Advice for main & rear spar replacement for a 7GCAA

[anejohn]

Hi all
A new FNG here to the forum looking for input on rebuilding an old set of wings with new wooden spars from Rainbow Flying Services. Through the over head inspection plates a compression fracture was found on the right (starboard) wing after the backing plate. Spoke with “Ron” from Rainbow Flying Service and he said it was probably caused by excessive loading to the wing from an aerial maneuvers gone bad. I don’t own a chute and haven’t participated in any aerobatics in this bird but I have literally cheated death in the 19 hours I’ve flown in this airplane before finding this fracture on a annual inspection. If it were not for Ron’s STC for the square inspection plates on top of the wing we would have never found this crack.
Anyway, I am a A&P with a good set of tools and a nice hanger to work in. I have a good understanding of wood working but not on wooden aircraft. I have worked with the Polyfiber process and comfortable with its application though I have never recovered an entire wing. Besides removing the fabric and the tedious process of removing the nails from the old spar. How difficult is it get the old spar out and the new spar back in and re-attached. Is it best to start with main spar then the rear spar or vice versa. Besides the AC43-13 is there other guidelines or videos I can reference?
Thanks for reading..

 

[Bartman]

Hi John and welcome to the site!

I did a conversion on my wings to Milman aluminum spars and there are a lot of photos in the thread here; https://champcitabriadecathlonforums.com/threads/68-citabria-7eca-milman-spar-conversion.23/

You're using Rainbow wood spars but some of the steps will be the same, like removing the nails from the ribs. If you just pry them out they'll destroy the rib flanges. My nails were extremely hard to pry out until I tried heating them, then they came right out and didn't damage the ribs at all.
Here's an explanation; https://champcitabriadecathlonforum...eca-milman-spar-conversion.23/page-3#post-705
and more pics; https://champcitabriadecathlonforum...eca-milman-spar-conversion.23/page-4#post-771

If you mark the new spars with the rib locations before you disassemble the wing panels, you can just cut up the old spars to get them out of the way but, either way, you'll have to fully disassemble the wing outboard of the doublers to get the new spars into place. My old wood spars didn't come out without a lot of persuasion but prying the rib flanges back away from the spar face a little but really helped.

Good luck!

 

[Bartman]

I saw your pics on Facebook, that is a crazy spar crack!

 

[Joe]

If your Fabric is still original, I would consider doing the Milman conversion and covering both wings. You will spend about 10K and a few hundred hours of labor, but it's a once in a lifetime investment. If just doing the right wing you can likely get away with keeping the rear spar as long as the smooth rib nail have not been replaced with the ring shank nails. You'll have to remove the ribs outboard of the strut attachment. The ring shank nails make a mess out of everything, and considerable cursing is required to coach them out. Going back together Ron has an STC to replace with screws. Used wings show up online every once in a while, but almost always require recovering, It's a great option if you can find one. Best of luck anejohn.

 

[anejohn]

Thank you Bartman & Joe for your insight, it is greatly appreciated and I will keep the forum abreast of what path the aircraft owner chooses to go. We do have spare wings at the hanger although they are un-airworthy at this time. (Bear with me, it’s a long story) We have the original set of wings from the guy who sold the aircraft. Here’s a brief history. The aircraft left gear bolts sheared, clasping onto the left wing during landing. The previous owner bought the aircraft and replaced damaged items to include the prop, engine, fabric, stringers and wings he purchased online. Upon hearing about the newly discovered crack he offered the original set of wings believing the right wing was airworthy since it sustained no damage during the accident. Once the we got a closer look though, we discovered two cracks along the grain line of the main spare at the wing attachment point. We can only assume this happened because of the dry climate of Nevada and the wings being in storage for maybe 10 years.
Not sure what the owner will decide to do. He’s not too happy at spending 120 man hours or more rebuilding one wing in addition to the costs involved for material. He just wants to fly it.
Find attached photos of the spare set of wings and a photo of the crack at the wing attachment point.
Thanks again!

 

[Bob Turner]

Might check AC 43.13 -2A. Might be Chapt. 1 or 2 - I was just in there studying wing splices. Look up cracks - some are allowable.

The crack I am dealing with is over six feet long - hence my first Stearman complete spar replacement. Big time learning event.

 

[anejohn]

Might check AC 43.13 -2A. Might be Chapt. 1 or 2 - I was just in there studying wing splices. Look up cracks - some are allowable.

The crack I am dealing with is over six feet long - hence my first Stearman complete spar replacement. Big time learning event.

Hi Bob
As I understand it, the first crack I showed is a “no go” because its location is adjacent to the spar doubler and wing attachment point.
In the second set of wings the crack is unrepairable because its location is at the wing attachment point of the main spar. With the time and effort I think this is a spar replacement project.
Below is part of the passage of AC43-13:
1-40. SPLICING OF SPARS. Unless otherwise specified by the manufacturer, a spar may be spliced at any point except under the wing attachment fittings, landing gear fittings, engine mount fittings, or lift and interplane strut fittings.
Also attached here I included Service letter 406 illustration which shows likely areas of cracking of the spar.

