This piece originally ran in Lisa’s Airworthy column in the April 2022 issue of EAA Sport Aviation magazine.

“What did you think was going to happen?” Lee said.

“You sound like Jay Leno.”

“Yes. When people become irrationally exuberant, suddenly they think they can do something having no forethought about the likely outcome,” Lee said, looking at the long crack leaving the hole in the corner of the new Plexiglas side window.

“I thought it was like drilling any other thing,” Alan said. “I guess it’s not.”

“Plexiglas is more sensitive than Lexan, but all plastics can crack. What happened?”

“I drilled a hole for the fastener,” Alan said. “I didn’t see the small crack there at the corner. Then, when I tightened up the bolt, boom, the crack propagated.”

“Where’s your copy of AC-43?”

“Where’s my what?”

“AC 43.13 — Acceptable Methods, Techniques, and Practices – Aircraft Inspection and Repair. It has an entire section on plastics.”

“Sounds like what I need,” Alan said as he looked at Lee with a sheepish grin.

“I have an extra copy from a class,” Lee said. “I’ll bring it over. You shouldn’t be repairing anything on an airplane without it.”

* *

Repair work. How hard could it be? There’s nothing wrong with trying to repair something by yourself. But when you’re repairing an airplane, some common sense needs to form the basis of the experiment. As I’ve chirped about over and over in this column, we need to set ego aside and consider that we may not know everything.

I love fixing things and coming up with what I think are clever repairs and solutions until they fall apart or stop working. For shower doors or car antennas it’s one thing; for airplanes it does take some extra care.

Over time I have found the same mistakes popping up in the materials we work with on airplanes. Whether you are building, restoring, maintaining, or repairing, here are the top issues and what to do about them.

Sheet Metal

Aluminum is a terrific light and strong material well suited for aircraft. Possessing a high strength-to-weight ratio, it’s also flexible and malleable. Aluminum is corrosion resistant and easy to work with.

Although aluminum is tough, if it’s damaged by overbending or deep scratching (creating cracks and stress concentrations), the potential for structural weaknesses goes up. Accumulated mistakes in assembly or repairs can add up to serious problems later.

The biggest problem I see is poor riveting during a kit build. Poor riveting usually happens when the builder didn’t get training or did not practice before starting. They don’t realize that some of the work can’t be used in the final assembly.

I visited a project where a builder had just bought a partially completed RV-4. Only the fuselage and a few panels had been assembled and riveted. When I arrived, the builder excitedly showed me the kit.

“It’s been sitting for a while,” Jeff said. “I’m not sure what I have.”

I took a look. My heart sank. The riveting work was dreadful. Clearly assembled by someone with no sheet metal experience, the holes were over-large and the rivets over-driven. Shreds of metal poked out of nondeburred hole edges, and the gun had slipped on most of the rivet heads. Many holes were too close to the edge, and extra holes were drilled next to them to make up for it.

I shook my head in disappointment.

“Jeff, you just bought this project sight unseen?”

“Well, yeah,” Jeff said. “I figured the guy barely started on it. It must be a good deal.”

“Okay, I have bad news,” I said. “We need to order new sheet metal and start over.”

For repairs, ask: “Is the problem confined to one or two small areas?” The assembly may keep its structural functionality, but you need to make the best repair you can with the least amount of cosmetic damage. This can include sizing up on the rivets if you have enough edge clearance.

The key to sheet metal work is getting the knowledge and skills before you begin repairs or building. As simple as this sounds, I’ve encountered enough situations where the mechanic or builder decided that the tinkering or the building would give them the experience they needed.

Always consult the factory or the manufacturer for repair procedures first in addition to reviewing AC-43.13.

If you’re about to begin a sheet metal kit, or you need sheet metal repair skills, attend an EAA SportAir Workshop. This training is fun, and I guarantee it will remove 80 percent of the heartache you will suffer if you’ve never worked with sheet metal and try to make it up as you go along.

Learning how to identify, chamfer, deburr, and drive rivets will save you hours of rework.

Fabric

Fabric is another amazing material. Most people don’t realize that the Vought F4U Corsair, a Navy fighter (1944) with a 425 mph cruise speed, had fabric on its wings from the spar back, and on its elevators and rudder. Tough stuff!

The biggest repair problems with fabric come from mixing system materials, not following directions for shrinking the Dacron, not understanding what attachments to the structure are needed, and spacing.

There are many systems for fabric covering. The two most well-known are Poly Fiber and Ceconite. They start with the same polyester fabric, but the chemicals for the coatings are different. Each system has a supplemental type certificate (STC) and must be used as a system rather than mixing and matching them.

Shrinking the fabric should happen in two specific stages with a calibrated iron. The “heat gun” method will ruin the fabric, and it’s not repairable once that happens. Let me repeat that. If fabric has been shrunk with a heat gun in a Poly Fiber or Ceconite system, it must be replaced with new fabric and installed correctly.

