TECH TIP: Restorations – Metal Parts and Pieces
Over the past few months we have been taking a look at the process of restoring an antique or classic airplane. Wood and metal are two primary types of materials used on many older airplanes. We have talked about inspecting and repairing wooden surfaces. Now let’s take a look at inspecting and repairing metal parts.
The fuselage structure on a large number of older aircraft was constructed using steel tubing. Landing gear assemblies, wing struts, and tail surfaces are usually made using steel. In addition, aluminum has been used on most fairings and cowlings. Steel and aluminum are subject to deterioration from rust and corrosion. It is essential that these surfaces be properly inspected and repaired during the restoration process.
Steel is particularly susceptible to rust. When inspecting your project, particular attention should be given to the rear of the fuselage assembly. Moisture will often accumulate in this area (particularly in a tailwheel airplane) creating rust over a period of time. The weakened tubing may not be easily detected. You should spend time looking for evidence of rust in the form of bubbles in paint, bare areas, and rusted areas. If in doubt, take a small knife or a pointed tool and poke around on the tubing. If a problem exists, you will be able to push a sharp object through the tubing quite easily. If this is the case that section of tubing will need to be replaced or repaired. If you are not a welder, then hire someone to come and make the repair. Use FAA Advisory Circular 43-13 as a guide for steel tubing repairs.
All metal tubing should be thoroughly inspected for damage and for rust. Often you will be able to simply remove the rust without having to repair the tubing. I would recommend when restoring an airplane that you consider removing all primer and paint from steel surfaces. This may be accomplished using a bead blaster or a sand blaster. Care must be taken to ensure that the proper amount of air pressure is used. Too much pressure can cause pitting of the metal or additional damage to steel surfaces. As you might imagine, removal of old primer can be a big job on an entire fuselage or other large part of the airplane.
If the tubing appears to be in good shape blasting off all of the primer may not be necessary. In any event, you will want to inspect the surfaces completely and then spray a new coat of primer over the tubing. This will protect the surface from rust in addition to providing a pleasing appearance. If you elect to remove all primer from the tubing be sure that you reprime the structure immediately. Rust will form on a bare steel surface within a few hours. Use a two-part epoxy primer on the tubing. Clean the tubing just prior to priming using a metal surface cleaner such as Poly-Fiber’s C-2200. This will remove oil, wax, and other contaminants from the surface. If you are priming an entire fuselage assembly, check for bare (unprimed) areas several times. It is very easy to miss sections of tubing with the primer. You may also want to go back over certain areas with a color topcoat. This is particularly true if the tubing will be visible to the pilot and passengers on the inside of a fuselage, as an example.
Another reason to use an epoxy primer is to prevent the fabric covering chemicals from removing the primer. Fabric covering chemicals contain solvents such as MEK (methyl ethyl ketone). These solvents will act as a paint remover on primers other than epoxy. After the proper curing time, these chemicals do not affect epoxy primers. That means when you apply the fabric covering over the metal surfaces you do not have to worry about subsequent corrosion problems caused by removal of the primer from the action of the solvent.
Certainly you will want to remove all metal fittings on the wings, center section, fuselage, etc. I recommend using a bead blaster (or other means) to remove and clean all fittings. After removal of primers and paint, the fittings need to be checked for cracks, damage, etc. After cleaning the parts, you should use a dye penetrant to inspect for cracks on the part itself. Ensure no cracks or suspect areas are present on any fitting. Removal of fittings can be a slow process but I feel it is necessary during the restoration procedure. It will ensure the primary fittings used to attach wings, control surfaces, etc. are safe to use again. You will often find fittings that are in need of being replaced. If replacement of a part is needed, you should be able to find a source from which to purchase the part. If not, you may be required to actually build a new part. We will discuss the legality of making parts for your airplane in a later article.
With steel pieces it is critical that they be primed soon after being blasted and inspected. Once the structure or piece has been stripped and the rust eliminated, the metal must be protected within 1-2 hours. Be sure to have the primer and spray equipment ready before you begin blasting or cleaning. Remember, rust will begin to form on a bare steel surface within a very short period of time. Again, use an epoxy primer to prime the part. Don’t forget to tag each part before you set it aside so that you will remember its proper place.
