Repairability of Sheet Metal Structure 5


INTRODUCTION TO AIRCRAFT STRUCTURAL REPAIR TECHNIQUES & SUBSTANTIATION

Target audience is A&P mechanics and young engineers. This blog is a tease but currently free because my original Boeing textbook “Liaison Engineering Discussion Series” is being rewritten. Formatting has been lost during cut and paste.

(CAUTION: The following is not approved data consult the SRM or contact a DER)

“Typical Repairs for Aircraft Structures
This section describes typical repairs of the major structural parts of an airplane. When repairing a damaged component or part, consult the applicable section of the manufacturer’s SRM for the aircraft. Normally, a similar repair is illustrated, and the types of material, rivets, and rivet spacing and the methods and procedures to be used are listed. Any additional knowledge needed to make a repair is also detailed. If the necessary information is not found in the SRM, attempt to find a similar repair or assembly installed by the manufacturer of the aircraft.

Corrugated Skin Repair
Some of the flight controls of smaller general aviation aircraft have beads in their skin panels. The beads give some stiffness to the thin skin panels. The beads for the repair patch can be formed with a rotary former or press brake.

ama_Ch04 99

Replacement of a Panel
Damage to metal aircraft skin that exceeds repairable limits requires replacement of the entire panel.

ama_Ch04 100A panel must also be replaced when there are too many previous repairs in a given section or area. (or contact a DER)
In aircraft construction, a panel is any single sheet of metal covering. A panel section is the part of a panel between adjacent stringers and bulk heads. Where a section of skin is damaged to such an extent that it is impossible to install a standard skin repair, a special type of repair is necessary.

The particular type of repair required depends on whether the damage is repairable outside the member, inside the member, or to the edges of the panel.

Outside the Member
For damage that, after being trimmed, has 81⁄2 rivet diameters or more of material, extend the patch to include
the manufacturer’s row of rivets and add an extra row inside the members.

Inside the Member
For damage that, after being trimmed, has less than 81⁄2 manufacturer’s rivet diameters of material inside the
members, use a patch that extends over the members and an extra row of rivets along the outside of the members.

Edges of the Panel
For damage that extends to the edge of a panel, use only one row of rivets along the panel edge, unless the
manufacturer used more than one row. The repair procedure for the other edges of the damage follows the previously explained methods.”

We will review this repair in detail as time goes by

This blog is a tease but currently free because my original Boeing textbook “Liaison Engineering Discussion Series” is being rewritten. Stay tuned for:
Structural Aircraft Repair Substantiation for non-Engineers
or
How to fix or not fix what’s broke and justify it

Feedback and questions encouraged. Comment here, email us at jim@callahan.aero, or visit our website at http://www.Callahan.aero or our  Facebook page, “Callahan Aircraft Services, LLC

Sincerely,

James W. Callahan, FAA DER

jim@callahan.aero

256-572-1438
540 N College St.
AUburn, AL  36830

Not responsible for anything below this line

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Repairability of Sheet Metal Structure 4


INTRODUCTION TO AIRCRAFT STRUCTURAL REPAIR TECHNIQUES & SUBSTANTIATION

Target audience is A&P mechanics and young engineers. This blog is a tease but currently free because my original Boeing textbook “Liaison Engineering Discussion Series” is being rewritten. Formatting has been lost during cut and paste.

“Repair of Stressed Skin Structure
In aircraft construction, stressed skin is a form of construction in which the external covering (skin) of an aircraft carries part or all of the main loads. Stressed skin is made from high strength rolled aluminum sheets. Stressed skin carries a large portion of the load imposed upon an aircraft structure. Various specific skin areas are classified as highly critical, semicritical, or noncritical. To determine specific repair requirements for these areas, refer to the applicable aircraft maintenance manual.

Minor damage to the outside skin of the aircraft can be repaired by applying a patch to the inside of the damaged sheet. A filler plug must be installed in the hole made by the removal of the damaged skin area. It plugs the hole and forms a smooth outside surface necessary for aerodynamic smoothness of the aircraft. The size and shape of the patch is determined in general by the number of rivets required in the repair. If not otherwise specified, calculate the required number of rivets by using the rivet formula. Make the patch plate of the same material as the original skin and of the same thickness or of the next greater thickness.

