Preflight Pass
Get started
Menu

Airframe · Lesson 13 of 40

Metallic Structures

How metal airframes carry loads, how to classify damage, choose repair material and rivets, lay out, form and drill a sheet metal repair, heat treat aluminum, and the basics of gas and inert-gas welding and steel-tube repairs.
Free preview13 min read

Last reviewed

By the end of this lesson you can

  • Classify sheet metal damage and choose between patching, insertion and replacement
  • Select repair material, rivet size, rivet length and rivet layout using the handbook's rules
  • Calculate setback, bend allowance and total developed width for a bend
  • Explain solution heat treatment, aging and why icebox rivets are refrigerated
  • Describe welding gases, flames, inert-gas welding and steel-tube sleeve repairs

Most airframes are aluminum sheet metal joined with rivets, and many light aircraft still have welded steel-tube frames. The numbers in this lesson are the handbook's general practices for study. For real work, the manufacturer's structural repair manual (SRM) comes first (FAA-H-8083-31B, p. 4-1), and AC 43.13-1B applies only when there are no manufacturer repair instructions (FAA-H-8083-31B, p. 4-92 to 4-93).

Types of structures

A truss fuselage is usually welded steel tubing in which every member can carry tension and compression, generally covered with fabric (FAA-H-8083-31B, p. 1-7). A monocoque (single shell) fuselage relies largely on its skin for the primary loads, so it struggles to be strong enough without becoming too heavy (FAA-H-8083-31B, p. 1-7 to 1-8). Semimonocoque construction reinforces the skin with longerons and stringers, and most modern aircraft are built this way(FAA-H-8083-31B, p. 1-8). Because loads are shared among the frame members and the stressed skin, no single piece is failure critical (FAA-H-8083-31B, p. 1-9). In a fail-safe structure, another part takes over the load if one member fails (FAA-H-8083-31B, p. 1-13).

A repair must carry the stresses across the damage and back into the original structure. The handbook lists six: tension, compression, shear and bearing (basic stresses) and bending and torsion (combination stresses)(FAA-H-8083-31B, p. 4-2). Abrupt changes in cross-section start fatigue cracks (FAA-H-8083-31B, p. 4-1 to 4-2), and a repair that is too strong makes the structure flex excessively at its edge, which also speeds up fatigue (FAA-H-8083-31B, p. 4-86).

Inspecting metal structure and classifying damage

Hidden damage often shows only as buckled or wrinkled skin or loose or working rivets. A working rivet moves under stress without visibly loosening and may leave a dark, greasy residue around its head (FAA-H-8083-31B, p. 4-89). A black residue of aluminum oxide around a rivet, called fretting corrosion or a smoking rivet, shows movement but not necessarily looseness (FAA-H-8083-31B, p. 4-91). Eddy current and ultrasonic inspection find hidden cracks that visual inspection cannot confirm (FAA-H-8083-31B, p. 4-89). After a fire, aluminum that looks sound can be checked with an eddy current conductivity tester, and steel with a hardness tester (FAA-H-8083-30B, p. 10-16).

Learn the defect terms: a dent is an indentation from a forceful strike, a nick is a local notch on an edge, a gouge is a groove made under heavy pressure, brinelling is shallow spherical dents from a high load, and galling is metal torn loose by friction between moving parts (FAA-H-8083-31B, p. 4-89 to 4-90). Damage falls into four classes (FAA-H-8083-31B, p. 4-90):

  • Negligible: surface damage that does not affect structural integrity, fixed by smoothing, sanding, stop drilling or hammering out, with no added material.
  • Repairable by patching: beyond negligible limits, bridged by splice members that overlap sound structure.
  • Repairable by insertion: too large to patch, or a patch would upset alignment (as at a hinge or bulkhead), so an identical member is spliced in.
  • Replacement: when repair is impractical, as with castings, forgings and hinges.

Stop drilling spreads the stress around a small hole instead of the crack tip (FAA-H-8083-31B, p. G-34); the handbook's typical repairs use a No. 40 drill (FAA-H-8083-31B, p. 4-103). Keep the corner radius of any rectangular cutout at least 1/2 inch(FAA-H-8083-31B, p. 4-86).

