TECH TIP – REPAIRING SHEET METAL
by Ron Alexander, VAA 27150
It is feasible for an aircraft owner or builder to effectively accomplish simple repairs on sheet metal. A simple repair is one that does not involve damage to structural members or damage that affects control surface balance, etc. This discussion will be limited to simple types of repair. Major sheet metal repairs should be accomplished with the advice and assistance of a professional or only after ensuring that you have adequate knowledge. What about legalities concerning doing sheet metal repair on your own airplane?
As the owner of a production plane you may legally accomplish a simple sheet metal repair if you work under the supervision of a licensed airframe mechanic. Repairing damage on a production airplane is covered under FAR Part 43. That regulation states that a licensed airframe mechanic or a FAA approved repair station must perform repairs. It also states that a person without a mechanic’s license may work on a production aircraft under the supervision of a certified airframe mechanic. That means if you own a production airplane and you do not have an airframe mechanic license, you are still allowed to work on your airplane under the supervision of a licensed mechanic. So, concerning simple sheet metal repairs on a production airplane, you are allowed to do the repair under the supervision of a licensed mechanic. The mechanic will tell you what to do, make sure you do it correctly, and then sign in the logbook that the repair has been completed. It is the direct responsibility of the certified mechanic to ensure the repair is done properly.
As with any type of repair, the first step is to assess the damage to the part. This will determine whether or not the repair needed will be major or minor. Listed are several examples of minor damage to a sheet metal structure:
- Missing or damaged rivets
- Scratch or small dent on a sheet metal surface
- A small crack
- Corroded sheet metal surface
Damage that exceeds the scope of the ones mentioned will most likely be considered as major and a professional mechanic should accomplish the repair if you have a production aircraft. Hidden damage may extend beyond the area that has apparent damage. Deformed rivets will often indicate damage to a sheet metal structure. If you have an indication of damage you should inspect the entire area well past any deformed rivets.
If you have damage involving a control surface the repair may be relatively simple but any change that occurs from the repair itself will have an impact on balance. In other words, rebalancing the surface to prevent any type of flutter occurring in flight should follow repairing a control surface. Damage to metal spars, formers, ribs, etc. will normally fall under the major damage category. We will confine the discussion in this article to the simple repairs outlined earlier.
Missing or Damaged Rivets
It is not uncommon to find rivets missing on an airframe. This could be the result of corrosion or it could be the result of some sort of underlying damage. Why the rivet is missing should first be determined. There is an obvious reason and you should investigate the area for further damage. Occasionally, a rivet will be damaged. Normally this will be apparent because the rivet will be slightly raised or slanted. If you can place a small feeler gauge (.020) under the head of the rivet there is a possibility that the rivet has been stretched. Replacing the rivet is a fairly easy task. Repairing any additional damage to the sheet metal structure itself possibly could be classed as a major repair.
A loose or missing rivet should be replaced with the exact same type of rivet originally used. You should be able to determine that from your plans if you are dealing with an amateur-built airplane or through the manufacturer or service manual if you are working with a production airplane. You will also find identification marks on the head of rivets. You will probably encounter mostly “AD rivets”. This type will have a dimple on the head of the rivet itself.
If the rivet is damaged, you will have to remove it. Removal of a rivet sounds simple enough but extreme care must be taken in the process to prevent further complications. You do not want to enlarge the rivet hole. Determining the proper size drill bit to use is essential.
During original installation, rivet holes are drilled slightly larger than the rivet diameter. When properly installed and driven, the rivet will expand to fill the hole. Drill sizes are based on numbers and letters. You will usually encounter a 3/32 or 1/8 inch diameter rivet. Drill sizes for each type of rivet are as follows:
Rivet Diameter Drill Size
The drill size of each of these is slightly larger than the rivet itself. This allows the rivet to go in place without forcing it thus scraping off any protective coating. During the process of driving or squeezing the rivet, the shank will swell up and completely fill the hole.
With this in mind, select the proper size drill bit to use in drilling out a defective rivet from the chart above.
