power pack repair

SteveG · #1 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 · #2 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 · #3 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 · #4 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!