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

9.2 Composites


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.

composites

Aircraft composites have been around since Orville, Wilbur, and Chuck. That’s Orville and Wilbur Wright, and Charles Taylor with Taylor being the first aircraft and power plant mechanic, although not certified since there was no regulatory agency at the time.

The Wright brothers used wood natures composite and plywood being a man-made solid laminate composite.

wright    figure 9.1

I heard Wilbur told Orville, “this has never been done before.” kitty hawk

Fiberglass is a lightweight, extremely strong, and robust material. Although strength properties are somewhat lower than carbon fiber and it is less stiff, the material is typically far less brittle, and the raw materials are much less expensive. Its bulk strength and weight properties are also very favorable when compared to metals, and it can be easily formed using molding processes.

Carbon fibers are lightweight and initially twice as strong as the same weight of steel. Carbon fibers find their way into bulletproof vests, high performance aircraft, automobile tires, and sports equipment.

history

langley-twinA composite is when two or more different materials are combined together to create a superior and unique material. The first uses of composites date back to the 15

00s B.C. when early Egyptians and Mesopotamian settlers used a mixture of mud and straw to create strong and durable buildings. Straw continued to provide reinforcement to ancient composite products including pottery and boats.

Plywood has been made for thousands of years; the earliest known occurrence of plywood was in ancient Egypt around 3500 BC when wooden articles were made from veneers glued together crosswise.

Wood was among the first materials used to construct aircraft. Most of the airplanes built during World War I (WWI) were constructed of wood frames with fabric coverings. Wood was the material of choice for aircraft construction into the 1930s.

will wood return and replace aluminum in aviation?

figure 9.2 langley twin

Engineers at the Owens Illinois Glass Company and Corning Glass Works develop several means to make glass fibers commercially viable. Composed of ingredients that constitute regular glass, the glass fibers produced in the 1930s are made into strands, twirled on a bobbin, and then spu

n into yarn. Combined with plastics, the material is called fiberglass.

Fiberglass, Fiber-reinforced polymer (FRP), Glass-reinforced plastic (GRP), Glass-Fiber Reinforced Plastic (GFRP) is a fiber reinforced polymer made of a plastic matrix reinforced by fine fibers of glass. It is also known as Glasfaserverstärkter Kunststoff (GFK in German), Gurasufaibā (in Japanese), زجاج مغزول (in Arabic), fibra de vidro (in Portuguese), fiberglas (in Turkish), and fibreglass (in English, the Queen’s).

Fiber-reinforced polymer (FRP) composite materials date back to the early 1940’s in the defense industry, particularly for use in aerospace and naval applications. The U.S. Air Force and Navy capitalized on Fiber-reinforced polymer (FRP) composites high strength-to-weight ratio capability, resistance to weather, and salt air corrosive effects.

British engineers in 1964 develop carbon (graphite) fiber by stretching synthetic fibers and then heating them.

“In the world of aviation, composites may be the most significant materials. Around 1960’s saw the need of carbon fiber composite embedded in resin. None other than U.S. Air Force and U.S. Navy used this material in many different applications. Major applications are particularly seen in aircraft control surfaces suseahawkch as rudders, primarily controlling the movement of the airplane’s nose to the left or right directions; and ailerons, responsible for the longitudinal movement of the airplane. The pinnacle of success for the material’s application was being brought to known in public with its major participation in the production of Boeing 787 Dreamliner.

Boeing 787 Dreamliner prides itself as the most innovative airplane ever invented in the history of mankind. This is the first ever airplane that uses carbon fiber in most of its parts. It is in this regard that its developer, Boeing Commercial Airplanes, boasts that the state-of-the-art airplane is the most fuel-efficient among all airliners in the world since it only consumes twenty percent less fuel than any similarly-sized planes. Since it is lighter in weight compared to other airliners, it has smaller engine, thus the need for minimal amount of fuel.” Wes Collins

figure 9.3 boeing 787                                       

Good stuff but say good bye to your old expectations of emergency field repairs of primary structure like pressurized fuselage skins keeping revenue aircraft in service. Damaged structural plies may not have simple temporary repairs. Heard of boiler plate proposals over the damaged plies but that may require specific approval meaning even more time.

workmanship

What are the downsides of carbon (graphite) fiber repairs that mechanics need to be aware of compared to fiberglass repairs? There is no difference except when it comes to primary structure. We don’t use fiberglass in primary structure and primary structure is critical therefore the repair has less margin for error. Carbon (graphite) fiber repairs on primary structure are critical and therefore must be done right.

Poor workmanship can not be remedied by any repair and every subsequent poorly repaired structure may require another repair approval costing more money and time.

The following list lays out some common concerns for composite repairs:

  • Repair facilities are expensive

  • Repair facilities must remain uncontaminated

  • Repair facilities must be environmentally controlled

  • Repair materials have a shelf life

  • Repair materials must remain uncontaminated

  • Repair materials are not cheap

  • Repair materials may be hazardous

  • Repairs take time, much more time

  • Repairs require holding fixtures

  • Repair technicians require training

  • Repair technicians require protective clothing

  • Repair technicians require protective gear

  • Repair processes are definitive and compliance is a must

But carbon (graphite) fiber is good stuff just no “metal hammers,” greasy fingers, or hap hazard slap it together attitude. That comment about greasy fingers is anyone’s fingers no matter how washed – use gloves! Humans are greasy, some more than others but we are all greasy.

Working the B-1B bomber’s flaps, I was directed to go through the flap assembly process and figure out why the flap couldn’t pass its leak test. Titanium ribs, aluminum honey comb core, carbon fiber spar and carbon fiber skins assembled on a steel tool and baked for hours led to warped ribs. All materials had a different coefficient of thermal expansion so each part expanded at different rates while in the autoclave or oven and then cured into place. But that different cause the leak.120327-F-UR602-001figure 9.4  USAF b1-b

My investigation continued into the clean room and discovered the source. Prior to any assembly, cutting, or ply orientation the geniuses picked up the individual plies with their greasy index finger and thumb at one corner. You guessed it, at the corner that leaked. After laying each ply down carefully they to put their gloves on.

SR22_960x480_1figure 9.5 cirrus sr22

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Just thought that would be nice.

Cirrus SR22, SRM 51-20 available for free online at http://servicecenters.cirrusdesign.com/techpubs/pdf/AMM/SR22/pdf/51-20.pdf

Carbon (graphite) fiber material varies significantly from aircraft make and model. Do not make an unapproved material substitute for plies or resin. Composite material is determined by the airframe or power plant manufacturer and not the material manufacturing.

Prepreg plies in lieu of ply and resin combination is not a material substitution but a process change. In most cases this is not only acceptable but preferred Check the OEM manual in case it states otherwise.

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 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