Patches (or Doublers)
Skin patches may be classified as two types:
• Lap or scab patch (or non-flush Doublers)
• Flush patch

(or flush, internal or external Doublers is the way I see it)

Lap or Scab Patch (or non-flush Doublers)
The lap or scab type of patch is an external patch where the edges of the patch and the skin overlap each other. The overlapping portion of the patch is riveted to the skin. Lap patches may be used in most areas where aerodynamic smoothness is not important (or not critical or cosmetics are not desired). Figure below shows a typical patch for a crack and or for a hole.

ama_Ch04 95

When repairing cracks or small holes with a lap or scab patch, the damage must be cleaned and smoothed. In repairing cracks, a small hole must be drilled in each end and sharp bend of the crack before applying the patch. These holes relieve the stress at these points and prevent the crack from spreading. (Do not leave crack, trim it out with generous radii – stop drilling dos not prevent the crack from spreading. It inhibits growth and if you must and you will use a 1/4″ diameter reamer and deburr the edges) The patch must be large enough to install the required number of rivets. It may be cut circular, square, or rectangular.

Flush Patch
A flush patch is a filler patch that is flush to the skin when applied it is supported by and riveted to a reinforcement plate which is, in turn, riveted to the inside of the skin. Figure below shows a typical flush patch repair. The doubler is inserted through the opening and rotated until it slides in place under the skin. The filler must be of the same gauge and material as the original skin. The doubler should be of material one gauge heavier than the skin.”

ama_Ch04 97 ama_Ch04 96

This blog is a tease but currently free because my original Boeing textbook “Liaison Engineering Discussion Series” is being rewritten. Posts on Instructions for Continued Airworthiness and Composites coming soon. Stay tuned for:
Structural Aircraft Repair Substantiation for non-Engineers
or
How to fix or not fix what’s broke and justify it

Feedback and questions encouraged. Comment here, email us at jim@callahan.aero, or visit our website at http://www.Callahan.aero or our  Facebook page, “Callahan Aircraft Services, LLC

Sincerely,

James W. Callahan, FAA DER

jim@callahan.aero

256-572-1438
540 N College St.
AUburn, AL  36830

Not responsible for anything below this line

Repairability of Sheet Metal Structure 3


INTRODUCTION TO AIRCRAFT STRUCTURAL REPAIR TECHNIQUES & SUBSTANTIATION

Target audience is A&P mechanics and young engineers. This blog is a tease but currently free because my original Boeing textbook “Liaison Engineering Discussion Series” is being rewritten. Formatting has been lost during cut and paste.

“Approval of Repair
Once the need for an aircraft repair has been established, Title 14 of the Code of Federal Regulations (14 CFR) defines the approval process. 14 CFR part 43, section 43.13(a) states that each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer’s maintenance manual or instructions for continued airworthiness prepared by its manufacturer, or other methods, techniques, or practices acceptable to the Administrator. (like a DER approval via Form 8110-3) AC 43.13-1 contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions.”

AC 43.13-1 contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions.

“This data generally pertains to minor repairs. The repairs identified in this AC may only be used as a basis for FAA approval for major repairs. The repair data may also be used as approved data, and the AC chapter, page, and paragraph listed in block 8 of FAA Form 337 when:

a. The user has determined that it is appropriate to the product being repaired;
b. It is directly applicable to the repair being made; and
c. It is not contrary to manufacturer’s data.

Engineering support (actually their DER or A/R)  from the aircraft manufacturer (or a DER) is required for repair techniques and methods that are not described in the aircraft maintenance manual or SRM. FAA Form 337, Major Repair and Alteration, must be completed for repairs to the following parts of an airframe and repairs of the following types involving the strengthening, reinforcing, splicing, and manufacturing of primary structural members or their replacement, when replacement is by fabrication, such as riveting or welding.

• Box beams
• Monocoque or semimonocoque wings or control surfaces
• Wing stringers or chord members
• Spars
• Spar flanges
• Members of truss-type beams
• Thin sheet webs of beams
• Keel and chine members of boat hulls or floats
• Corrugated sheet compression members that act as flange material of wings or tail surfaces
• Wing main ribs and compression members
• Wing or tail surface brace struts, fuselage longerons (or stringers)
• Members of the side truss, horizontal truss, or bulkheads
• Main seat support braces and brackets
• Landing gear brace struts
• Repairs involving the substitution of material
• Repair of damaged areas in metal or plywood stressed covering exceeding six inches in any direction
• Repair of portions of skin sheets by making additional seams
• Splicing of thin sheets
• Repair of three or more adjacent wing or control surface ribs or the leading edge of wings and control surfaces between such adjacent ribs”

