Avid MK4 Wing Strut Inspection

[bcbushcaddy 27]

Hello, 

So here we, Willis and me, have started removing the wings to upgrade them with extensions, new fabric and some minor repairs. Upon pulling the struts from the wing attach fittings I noticed a minor gap between the wood fairing (rear) and 4130 pipe (front) that forms the strut. Just wondering if anyone has ever encountered any moisture and/or rust issues in this area or has anyone ever removed the fairing, I know its glued, to inspect for possible corrosion. Those struts are now 20 to 30 years old, ..just wondering. 

[EDMO 1078]

I pulled the wood fairings off of some 20+ year old Kitfox struts that seemed to have been well glued - other than the glue, there was some slight surface rusting which cleaned up easily with hand sanding - nothing deep enough to be concerned about.  These were not powder coated, and appeared to just have primer on them before gluing the fairings on, and painted later.   EDMO

Edited 19 May 2016 by EDMO

[dholly 718]

IIRC, Leni's were rusted to the point he had to scrap them due to moisture retained by the faring covers.

[EDMO 1078]

Untreated wood is going to gather moisture, or have moisture in it - Somehow, either epoxy coating the wood, or putting a solid barrier of powder coating, epoxy paint or glue on the struts, or all of the above,  might eliminate that problem.   EDMO

Edited 19 May 2016 by EDMO

[akflyer 1427]

Hello, 

So here we, Willis and me, have started removing the wings to upgrade them with extensions, new fabric and some minor repairs. Upon pulling the struts from the wing attach fittings I noticed a minor gap between the wood fairing (rear) and 4130 pipe (front) that forms the strut. Just wondering if anyone has ever encountered any moisture and/or rust issues in this area or has anyone ever removed the fairing, I know its glued, to inspect for possible corrosion. Those struts are now 20 to 30 years old, ..just wondering. 

Mine were pitted past the point of being safe to fly, so were the struts on Bobs KF I.  I replaced with 1" X.058" tubing.  I cut the original struts about 6" from each end and slid them inside the 1" tubing.  Then I welded the new tubing on using fillet welds around the ends and a few rosette welds.  I notched the inboard ends of the tubing so they would slide up tight to the strut mount on the fuse end so I have full length 1" struts.  I went with the plastic fairings from KF so I can pull them off and inspect the tubing.  The entire length of the original tubing was pitted where the wood fairings were glued on. 

I think a big reason for it was the tubes were not blasted or prepped before they glued the wood on and it had no anchor profile so the epoxy just pulled the mill scale loose from the tubing and was not bonded to it.

 

[bcbushcaddy 27]

Thanks for the enclosed info. I guess we better take a peek...

Stay tuned! 

[C5Engineer 1170]

I was in the same boat with mine. Used the overall condition and decided to fly em. Now that it's been a few years I've thought of cutting into them to take a peek. Problem is there's really no good way I could see to do it for just an inspection. Let us know what you find. 

[akflyer 1427]

Joey... see if the NDE guys on base can xray them for you..

[marshawk 82]

Well we open up the wood fairings on the struts today and found a whole lot of rust going on, may be worth a check!The top front strut was were most of the rust was

[Av8r3400 1178]

Wow!  That's scary...

 

[C5Engineer 1170]

Wow thanks for posting the results!

[bcbushcaddy 27]

Let me elaborate a little to Willis' post...

There was no obvious signs of any corrosion other than a few small specks around the very top and Jury strut brackets while they were sealed in the fabric. It was the slight separation of the wood (and a hunch) that lead to this inspection. There was also no discolouration of the fabric on the outside. However there is discolouration present in the wood, see center photo, for almost 2' down the tube. There is little rust on the top covered 1" where it is able to dry out, through the crack, mind you. The issue arises further down where the water collects and remains for periods of time. The top 2' of the wood filler was no longer "glued" to the strut but had separated with copious amounts of rust. Punch tests along the rear center line in 1" increments did not lead to any protrusions, ...as of yet! Not sure how much longer the tooth of time could gnaw away before failures would occur? 

