# U.S. Department of Transportation![](media/image1.wmf){width="0.5506944444444445in" height="0.5708333333333333in"}

## Federal Aviation Administration

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  Subject:   **[INFORMATION]{.underline}**: Bonded Date:   **September 2, 2005**
             Joints and Structures - Technical             
             Issues and Certification                      
             Considerations; PS-ACE100-2005-10038          

                                                           

  From:      Acting Manager, Small Airplane        Reply   Lester Cheng;
             Directorate, ACE‑100                  to      316-946-4111
                                                   Attn.   
                                                   of:     

                                                           

  To:        See Distribution                              
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**1.0 General**

Many manufacturing and repair applications for aircraft structures in
small airplanes, transport airplanes, rotorcraft, and propellers use
bonding. Many technical issues for bonding are complex and require
cross-functional teams for successful applications. Government agencies
and the aircraft industry combined their adhesive bonding experiences
and technical insights to gain mutual safety benefits. In 2004, the FAA
conducted a survey and two workshops to benchmark industry practices for
structural bonding.

In general, bonded structures may include composite-to-composite,
composite-to-metal, and metal-to-metal. The nature and technical
parameters that govern these three types of bonded structures are, in
essence, the same or closely related. This policy statement is
applicable to these three types. Both manufacturing and maintenance
(repair) can use structural bonding applications.

1.1 Purposes and Scope

Building on data collected through the survey and workshops, the Small
Airplane Directorate developed this policy statement. The purposes of
this policy statement include: (1) to review the critical
safety/technical issues, (2) to highlight some of the successful
engineering practices employed in the industry, and (3) to present
regulatory requirements and certification considerations pertinent to
bonded structures. This policy statement also identifies some available
guidance and technical resources for reference purposes. This policy
statement applies to part 23 airplanes.

To achieve continued operational safety, the bonding applications
require integrated considerations involving design, manufacturing and
maintenance. The scope of this policy statement includes (1) material
and process qualification and control, (2) design development and
structural substantiation, (3) manufacturing implementation, and
(4) repair implementation.

Section 3 identifies key technical issues for bonded structures. Section
4 addresses specific certification considerations for the same technical
issues, providing regulatory policy for bonded structures. Section 3
also summarizes some proven engineering practices used to address the
key technical issues for bonded structures. Consider such information as
guidelines or recommendations, which are documented in greater detail in
an FAA Technical Report \[DOT/FAA/AR-05/13\].

1.2 Bonded Structure Applications

Traditionally, the general aviation (small airplanes) industry often led
in developing bonding applications. These applications extended from
metal bonding in primary load bearing applications to the extensive use
of composite bonding in new prop-driven airplanes (for example, sandwich
skin panels, attachments and major splices). Transport airplanes widely
employed bonded attachments for some metal and most composite structures
(for example, stringers and sandwich panels). Rotorcraft and propellers
used bonded structures in some airframe and dynamic parts (for example,
rotor blades and propellers have bonded metal and composite structure).
For maintenance functions, bonded repairs are typically used with
sandwich panels for all types of products.

Using bonded structure has advantages such as cost saving and weight
reduction; therefore, new applications are expanding and challenging the
qualified workforce. Development of guidance and training is a high
priority to maintain the required level of aviation safety both for
initial and continued airworthiness.

The application criticality of bonded structures may be assessed by a
few key parameters. These parameters include (1) structure
function/configuration (for example, primary versus secondary, single
versus multi load path), (2) loading types/characteristics,
(3) environment of operation, and (4) service experience. In general,
more critical applications require more stringent process control and
substantiation for ensuring structural integrity.

1.3 Composite Safety and Certification Initiatives

In 1999, the Federal Aviation Administration (FAA) established the
Composite Safety and Certification Initiatives (CS&CI) program for work
with industry, government agencies, and academia to update certification
guidance materials and support standardized composite engineering
practices. To support this mandate, the Small Airplane Directorate has
developed several guidance materials through the years. The guidance
includes \"Materials Qualification and Equivalency (Policy, 2003),\"
\"Static Strength Substantiation (Policy, 2001),\" \"Substantiation of
Secondary Composite Structures (Policy, 2005),\" and \"Acceptance
Guidance on Material Procurement and Process Specifications (AC,
2003).\" These guidance materials are listed under Sections 2.2 and 2.3.

Under CS&CI, a parallel effort was also performed in the Rotorcraft
Directorate. From 2000 to 2002, Aviation Rulemaking Advisory Committee
(ARAC) efforts for a new rule (i.e., Sections 27.573 and 29.573) and
advisory circular materials were conducted. This activity resulted in an
updated AC 29-2C-MG 8 (2005). In this AC, process quality control is
highlighted as a primary focus to ensure the long-term performance of
bonded joints. Additional thoughts on bonding are also presented
therein.