There are some covering systems, specifically Oratex, that do use a heat gun, set to 480 degrees Fahrenheit. It’s important to identify what you’re working with. With certified aircraft, you’ll be dealing with an STC’d covering system.

Repairs to rips or holes in a fabric-covered aircraft are easy. Follow the directions from the manufacturer and for the specific system you are using. If you’re about to begin a tube and fabric aircraft kit and haven’t worked with the system, attend a workshop first.

Both Poly Fiber and Ceconite systems come with excellent manuals. They describe how to best attach the fabric and the spacing (based on never exceed speed) for them.

Composites

Composite material benefits include a high strength-to-weight ratio, high impact resistance, thermal stability, and resistance to corrosion. It wasn’t until the 1960s that fiberglass became popular in small aircraft. Now, even the Airbus A350 is built of 53 percent carbon fiber reinforced polymer. And the Boeing 787 is 50 percent composite.

“Composite” as a descriptor means that several different materials are combined to gain greater functionality, so this is a perfect adjective for what designers came up with. Today’s composite aircraft use a sandwich process with fiberglass skins on the outside and foam multicellular honeycomb, or other exotic materials, as a core.

But for repairs, this can be a problem. Years ago, a friend donated their composite low-wing airplane to a high school shop class. The airplane had spent eight months in a field with the canopy cracked open. The class set out to disassemble, repair, and reassemble the aircraft and fly it.

A year later they were ready to flight test the airplane. The designated airworthiness representative arrived and performed an inspection. He was shocked to discover that the class had power-sanded through the fuselage’s outer fiberglass layer and filled the foam core areas with Bondo. The airplane never flew again.

The top issues I find with composite repairs are adding too much resin (makes it heavy), not following the manufacturer’s system (type of epoxy and cloth), and not realizing there are specific instructions for scarfing and layering. Airplanes are not boats.

Wood

The Wright brothers used spruce and ash with fabric lashed together with twine for their flights in 1903. Wood as a building material is durable and easy to repair.

The biggest problems I see with wood on aircraft repairs are that hidden damage is not identified. And when it is found, the repairs are not done according to the standards in AC-43.13. Is this sounding familiar?

In older certified aircraft, such as the Stearman, owners decide to re-cover the airplane without checking the condition of the metal and wood structures underneath. Don’t assume that because the airplane has been flying that everything is okay.

If you dig down and find wood needing repair, consult the manufacturer’s repair information if available and AC-43-13. The type of glue, clamping pressures, and what type of splicing techniques you use are critical to the strength of the repair.

Some Surprising Tools

Mechanics are always looking for reasons to buy tools. When you are inspecting and repairing, there are a few tools you should keep handy that won’t break the bank. One is a tap tester, and the other is a high intensity flashlight.

Typically, we think of a tap tester in the context of composite inspections. While you can use a coin to tap test, there is a metal version of the tester called a Tap Hammer. It is machined of 1/2-inch diameter aluminum and features a rounded end on one side and a tapered blunt end on the other side. This tool will put you out $20, but you’ll suddenly find that it works on lots of other material inspections besides composites. You can use it on frame chromoly to identify rusted, thinning tubing. You can also use it to check for delamination in other materials, such as wood. The key to using the tap tool is experience. Educate yourself by experimenting with lots of different material conditions so you’ll know what you are hearing and feeling.

The flashlight can be used from the inside of frames or the inside of parts to see through layers.

Once again, you need to get experience with many conditions before you know what you are seeing.

For a little more money, invest in a borescope. This tool will round out your ability to see, hear, and feel material condition in nearly all materials.

Are You Up to It?

There are a few things to keep in mind as you consider performing repairs on your airplane. The first one is to determine how much work you can legally do without a repairman certificate. This is straightforward if you’re repairing a certificated aircraft, since the FAA has made the items pretty clear.

It is less clear for homebuilts and light-sport aircraft. See a mesh chart of who can do what in my column in the September 2021 issue.

In either case, make sure your skills and knowledge are up to the repair tasks. If you are uncomfortable, get training and help from certificated mechanics. Be honest with yourself about how much you can do on your own. One way to get experience is to talk an A&P mechanic into taking you under their wing to oversee inspections and repairs.

One thing that A&Ps and A&P/IAs learn early on is that the resources available to us are very specific and shorten the learning curve. From advisory circulars to AD notes and methods of finding information, we can use the same techniques to learn how to work on our own aircraft.

Understanding the quirks of materials is half the battle for repairs. The other half is knowing where to go for knowledge.

Lisa Turner, EAA Lifetime 509911, is a manufacturing engineer, A&P, EAA technical counselor and flight advisor, and former DAR. She built and flew a Pulsar XP and Kolb Mark III, and is researching her next homebuilt project. Lisa’s third book, Dream Take Flight, details her Pulsar flying adventures and life lessons. Write Lisa at Lisa@DreamTakeFlight.com and learn more at DreamTakeFlight.com.