Repair material and aluminum heat treatment

A repair must duplicate the original strength. If a weaker alloy must be used, use a heavier gauge; never use a lighter gauge, even of a stronger alloy, because buckling strength depends mainly on thickness (FAA-H-8083-31B, p. 4-86). A buckled part must not be relied on to carry its load again (FAA-H-8083-31B, p. 4-86), and annealed alloys are not recommended for structural repairs (AC 43.13-1B, par. 4-53, p. 4-15). Alloy 2024 is used for skins, structure and rivets; 7075 is stronger but less tough (FAA-H-8083-31B, p. 4-30). Alclad has a thin layer of pure aluminum on each side (FAA-H-8083-30B, p. 7-21), and a scratch through it lets corrosion attack the alloy beneath (FAA-H-8083-31B, p. 4-89).

Solution heat treatment means heating to a set temperature, soaking, then quenching rapidly; aging follows, either naturally at room temperature or artificially (precipitation heat treatment) (FAA-H-8083-30B, p. 7-21). No more than 10 seconds may pass between furnace and quench for 2017 or 2024 sheet (FAA-H-8083-30B, p. 7-22). Alloys 2017 and 2024 reach full strength by natural aging in about four days, while 2014 and 7075 also need precipitation heat treatment. Temper codes: W is solution heat-treated and unstable, T3 is solution heat-treated then cold-worked, T4 is solution heat-treated, and T6 is solution heat-treated then artificially aged (FAA-H-8083-30B, p. 7-21). Reheating 2017 or 2024 above 212 °F, including baking primers, impairs the heat treatment (AC 43.13-1B, par. 4-54, p. 4-16).

Alloy 2117 (AD) rivets are heat-treated once by the manufacturer and can be driven any time. Alloy 2024 rivets harden about 10 minutes after quenching, so they are kept in a refrigerator below 32 °F to stay drivable (FAA-H-8083-30B, p. 7-24). That is why 2024 (DD) rivets are called icebox rivets; most have been replaced by E-type rivets driven as received(FAA-H-8083-31B, p. 4-31).

Layout, forming and drilling

  • Bend radius is measured to the inside of the bend; the minimum comes from a table for the alloy, temper and thickness (FAA-H-8083-31B, p. 4-58 to 4-59).
  • Bend allowance (BA) is the length of the curved neutral line between the bend tangent lines(FAA-H-8083-31B, p. 4-58).
  • Setback (SB) is how far the bend starts back from the mold line. For a 90° bend, SB = R + T; otherwise SB = K(R + T), where K is the tangent of half the bend angle (FAA-H-8083-31B, p. 4-59).
  • Flat = mold line dimension minus the setback, or minus two setbacks with a bend at each end(FAA-H-8083-31B, p. 4-59).

For any bend, BA = (0.01743R + 0.0078T) × N, where N is the degrees of bend (FAA-H-8083-31B, p. 4-64). Total developed width (TDW) = flats + (bend allowance × number of bends), always less than the sum of the mold line dimensions(FAA-H-8083-31B, p. 4-66). Draw the sight line one bend radius inside the bend tangent line under the brake nose bar, not in the middle of the bend allowance (FAA-H-8083-31B, p. 4-66). Bend lines should run at 90° to the grain (FAA-H-8083-31B, p. 4-58). Where bends meet, drill relief holes at the intersection of the inside bend tangent lines, at least 1/8 inch in diameter for aluminum up to 0.064 inch thick (FAA-H-8083-31B, p. 4-71 to 4-72).

To drill a rivet hole, center punch, drill an undersized pilot hole, then open it to size: No. 40 for a 3/32-inch rivet, No. 30 for 1/8 inch (FAA-H-8083-31B, p. 4-39). Hold the drill at 90°, remove burrs and clean away chips (FAA-H-8083-31B, p. 4-40). Mark aluminum with a fiber-tipped pen, because pencil graphite can cause corrosion (FAA-H-8083-31B, p. 4-4). Dimple sheet that is too thin to countersink (FAA-H-8083-31B, p. 4-40); 7075-T6, magnesium and titanium must be hot dimpled (FAA-H-8083-31B, p. 4-43).

Rivets

Selecting rivets

In AN470AD3-5, 470 is a universal head, AD is 2117-T alloy, 3 is the diameter in thirty-seconds of an inch and 5 the length in sixteenths; 426 is the 100° countersunk head (FAA-H-8083-30B, p. 7-56). Head markings: a dimple for AD (2117), a raised dot for D (2017), two raised bars for DD (2024) and a raised ring for E (7050) (FAA-H-8083-31B, p. 4-33). Use countersunk heads where a smooth surface is needed (FAA-H-8083-31B, p. 4-31).