An international working group under \"The Technical Cooperation Program
(TTCP)\" drafted a \"Certification of Bonded Structure\" document in
2001, following three years of coordinated efforts. This document
provided a good basis in general guidance for certification and
recommended future industry interface to establish more guidance. We
followed this recommendation in defining the FAA\'s industry survey and
bonded structures workshop as outlined below.

1.4 Industry Survey and Bonded Structures Workshop

In 2004, tasks were initiated to benchmark industry practices for
structural bonding. The efforts included a survey and two bonded
structures workshops (June 2004 in Seattle, Washington and October 2004
in London, United Kingdom) to engage experts from around the world.
These efforts addressed the full scope of the continued operational
safety. Technically, it covers (1) material and process qualification
and control, (2) design development and structural substantiation,
(3) manufacturing implementation and experience, and (4) maintenance
implementation and experience. FAA Technical Report \[DOT/FAA/AR-05/13\]
documents the results of these efforts.

The bonded structures survey and workshops resulted in a large amount of
data to benchmark industry practices. This information forms the
technical foundation for this policy statement.

**2.0 Regulations, Guidance and Supporting References**

2.1 Federal Regulations

The regulations closely related to this policy statement include:

**14 CFR Part 21, Subpart G - Production Certificates**

Section 21.139 Quality control.

Section 21.143 Quality control data requirements; prime manufacturer.
Section 21.147 Changes in quality control system.

**14 CFR Part 23, Subpart C -- Structure**

Section 23.305 Strength and deformation.

Section 23.307 Proof of structure.

Section 23.573 Damage tolerance and fatigue evaluation of structure.

Section 23.575 Inspections and other procedures.

**14 CFR Part 23, Subpart D - Design and Construction**

Section 23.601 General.

Section 23.603 Materials and workmanship.

Section 23.605 Fabrication methods.

Section 23.609 Protection of structure.

Section 23.611 Accessibility provisions.

Section 23.613 Material strength properties and design values.

Section 23.865 Fire protection of flight controls, engine mounts, and\
other flight structure.

**14 CFR Part 23, Subpart G - Operating Limitations and Information**

Section 23.1529 Instructions for Continued Airworthiness.

> **14 CFR Part 43 - Maintenance, Preventive Maintenance, Rebuilding\
> and Alteration**

Except for § 23.573(a), these regulations are generic in nature and
applicable to both metal and composite structure. Section 23.573(a) sets
forth the requirements for substantiating the primary composite airframe
structures, including considerations for damage tolerance, fatigue, and
bonded joints. For any bonded joint, § 23.573(a)(5) prescribes that
\"the failure of which would result in catastrophic loss of the
airplane, the limit load capacity must be substantiated by one of the
following methods\--

\(i\) The maximum disbonds of each bonded joint consistent with the
capability to withstand the loads in paragraph (a)(3) of this section
must be determined by analysis, tests, or both. Disbonds of each bonded
joint greater than this must be prevented by design features; or

\(ii\) Proof testing must be conducted on each production article that
will apply the critical limit design load to each critical bonded joint;
or

\(iii\) Repeatable and reliable non-destructive inspection techniques
must be established that ensure the strength of each joint.\"

2.2 Advisory Circulars

The following advisory circulars (ACs) relate to this policy statement.
They are available on the Internet at [www.faa.gov](http://www.faa.gov/)
or you may request a copy at no cost from the U.S. Department of
Transportation, Subsequent Distribution Office, Ardmore East Business
Center, 3341 Q 75^th^ Avenue, Landover, MD 20785.

AC 20-107A \"Composite Aircraft Structure\" (April 1984)

AC 21-26 \"Quality Control for the Manufacture of Composite\
Structures\" (June 1989)

AC 23-20 \"Acceptance Guidance on Material Procurement and\
Process Specifications for Polymer Matrix Composite Systems\" (September
2003)

AC 145-6 \"Repair Stations for Composite and Bonded Aircraft\
Structure\" (November 1996)

2.3 Policy Statements

The policy statements closely related to this policy include:

> "Policy on Acceptability of Temperature Differential between Wet Glass
> Transition Temperature (Tgwet) and Maximum Operating Temperature (MOT)
> for Epoxy Matrix Composite Structure" PS-ACE100-2-18-1999, February
> 1999
>
> \"Static Strength Substantiation of Composite Airplane Structure\"
>
> PS-ACE100-2001-006, December 2001
>
> "Material Qualification and Equivalency for Polymer Matrix Composite
> Material Systems\" PS-ACE100-2002-006, September 2003
>
> "Substantiation of Secondary Composite Structures\"
> PS-ACE100-2004-10030, April 2005

The policy statements are available on line at
[www.faa.gov](http://www.faa.gov/) or you may request a copy from the
Small Airplane Directorate at 901 Locust, Room 301, Kansas City, MO
64106.

