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November 11, 2009
February 15 What Lies Beneath Getting into your Cardinal
No sledgehammers were involved in the refurbishment of your Cardinal.
Okay, maybe a little persuasion was required in spots, to remove hardware large and small that hadn't budged in decades. In fact, there are several useful techniques involved in taking an airplane apart - techniques that are easy to learn, but can take years to master. And the A&Ps working on your airplane are masters! Among the key disassembly processes: unthreading a maze of wires and identifying and securing them, and, my personal favorite, drilling out rivets.
Drilling out rivets? Why did we need to drill out rivets? Actually, in a number of places we used this technique to get to places that hadn't been visited in a while. Although it's relatively easy in many airplanes to drop a headliner and view the spar, or pull control surfaces and get a better view into the wings and stabilizers, it required the removal of several hundred rivets to get to the places we really wanted to check out - simply because they aren't easy to get to, and thus likely hadn't seen daylight for decades.
This included pulling up the seat tracks and removing the floor boards underneath, removing the vertical stabilizer from the empennage, and removing skins from a couple of control surfaces that needed attention.
To drill out a rivet, you essentially just take a drill to the head of the rivet until you get down to the point where the head meets the rivet shank or break line. This allows you to pop off the top of the rivet, at which point you can punch out the shank of the rivet - or continue drilling if the shank refuses to budge.
The technique is simple: Drill down into the rivet head as cleanly as you can, as straight as you can, without expanding the size of the original hole or otherwise damaging the aluminum nearby. Good lighting is essential, as is the right-size drill bit and a steady hand. Now is not the time for another Starbucks run.
A professional can do it in one nearly seamless operation - almost a single move - by drilling the appropriate depth into the rivet, stopping the rotation of the drill bit, then quickly tilting the drill and popping off the head.
What's in there? Brian Hubbard and Anja Cook, two of Air Wrench's top-notch technicians, worked intensively over a couple of afternoons to bring up the seat tracks and floorboards so that we could inspect underneath the rails themselves, and underneath the floor.
I was on site when they pulled up the last of the floor, and I could see for myself exactly what they found. I expected gum wrappers, a couple of ancient pools of dried up Coca-Cola, maybe a missing earring or two (I once lost my pilot certificate for a week under the seat tracks of one of the stalwart 172s on the flight school flight line - so you never know what you'll find). There wasn't much, honestly. Just some dirt and a loose screw or two. Oh, and Clyde Cessna's missing logbook, which I'm selling on eBay.
I'm kidding. Do you think I'd sell that?
And as for corrosion? We found a couple of minor blooms of superficial surface corrosion along the rear portions of the floorboards where they attach to the longerons in the belly of the airplane. There were also localized halos of light corrosion under the rivet heads where the seat tracks mated to the floor boards. Whether we decide in the end to polish out these areas or replace the aluminum, the floor will be clean.
We found areas where the seat tracks had worn and warped slightly with time - nothing to be alarmed about, but an opportunity for us to take those tracks back to square one. The winner of this airplane is going to have brand new tracks, and while they'll still need periodic inspection, they'll be in a much better place, functionwise. Buried in the second incredible installment of our parts shipments from Cessna, we had four new seat tracks, ready to go. It has been one of the coolest aspects of this project so far - that Cessna keeps such an exhaustive inventory of parts, and for an airplane that left its factory 30 years ago.
By pulling the floorboards, we also had a clear view of the control cables to the rudder and elevator control surfaces, as well as the trim cables, and we checked them for wear, slop, or abrasion. We also took a good look at another important circulatory system - the fuel lines and sumps. When we did (and had a good opportunity to clear out the system with the wings off and the tanks drained), we determined that they were in excellent condition to be cleaned up and readied for many more years of service. We did however decide to replace the fuel gascolator assembly, as we noticed pitting in the bowl, a common finding.
We found almost no corrosion (or anything else of concern) when we removed the vertical stabilizer, stabilator, and tail cone - good news! But this portion of the disassembly served another purpose: While the pieces were apart, our dynamite paint team at Advanced Aircraft Refinishers could strip, prime, and paint the individual pieces separately, an unusual and highly detailed process we'll cover in an update down the road.
And that brings up a critical thing to keep in mind about the project as a whole - no single event happens in a vacuum. On the contrary! While one part of the team is busy working hard on the airframe, other critical events take place in concert. One of paramount importance came to pass on January 30, the day that Lycoming delivered to us the heart of our Cardinal - the freshly factory overhauled O-360. So many of us fly behind O-360s and never get a chance to see one shiny and new - it's an impressive sight. And the folks at Lycoming pulled many strings to get us the engine in plenty of time to start the prep work necessary to hang it at just the right time. You'll read a lot more about the firewall-forward side of the project, so stay tuned.