 

[Slipinonfinal]

anejohn,

Thank you for those pics here, they are extremely valuable to have. On a separate note in the pics it also shows the rib inboard of the spar crack, cracked through a screw hole. With the shear size of that crack in the wooden spar it makes one have to ask how far did this wing flex and not break? Scary thought.

In really examining these pics it initially makes me wonder just exactly how hard of a Landing was this? Given the right wing spar crack and the angle of the crack from top going down and outboard it makes me wonder if this was a wing flex going down from the whole airframe hitting the runway really really hard and perhaps the landing gear bolts on the left side as not so much the issue but the weak link in the system when the aircraft really crashed into the runway. To me it looks like that right wing flexed really hard in a down ward direction.

That is defiantly one for the books and prob should get archived here on the site. Thanks again. Really good stuff about a really horrible issue.

On a seperate note the wing root cracks are all too common on the spars of these aircraft and not at all unusual and I wouldn't blame the Nevada climate for those even thou pretty big in size and yes def not airworthy. There is a repair for the really small cracks at the wing spar attach fitting area at the fuselage. I tell folks that it is a legal repair but at that point you have to wonder how much are you not seeing on these wings and how munch safety and security will you have from that point.

As a general subject these spars are going to get to be a larger area of concern as time goes on. Cared for correctly they stand the test of time and Rainbow Ron sure has the data to show that. Any structure has limits including the Millman and American Champion metal spars and specifically the attach fittings. Radical aerobatics in these aircraft and under reported ground loops with resulting wing tip strikes will continue to be a concern's as well as what does not make it in the log books. Pre-buy inspections of these wings can not be emphasized enough.

Finally starting to thaw here a bit in Alaska, been a tough winter. Cheers, Brian

 

[anejohn]

Great feed back Slipinonfinal

The cause of the damage can only be know by the guy who put it there and even if they are reading this would never tell us. I’ve heard suggestions that it ground looped and caught the wing tip. It flipped over on its back or it was involved in an inverted negative G maneuver. With a really hard landing I would have thought the spring gear would have absorbed most of the downward force but that too is a good assumption.

Swapping ideas about what happened is great but I need to focus on the current options available.

. Rebuild these wings using wood from Rainbow Flying Service

. Rebuild using Milman metal spars

. Buy ACA’s premade wings.

My choice is Rainbow’s wooden spar because that’s what these airplanes were certified with, it’s less expensive and I’m a cheap pilot. ($1000 per spare)

.Milman metal spars for its strength and durability. It also increases the gross take off weight by 100 lbs I’m told and that would increase the MGTW from 1650 to 1750 lbs. This is certainly a plus considering we can stop bumping gas to stay with in GW takeoff limits when flying with two seats occupied. ($7000 per spar)

. Or bite the bullet and buy a complete set of wings ready for paint from ACA and forget the rebuild hassle altogether. The discussion about ridge nails vs smooth nail extraction and damaging ribs is concerning. ($29,000 )

While we are on the topic, what would be the round about price of a set of wings already rebuilt using Ron’s wooden spars vs Milman metal spars?

Thanks again for your responses!

 

[Slipinonfinal]

anejohn,

While I do like the wooden spars from Ron as they are per se new and improved over the original, the down side is in the resale of these aircraft with wood spars. I would argue as some one whom deals with these aircraft all the time that they do best with wood and especially Ron's spars, they are worth the money, but the bottom line is it doesn't matter what you know about these aircraft because if you're going to sell it down the road the millman metal spars will pay for them selfs if they are done well because pilots want metal spars and elimination of the wood spar A.D. If you are not a A&P you will also have to build into the annual the time (cost) to inspect the new wood spars every year.
My vote is Millman metal spars by a A&P who has already done at least one set of wings and can show you a pic of his work.
The company wings are of course great and as far as the gross weight increase with the ACA wings it's not 100 lbs increase because the wings are already something like 50 to 60 pounds heaver, so you are actually only getting 40 to 50 lbs gross weight increase but they sure like to tell you 100 lbs.


Cost is another tough question because it depends a lot on several choices. One which fabric and paint covering process will you use? Two what A&P is going to do it? A cheap A&P or an experienced one. Generally speaking you get what you pay for. I would start by asking them how many hours is it to do a wing with the process they use? Expect there to be more damage in the wings not already found.

Good Luck, Cheers, Brian

 

[Slipinonfinal]

anejohn,

The ACA wings give you roughly another 40 to 50 lbs useful load. Which is probably a better way to say what I wrote.

Cheers, Brian

 

[Bartman]

There is no gross weight increase with the Milman spars and whether or not the plane is heavier or lighter after they're installed depends on the installer. My 7ECA lost weight when the Milman spar conversion was done.

As @Slipinonfinal said, the metal spars make for a better resale experience but they also eliminate the need for all of the extra inspection plates. I think wood spars are fine so long as you document them very well before putting them back under fabric. It's the resale a few owners down the road where it gets harder and harder to prove they're still fine.

If someone is paying a shop to do the Milman installation, it's probably better to just go the factory route. If it's being done in a garage by an owner with IA support, the Milman kit is pretty darn nice.

The ultimate kicker might be though that the factory wings have spades available, not so for wood/milman spar wings.