2.4 Supporting References

The FAA Technical Center publishes the following technical reports, and
they are available through the National Technical Information Service
(NTIS), Springfield, Virginia 22161. They are also available in Adobe
Acrobat portable document format (PDF) at the FAA William J. Hughes
Technical Center\'s Full-Text Technical Reports page:
actlibrary.tc.faa.gov.

> \"Certification Testing Methodology for Composite Structures, Volumes
> I and II\" DOT/FAA/CT-86/39, October 1986
>
> "Handbook: Manufacturing Advanced Composite Components for Airframes"
> DOT/FAA/AR-96/75, April 1997
>
> \"Advanced Certification Methodology for Composite Structures\"
> DOT/FAA/AR-96/111, April 1997
>
> "Effects of Surface Preparation on the Long-Term Durability of
> Adhesively Bonded Composite Joints" DOT/FAA/AR-03/53, July 2003
>
> "Bonded Repair of Aircraft Composite Sandwich Structures,"
> DOT/FAA/AR-03/74, February 2004
>
> \"Assessment of Industry Practices for Aircraft Bonded Joints and
> Structures,\" DOT/FAA/AR-05/13, July 2005

The Department of Defense (DOD) and the FAA jointly maintain
MIL-HDBK-17. The industry has helped to develop MIL-HDBK-17. The
handbook provides guidance for composite applications, and has three
volumes of information on [p]{.underline}olymer [m]{.underline}atrix
[c]{.underline}omposite (PMC) materials. The current PMC handbooks are
at Revision F (Year 2002). Volumes 1, 2, and 3 contain valuable
information closely related to this policy statement. This handbook can
also serve as a reference for most of the terms used in this document.
You may either get MIL-HDBK-17 from the Department of Defense Single
Stock Point (DODSSP), 700 Robbins Avenue, Building 4D, Philadelphia, PA
19111, or buy this handbook through ASTM International\'s website at
\"www.astm.org\" \[note: search using keyword \"MIL-17\"\].

Consistent and repeatable industry engineering practices would help to
assure compliance with the requirements related to bonded joints and
structures. MIL-HDBK-17, ASTM International, and Society of Automotive
Engineering (SAE International) Aerospace Materials Specifications (AMS)
Committee P-17 are jointly pursuing standardization. The SAE Commercial
Aircraft Composite Repair Committee (CACRC) has also been a partner in
standardizing engineering repair and maintenance procedures. For
example, the CACRC has published "Design of Durable, Repairable, and
Maintainable Aircraft Composites," (SAE AE-27, 1997).

**3.0 Technical Issues**

[]{.mark}

Key technical issues for bonded structures are complex and depend on
many factors. As a result, successful applications rely on coordinated
engineering work, which crosses several disciplines including material,
process, design, analysis, manufacturing and repair. Much of the
complexity relates to the bonded interface between substrate and
adhesive materials. Consider this interface a unique material because
the substrate is altered by the selected surface treatment to chemically
bond to the adhesive during cure. Surface treatments also remove or
abrade the substrate to increase the surface area for mechanical
interlocking with the adhesive. As with any material or process used in
aircraft structure, the goal is to develop controls that yield a bond
with reliable and repeatable performance. In general, the material and
process controls for bonding are more stringent than those for other
fabrication processes.

> Materials and processes used in bonding meet qualification standards
> related to structural integrity and long-term durability. Once
> qualified, material and process controls are used to ensure these
> standards are met in subsequent production and maintenance activities.
> Recognition of the load paths and local stress distribution is needed
> to design bonded joints, attachments or repair details that will meet
> load and environmental requirements without significant degradation
> over time. Manufacturing, tooling and maintenance considerations are
> also important to design. Bond process scale-up, design data
> development, and structural substantiation are coordinated for proof
> of concept during certification. Ultimately, bonding procedures,
> process controls, documentation, and training are implemented in
> either manufacturing or repair, or both, to reliably produce the
> proven structural concept.

3.1 Material and Process Qualification and Control

Qualification demonstrates the stiffness, strength, durability and
reliability of bond materials and processes for aircraft applications.
Such efforts start with defining bonding processes and selecting
compatible substrate, adhesive, and ancillary materials.

Qualification tests are carefully selected to evaluate structural
performance, environmental effects and long-term durability. In
coordination with qualification, it is also important to establish (1)
processing tolerances, (2) material handling and storage limits, and (3)
key characteristics and process parameters to monitor in quality
control. Most bond failures and problems in service have been traced to
invalid qualifications or insufficient quality control of production
processes.

