Sheet Metal 5: Structures: Damage and Repair SM5

Sheet Metal Structures Repair Overview

• A rough landing can cause landing gear overload and become sprung
• Visual inspections may not reveal all damage types
• Internal injuries may show external evidence like buckled or wrinkled skin, loose rivets or working rivets
• External indications of internal injury must be heeded
• Warped wings show parallel skin wrinkles diagonally across wings, caused by violent maneuvers, rough air or hard landings
• The structure may be distorted and weakened without rupture
• Visually inspecting cannot accurately determine cracks in structural members
• Eddy current and ultrasonic inspection can find hidden damage

Damage Classification

• Negligible damage is surface damage that does not affect structural integrity
• Negligible repairs are smoothing, sanding, stop drilling, hammering and no added materials
• Negligible damage may be left or corrected without restricting flight
• Corrective action is required to prevent damage spread and frequent inspections
• Permissible negligible damage limits vary and must be researched on an individual basis
• Failure to research negligible limits can lead to insufficient structural strength
• Patching repairs damage exceeding negligible limits by installing a patch over the damaged structural part
• Patches span and overlap damaged areas and surrounding structure
• Structural fasteners are applied to restore original load-carrying traits
• Patching depends on damage extent and component accessibility
• Patch material is usually the same as the damaged part, but thicker

Stressed Skins Patches

• Stressed skin is a construction where the external covering carries some or all of the main loads
• Stressed skin uses high strength rolled aluminum sheets
• Stressed skin carries a large part of the aircraft structure load
• Minor skin damage can be patched
• Skin patches are classified as lap/scab or flush

Lap or Scab Patch

• Lap/scab patch is an external patch with overlapping edges, riveted to the skin
• Lap patches are for areas where aerodynamic smoothness is not important
• It can be cut circular, square, or rectangular however square or rectangle corners are rounded to a radius of 1/4-inch
• Edges are chamfered to 45° for 1/2 the material thickness and bent down 5° for sealing, reducing airflow impact
• For small hole repairs, damage is cleaned and smoothed
• Cracks must be stop drilled to relieve stress and prevent spreading, the patch large enough for rivet installation

Flush Patch

• Flush patches repair skin damage requring a smooth skin surface
• It is a filler patch flush to the skin, supported and riveted to a reinforcement plate (doubler) inside the skin

Flush Patch Steps

• Remove the damage and cut the damage to a round, oval, or rectangular shape
• Round all corners of a rectangular patch to a radius of 0.5-inch
• The doubler is the same material as skin but one thickness greater
• The size of the doubler depends on the edge distance and rivet spacing
• The doubler is inserted through the opening, rotated, and slid under the skin
• The filler has the same material and thickness as the damaged skin

Pressurized Area Repairs

• Pressurized aircraft skins are highly stressed, pressurization cycles apply loads
• Repairs require more rivets than non-pressurized skins

Pressurized Area Repair Steps

• Remove damaged skin section and radius all corners to 0.5-inch
• Fabricate a doubler of the same material, but one size greater
• Doubler size depends on the number of rows, edge distance, and rivets spacing
• Fabricate an insert of the damaged skin material and thickness, skin-to-insert clearance 0.015-inch to 0.035-inch
• Drill holes through the doubler, insertion, and original skin
• Spread a sealant layer on the doubler, securing it to the skin with Clecos
• Fasteners used must be the same in the surrounding area and install them through the doubler to the skin and the insertion to the doubler and dip all fasteners in sealant prior to installation

Repair Layout, Rivet Selection

• Material, temper condition, and strength of metal
• Proper size, material, and style of rivets
• The number of rivets
• The distance between the holes and edges of the patch

Distances

• Distances are measured in terms of rivet diameter
• Use the original material for the repair
• Always consult the aircraft maintenance manual or SRM for rivet type, size, spacing, and number
• Use blind rivets if structural access is impossible
• Match the existing rivet pattern wherever possible

Rivet Diameter

• If no repair information exists, rivets should be the same as similar joints on the aircraft
• Determine rivet size by referring to the size within the surrounding area by the manufacturer
• Or by using the 3X rule

Rivet Material & Type

• Always consult the maintenance manual
• Determine rivet material to use for the repair job can is obtained from the surrounding joint
• The 2117-T AD rivet is used for general repair since it doesn't need heat treatment, is soft, strong, and corrosion resistant with most alloys
• Use flush rivets for flush-riveted aircraft wings/stabilizers upper surfaces and the lower leading edge
• Use universal head rivets in all other surface areas
• Rework larger rivet holes, ensure the edge distance is maintained

Rivet Length

• Rivet total length calculation needs the combined thickness of materials
• The combined thickness of materials is the grip length
• The rivet total length equals the grip length plus the rivet shank needed to form the shop head
• The latter equals 1.5x the rivet shank diameter
  • Where A = total rivet length, B = grip length, and C = the shop head material length, calculated as A = B + C

Damage Classification (Part 2)

• Insertion replaces damaged area when too large to patch or interferes structural alignment with an identical member
• Splice connections at each end transfer load to the original structure
• Replacement occurs when damage is impractical to repair, more economical or easier to replace
• Cases of replacements are damaged castings, forgings, hinges, and small structural members.
• Highly stressed members must be replaced as repair would affect on safety

Approval

• 571.06 major reportable repairs and modifications include spar repair or modification and pressured fuselage repair or modification
• When aircraft undergoes a major repair or modification, CARs section 571.12 requires actions reported to the Minister
• Routine changes of landing gear configuration, or role equipment are exempt from this requirement

Repairing Cracks

• Vibration extends cracks if not repaired
• Cracks must be stop drilled with a small hole at each end of the crack to spread out stresses
• A small patch should be riveted over the stop-drilled crack to stiffen the area and stop vibration
• The patch material used should be same as original material or heavier grade not less then two times the length of the crack
• Figure out rivet size by using the 3X rule; edge distance, and rivet spacing