This blog is a tease but currently free because my original Boeing textbook “Liaison Engineering Discussion Series” is being rewritten. Stay tuned for:
Structural Aircraft Repair Substantiation for non-Engineers
or
How to fix or not fix what’s broke and justify it

Feedback and questions encouraged. Comment here, email us at jim@callahan.aero, or visit our website at http://www.Callahan.aero or our  Facebook page, “Callahan Aircraft Services, LLC

Sincerely,

James W. Callahan, FAA DER

jim@callahan.aero

256-572-1438
540 N College St.
AUburn, AL  36830

Not responsible for anything below this line

Repairability of Sheet Metal Structure 2


INTRODUCTION TO AIRCRAFT STRUCTURAL REPAIR TECHNIQUES & SUBSTANTIATION

Target audience is A&P mechanics and young engineers. This blog is a tease but currently free because my original Boeing textbook “Liaison Engineering Discussion Series” is being rewritten. Formatting has been lost during cut and paste.

“Inspection for Corrosion
Corrosion is the gradual deterioration of metal due to a chemical or electrochemical reaction with its environment. The reaction can be triggered by the atmosphere, moisture, or other agents. When inspecting the structure of an aircraft, it is important to watch for evidence of corrosion on both the outside and inside. Corrosion on the inside is most likely to occur in pockets and corners where moisture and salt spray may accumulate; therefore, drain holes must always be kept clean. Also inspect the surrounding members for evidence of corrosion.

Damage Removal
To prepare a damaged area for repair:”  STOP HERE IF YOU PLAN ON USING A DER or at least me.

Contact me before removing material to any standard practice.  Most likely I will contour with your approach but sometimes it is salvaging the part or drastically minimizing the repair.  DERs do not work to any standard practices but we generate new data analyzing the repair.  There have been many times I saved thousands, tens of thousands of dollars from deviating from published or standard practice actually making the aircraft safer. A good engineer may save structure, a poor engineer replaces parts. A good doctor may save limbs, poor doctors amputate. But we can’t save the part if not called early enough and remember we represent FAA engineers not inspectors.  We shouldn’t kiss and tell as long as the aircraft will be safe when it returns to flight.

“Damage Removal
To prepare a damaged area for repair:

1. Remove all distorted skin and structure in damaged area.
2. Remove damaged material so that the edges of the completed repair match existing structure and aircraft lines.
3. Round all square corners.
4. Smooth out any abrasions and/or dents.
5. Remove and incorporate into the new repair any previous repairs joining the area of the new repair.

Repair Material Selection
The repair material must duplicate the strength of the original structure. If an alloy weaker than the original material has to be used, a heavier gauge must be used to give equivalent cross-sectional strength. A lighter gauge material should not be used even when using a stronger alloy. (unless a DER will approve it)

Repair Parts Layout
All new sections fabricated for repairing or replacing damaged parts in a given aircraft should be carefully laid out to the dimensions listed in the applicable aircraft manual (or DER instructions) before fitting the parts into the structure.

Rivet Selection
Normally, the rivet size and material should be the same as the original rivets in the part being repaired. If a rivet hole has been enlarged or deformed, the next larger size rivet must be used after reworking the hole. When this is done, the proper edge distance for the larger rivet must be maintained (I respectively disagree). Where access to the inside of the structure is impossible and blind rivets must be used in making the repair, always consult the applicable aircraft maintenance manual (or DER instructions) for the recommended type, size, spacing, and number of rivets needed to replace either the original installed rivets or those that are required for the type of repair being performed.

Rivet Spacing and Edge Distance
The rivet pattern for a repair must conform to instructions in the applicable aircraft manual (or DER instructions). The existing rivet pattern is used whenever possible.

Corrosion Treatment
Prior to assembly of repair or replacement parts, make certain that all existing corrosion has been removed in the area and that the parts are properly insulated one from the other.”

This blog is a tease but currently free because my original Boeing textbook “Liaison Engineering Discussion Series” is being rewritten. Stay tuned for:
Structural Aircraft Repair Substantiation for non-Engineers
or
How to fix or not fix what’s broke and justify it

Feedback and questions encouraged. Comment here, email us at jim@callahan.aero, or visit our website at http://www.Callahan.aero or our  Facebook page, “Callahan Aircraft Services, LLC

Sincerely,

James W. Callahan, FAA DER

jim@callahan.aero

256-572-1438
540 N College St.
AUburn, AL  36830

Not responsible for anything below this line