Adhesive and substrate material selections should consider the bonded
joint design configuration, loading requirements, manufacturing process
constraints, environmental conditions and chemical resistance to fluids
found in service. Reduction in strength properties at the maximum
operating temperature (MOT) should be known for each application. The
properties of polymer composite substrate and adhesive materials are
affected by both temperature and moisture content, with a significant
decrease in strength above the wet glass transition temperature
(T~gwet~). A simple guideline often used in selecting composite
substrate materials is for the T~gwet~ to be 50 °F greater than the MOT
of structural applications. The analogous guideline for adhesive
materials is for the T~gwet~ to be 30 °F greater than the MOT. As
discussed in the ACE-100 1999 Policy Statement (see Section 2.3), more
rigorous environmental testing for specific design detail is expected
when selected polymer materials do not meet such guidelines. For
example, adhesives used for windshields have relatively low T~gwet~ but
meet the requirements for such an application when subjected to extreme
temperatures. Extensive field experience showing the suitability of a
given adhesive applied in a particular application would supersede the
need for additional tests. Regardless of meeting guidelines, any changes
in material properties at temperatures above the MOT (for example,
runway sun exposure) also need to be understood. An understanding of the
intended manufacturing or repair facility's tooling concepts and process
steps is also useful in material selections. Material suppliers often
have data that can aid in selecting compatible materials for use in
bonding.

Bond fabrication procedures should be well defined to perform trials
that demonstrate compatibility between the substrates, adhesive, and
processes. Some of the most important process steps relate to substrate
storage, handling, and surface preparation. Reliable adhesive bonding
depends on (1) a surface free of contamination, (2) a chemically active
surface, and (3) dry surface.

The storage, handling, and surface preparation procedures needed to
accomplish these characteristics will depend on the specific substrate
and adhesive materials. There are distinct differences for composite and
metal substrate materials. Sandwich core materials such as honeycomb and
foam will also have unique process differences, depending on the base
material type. Film adhesives have storage and handling requirements
similar to pre-impregnated, uncured composite materials. Paste adhesives
will likely have different components that require precise mixing before
application.

Another important aspect of the bonding process is adhesive cure. The
bonding surfaces must contact the adhesive with sufficient pressure and
temperature to accomplish cure. The sequence, ramp rates, and dwell
times for temperature and pressure application are defined in the cure
cycle. Bond process tolerances should be explored and defined for resin
mixing, surface preparation, and cure processing steps to document
preliminary specifications during material compatibility trials. Such
trials are carried out before proceeding with complete material and
process qualifications. It is also wise to perform some bond process
trials with representative structural geometry and tooling to ensure the
bonding procedures can be taken beyond the laboratory scale. Material
suppliers should help define bond process procedures.

Bonding materials and processes are qualified for use in structural
applications. The overall goal of qualification testing is a basic
characterization, which best represents the materials and processes in
the application. As a result, specific specimen types and test
conditions used in qualification will depend on the application. Most
programs use some qualification tests to help set benchmarks in
chemical, physical, process, and mechanical properties, which are used
with other quality checks for subsequent material and process control.
Enough unique batches of materials, independent bonding process trials
and test repetitions are used to ensure a representative population for
reliable benchmarks.

Substrate materials used in aircraft products are typically qualified
before bond processes. Adhesive material qualification may also occur
separate from bond process qualification. This is a common industry
practice when an adhesive is going to be used with several different
substrate materials because it is desirable to control the material with
a set of properties that are not dependent on the specific bonding
application. A separate bond process qualification is needed for each
unique combination of materials because of the complex interface between
substrate and adhesive, which depends on many variables (for example,
material compatibility, surface preparation and cure kinetics).
Sometimes, the adhesive and bond process qualification are combined for
a specific set of materials.

The industry uses several different physical, chemical, and mechanical
tests for adhesive material and bond process qualification. Physical and
chemical tests may be used to control surface preparation, adhesive
mixing, viscosity, and cure properties (for example, density, degree of
cure, glass transition temperature). Industry uses lap shear stiffness
and strength as the most common bonded joint mechanical tests for
adhesive and bond process qualification. In addition to shear testing in
an ambient environment, variables include extreme service temperature
and moisture content conditions.

Although useful for material characterization and design, shear tests do
not provide a reliable measure of long‑term durability and environmental
degradation associated with poor bonding processes. Some form of a peel
test has proven more reliable for evaluating proper adhesion. This
relates to the mechanical interlocking that occurs when adhesive
penetrates the roughened surface of the substrate after preparation. The
apparent bond static shear strength and stiffness may appear sufficient
with mechanical interlocking, even if little or no chemical bond has
formed during the process. Without chemical bonding, the so-called
condition of a "weak bond" exists because mechanical interlocking is not
adequate when the bonded joint is either loaded by peel forces or
exposed to the environment over a long period of time, or both.

