Aircraft Metal Structural Repair

Aircraft Metal Structural Repair

Aircraft require continuous maintenance of structural integrity, and metal structural repairs must adhere to the best available techniques to avoid immediate or potential danger. Repairs should maintain strength with the required safety factor without adding unnecessary weight that could cause stress risers and cracks. Sheet metal, often made of aluminum alloys, is used extensively in aircraft construction for both structure and outer covering.

Damage and Repair Methods

Damage to metal aircraft structures arises from corrosion, erosion, stress, accidents, and mishaps. While diverse repair methods exist, the ideal repair duplicated the original part in strength, material, and dimensions. Technicians need expertise in sheet metal forming, typically done at room temperature for aluminum. Forming methods include bending, brake forming, stretch forming, roll forming, and spinning. Consult manufacturer's manuals (SRM) and AC 43.13.1 for specific repair instructions.

Stresses in Structural Members

Aircraft structures must withstand flight and ground loads without permanent deformation. Repairs must effectively transfer stresses back into the original structure, ensuring a continuous path without abrupt changes in cross-sectional areas to avoid stress concentrations and fatigue cracking. The six types of stress are tension, compression, shear, bearing, bending, and torsion. Tension resists pulling forces, compression resists crushing forces, and shear resists sliding forces. Bearing stress resists rivet or bolt force on a hole. Torsion produces twisting, while bending combines compression and tension.

Tools for Sheet Metal Construction and Repair

Specialized tools, powered by various means, are essential for efficient sheet metal work. Layout tools like scales, combination squares, and dividers are used for accurate measurement and marking. Scales are used for measurement, combination squares for angles, and dividers for transferring measurements. Rivet spacers aid in rivet pattern layout. Marking tools include fiber-tipped pens (preferred over pencils due to corrosion risks) and scribes (used cautiously to avoid weakening material).

Punches

Punches, made of hardened and tempered carbon steel, come in solid and hollow forms. A prick punch is used for reference marks, while a center punch makes indentations for drilling. Automatic center punches use spring tension. Transfer punches utilize templates for marking hole locations. Drive punches remove damaged rivets, and pin punches drive out pins or bolts. Chassis punches create holes for instruments, and awls scribe marks or align holes.

Cutting Tools

Cutting tools include circular saws (like the Kett saw, useful for cutting metal), reciprocating saws (for push-pull cutting, good in tight spaces), and cut-off wheels (for cutting damage on skin and stringers). Nibblers are used for cutting sheet metal with a blanking action.

Shop Tools

Shop tools include squaring shears (for cutting and squaring sheet metal), scroll shears (for cutting irregular lines), and rotary punch presses (for punching holes). Band saws cut various materials, and disk/belt sanders smooth surfaces. Notchers cut out metal parts, and grinders sharpen tools.

Hand Cutting Tools

Hand cutting tools include straight snips (for straight cuts), aviation snips (for curved cuts), files (for shaping metal), and burring tools (for removing burrs). Die grinders with mounted wheels are used for cutting, grinding, and polishing.

Hole Drilling

Drilling involves portable power drills (pneumatic drills recommended around flammable materials), drill presses (for accuracy), and specialized motors like right angle drills. Accessories include extension drill bits and angle adapters. Drill bits come in various types (HSS, step drill, cobalt alloy, twist drill).

Drill Bit Sizes and Lubrication

Drill diameters are grouped by number, letter, and fractional sizes. Lubrication aids in chip removal and finish quality; cetyl alcohol is recommended for aluminum and titanium, and sulfurized mineral oil for steel. Reamers enlarge and smooth holes. Drill stops prevent excessive penetration. Drill bushings and guides aid in perpendicular drilling.

Hole Drilling Techniques

Techniques for accurate hole drilling include careful layout, prick punching, center punching, pilot drilling, and controlled drilling speed and pressure. For large holes, step drill bits and reamers are used. Chip chasers remove lodged chips and burrs.

Forming Tools

Sheet metal forming shapes metal using powered and hand-operated tools. Forming methods include bending, folding, and rolling, often done cold (at room temperature). Spring-back requires adjustments in bending. A bar folder makes bends or folds on sheet edges. Cornice brakes bend sheet metal, while box and pan brakes form boxes and pans. Press brakes form thicker parts. Slip roll formers shape sheets into cylinders. Rotary machines shape edges or form beads.

Stretch Forming and Drop Hammer

Stretch forming tensions metal over a form. Drop hammer forming shapes metal between matched dies under repetitive blows. Hydropress forming uses rubber pads to press metal against form blocks. Spin forming rotates metal at high speeds to shape it against a form.

English Wheel and Piccolo Former

The English wheel creates double curves by stretching metal between wheels. The piccolo former is used for cold forming and rolling sheet metal and profile sections.