Taking the flap In previous projects, we discovered (or rather, uncovered) problems when the airplane went into the paint shop, by virtue of the stripping process. In fact, this was a driving factor in constructing the refurbishment process in the way we did for this year's project. Could we strip the airplane of paint early enough in the timeline to address airframe issues - and be in a good position to remediate those issues on the spot?
With our paint shop right on the same airport as our airframe disassembly crew - and ready to strip (paint) at a moment's notice - we could do both.
Tools from the trenches 5 steps to airframe refurbishment
1. Know your goals before the project starts. Don't go into an airframe just to see what you'll find. You're going to find some issues, no matter the airplane's age. Aside from safety of flight items, the scope of your project depends on your focus. 2. Find the right people to do the job. If an annual inspection is akin to your yearly physical, an airframe disassembly is major surgery. Interview potential A&Ps not only for their experience, but also for their enthusiasm and time available for such a restoration. The "guru" for your airframe may not be the best choice if the shop is full of unfinished projects. 3. Create an inventory process. You are now an aircraft manufacturer of one, and you need to know that each piece is accounted for - so it can go back on the airplane, at the very least. Obtain a parts catalog of your own, as well as a service manual, so you can stay informed throughout the process. You can computerize your inventory, or you can use masking tape, markers, and a three-ring binder. Sometimes simple is best so that there are no barriers to recording everything. 4. Assess the parts. When a piece comes off the airplane, it should be assessed for clean-up, repair, or replacement. Depending on the serviceability of the part, you may have to choose from all three options - or a replacement will be required. Though unlikely, parts may also be damaged in the disassembly process. Prepare your budget with room for these unexpected replacements and repairs. 5. Make a list, and check it twice. Your replacement parts list should include the name and location of the item, the aircraft manufacturer's part number, a reference to the appropriate figure in the parts catalog, and the quantity of items needed. Once you have your list, there are several sources for new or refurbished parts. These include the aircraft manufacturer or type certificate holder; major parts sources such as Univair, Chief Aircraft, Wag-Aero, and Aircraft Spruce & Specialty; specialty retailers for your model; aircraft salvage companies; your aircraft's type club; and, yes, eBay.
When we stripped the control surfaces, the team found hangar rash on one aileron, and on both flaps, that was unacceptable - and that meant reskinning all three. Larry Gobble looked upon this task with relish - he's one of our sheet-metal experts (and also a part of the Air Wrench crew) - so we happily gave the job to him.
After receiving beautiful new skins from Cessna, Gobble removed the old skins, which also gave us a chance to look inside the flaps, to see what might have been hiding for 30 years. What we found matched the general condition of the metal inside the wings - a dusting of surface corrosion on the upper skin (the result of condensing moisture over the years - dramatically more apparent on the upper skins than on the lower skins, which I found counterintuitive at first), and similar halos of corrosion around rivet heads.
One theory on this kind of minor corrosion is that in small amounts it forms and then serves as a protective layer against further corrosion. Of course, that's just a theory, and most aircraft owners prefer to remove corrosion when they can, or replace the metal that has begun to oxidize. And corrosion of this kind shouldn't be confused with more aggressive kinds of intergranular corrosion, such as that caused by the reaction between certain dissimilar metals or improper heat treat techniques.
Gobble replaced the old skins with the new ones, already primered green and ready for paint. Upon close inspection, the man's talent becomes clear. To understand how, you need to know the difference between two styles of rivets. Regular solid shank rivets require the use of a bucking bar placed against the shank end, while a pneumatic rivet gun is used on the head to drive the rivet into the bar, causing the rivet to compress and widen to fill the hole tightly and flatten the rivet tail. Structural blind fasteners (also known by the brand name Cherrymax) work without the use of the bucking bar, and are typically used in tight places where it's difficult to place the bar correctly - or at all. Solid shank rivets are preferred in most situations.
The original flap came from the Cessna factory in 1977 with all seams closed up using solid shank rivets but for three on each side. How they pulled this off can be viewed as a minor magical act, but that's their job. Gobble matched it: Out of 173 rivets on the flap, he used just three blind rivets on each side to finish it off. Gobble had to manufacture two bucking bars on site to get into the hard-to-reach places as the final skin was shot into place - duplicating Cessna's original feat. The man just makes me want to be a better riveter.
I'm working on it, but they still won't let me near your airplane with a rivet gun.
E-mail the author at firstname.lastname@example.org.
For more information on the myriad types of hardware, see " Airframe & Powerplant: Aircraft Quality Hardware," November 2006 Pilot.
Freeman's can provide all the specialty types of rivets for virtually any kind of project - just as the folks there did for us for the 2007 Catch-A-Cardinal's control surfaces, along with a lengthy list of other certified hardware for the rest of the airplane. You can download the company's entire catalog via the Web site or tap into its wealth of knowledge by calling 800/635-5631.
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