Aircraft Metal Structures As Opposed to Aerospace Composite Structures
Repairs to riveted or bolted steel truss type structures should be made employing some general principles.
Methods for repair of vital members should be specifically approved by a representative of the Federal Aviation Administration.
Aluminium alloy aerospace structures.
Extensive repairs to damaged stressed skin on monocoque types of aluminium alloy structures should preferably be made in accordance with specific recommendations of the manufacturer of the aircraft.
In many cases, repair parts, joints, or reinforcements can be designed, and proof of adequate strength shown, without the calculation of the design loads and stresses, by properly considering the material and dimensions of the original parts and the riveted attachments.
Examples illustrating the principles of this method as applied to typical repairs may be found in textbooks on metal structures.
An important point to bear in mind in making repairs on monocoque structures is that a repaired part must be as strong as the original with respect to all types of loads and general rigidity.
Avoid drilling oversized holes or otherwise decreasing the effective tensile areas of wing-spar capstrips, wing, fuselage, or fin longitudinal stringers, or other highly stressed tensile members.
Make all repairs or reinforcements to such members in accordance with factory recommendations or with the specific approval or a representative of the Federal Aviation Administration.
Disassembly Prior to Repairing.
If the parts to be removed are essential to the rigidity of complete aircraft structures, support the remaining structure prior to disassembly in such a manner as to prevent distortion and permanent damage to the remainder of the structure.
When rivets are to be removed, weaken the rivet head by drilling.
Use a drill of the same size as the rivet.
Drilling must be exact center and to the base of the head only.
After drilling, break off the head with a pin punch and carefully drive out the shank.
Removal of rivet heads with a cold chisel and hammer is not recommended because skin damage and distorted rivet holes will probably result.
Care must also be taken whenever screws must be removed for disassembly or removal of stress plates, access plates, fillets, etc.
, to avoid damage to adjoining structure.
When properly used, impact wrenches can be effective tools for removal of screws; however, damage to adjoining structure may result from excessive vertical loads being applied through the screw axis.
Excessive loads are usually related to improperly adjusted impact tools or attempting to remove screws that have seized from corrosion.
Remove seized screws by drilling and use of a screw extractor.
Structural cracks may appear in the doubler that runs parallel to the line of anchor or plate nuts installed for securing access doors or plates.
Inspect rivet joints adjacent to damaged structure for partial failure (slippage) by removing one or more rivets to see if holes are elongated or the rivets have started to shear.
Aluminium for replacement aircraft structures.
In selecting the alloy, it is usually satisfactory to use 2024 in place of 2017 since the former is stronger.
Hence, it will not be permissible to replace 2024 by 2017 unless the deficiency in strength of the latter material is compensated by an increase in material thickness, or the structural strength is substantiated by tests or analysis.
Information on the comparative strength properties of these alloys, as well as 2014, 6061, 7075, etc.
, is available.
The choice of temper depends upon the severity of the subsequent forming operations.
Parts having single curvature and straight bend lines with a large bend radius may be advantageously formed from heat-treated material, while a part, such as a fuselage frame, would have to be formed from soft, annealed sheet and heat-treated after forming.
Make sure sheet metal parts which are to be left unpainted are made of clad (aluminum coated material.
Make sure all sheet material and finished parts are free from cracks, scratches ( kinks, tool marks, corrosion pits, and other defects which may be factors in subsequent failure.
Forming Sheet Metal Parts.
Bend lines should preferably be made to lie at an angle to the grain of the metal (preferably 90°).
Before bending, smooth all rough edges, remove burrs, and drill relief holes at the ends of bend lines and at corners, to prevent cracks from starting.
For material in the heat-treated condition, the bend radius should be large.
Alclad sheet may be bent over slight smaller radii than the corresponding tempers of uncoated alloy.
after quenching, this alloy may be formed over appreciably smaller radii.
Heat treatment of aluminium alloy parts.
All structural aluminium alloy parts are to be heat-treated in accordance with the heat treatment instructions issued by the manufacturers of the materials.
If the heat-treatment produces warping, straighten the parts immediately after quenching.
Heat-treat riveted parts before riveting, to preclude warping and corrosion.
When riveted assemblies are heated in a salt bath, the salt cannot be entirely washed out of the crevices, thus causing corrosion.
Quenching in Hot Water or Air.
Quench material from the solution heat-treating tempera ture as rapidly as possible, with a minimum delay after removal from the furnace.
Quenching in cold water is preferred, although less drastic chilling (hot or boiling water, air blast) is sometimes employed for bulk sections, such as forgings, to minimize quenching stresses.
Transfer of 2017 alloys from the heat-treatment medium to the quenchtank should be accomplished as quickly as possible.
An elapsed time of 10 to 15 seconds will, in many cases, result in noticeably impaired corrosion resistance.
Reheating of 2017 and 2024 alloys at temperatures above that of boiling water after heat treatment, and the baking of primers at temperatures above that of boiling water, will not be considered acceptable without subsequent complete and correct heat treatment, as such practice tends to impair the original heat treatment.