Shrinking and Stretching Tools

Shrinking tools compress metal; stretching tools expand it. Hand-operated shrinker/stretchers manually apply force. Dollies and stakes support metal during forming. Hardwood form blocks and V-blocks are also used. Sandbags offer support during bumping.

Sheet Metal Hammers and Mallets

Sheet metal hammers and mallets shape metal without marring it. Clamps and vises hold materials in place.

Reusable Sheet Metal Fasteners

Reusable fasteners like Cleco fasteners temporarily hold parts together. They come in various sizes, identified by color codes. Hex nut and wing nut fasteners provide higher clamping force.

Aluminum Alloys

Aluminum alloys are commonly used for aircraft repair. Various alloys (1100, 3003, 2014, 2017, 2024, 2025, 2219, 5052, 5056, 6061, 7075) offer different properties for applications, from fuel tanks to structural components. Alclad™ aluminum has a protective cladding.

Structural Fasteners - Solid Shank Rivet

Solid shank rivets are common fasteners in aircraft construction. They are low-cost and easy to install but have low tensile strength. Rivet head shapes include universal and countersunk. Rivets come in different alloys (2017-T4, 2024-T4, 2117-T4, 7050, 5056) and sizes, identified by code markings. Rivet length is determined by grip length plus shank length needed to form a shop head.

Rivet Strength and Spacing

Rivet strength depends on the material (e.g., 2117-T4 is a common repair rivet). Rivet spacing is measured between centerlines and must meet minimum requirements. Edge distance (distance from rivet center to edge) should be within 2 to 4 rivet diameters. Rivet pitch is the distance between rivets in a row. Transverse pitch is the distance between rivet rows.

Rivet Installation Tools

Rivet installation tools include rivet cutters, bucking bars, hand rivet sets, countersinks, and dimpling dies. Pneumatic rivet guns are also used with interchangeable rivet sets. Reduced vibration tools enhance operator comfort.

Rivet Sets and Compression Riveting

Rivet sets or headers must match the rivet being driven. Compression riveting (squeezing) is a quick method using hand, pneumatic, or pneudraulic squeezers. Microshavers can obtain required flushness.

Riveting Procedure and Hole Preparation

The riveting procedure includes hole transfer (using existing holes as guide), careful hole preparation (proper size, shape, and deburring), pilot drilling, and countersinking if required. Drill at a 90° angle and remove burrs.

Pneumatic Rivet Gun

Pneumatic rivet guns need to be held firmly and the force of the gun needs to be absorbed by the bucking bar and not the structure being riveted. The driving force of the rivet gun is adjusted by a needle valve on the handle. A rivet set must be selected that allows slight upsetting of the head driving. All rivets need to be driven within 3 seconds.

Countersinking

Countersinking creates a conical recess for flush rivet heads. Proper countersinking is critical to avoid weakening the joint. Dimpling (making indentations) is used for thinner sheets. Various tools such as microstop countersinks and dimpling dies are used.

Rivet Spacing and Hole Preparation

Rivet spacing is measured between the centerlines of rivets in the same row. Edge margin is the distance from the center of the first rivet to the edge of the sheet. Use a sharp prick punch and magnifying glass to mark the holes. For holes 3⁄16-inch and larger, pilot drilling is recommended. Enlarge each pilot drilled hole to final size.

Dimpling Techniques

Dimpling techniques include radius dimpling, coining the dimpling (using a countersunk rivet as the male dimpling die), and hot dimpling (using heated dimpling dies). Combination predimple can be used with a 100° countersink method. Make sure to consult the manual on dimpling procedures.

Rivet Evaluation

After installation, rivets and surrounding areas must be inspected for deformities. Cracked dimples can be radial or circumferential. Some common causes of unsatisfactory riveting are improper bucking, rivet set slipping off or being held at the wrong angle. Remember to always examine adjacent parts during repair.

Removing of Rivets

Rivet removal involves carefully drilling off the manufactured head and punching out the shank, avoiding damage to the hole. The shank or rivet head should be rotated before turning on the power and to the depth of the head while holding the drill at a 90 degree angle when you are drilling the head of the rivet. When removing a countersunk or flush head use a punch as a lever.

NACA Riveting

NACA riveting is a specialized technique for fuel tank areas, where the shank is upset into an 82° countersink.

Special Purpose Fasteners

Special purpose fasteners are designed for applications in which fastener strength, ease of installation, or temperature properties of the fastener require consideration. Because these fasteners are often inserted in locations where one head, usually the shop head, cannot be seen, they are called blind rivets or blind fasteners. Always consult the manual about using blind rivets for your specific repair procedure.

Blind Rivets (Friction and Mechanical Lock)

Mechanical-lock blind rivets are designed with the locking (foil) collar to not lose the center stem due to vibration. Friction-locked blind rivets have a multiple-piece construction and rely on friction to lock the stem to the sleeve. A wide variation of tooling exists for these sorts of rivet applications.

