Maintenance Matters

AC aluminum: characteristics and simple repairs.


The aluminum parts of the most popular metal construction kit airplanes seldom require smoothing or detailing prior to fitting before assembly. So when metal is bent from a bird strike, dragging a wingtip during a botched crosswind landing, or from a violent ground loop, most owners face a decision: Repair the damage or replace the damaged parts? Some kit manufacturers encourage the owner to send digital photos of the damaged and adjacent parts to the company’s customer service department. Then the manufacturer will often send the owner a repair scheme created by the company’s engineering department. I called customer service at Van’s, Zenith and Sonex Aircraft and posed a theoretical question: What steps should one of your customers take if a bird hits and dents the leading edge of a wing?

Commonly used pneumatic sheet-metal tools include (top to bottom) a 90° drill motor, a straight motor with a 3⁄8-inch chuck and a high-speed die grinder. Sanding drums and a 3-inch, quick-change sanding disk holder are used in both the straight drill motor and the die grinder. Two Unibits are bottom center. The twist drill below the 90° motor has a drill stop installed.

Van’s and 2024-T3 Alclad

Gus Funnell at Van’s customer service said that the company doesn’t have the resources to provide individual repair schemes for each accident or incident. In response to the bird strike scenario, he said the owner could either patch the damage in accordance with the data and information in FAA Advisory Circular 43.13.1B or remove the damaged parts and install new parts purchased from the factory. This answer seems to be a non-answer, but it’s not. Richard VanGrunsven, the founder of Van’s Aircraft, elected from the beginning to use time-tested traditional aircraft sheet-metal construction methods in his designs. The exception is Van’s LSA aircraft, the RV-12, which uses blind (high quality “pop” type) rivets instead of solid rivets in the airframe fabrication. Traditional techniques mean that Van’s builders have already practiced the same techniques and used the same tools they will need to make sheet-metal repairs. Van’s builders learn to dimple skins before driving or squeezing countersunk solid rivets. They also learn to use 2X and 3X rivet guns and specially shaped bucking bars or rivet squeezers when driving individual solid brazier head or countersunk rivets. Builders also become skilled in driving out a poorly driven rivet without enlarging the rivet hole. They have a feel for maintaining skin quality.

According to Nick Bonacci, author of Aircraft Sheet Metal, the use of aluminum alloys in aircraft construction dates back to 1925. Following in this long tradition, 2024-T3 alclad aluminum alloy is the metal of choice for Van’s Aircraft airframe construction, though small amounts of 6061-T6 are also used. The 2024-T3 alclad alloy is almost exclusively the alloy of choice in most FAA-certified light aircraft aluminum airplane designs as well.

The name alclad indicates that each side the alloy sheet (2024-T3 alloying elements include copper, silicon and manganese) has a very thin layer of pure aluminum (5% thickness per side) pressed onto each side of the aluminum alloy after the sheet has been rolled and drawn into the final shape. Pure aluminum is very resistant to corrosion, much more so than the uncoated 2024 alloy. Scratches and tool marks that break through the alclad layer increase the possibility of skin corrosion.

A standard sheet of aircraft aluminum is 4-feet wide by 8-feet long; it can also be ordered in lengths up to 12 feet from standard aircraft supply houses such as Aircraft Spruce & Specialty, Airparts, Inc. and Wicks Aircraft. Aircraft sheet and roll aluminum is sold in standard thicknesses. The most common are 0.016, 0.020, 0.025, 0.032 and 0.040 inch. I found a quote for one 4×8 sheet of 2024-T3 alclad 0.020 for $111. The same size sheet of 6061-T6 (non clad) was $66. There are additional charges for crating and shipping.

Riveting tools shown in the lower half of this photo include a 3X rivet gun, two bucking bars and a variety of straight and offset flush and brazier head rivet sets.

6061-T6 Aluminum

6061-T6 aluminum alloy is the aluminum alloy used in both Sonex and Zenith airplanes. The 6061 alloy contains copper, silicon, magnesium and chromium, and is more corrosion resistant than 2024, so alclad 6061 is rarely used. It is also easier to polish to a shiny finish, it’s about 30% cheaper than 2024, and it can be bent in a smaller radius without cracking. The big drawback is that 6061 has approximately 35% less tensile strength than 2024 for the same thickness. The 6061 is still a good aircraft aluminum, and in fact it is in some ways better than 2024. But for the purposes of this article, the most important point is the repairman’s responsibility in determining the correct alloy for any repair that’s undertaken. A phone call to the kit manufacturer is the easiest way to get this information.