A wedge specimen that combines peel loads and extreme environmental
conditions has proven to be a good accelerated lab test for detecting
unacceptable metal bond processes, which degrade over time and lead to
adhesion failures in service. Other specimen types that have been used
to evaluate weak bonds for composites include double cantilever beam
(DCB) and flatwise tension. Bonded sandwich specimens often use rolling
drum peel or flatwise tension tests for similar assessments.

Adhesion failures, which indicate the lack of chemical bonding between
substrate and adhesive materials, are considered an unacceptable failure
mode in all mechanical test types. It is important to examine bonded
joint failures to make an accurate assessment of whether adhesion
failures are evident. In peel testing such as a metal bond wedge test,
adhesion failure is the primary assessment. Material or bond process
problems that lead to adhesion failures are solved before proceeding
with qualification testing.

Material procurement specifications are used to control substrates,
adhesives, and other materials important to the bonding process. This
includes ancillary materials such as peel ply, which are known to affect
the bond surface. The material specification will include requirements
used for qualification and subsequent acceptance testing. A film
adhesive specification has storage and handling requirements. A paste
adhesive specification has such information for all components. The
volatile content of adhesives is controlled for repeatable processing.

Users and suppliers should document agreements on what constitutes
significant material changes, which may occur over time. Procedures
should be available to judge whether a minor change can be validated by
equivalency sampling tests or whether a complete requalification is
needed for major changes.

Process specifications are needed to control adhesive bonding.
Cleanliness and environmental controls are defined for shop areas used
in bonding. Substrate storage and handling requirements are also
documented in the specification or equivalent documents. Some key bond
fabrication steps that use stringent process control include (1) surface
preparation, (2) adhesive mixing and application, (3) bondline thickness
tolerances, (4) cure cycle, and (5) post-bond inspection.

A "process control mentality," which includes a combination of
in-process inspections and tests, has proven to be the most reliable
means of ensuring the quality of adhesive bonds in successful
applications. As for qualification tests, any process control test that
reveals adhesion failures indicates a problem that needs to be resolved.
The importance of in-process inspections and tests is magnified by the
lack of reliable non-destructive inspections (NDI) to detect weak bonds
or conditions that ultimately lead to adhesion failures in service.
Although NDI methods (for example, ultrasonics) have limits in detecting
weak bonds, they provide an important, final process control check of
other defects found with bonded joints (for example, porosity,
de-bonding and foreign material inclusions).

Documented procedures are needed to control process changes, which may
be desired over time. If a change is minor, it may be validated through
equivalency sampling tests. A complete bond process requalification is
needed for major changes.

3.2 Design Development and Structural Substantiation

Most bonded joints and attachments are designed to transfer shear loads.
Local peel forces are also present in a bond stress distribution, but
structural details are typically designed to minimize such stress. The
stress distribution and strength of a bonded joint or attachment relates
to the substrate geometry (for example, thickness, taper angles),
bondline thickness, bond overlap length and, if using composite
substrates, laminate lay‑up or fiber architecture. Residual stresses can
be an important design consideration, depending on the directional
differences in thermal expansion and stiffness of substrate materials.

Many bonded joints and attachments are designed to be fail-safe as
related to small airplane regulations for composite damage tolerance.
Alternate load paths are achieved by using fasteners or additional
splices attached in a second bonding operation. Due to the mechanisms of
bonded and bolted joints, the joint or attachment capability is designed
assuming only the bond or bolts are transferring loads.

Analysis methods used by the industry to design bonded joints and
attachments range from crude models to simple two-dimensional analyses
to software-based tools, which include more refined structural
definition. Crude models, which convert shear flow and other loads to an
average shear stress, usually calculate bonded joint capability using
very conservative design values. Despite the inherent conservatism, such
an approach requires considerable testing of specific design detail in
structural substantiation.

Both simple analyses and software-based tools predict local shear and
peel stress distributions, which help design joint parameters for
optimal performance. Some models include nonlinear elastic and plastic
adhesive behavior for further joint optimization. Sufficient bond
overlap length should be designed to ensure plastic deformation occurs
without a risk to bond integrity or damage accumulation. In one
guideline, the overlap length is designed to carry all loads by adhesive
plastic deformation, with sufficient elastic trough away from the joint
ends to provide creep resistance. Software models of adhesive joint
geometry and load conditions allow further design refinement (for
example, analysis of joggles).

An integrated product team (IPT) helps address important manufacturing,
tooling, and maintenance considerations in bonded joint design. One
related IPT goal is to design structural detail that can be reliably
produced in the factory to meet the performance requirements. Similarly,
the IPT needs to recognize the maintenance implications of future
inspection and repair activities for bonded structural details.

