embarking on this adventure keep in mind it is not, as Jeff said, for the faint of heart. If you hire it done, whatever they quote you cannot possibly be enough if they have never done it before.

Regards to all, Steve G.

JAG · #9 02-01-21, 09:04 AM

Steve - thank you for your detailed response and tips. I feel better about myself now for struggling with this damn thing, and truly appreciate the "pro-tips". Disappointing to hear that your power pack was not "plug and play" like they describe; you have given some good advice that I will use on the install of mine.

Cheers,
Jeff

P.S. the scars on the back of my left hand I will wear with pride - for the memory of that damned 4th bolt...

SteveG · #10 02-01-21, 06:51 PM

A few random additional thoughts on this project by someone who has been there and learns everything the hard way, in the hopes that they may be of some value to others working in the same area. Employ at your peril, your mileage may vary.

Start threads on all fittings and bolts while assembly is loose. Misalignment can be pulled into position with mechanical advantage of threads but immovable misalignment will prevent fitting nuts from starting without the risk of cross-threading. Hand tighten in sequential order before wrench application.

Verify actuation linkage aligns with power pack shaft before final torque of fitting nuts and mounting bolts. The rod assembly passes through a bushing before engaging the manifold shaft and there is no adjustment for this alignment, it is fixed by the structural geometry.

It is likely that the clevis end of the control linkage will require adjustment once power pack position is finalized. While moving the selector handle from full down to full up observe that stop to stop rotation on the manifold shaft is obtained without panel interference.

Observe that cam on manifold shaft ,when rotated into position, depresses roller arm of switch which in turn operates switch correctly. This should have been adjusted by the overhauler but.... trust but verify.

Cox will return the unit without safety wire in the mounting screws of the switch and switch shroud. This is intentional on their part, using lock washers in lieu of safety wire.

mshac · #11 02-04-21, 12:34 PM

Quote:

Originally Posted by SteveG

A few random additional thoughts on this project by someone who has been there and learns everything the hard way, in the hopes that they may be of some value to others working in the same area. Employ at your peril, your mileage may vary.

Start threads on all fittings and bolts while assembly is loose. Misalignment can be pulled into position with mechanical advantage of threads but immovable misalignment will prevent fitting nuts from starting without the risk of cross-threading. Hand tighten in sequential order before wrench application.

Verify actuation linkage aligns with power pack shaft before final torque of fitting nuts and mounting bolts. The rod assembly passes through a bushing before engaging the manifold shaft and there is no adjustment for this alignment, it is fixed by the structural geometry.

It is likely that the clevis end of the control linkage will require adjustment once power pack position is finalized. While moving the selector handle from full down to full up observe that stop to stop rotation on the manifold shaft is obtained without panel interference.

Observe that cam on manifold shaft ,when rotated into position, depresses roller arm of switch which in turn operates switch correctly. This should have been adjusted by the overhauler but.... trust but verify.

Cox will return the unit without safety wire in the mounting screws of the switch and switch shroud. This is intentional on their part, using lock washers in lieu of safety wire.

SteveG, thanks for your tale of the emotional roller-coaster that is DIY repair work! Your story reminded me of replacing my rear alternator, and having to remove it AGAIN due to a "bad" OH'ed unit. It may not be quite the job the powerpack R&R is, but the emotions were very similar! I do believe I had an extra finger or two of scotch that evening!

Learjetter · #12 07-24-24, 01:00 PM

July 2024:

Cox airparts will overhaul 9881200-1 ('77 G-model) powerpack for $4242 + parts with a 3-6 week lead time.

Power Packs Plus will exchange for $4975 (+ $10K qualifying core)

Aerospace Turbine Rotables says they've never heard of part number 9881200-1.

QAA in Tulsa say they cannot IRAN or OVHL.