Rivet diameter should be about 2 1/2 to 3 times the thickness of the thicker sheet: multiply the skin thickness by 3 and use the next larger size. Rivets under 3/32 inch are not used on stressed structure (FAA-H-8083-31B, p. 4-32). Rivet length is the grip plus about 1 1/2 diameters for the shop head, which ends up about 1 1/2 times the shank diameter(FAA-H-8083-31B, p. 4-31 to 4-32). If a hole is enlarged, use the next larger rivet (1/32 inch larger) and keep minimum edge distance (FAA-H-8083-31B, p. 4-44 to 4-45). Blind rivets are never used in fluid-tight areas, air intakes, control surfaces, hinges or other highly stressed places, and metal airframe repairs need manufacturer authorization or FAA approval to use them (FAA-H-8083-31B, p. 4-45 to 4-47).

Rivet layout

Rule (D = rivet diameter)Value
Edge distance, universal headAt least 2D; preferred 2D + 1/16 inch; no more than 4D
Edge distance, countersunk headAt least 2 1/2D
Rivet pitch (same row)At least 3D (4D in a two-row layout); typically 4D to 6D
Transverse pitch (between rows)Usually 75 percent of pitch; at least 2 1/2D

These values come from (FAA-H-8083-31B, p. 4-33 to 4-34). A rivet too close to the edge can crack the sheet; too far away and the edge turns up (FAA-H-8083-31B, p. 4-34). Whenever possible, copy the manufacturer's spacing (FAA-H-8083-31B, p. 4-33).

Installing and removing rivets

Bad rivets come from improper bucking, a slipping or tilted set, wrong hole or rivet size, burrs, hard rivets, and too much or too little driving; a joggled shank in a removed rivet means partial shear failure (FAA-H-8083-31B, p. 4-43). To remove a universal head rivet, work from the manufactured head: file a flat, center punch it, drill to the depth of the head with a drill one size smaller than the shank, break off the head with a punch, then drive out the shank while supporting the sheet (FAA-H-8083-31B, p. 4-44 to 4-45). Drilling too deep lets the shank turn and tear the metal (FAA-H-8083-31B, p. 4-44).

Welding processes

Gas welding: gases, storage and flames

Acetylene is unstable: its cylinders hold acetone in a porous filler, free acetylene can explode at 29.4 psi, and the acetylene gauge is never set above 15 psi (FAA-H-8083-31B, p. 5-6). Keep cylinders upright and secured; if an acetylene cylinder has been on its side, stand it upright at least twice as long before use (FAA-H-8083-31B, p. 5-10). The acetylene hose is red with left-hand threads and the oxygen hose green with right-hand threads (FAA-H-8083-31B, p. 5-7). Flashback arrestors stop a flashback from reaching the cylinders (FAA-H-8083-31B, p. 5-7). Oil or grease can ignite violently with oxygen(AC 43.13-1B, par. 4-114, p. 4-95), and leaks are never checked with a flame (FAA-H-8083-31B, p. 5-10).

  • Neutral flame (balanced): most welding, because it does not change the base metal (FAA-H-8083-31B, p. 5-12).
  • Carburizing flame (excess acetylene, with a feather): high-carbon steel, aluminum, nickel and Monel (FAA-H-8083-31B, p. 5-12).
  • Oxidizing flame (excess oxygen): brazing and welding brass and bronze; on steel it makes the metal foam and spark (FAA-H-8083-31B, p. 5-12).

Shut down by closing the torch acetylene valve first (FAA-H-8083-31B, p. 5-14). Weld 4130 chrome-moly tubing with a soft neutral flame, preheating sections over 3/16 inch to 300 °F to 400 °F (FAA-H-8083-31B, p. 5-16). A good weld is smooth, slightly convex and tapers into the base metal; never file a weld or fill it with solder (FAA-H-8083-31B, p. 5-15). Do not weld 2024 or 7075 aluminum, heat-treated alloy steel parts such as bolts, or brazed parts (AC 43.13-1B, par. 4-89, p. 4-75).

Inert-gas welding and shielding gases

Gas tungsten arc welding (GTAW or TIG) uses a nonconsumable tungsten electrode, a hand-fed filler rod and an inert shielding gas such as argon or helium that keeps oxides out of the puddle. It is preferred for stainless steel, magnesium and thick aluminum: DC straight polarity for steel and titanium, AC for aluminum and magnesium(FAA-H-8083-31B, p. 5-4). Gas metal arc welding (GMAW, formerly MIG) feeds a consumable wire under argon, helium or carbon dioxide but suits production rather than repair work (FAA-H-8083-31B, p. 5-2, 5-4).