Design criteria, analysis, and test data are needed for timely
disposition of manufacturing defects and service damage, which are found
in the factory and field, respectively. Sophisticated analysis methods
to predict the effects of bond defects and damage scenarios continue to
evolve; however, most applications to date depend on test data. This is
particularly true when considering damage as complex as that caused by
foreign object impact.

The design of bonded repairs uses many of the same procedures and tools
applied to bonded joints and attachments. A typical bonded repair
considers the patch geometry, scarf angle, and bondline thickness.
Residual stresses resulting from a difference in the laminate lay‑up,
stiffness, and thermal expansion properties between the bonded patch and
base part also need to be considered in repair design and analysis.

As discussed in PS-ACE100-2001-006, the overall approach applied for
composite structural substantiation dictates design data development.
The same philosophy applies for bonded structures. Some benefits are
possible using a building block approach with refined analyses and test
correlations for structural details, which range in size and assembly
completeness (coupons, elements, subcomponents and components).
Alternative approaches, based on crude analyses and a conservative
demonstration of strength at the large scale, typically have more
constraint. There is more freedom to expand beyond the specific
structural details, damages, defects, and repairs addressed in
large-scale tests if there is a more refined correlation between the
analysis and tests within the building block approach.

Design data development includes characterization for minimum and
maximum service temperatures, as well as the moisture content possible
after years in service. A service damage threat assessment is also
needed to define the full scope of structural tests and analyses. The
materials, conditions, and processes used for repair in the factory and
field often differ and need to be substantiated separately within the
building block approach. Technicians involved in bonded joint testing
should be trained to identify adhesion failures. Any adhesion failures
noted through the course of building block testing are considered
unacceptable.

The long-term durability of bonded joints also needs to be addressed. As
discussed in Section 3.1, bond process qualifications usually initiate
these efforts using accelerated test procedures that force an assessment
of the chemical bonding. The long-term environmental exposure to
temperature, moisture and other fluids found in service is also
characterized by testing, as discussed in Section 3.1. Element and
subcomponent tests provide further proof of acceptable bonding processes
applied at larger structural scales.

Final substantiation of static strength, fatigue, and damage tolerance
relies on large-scale test components manufactured using production
processes. Building block analyses and tests can be used to determine
most of the effects of structural details, damage, defects and repair.
However, large-scale tests are still needed for final proof of the
design and manufacturing characteristics of configured structures, which
include bonded joints, cutouts, damage, repairs and combined loads. This
includes validation of load paths and a final strength assessment. For
example, integrally bonded airframes often have secondary load paths and
complex failure modes that are difficult to predict.

Moisture and temperature property degradation is often handled in
large-scale tests using overload factors derived from smaller scale
tests. One issue that is difficult to directly address in large-scale
tests is the completed cure of the adhesive and the availability of
related peak moisture and temperature-dependent properties at the
structural level. In addition to the process controls used to ensure
sufficient temperature conditions for the required cure cycle during
fabrication, some laboratory testing of samples cut from the bonded
structure can provide the necessary substantiation of manufacturing
processes.

3.3 Manufacturing Implementation

When implementing new bonding facilities and fabrication processes,
manufacturing trials are needed to develop an understanding of
structural details that can be reliably produced. Scaling issues, which
directly relate to structural details, are likely for several bond
process steps including surface preparation, adhesive application, and
cure control. Tooling used for bond assembly and cure will depend on
mating part geometry, cured tolerance controls, and other factors that
relate to design details. An iterative process of defining structural
details and performing manufacturing trials, and testing for performance
has proven successful for new design development in the past. Such
efforts become more efficient as a manufacturer gains experience in
bonding applications.

Special facilities and procedures are needed to manage and control
materials and key bonding process steps. The associated issues relate to
cleanliness, environmental conditions, storage, material life,
processing records, staff training, and maintenance. Material control
discussed in Section 3.1 as pertaining to procurement specifications is
applied to ensure batch-to-batch consistency. Factory procedures and
records manage the storage conditions, shelf‑life, out‑life and handling
of all uncured materials to ensure they meet the associated
specification requirements. Cold storage conditions and handling
procedures (for example, time required in the ambient environment before
unpacking for use to avoid condensation) are needed for some types of
adhesives. The environmental storage conditions of some substrate
materials may also need to be controlled (substrate materials and
bonding processes affected by pre-bond moisture). Facilities and
procedures for control of the bond assembly environment, equipment,
tooling and factory personnel are needed to manage all sources of
contamination. Expendable materials used in bonding processes are also
controlled to manage sources of contamination and avoid changes in bond
process steps. For example, chemicals and blast media used for different
surface preparation techniques are controlled, discarded, and replaced
as needed to maintain process standards. Factory maintenance of
facilities, equipment, and tooling is used to ensure the cleanliness and
material and process controls needed for repeatable production of bonded
structure.