Aluminum pieces should be stripped and inspected for corrosion. Stripping off the old paint and primer may be a difficult and time-consuming task. It is sometimes advisable to take them to a paint shop where they use a commercial stripper to remove paint. After removing all paint and primer inspect it for damage and corrosion. If corrosion is present it will have to be removed and treated. Use fine sandpaper, Scotchbrite pads, or aluminum wool to remove the corrosion. Never use steel wool or a steel brush. After the corrosion is removed the old aluminum should be acid etched. This is simply a process of washing the aluminum with a product such as Poly Fiber’s E-2310 Acid Etch diluted with water. Acid etching removes oil and light corrosion while roughening the surface to provide a firm primer bond. The part is then thoroughly rinsed. Next wash the surface with E-2300 Conversion Coating that inhibits corrosion and further enhances primer adhesion. After this step the part is rinsed and allowed to completely dry before priming.
If you are replacing certain parts with new aluminum, or making a repair using new aluminum, then it must be properly treated before priming. Paint must be able to “grip” or adhere to the surface onto which it is applied. Most new aluminum surfaces have a layer of pure aluminum on the surface called alclad that protects the metal from corrosion. It is very smooth and not favorable to paint adhesion. Therefore, the surface must be adequately prepared by cleaning and slightly roughening to guarantee primer adhesion. This is accomplished by using a conversion coating such as alodine. This chemical process creates a ceramic layer over the aluminum that coats the surface and provides tooth adhesion.
After the aluminum (new or old) has been properly cleaned and treated, it is then primed. Again use a two-part epoxy primer. An epoxy primer will ensure corrosion protection and also provide a bonding surface for most topcoat paints. Very often, polyurethane topcoats will lift or wrinkle primers other than epoxies much as a paint stripper will do. A primer is necessary to provide a bond between the metal and the final topcoat paint. The primer coat should be applied according to the manufacturers directions. Usually, two light coats will be applied. Heavy coats should be avoided.
Certainly as you dismantle your airplane you will be removing bolts, nuts, etc. What about these pieces? Should they be replaced? Personally when I rebuild an airplane I go back with all new hardware. To me it is a safety issue in addition to a cosmetic issue. If I elect to use any old hardware it will not be in a critical area. Wing attach bolts, gear attach bolts, etc. should have new hardware.
The quality of our workmanship in restoring an airplane is very important. We all take the needed time and spend the necessary money to ensure a high quality airplane. We want it to not only look attractive, but also to be safe. The hardware used to assemble your airplane should be nothing but the best. Why take the time to completely restore a perfect wing only to attach it to the fuselage with used hardware? It makes no sense. To quote the Airframe and Powerplant Mechanics General Handbook. “The importance of aircraft hardware is often overlooked because of its small size; however, the safe and efficient operation of any aircraft is greatly dependent upon the correct selection and use of aircraft hardware.”
During the restoration process it is absolutely imperative that you use nothing but aircraft grade hardware. Commercial grade hardware found in hardware or automotive stores should not be considered for use on your airplane. Why? Let’s look at bolts as an example. Common steel bolts purchased from a hardware store are made of low carbon steel that has a low tensile strength usually in the neighborhood of 50,000 to 60,000 psi. They also bend easily and have little corrosion protection. In contrast, aircraft bolts are made from corrosion resistant steel and are heat treated to strength in excess of 125,000 psi. The same comparison applies to most hardware items. So, use only aircraft quality hardware on your airplane. Save the other hardware for your tractor.
Where do you find information concerning selection and use of aircraft hardware? FAA Advisory Circular 43-13-1B is an excellent reference source. The Airframe Mechanics General Handbook also has a very good section on the selection and use of hardware. These two books are considered the primary authority on the proper use of hardware. In addition, I would recommend two other small reference books: the Standard Aircraft Handbook and the Aviation Mechanic Handbook. Both of these provide a good reference source. The Aircraft Spruce & Specialty catalog also contains good reference material on hardware. If you have any doubts about the quality of the aircraft hardware you are purchasing, request a copy of the manufacturer’s specifications. These specifications along with a specific manufacturer’s lot number should be available from the supplier.
Proper inspection of all metal parts during the restoration process is essential. Replacement may be necessary in some instances. Take the time to thoroughly inspect fittings in particular. Use only aircraft quality hardware when you reassemble the airplane.