Steel-tube weld repairs

  • Dent at a cluster: a patch plate of the same material and thickness as the heaviest damaged tube, with fingers extending at least 1 1/2 tube diameters along each tube (FAA-H-8083-31B, p. 5-31).
  • Dent or crack between clusters: stop drill cracks with a No. 40 drill and weld on a split sleeve cut at 30°, reaching at least 1 1/2 tube diameters past the damage (FAA-H-8083-31B, p. 5-31 to 5-32).
  • Partial tube replacement: an inner-sleeve splice with rosette welds, or an outer-sleeve splice(FAA-H-8083-31B, p. 5-32 to 5-33).

A welded crack still needs reinforcement (FAA-H-8083-31B, p. 5-30). Bolted sleeves are not used on welded steel-tube structure without manufacturer or FAA authorization (AC 43.13-1B, par. 4-93, p. 4-79), and heat-treated landing gear must be reheat-treated after welding (FAA-H-8083-31B, p. 5-33).

Shop safety

Wear goggles when drilling and grinding (FAA-H-8083-31B, p. 4-12, 4-14). Never point a rivet gun at anyone, never pull the trigger unless the set is held against a rivet or a block of wood, and disconnect the air hose when it is idle(FAA-H-8083-31B, p. 4-38). For gas welding, use shade 3 or 4 green lenses and keep a fire extinguisher nearby(AC 43.13-1B, par. 4-114 to 4-115, p. 4-95).

Worked example

You are repairing 0.040-inch 2024-T3 skin and must also form a 90° angle with a 0.16-inch bend radius and two 1.00-inch legs (mold line dimensions). These are practice numbers only.

  1. Rivet size: 3 × 0.040 = 0.120 inch, so use 1/8 inch (0.125 inch) (FAA-H-8083-31B, p. 4-32) with a No. 30 drill (FAA-H-8083-31B, p. 4-39).
  2. Layout: minimum edge distance 2 × 0.125 = 0.250 inch; minimum pitch 3 × 0.125 = 0.375 inch in one row (4 × 0.125 = 0.50 inch in two rows). With a 0.75-inch pitch, transverse pitch is about 0.75 × 0.75 = 0.56 inch (FAA-H-8083-31B, p. 4-33 to 4-34).
  3. Setback: SB = 0.16 + 0.040 = 0.20 inch, so each flat = 1.00 − 0.20 = 0.80 inch.
  4. Bend allowance: (0.01743 × 0.16 + 0.0078 × 0.040) × 90 = (0.00279 + 0.00031) × 90 ≈ 0.279 inch(FAA-H-8083-31B, p. 4-64).
  5. TDW: 0.80 + 0.80 + 0.279 ≈ 1.88 inches, less than the 2.00-inch total of the mold lines, as it must be (FAA-H-8083-31B, p. 4-66).

Common mistakes

  • Substituting a thinner sheet of a stronger alloy.
  • Reusing the same rivet size in an elongated hole.
  • Drilling a rivet out from the shop head, or deeper than the head.
  • Drawing the sight line in the middle of the bend allowance.
  • Letting 2024 icebox rivets warm up before driving them.
  • Laying an acetylene cylinder on its side, or setting acetylene above 15 psi.
  • Filing a weld smooth, which removes strength.

Key points

  • Semimonocoque structure shares the loads between frames, stringers, longerons and a stressed skin.
  • Damage classes: negligible, repairable by patching, repairable by insertion, or replacement.
  • Repair material: same alloy and gauge, or a heavier gauge if the alloy is weaker; never thinner.
  • Rivet diameter about 3 × the skin thickness (next size up); length = grip + about 1.5 diameters.
  • Edge distance at least 2D (2.5D countersunk); pitch at least 3D; transverse pitch about 75% of pitch.
  • SB = R + T for a 90° bend; BA = (0.01743R + 0.0078T) × N; TDW = flats + bend allowances.
  • Solution heat treat, quench within 10 seconds (2017/2024 sheet), then age naturally or artificially.
  • Acetylene never above 15 psi; neutral flame for most welding; TIG uses AC for aluminum and magnesium.
  • Steel-tube patch fingers and sleeves extend at least 1.5 tube diameters past the damage.

Lesson quiz in the full course

10 questions on this lesson, each with an explanation and its FAA source.

See what's in A&P Complete

Sources for this lesson