I have just checked the Kitfox's webpage for SB's  http://www.kitfoxaircraft.com/index.php/service-bulletin  but could not find any. Maybe I've missed it or possibly misread it? Needless to say AVID will no longer issue any, so it is up to the community to raise the awareness to this fact. I am certain that your garage kept aircraft in Tonopah NV will most likely be fine, but I am also certain that your parked/tied down Vancouver Island bird will show the same thing. Well, how much rust is reasonable? I don't want to answer that one, but how about none as a start to the conversation. 

In either case I highly recommend an inspection, some of these aircrafts are now 30 - 40 years old and having looked at a lot of PA-18s, 20's, 22's Citabria's and you name it, it's not uncommon to find hidden rust. The ultimate call is yours as the owner, pilot and mechanic. 

Edited 19 Jul 2016 by bcbushcaddy
missing word, grammar,

[dholly 718]

Oh my. Obviously good call taking a peek, some guys would be hesitant to do it. Always listen to your gut on the safety issues! Omitting the wood fairings is problematic on the early models with lighter duty lift struts where the wood lift strut fairings are mandatory for structural integrity, as noted in the Avid Flyer Service Bulletin. I can't see thin wall metal cover being their equal. Maybe it's time to bite the bullet and upgrade, ie. buy or build some heavy duty 1" lift struts like Leni did. If I was in a wet environment I think I would go that route and add the streamlined pvc snap-on fairings to allow ventilation.

Edited 19 Jul 2016 by dholly
sp correct

[bcbushcaddy 27]

BIG THANKS for the heads up on the structural integrity! I guess we'll be exploring a few "strutting" options on this one. Don't see the point on re-doing it the same way. Certainly going to have to take a look a Leni's upgrades. 

[marksires 484]

Any info on the beefed up struts?  I need to rebuild mine - they looked pretty much like the pictured ones after a runway light jumped out in front of one and knocked the fairing off.  The other one looked as bad after i had the heads up to look at them.  Neither showed any outward signs during preflight or annual condition inspection.  I'd like to rebuild them with the heavier struts so I don't nee the fairing for structural integrity, just a fairing for aerodynamics.

 

Mark

 

[wypaul 448]

I am wondering if the struts were properly primed and painted to begin with.  Mine were powdered coated at the factory and I hope that they will fair better than those.   I guess another advantage of living in a dry, salt free climate is that things don't rust at the drop of a hat.  Many of the Avids gear legs were faired the same way so I guess those may have the same problems.  Thanks for posting your findings maybe it will save someone else.

[EDMO 1078]

I believe that Kitfox used 1" x .049" wall 4130 tubing for the later struts - Leni and I used .058" wall - I forget if I used it because it was either cheaper or more available locally than the .049" wall 1" OD 4130 tubing, and 3/4"OD inserts would fit inside of it tighter.   The rod ends are normally 5/16 -24 thread, with 1/4" diameter crossbolt, even on the 1700 GW Magnum - I had some 7/16-20 thread with 5/16" crossbolt rod ends, so I made 3/4" OD x 4" long 4130 threaded inserts to weld into the scarf-cut ends of the tubing, and had to modify my spar attach fittings to take the larger rod ends.  I think Leni got the Kitfox plastic fairings for his, and he used washers to take up the space in the spar strut fittings.  I think Leni used the original strut to fuselage fittings by welding the new tubing over the stubs of the old struts, but I made new strut to fuselage fittings because the angle of my struts had changed since I raised the wings up 6".    EDMO

Edited 19 Jul 2016 by EDMO

[TJay 1041]

If you guys want to build new ones the way ED is describing I bought 12 rod ends because they were cheaper that way and I only need 4, they have a 3/8x24 rolled thread with special reducers to get the bore down to a 1/4 bolt. Very strong that way, They are a RBC Heim MT6 bearing