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Thread: Spliced short-wing spar in Super Cub?

  1. #1

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    Spliced short-wing spar in Super Cub?

    Pulled the fabric off of a '79 model PA-18-150 today, and found this front spar in the left wing. Anyone seen this before? It looks to be a short-wing spar that was re-drilled and spliced. See photos of lift strut holes and doubler rivet holes, as well as other unused holes. No record in the logs or 337's of course, but I do know this fabric has been on since 1989. I know the splice is a legit process and have seen the Piper data for doing it, but can it be legally done from a short-wing spar? Is it possible this came from Piper? Only reason I ask is the tape/sticker with the "PA-18" and "12/2x/1978". This red marker/pencil looks similar to writing Ive seen on other spars from the factory, but obviously anyone can write anything on a piece of masking tape. I don't mind changing the spar, I'm just curious if anyone else has come across this...
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    Last edited by ak49flyer; 01-14-2022 at 12:44 PM. Reason: Rechecked mfr. date- signed off Jan. 11, 1979

  2. #2
    Steve Pierce's Avatar
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    Are there holes above the inboard strut attach fitting holes? A short wing Piper spar has a doubler riveted about the strut attach fittings. I thought there would be a complete bay difference between a SC spar strut attach fitting and a Short Wing. I have never seen Piper write the numbers on tape affixed to the spar, always written on the spar itself.
    Steve Pierce

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    Edit: yes you can see the doubler holes in the photos- also unused lift strut attach holes one bay in from current ones. Notice spar is “upside down” from the short-wing perspective.

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    Steve Pierce's Avatar
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    Here are some Clipper, Pacer and Tri-Pacer spars. The angle is longer on the Pacer and Tri-Pacer than the Clipper.
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    Steve Pierce

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    Yep, that’s exactly what it looks like, in the first pic...

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    Another weird thing: the unused lift strut fitting holes in this spar number 6- Steve’s pics only have 4, and Cub typically has 5. I do have the holes for the doubler, though...
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    Sounds like it would work fine and be quite safe, and maybe even be classed as a minor alteration - but I bet there are folks who would freak out with the extra holes. My understanding is that the holes do not weaken the spar.

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    Yeah I think that's the big question: Is there any guidance for how many extra holes can be in the spar, and where? The splice is very well done, and exactly in accordance with Piper S.M. 3A, so I have no problem with that, I'd just like some reference to show anyone down the road that the extra holes are ok...

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    wireweinie's Avatar
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    Ok, passing along some of that knowledge (it was just given to me, too)

    Seems that Piper ran low on spars at the end of PA 18 production. So their answer was to fab up some (around thirty or so) using Pawnee spars. These were all painted black at the factory. If you look at those unused strut attach holes, you'll note that they seem to run upside down. But that angle is correct for the Pawnee and I bet that a Pawnee uses six bolts to attach the strut to the spar. The guy that passed this on was trying to extend a set of wings, years ago, but found the black splice already in place. FAA got involved, yada yada, and Piper finally admitted these were done at the factory.

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    Wow! Exactly the type of scenario I was suspicious of... Now if we could get that in writing from someone "official". I talked to Clyde Smith, and he doesn't know anything about it, and Piper engineering is no help at all...

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    Steve Pierce's Avatar
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    Could be a Pawnee spar. Have seen that before.
    Steve Pierce

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    You’ve seen them from the factory like this? That must be the deal- thanks!

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    Do the drawings exist that allow spliced PA-25 spars to be used to build the PA-18 or was this a "hope no one will notice" production line change.

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    Steve Pierce's Avatar
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    Quote Originally Posted by ak49flyer View Post
    You’ve seen them from the factory like this? That must be the deal- thanks!
    No, some Jake legged mechanic.
    Steve Pierce

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    Suggest that you take a survey of other PA_18s around the same serial number and look in the wings to see if similar spars are out there. That would help solve the mystery of "done at the factory".

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    Quote Originally Posted by frequent_flyer View Post
    Do the drawings exist that allow spliced PA-25 spars to be used to build the PA-18 or was this a "hope no one will notice" production line change.
    Exactly what I’d like to know- did someone run upstairs, have an engineer ok the idea, run back down and start banging spars together, or did they actually have drawings?

  17. #17
    Steve Pierce's Avatar
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    Spars didn't cost Piper much, I would bet a large sum of money that was not done at the factory.
    Steve Pierce

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  18. #18
    skywagon8a's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    Spars didn't cost Piper much, I would bet a large sum of money that was not done at the factory.
    I bought a new wood spar from Bellanca for a 7GCB which was made of four short pieces of spruce scarf spliced end to end without gussets instead of being one long piece. There wasn't any mention of the splices in the paperwork. I asked my FAA PMI about it and he said "They" can do it without gussets but you can't. So I used the spar. I've never seen an approved spar splice method without using gussets. I can imagine the next mechanic who opened up the wing for recover may see the spar and question the legality about it. That was long ago and no one ever called me to complain, so either no one caught it or they didn't care.