Many experts consider surface preparation to be the most important
process step for structural bonding. There are distinct differences in
the surface preparation techniques used for metal and composite
substrates. Phosphoric acid anodizing and grit-blast/silane processes
are examples of surface preparation techniques that have worked for
aluminum substrates. Sanding, media blasting, and peel ply surface
preparation techniques have been successfully used for composite
bonding, depending on the specific substrate and adhesive combinations.
Bonded sandwich construction, which uses honeycomb or foam core material
(composite or metal), requires specific procedures for core surface
control applied to achieve successful bonding. The use of some solvents
on honeycomb core surfaces can cause subsequent bonding problems. The
surface preparation procedures used for wood sandwich core materials
(e.g., balsa) typically include abrasion.

Manufacturing scaling issues for a particular bonded structure need to
be considered when selecting a surface preparation for bond process
qualification. The substrate surface morphology and chemistry, which are
created by a qualified surface preparation process, are not changed in
production implementation. Since specific bonded part geometries are
often more complex than specimens used for qualification, additional
processing challenges exist. Some production process controls used to
monitor surface preparation include visual checks, polarized light
checks, water break tests, chemical analysis, and mechanical tests using
samples from bonded witness panels. Once a surface is prepared, some
processes rely on time constraints during bond assembly, requiring
surface preparation to be repeated if the bonding operation does not
occur within the specified time.

There are geometric, fit-up and other timing issues to consider in bond
assembly. Cured dimensional tolerances and warpage are controlled for
mating parts. Since the bondline thickness affects the local bond stress
distribution and strength, it is also monitored using process checks.
Assembly jigs and procedures can provide pre-bond gap assessment.
Processing aids (for example, verafilm) are used to assess the tight
tolerances needed for some bonding processes. Handling requirements and
procedures exist to control the film adhesive lay-up process. The mixing
and application of paste adhesive require additional processing steps
and controls. The mixing ratios of paste adhesive constituents and
filler content are controlled and monitored. Tight controls are also
applied to determine the completeness of the mixing process. Depending
on the type of paste adhesive used, bond assembly time constraints from
adhesive mix to mated surface contact are implemented. The minimum
bondline thickness can be controlled using a number of different spacer
types (for example, scrim cloth, glass beads and micro balloons). The
tolerances of assembly jigs are monitored and maintained over time.

The cure of bonded structures is controlled to ensure that the adhesive
properly wets the substrate surfaces and dwells at temperatures needed
to fully develop the properties for intended applications, without
overheating. As discussed previously, the scaling issues associated with
bonding large-scale airframe structures are complex. Heat transfer, bond
surface contact pressure, and adhesive characteristics during different
stages of the cure cycle all combine to affect the final state of the
bond at local points throughout the structure. Manufacturing trials are
typically needed for new combinations of parts, tooling, and equipment.
Tolerances are established and in-process controls are implemented to
locally manage the bond cure cycle and avoid overheating. Cure tooling
and the equipment used to apply temperature to structures during bonding
is controlled and maintained. Procedures used for in-process monitoring
are validated and regularly calibrated.

Manufacturing quality management is important to many aspects of
bonding. A combination of strict in-process controls and post‑bond
inspections is used. The NDI of bonded structure provides necessary, but
not sufficient, evidence that proper bonding has been achieved. Current
NDI methods, such as ultrasonic methods, can locate areas of de-bonding,
porosity and foreign inclusions (for example, peel ply or backing paper
left in the bond line) but are unable to reliably detect defective bonds
resulting from contamination or incorrect materials or processing. The
latter condition is best controlled through in‑process checks.
In-process quality controls are usually applied to surface preparation,
adhesive mixing, bond assembly and cure. Witness panel tests consider
the combined affect of these process steps but for a simplified
geometry. As a result, other quality measures and controls provide
supplemental checks on the real structure.

Another area of manufacturing and design integration that is needed
moving into production is the disposition of processing flaws. As
discussed previously, design data development and structural
substantiation support this technical issue.

Factory technicians and quality personnel involved in various bond
process steps are trained in specific areas of responsibility. Adhesion
failures and the associated cause are properly identified and corrected.
Records are kept on material usage, process steps, and quality checks
applied to each bonded structure. Factory or service problems associated
with weak bonds or de‑bonding that cannot be traced to specific material
batches or manufacturing mistakes indicate unreliable bonding processes.

3.4 Repair Implementation

Many of the technical issues, which were discussed in the previous three
subsections, are important to bonded repair implementation. This
subsection will briefly review the issues and emphasize the unique
aspects related to bonded repair. An assessment of the full extent of
damage that requires a bonded repair is the starting point for current
discussions. In most cases, this may require NDI of the damaged area.

