Grounded after annual...

We bought a PA-18 135 August last year which has passed the annual just a month before.
This years annual turned out into grounding the plane. Cause: the right wings lower fabric isn't stitched properly to the ribs. Some of the ribs already moved an inch...
Does anyone has, or know where to get an explosion plan of the wings, so we can check if it has been put together correctly after the last major recondition (e.g. do the ribs need to be diagonaly stayed)?

Thanks a lot for any hint

Ribs get braced by using a non-adhesive 1/2" fabric tape, wrapping around one rib's top, then the next rib's bottom, back to the top, etc. You do two runs, so every rib is braced to the neighboring ribs at both the top and bottom.

Are you sure your ribs moved? It's more likely that the problem was overlooked at the last annual. If the ribs moved, they probably did so when the fabric was shrunk, prior to rib stitching. You need to get a copy of a covering manual, like Polyfiber's, and read it. It sounds like the fabric was shrunk incorrectly. If this is what happened, the bottom of the ribs would be bowed sideways, since they attach to the leading edge and the false spar/trailing edges, and the fabric doesn't shrink there. If they aren't bowed, then you really need a wing construction diagram.

SB

stewartb - thanks a lot for your prompt answer - on my side, I had to wait for the confirmation about the points you mentioned in your posting.

The ribs have not been braced Someone seems to have done a bad job a couple of years ago...

So is there any help available out there how I can get a hold of the wing construction plan for our PA-18 135?

Thanks in advance!

Here's a website that may help. Other guys talk about the Northland Cub drawings on CD, but I don't own it, so I don't know what's on it.

http://www.dakotacub.com/technical_info.htm

SB

Try these folks, if I remember correctly their CD has the wing drawings.
http://www.northlandaircraft.com/ or you could send them an e-mail and inquire.

delete

TJ is right, if the ribs are stitched in place the stitches will still hold the bottom of the rib up even if it isn't straight. Does this look like they stitched the ribs before shrinking the fabric or a tramel problem after the fact or just no support then stitched in place where they fell?

Rib bow

Sounds like you have a spam can A&P doing your annual that dosn't have any common sense. I doubt your ribs could be bowed that much to make any difference unless the cap strip is kinked and that would take some doing. Yes, perfectly straight is nice, but a little bow will not hurt a thing. I would take it to another A&P that knows Cubs. Crash

Thanks djfraudman! The CD content says something about 117 wing drawings. The CD is ordered - I wonder how long it will take to make it's way from Alaska to Switzerland... ;-)

Well, the right wing's ribs you can see through the maintenance holes are bowed outside - that much, the stitches which should be on the right of the ribs are far (about 3 times ribs wide) on the left. Finaly, we also found some cotton (?) left overs which could have been from rib bracing.
The report of last year's annual shows, stitching was repaired. So it wasn't overseen. However, what has been used to repair the stitches looks more like fishing line than what you would normaly use for this purpose (and has been used for the left wing's stitches).
Last year's annual was done by a different A&P. Today, I've no idea who did repair the stitches. IMHO: If it was the last year's A&P, I'd rather trust the current one - whom at least two of our team know for more than 20 years and had all annuals done by (and some together with) him on old aircrafts like a Bo207 and a PA-12 the time we did own these aircrafts.

Thanks for the hints towards the material. We'll have to check what we can get a hold of in Germany/Switzerland...

ooooo watch out, I inspected a Citabria that had monofilimant line for rib stitching. It was slowly cutting through the ribs. It did a lot of damage. Needless to say I got as far away from that airplane as possible........