    I would not bet the factory didn't do that spar. They can and do do things within the factory which they don't tell others they do. I do question this particular spar's open holes which appear to be inboard of the lift strut fitting where the high bending loads occur. It seems to me, at the very least there ought to be a doubler plate over those PA-25? lift strut fitting holes to strengthen the web from buckling.
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    Steve Pierce's Avatar
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    I have a J3 spar from the late 30s with a splice and a doubler on one side. Ribs were modified with the factory rivets and looked factory. The splices we're signed and dated by the factory employees and the dates coincide with the manufacture date.

    I still doubt in 1978 Piper would have their employees take the time to scab a Pawnee spar together with another piece of spar vs just installing a new spar. That is about the time they started using the metal flaps and ailerons to save time.
    Steve Pierce

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  20. #20

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    Lots of factories built wood spars either with splices, or laminated. Aeronca did it a lot. I have a copy of the Howard process spec for how they did splices and laminations. Even in the ‘30s and 40s they were having issues finding spruce pieces big enough to mill spars from. Of course this is off topic on the spliced metal spars.


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  21. #21
    kase's Avatar
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    There was a factory 79 cub here with Pawnee spars painted black. I'll see if I can find the n number later today.
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    SuperCub MD's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    Spars didn't cost Piper much, I would bet a large sum of money that was not done at the factory.
    I might take that bet.. I found a spliced 18 spar once, after looking onto it was reasonable sure it left the factory that way. This was near the end of the Pawnee run, if they had a pile of pa 25 spars left, I would not be surprised they used them up. Tape on it might be so red shows up on the black. People forget Piper was a production line trying to make money, not building custom built masterpieces.
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  23. #23
    kase's Avatar
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    This is the cub before it got wrecked. Only had 600 some hours since new. SN 79-09064. You should see how close that is to the other one with Pawnee spars
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    18-7909056 here- found two others in the range that are semi-local I might look at just for curiosity sake...

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    wireweinie's Avatar
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    Quote Originally Posted by ak49flyer View Post
    18-7909056 here- found two others in the range that are semi-local I might look at just for curiosity sake...
    Your serial number is only five away from the aircraft referred to in post #9. Also see the attached pic from the PA25 parts manual. It shows what looks like six attach holes in the forward strut attach fittings.

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  26. #26
    Steve Pierce's Avatar
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    Quote Originally Posted by SuperCub MD View Post
    I might take that bet.. I found a spliced 18 spar once, after looking onto it was reasonable sure it left the factory that way. This was near the end of the Pawnee run, if they had a pile of pa 25 spars left, I would not be surprised they used them up. Tape on it might be so red shows up on the black. People forget Piper was a production line trying to make money, not building custom built masterpieces.
    When did they stop building Pawnees? Was the spar solved or a modified Pawnee spar?
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    Last Piper produced Pawnee was 8156024, so 1981 would be the last PA-25 with a Piper data plate.


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    Steve Pierce's Avatar
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    Getting very interesting. Too many consistencies. Went back and looked at this one I was involved in several years ago.
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    Steve Pierce

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    That looks like a right wing, but otherwise identical to this one... Was that a ‘79?

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    Steve Pierce's Avatar
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    I couldn't sleep early this morning and remembered this and dug up the pictures. Gotta find the aircraft data now.
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    Just got off the phone with the owner of a ‘79 here in-state. Has a spar exactly like that- spliced and painted black. He was told Piper did something like 28 that way to use up Pawnee spars...
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    Sounds like a lot of effort but once you have a template and system I bet you could do that splice in very little time. Looks like the plate in the spar splice kit so parts might have been on the shelf. https://www.univair.com/content/PIP_SM003A.pdf Just cut drill Bang, employees tend to like a paycheck so getting the job done works for everyone. I will vote for factory job now that you guys have done all the work.
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    Steve’s pictures are the 4th one done like this that I’ve been able to confirm existed- talked to a couple more guys with anecdotal info about it this morning- pretty much what Web said above. Sounds like no one actually ever saw any paper from Piper backing it up...

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    One theory was that they were painted black to make it harder to see all the extra holes.. all in all, a lot of extra effort...

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    frequent_flyer's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    I would bet a large sum of money that was not done at the factory.
    A safer bet is starting to look like "I would bet a large sum of money there is no documentation that was done at the factory".

    Can full strength be achieved by splicing only the shear web? Were the caps also spliced with a doubler?

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    No doublers needed on the caps- it’s in a Piper service memo- 3A I think. Just like the splices in the pics here.

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    frequent_flyer's Avatar
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    Quote Originally Posted by ak49flyer View Post
    No doublers needed on the caps- it’s in a Piper service memo- 3A I think. Just like the splices in the pics here.
    Ok, thanks. I should have read the service memo before posting. Now I have read it I see that it is not expected to give full spar strength as it can only be applied outboard of the lift strut.
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