Mark Schaible at Sonex Aircraft said owners can send pictures of the damage to the company’s customer service department for a repair scheme. Sonex uses 0.025 and 0.032 6061-T6 exclusively in its airplanes. When asked if Sonex compromised strength by using 6061, Shaible reminded me that all Sonex airplanes are strong enough for aerobatic maneuvers (at reduced maximum takeoff weights), meaning the structure is built to the +6/-3 G standards the FAA requires for the aerobatics certification.

Schaible also said that Sonex aircraft are assembled using high-quality (Cherry N-type) blind rivets that don’t require the use of rivet guns or bucking bars. (Though a pneumatic rivet puller is a builder’s best friend.)

When asked about a repair scheme for wing leading-edge damage, Schaible said repair schemes vary and are entirely dependent on the extent of the damage. He suggested that a small dent might require nothing more than cutting an additional inspection-type hole in the lower wingskin to get access to tap the dent back out, followed by smoothing and reshaping the leading edge profile using a small amount of dent filler. This type of repair is perfectly satisfactory in most instances but would be readily visible unless the wing leading edge was painted after the repair.

One of the drawbacks of aircraft aluminum is that once it’s stretched, it can’t be shrunk back to its original size. Dents and bends can be straightened, but load-bearing parts must be reinforced properly or replaced with undamaged new parts to return the structure to design strength.

Sebastien Heintz at Zenith said that the company uses 6061-T6 exclusively. Zenith Aircraft also uses blind (Cherry N-type) rivets. “There aren’t any special tools or skills required,” said Heintz. “Anything requiring special tools such as driving solid bucked rivets is done at the factory.”

Heintz echoed Schaible’s advice when the bird strike question was posed. “We try to supply a repair scheme, but R and R [removal and replacement] of the damaged parts provides the best and most satisfactory solution.”

The micro-stop countersink cage is adjustable. Once set, this feature cuts metal to the desired depth when using flush (countersunk) rivets. Piloted cutters in common sizes are interchangeable.

Cutting Metal

The most commonly used aluminum cutting tools are the floor-mounted shear and the bandsaw for cutting sheets to rough sizes. Hand tools such as offset left, offset right and straight snips, nibblers and files are used to cut metal to the final shape needed. (I prefer the snips made by Irwin.) Builders doing a lot of sheet-metal work should purchase air- or electric-powered hand shears and nibblers (a nibbler is a reciprocating punch that knocks out a line of overlapping small holes from a metal sheet).

Other often-used tools include Unibit step-drills. I have two: one for holes from 1⁄8-inch diameter up to ¾-inch, in 1⁄16-inch steps, and one for larger holes up to ¾-inch diameter, in 1⁄8-inch steps. I find a mini-sized, compressed-air-powered drill motor and a mini-sized, high-speed die grinder to be indispensable. A selection of hand files, small barrel-shaped sanding drums and small sanding disks are helpful for fine material removal and finishing edges, and for removing tool marks and stress risers.

One of the keys to success in working aluminum is eliminating sharp edges and tool marks. For this purpose, fine-cut files, sanding blocks and disks are the tools of choice.

Unibit step drills are handy and will quickly cut a variety of different hole sizes in aluminum. They are also excellent for cutting holes in plexiglass.

Making Repairs

The first step in making repairs is to determine the extent of the damage and whether it’s negligible, repairable or severe enough to require parts replacement. If it’s repairable, straighten and reinforce the damaged part using standard repair schemes.

Airframe Volume 1: Structures published by Aviation Supplies & Academics (ASA) in its Aviation Maintenance Technician Series, includes a well-illustrated sheet-metal reference and repair section as well as step-by-step instructions on the three most common types of repairs. It covers the information in AC43.13-1B.

A good sheet-metal worker is a skilled technician with the critical eye of an artist. Heintz encapsulated the glory and precision needed when working with aluminum when he said, “I like sheet metal because if it looks right, it usually is. You can see every rivet on a sheet-metal airplane, which means that errors and inconsistencies are very apparent.” Repairs on sheet-metal structures are usually visible. Given this fact along with the investment in tools and time required to produce a professional-looking repair, many owners opt to replace damaged parts, thereby returning their airplane to its original condition.

In the next installment of “Maintenance Matters,” we will look further into sheet aluminum repairs.


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