Pin Fastening Systems

Pin fastening systems have a threaded pin and a collar. The metal collar is swaged onto the grooved end, effecting a firm tight fit. These have a hex lock to engage and prevent rotation of the pin while the collar is being installed. The pin comes in two head styles. Both high shear and tensile strength pin systems exist.

Lockbolt Fastening Systems

Lockbolt fastening systems combines the features of a high strength bolt and a rivet with advantages over each.In general, a lockbolt is a nonexpanding fastener that has either a collar swaged into annular locking groves or a threaded collar. Three types of lockbolts are commonly used: pull type, stump type, and blind type. Use hand air pressure and power tools during their installation.

Blind Bolts

Blind bolts have a higher strength than blind rivets and are used for joints that require high strength. When determining the correct length of the fastener note that the grip length of a bolt is the distance from the underhead bearing surface to the first thread. Blind bolts are available in pull type and drive type.

Rivnut

The rivnut is a blind installed internally threaded rivet. Installed by crimping from one side, the rivet nut provides a threaded hole into which machine screws can be installed. Select the rivnuts size based on the thickness of the parent material and the size of the screw to be used.

Rivetless

With rivetless, new Cherry rivitless features a retainer that does now require flaring. This helps to eliminate the need or two additional rivet holes. This product replaces standard riveted nutplates.

Stretch or Shrink Forming

Forming requires either stretching or shrinking the metal, or sometimes doing both. Other processes used to form metal include bumping, crimping, and folding. Stretch forming involves to lengthen or increasing a particular area of metal while shrinking means to reduce an area.

Flat Pattern Development and Layout

To prevent any waste and get a greater degree of accuracy use a layout or flat pattern of a part to be cut before forming. Construction of interchangeable structural and nonstructural includes channel, angle, zee, or hat section members. Make a flat pattern to show how much material is required in the bend areas. K-Factor and charts need to be considered when dealing with angles that need to be accounted for during layout.

Hand Working of SheetMetal

Proper aircraft sheetmetal working involves both stretching and shrinking sheet metal. Use extreme care with thin gauge alloys and be aware that wrong steps may leave the part in a strained condition leading to fatigue or structural failure. Both hand and machine working of sheet metal have applications for repairs.

Bending of U-Channels

When calculating the bend allowance follow this procedure. Find the tangent of one half the value of the bend angle to fine the k-factor. The bend area is 10 to 15% thinner and it is often satisfactory to assume the natural axis center.

Forming operations

Several forming operations occur. Stretching occurs my hammering or rolling metal under pressure. Shrinking of metal occurs if metal is compressed into a smaller metal area. Bumping is shaped through hammering. Crimping is folding or pleating.

Squaring Shear Operations

The Squaring Shear performs three distinctly different operations: cutting to a line, squaring, and multiple cutting to a specific size. Squaring requires securing one edge and using the squared edge to make other edges square. When multiple pieces must be cut to the same dimensions, use the backstop, located on the back of the cutting edge on most squaring shears.

Cutting and Adjustments for the Shop

Circular-Cutting Saws should be kept from grabbing through maintenance of the handle. Airframe Parts need to be brought to a fixed location in the shop for part construction. Airframe Technicians use aluminum sheets up to .063 inches with the throatless shear through moving the metal during cutting. Index pins release when rotating the turret to change punches and require proper alignment.

Bend Allowance Notes

The tangent of one half the value of the bend angle is used to find the k-factor. The bend area is 10 to 15% thinner. It is often satisfactory to assume the natural axis center.

Crimping, and Folding and Bending

Crimping helps when folding sheet metal. Sheet metal are folded to make a bend or crease in sheets, plates, or leaves, Folds are sharp angular bends. Brakes are similar to the bar folder can make a bend or edge only as wide as the depth of its jaws. When using ordinary bends set right side of the shear.

Hole Transfers and Prepair

Hole transfers are for a metal part. Prepair can use various drill bits. Care of the hole’s shape should occur for best fit. During transfers a block of wood or a bucking bar should be under rivets.

Standard Rivent Alloy Code Markings

The rivets that are the right alloy in the repair should be correct for each of these circumstances. Stresses may be involved. Spacing for rivets matter, and spacing and edge damage is taken into consideration.

Installation Removal

Rivet Install tool. and removal proceeders are needed in prepairing these tools.

Inspection

There is a need to be visually inspected for these traits. As a reminder always perform inspection with safety in mind. Look for stress and structural damage.

Dimpling Inspection

Poor dimples should not be used. To determine, inspect visually. Then use features to be used upon each piece.

Different Damage to the Shop

This includes different situations that may occur. Take time to go about and figure the best procedure.