Aircraft Structural Repair Basics

Introduction to Aircraft Structural Repair

• Aircraft structural integrity requires continuous maintenance for satisfactory performance.
• Metal structural repairs or modifications should adhere to the best available techniques to avoid immediate or potential dangers.
• Aircraft reliability is dependent on design quality and workmanship in repairs or modifications.
• Aircraft metal structural repair designs must ensure the aircraft remains as light as possible.
• Repairs must be strong enough to carry all loads with the required safety factor, but avoid excessive strength to prevent stress risers.
• Damage to metal aircraft structures is often results from corrosion, erosion, normal stress, and accidents.
• Modifying aircraft structures may require extensive structural rework, such as reinforcing the wing structure to support additional stresses when installing winglets.
• Repairing a damaged section involves duplicating the original part in strength, material, and dimensions.
• Aircraft technicians require knowledge of sheet metal forming methods and techniques for structural repairs.
• Forming is changing the shape of solid metal through bending.

Forming of Metal

• Aluminum forming is usually done at room temperature.
• Repair parts are shaped to fit before being attached to the aircraft or component.
• Forming can range from simple operations like a single bend or curve to complex operations requiring compound curvature.
• Technicians need working knowledge of tools and equipment for forming metal.
• Considerations before forming include bend complexity, material type/thickness/ temper, and part size.
• These factors determine which forming method should be used.

Repair Layout

• Repair layout considerations include:
  • Determining the required number of rivets
  • Selecting the proper size and style of rivets
  • Assessing material, temper, condition, and strength
  • Determining the size of the holes
  • Spacing between the holes
  • Distance between the holes and patch edges
• Distances are measured in rivet diameter terms.

Rivet Selection

• Always consult the aircraft maintenance manual for rivet type, size, spacing, and quantity.
• Repair rivet patterns must follow instructions in the applicable aircraft manual, using the existing rivet pattern if possible.
• Rivet size and material should match the original rivets in the repaired part.
• If a rivet hole is enlarged or deformed, use the next larger size rivet after reworking the hole and maintain proper edge distance.
• The 2117-T rivet is used for general repair work due to it not requiring heat treatment, being fairy soft and strong, and is highly corrosion resistant.
• Check maintenance manual for correct rivet type and material.
• Select rivet head type based on the surrounding area and manufacturer's specifications.
• On flush-riveted aircraft, apply flush rivets to:
  • Upper wing and stabilizer surfaces
  • Lower leading edge back to the spar
• Use universal head rivets for all other surface areas.
• Whenever possible, choose rivets of the same alloy number as the material being riveted.
• Blind rivets must be used when access to the inside of the structure in impossible.

Stresses Applied to Rivets

• Shear is one stress applied to rivets involving the amount of force required to cut a rivet holding two or more sheets together.
  • Single shear occurs when a rivet holds two parts together.
  • Double shear occurs when a rivet holds three sheets or parts together.
• Tension is an additional stress applied to rivets.
• Bearing strength is the resistance to tension, or the tension amount required to pull a rivet head off.

Rivet Spacing

• Use the same rivet spacing and edge distance as the manufacturer in the area surrounding the damage.
• Consult the Structural Repair Manual (SRM) for the particular aircraft.
• There is no specific set of rules governing rivet spacing in all cases.
• Minimum requirements must be observed.
• Rivet pitch is measured between the centerlines of rivets in the same row.
  • Minimum spacing for protruding head rivets is 3 times the rivet diameter.
  • Minimum spacing for flush head rivets is 3 1/2 times the rivet diameter.
• Edge distance from the rivet center should never be less than 2 times the shank diameter.
• Gauge is the distance between rivet rows.
  • Gauge should never be less than 2 1/2 times the diameter.

Rivet Pitch Specs

• The smallest allowable rivet pitch is 3 rivet diameters.
• Typical rivet pitch ranges from 4 to 6 rivet diameters.
• Can be up to 12 rivet diameters in some cases.
• Minimum pitch depends on the number of rivet rows.
  • One-and three-row layouts have a minimum pitch of 3 rivet diameters.
  • Two-row layouts have a minimum pitch of 4 rivet diameters.
• Countersunk rivets have a larger pitch than universal head rivets.
• Increase rivet spacing by at least 1/16-inch to allow for hole oversizing without violating minimum spacing.
• Transverse pitch is the perpendicular distance between rivet rows, typically 75% of the rivet pitch.
  • The smallest allowable transverse pitch is 2 1/2 rivet diameters.
• Rivet pitch and transverse pitch can have the same dimension and are referred to as rivet spacing.
• Edge distance is the distance from the first rivet center to the sheet's edge. It should not be less than 2 or more than 4 rivet diameters

Edge Distance Specs

• Recommended edge distance is about 2 1/2 rivet diameters.
• The minimum edge distance:
  • Universal rivets is 2 times the rivet diameter
  • Countersunk rivets is 2 1/2 times the rivet diameter
• The sheet may crack or pull away from rivets if they are too close to the edge.
• The sheet may turn up at the edges when rivets are spaced too far from the edge.
• Add 1/16-inch to the minimum edge distance or use the next size of rivet diameter.
• Center-to-center distance between adjacent rivet lines or rows is the rivet gauge.

Rivet Layout Examples

• Find the edge distance at each end of a one-row layout, then lay off the rivet pitch.
• Place the second row at a distance equal to the transverse pitch in a two-row layout.
• Alternate rivet spots in the second row that falls midway between spots in the first row
• Lay off the first and third rows for a three-row layout by using a straightedge to determine the second row rivet spots.

Rivet Length

• Determine total rivet length by combining thickness of materials to be joined.
• This measurement is the grip length.
• Add the grip length to the amount of rivet shank needed to form a proper shop head.
• The amount of the rivet shank = one and a half times the diameter of the rivet shank.
  • A = B + C.
  • A = total rivet length
  • B = grip length
  • C = material to form a shop head