Drilling the Hole
Use a small hand drill (a pneumatic drill is usually preferred) and insert the proper size drill bit. In removing a defective rivet use a bit that is slightly smaller than the rivet. Be sure the point of the drill bit is sharp. Split point drill bits are very helpful for this purpose. They will make it easier for you to begin drilling without the bit moving off the rivet head. Be sure you are wearing eye protection. Drill only through the head of the rivet. Do not attempt to drill into the sheet metal itself. After drilling into the head, use a pin punch to pry off the rivet head. Next, using a small punch slightly smaller than the rivet shank drive the rivet out of the hole.
The objective is to remove the rivet without damaging the sheet metal or enlarging the hole.
The strength of a riveted piece is based upon the expanded diameter of the rivet. With this in mind, it is very important that you drill out the rivet using the proper size drill bit and that you replace the rivet with the proper size. This means determining the proper diameter and length of the rivet. The length must be such that the shop head (driven head of the rivet) must expand to form a head that is one and one-half the diameter of the shank. Rivets may be cut to proper length using a rivet cutter.
Installing the new rivet
This may or may not be a simple task. It depends upon whether or not you can reach the backside of the rivet. After placing the rivet in the hole, determine whether or not you will be able to drive or squeeze the rivet. You must be able to reach inside the structure with a bucking bar or be able to apply a rivet squeezer. If you cannot do this, you will have to use a blind rivet. Using a blind rivet will eliminate the possible need to cut an access hole to buck a standard rivet. In order to properly buck a rivet, you must be able to reach the backside. If there is no way to do this an access hole is often cut to allow bucking of the rivet.
There are many different types of blind rivets available such as friction-lock cherry rivets, CherryMax rivets, mechanical lock cherry rivets, etc. A special tool must be used to install most of these rivets.
Scratch or Small Dent
Often you will find a piece of sheet metal that has a scratch on its surface. This scratch should be repaired to prevent any corrosion from forming. A scratch can eventually cause the piece to form a crack. Most scratches can be burnished or polished smooth. This will prevent further development of a crack. The best way to polish out a scratch or gouge is to use a high-speed grinder with a Cratex abrasive wheel attached. This special wheel is rubberized and designed for use on sheet metal. It will allow you to easily remove the damaged area without further damage. These wheels may be obtained through industrial supply houses.
A small dent can be repaired using a filler material. Poly-Fiber’s SuperFil is an ideal filler material to use because it will not shrink with time. Bondo tends to shrink after being applied. Apply the SuperFil using a squeegee and allow it to dry overnight. You can then sand it smooth and touch up the paint.
This is a very common problem on sheet metal airplanes. You will often find a crack developing in an area such as an engine cowling. The common fix is to use a small drill bit and “stop drill” the crack. That means you drill at each end of the crack in the hopes of stopping further development. Stop drilling the crack should be followed with a repair. Vibration will normally cause the crack to extend if proper repairs are not made. To prevent vibration from further acting on a crack, after stop drilling you should then rivet a small patch over the area. The patch should be completed using the same type of metal found on the original piece. Remember that this patch must restore the strength of the piece that was lost due to the damage.
As an example, if you have a piece of .032 skin that has a crack, first stop drill both ends of the crack. You can then cut a small piece of .032 aluminum to use as a patch. You should cut the patch so that no abrupt changes occur. A rounded or several-sided patch is preferable over a square patch. Use Advisory Circular AC43-13 1B as a reference to determine proper rivet layout for the patch. After determining and marking proper rivet layout you can then drill out the holes and fasten the patch using clecos. After you are satisfied with the fit, remove the clecos and apply zinc chromate to the backside of the patch. This will prevent any corrosion from forming. You may then rivet it in place.
Corroded Sheet Metal
It should be very obvious that corroded sheet metal will weaken a structure. Because of this fact all corrosion should be removed. Use fine sandpaper, Scotch Brite pads, or aluminum wool to remove corrosion. Never use steel wool or a steel brush. The Cratex abrasive wheels mentioned earlier are also very helpful in removing corrosion.
After the corrosion is removed the 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. An acid etch removes oil and light corrosion while etching or roughening the surface to provide a firm primer bond. The part is then thoroughly rinsed. Next wash the surface with E-2300 Conversion Coating. This will inhibit corrosion and further enhance primer adhesion. After this step the part is rinsed and allowed to completely dry. You can then prime the damaged area and repaint the surface.
Removal and replacement of rivets around a corroded area is usually necessary. In some cases, the corroded area itself may have to be entirely replaced. This comes under the category of a major repair.