Methods for repair of vital members should be specifically approved by a representative of the Federal Aviation Administration.
Aluminium alloy aerospace structures.
Extensive repairs to damaged stressed skin on monocoque types of aluminium alloy structures should preferably be made in accordance with specific recommendations of the manufacturer of the aircraft.
In many cases, repair parts, joints, or reinforcements can be designed, and proof of adequate strength shown, without the calculation of the design loads and stresses, by properly considering the material and dimensions of the original parts and the riveted attachments.
Examples illustrating the principles of this method as applied to typical repairs may be found in textbooks on metal structures.
An important point to bear in mind in making repairs on monocoque structures is that a repaired part must be as strong as the original with respect to all types of loads and general rigidity.
Avoid drilling oversized holes or otherwise decreasing the effective tensile areas of wing-spar capstrips, wing, fuselage, or fin longitudinal stringers, or other highly stressed tensile members.
Make all repairs or reinforcements to such members in accordance with factory recommendations or with the specific approval or a representative of the Federal Aviation Administration.
Disassembly Prior to Repairing.
If the parts to be removed are essential to the rigidity of complete aircraft structures, support the remaining structure prior to disassembly in such a manner as to prevent distortion and permanent damage to the remainder of the structure.
When rivets are to be removed, weaken the rivet head by drilling.
Use a drill of the same size as the rivet.
Drilling must be exact center and to the base of the head only.
After drilling, break off the head with a pin punch and carefully drive out the shank.
Removal of rivet heads with a cold chisel and hammer is not recommended because skin damage and distorted rivet holes will probably result.
Care must also be taken whenever screws must be removed for disassembly or removal of stress plates, access plates, fillets, etc.
, to avoid damage to adjoining structure.
When properly used, impact wrenches can be effective tools for removal of screws; however, damage to adjoining structure may result from excessive vertical loads being applied through the screw axis.
Excessive loads are usually related to improperly adjusted impact tools or attempting to remove screws that have seized from corrosion.
Remove seized screws by drilling and use of a screw extractor.
Structural cracks may appear in the doubler that runs parallel to the line of anchor or plate nuts installed for securing access doors or plates.
Inspect rivet joints adjacent to damaged structure for partial failure (slippage) by removing one or more rivets to see if holes are elongated or the rivets have started to shear.
Aluminium for replacement aircraft structures.
In selecting the alloy, it is usually satisfactory to use 2024 in place of 2017 since the former is stronger.
Hence, it will not be permissible to replace 2024 by 2017 unless the deficiency in strength of the latter material is compensated by an increase in material thickness, or the structural strength is substantiated by tests or analysis.
Information on the comparative strength properties of these alloys, as well as 2014, 6061, 7075, etc.
, is available.
The choice of temper depends upon the severity of the subsequent forming operations.
Parts having single curvature and straight bend lines with a large bend radius may be advantageously formed from heat-treated material, while a part, such as a fuselage frame, would have to be formed from soft, annealed sheet and heat-treated after forming.
Make sure sheet metal parts which are to be left unpainted are made of clad (aluminum coated material.
Make sure all sheet material and finished parts are free from cracks, scratches ( kinks, tool marks, corrosion pits, and other defects which may be factors in subsequent failure.
Forming Sheet Metal Parts.
Bend lines should preferably be made to lie at an angle to the grain of the metal (preferably 90°).
Before bending, smooth all rough edges, remove burrs, and drill relief holes at the ends of bend lines and at corners, to prevent cracks from starting.
For material in the heat-treated condition, the bend radius should be large.
Alclad sheet may be bent over slight smaller radii than the corresponding tempers of uncoated alloy.
after quenching, this alloy may be formed over appreciably smaller radii.
Heat treatment of aluminium alloy parts.
All structural aluminium alloy parts are to be heat-treated in accordance with the heat treatment instructions issued by the manufacturers of the materials.
If the heat-treatment produces warping, straighten the parts immediately after quenching.
Heat-treat riveted parts before riveting, to preclude warping and corrosion.
When riveted assemblies are heated in a salt bath, the salt cannot be entirely washed out of the crevices, thus causing corrosion.
Quenching in Hot Water or Air.
Quench material from the solution heat-treating tempera ture as rapidly as possible, with a minimum delay after removal from the furnace.
Quenching in cold water is preferred, although less drastic chilling (hot or boiling water, air blast) is sometimes employed for bulk sections, such as forgings, to minimize quenching stresses.
Transfer of 2017 alloys from the heat-treatment medium to the quenchtank should be accomplished as quickly as possible.
An elapsed time of 10 to 15 seconds will, in many cases, result in noticeably impaired corrosion resistance.
Reheating of 2017 and 2024 alloys at temperatures above that of boiling water after heat treatment, and the baking of primers at temperatures above that of boiling water, will not be considered acceptable without subsequent complete and correct heat treatment, as such practice tends to impair the original heat treatment.
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