As discussed in Section 3.2, bonded repairs have many of the same design
considerations as bonded joints and attachments. Adhesive and patch
materials selected for bonded repairs are substantiated for specific
design and process details. Factory and field repairs using different
materials, designs, or processes require separate substantiation.

A building block approach that includes approved data from analysis and
test correlation for structural substantiation of bonded repairs has
many advantages. Such an approach provides both a strong basis for
maintenance documents and additional freedom to develop more repairs for
damage not previously covered, without returning to large‑scale tests.
Benefits are also possible by validating analysis tools that use the
actual repair geometry and load transfer characteristics. Maintenance
documents with details on approved inspection, damage disposition, and
repair procedures save time. Allowable damage or defects and bonded
repair limits should also be documented. Repair designs that use bonding
and fasteners consider the capability of each joining method separately
(fasteners will not be effective until bond failure occurs).

Materials and processes used for bonded repair are qualified. Section
3.1 discusses the qualifications and associated material acceptance and
process controls used for bonding. Section 3.3 expands discussions on
the management and control of materials and key bonding process steps as
related to manufacturing implementation. The critical technical issues
remain the same for repair implementation, but facility requirements are
typically posed at a smaller scale.

Specification requirements for storage conditions, shelf‑life, out‑life,
and handling of materials are managed by using repair shop procedures
and records. Procedures for control of the bonded repair environment,
equipment, tooling, and shop personnel are needed to manage all sources
of contamination. Any expendable material used in bonding processes is
also controlled to manage sources of contamination and avoid changes in
the bond process steps. Shop facilities, equipment, and tooling are
maintained to ensure the cleanliness and the material and process
controls needed to fabricate acceptable bonded repairs.

Important process steps (for example, damage removal, surface
preparation, adhesive application, and cure) will depend on issues
related to the specific damage and structural details that require
bonded repair. Additional issues become important when structure is such
that the bonded repair must be performed on‑airplane. Maintenance
documents need to take part-specific issues into account. Damage removal
for composite substrates often requires the creation of a surface with a
scarf angle, which is held within specified tolerances. Damaged core
removal and replacement, or filling with a suitable polymeric material,
is common for sandwich construction. Bond surface preparation is still
one of the most important process steps.

The field state of substrate materials will likely require more
attention than new parts stored in a controlled factory environment.
Water, oil, and hydraulic fluids, which may be present, need to be
removed in the area of the structural repair to avoid contamination and
curing problems. Moisture or other fluids that absorb into composite or
core materials (honeycomb or foam) over time in the field may also cause
bonding problems at cure temperatures. Field procedures to remove
sources of contamination, moisture, and other fluids need to be defined
and documented.

Cure of bonded repairs in the field requires special attention,
including methods of applying heat and pressure (for example, heat
blankets, vacuum bagging and control devices). Depending on the method
of applying heat, structural details, adjacent systems, and whether or
not the repair is performed on the airplane, the heat transfer to cure a
bonded repair may be very complex. Structures on an airplane have
different framing elements, systems, and entrapped spaces below the
outer surface that alter heat flow versus that of the bonded repair to a
part removed from the airplane and cured in factory or shop equipment
(for example, autoclaves and ovens). In-process measures and controls
are needed to ensure proper heat-up rate and dwell at the cure
temperature over the entire bonded surface of the repair.

Overheating the repair or adjacent structures to cause structural damage
or degradation is avoided. Proper placement of thermocouples or other
thermal measuring devices to monitor the lowest and highest temperatures
during cure of bonded repairs is important to process control. Unique
heater configurations and repair scenarios for specific structures
benefit from pre-bond heating trials to ensure proper placement of the
devices used for thermal control. If it is found that a bonded repair
cannot be performed properly on‑airplane, the manufacturer should be
contacted for alternate methods (for example, approved bolted repair).

Quality management is important to many aspects of bonded repair.
Therefore, some combination of in-process controls and post‑bond
inspections is used. As is the case for structural bonding in a factory,
NDI alone provides necessary, but not sufficient, evidence that proper
bonding has been achieved. Other repair quality controls are usually
applied to surface preparation, adhesive mixing, bond assembly, and cure
to complement NDI.

Another area of bond process and design integration that occurs is the
disposition of repair mistakes and processing flaws. As discussed
previously, design data development and structural substantiation
support this technical issue.

Technicians and inspectors involved in various bonded repair process
steps are trained in specific areas of responsibility, including the
proper use of equipment. Records are kept on material usage, process
steps, and quality checks applied to each bonded repair. Adhesion
failures and the associated cause are properly identified and corrected.
Such problems that cannot be traced to specific material batches or a
repair process mistake indicate unreliable bonding processes. Bonding
problems are reported to engineering staff.

3.5 Service Experience

Service experience of bonded structures and repairs provide the final
proof of a reliable bonding process that has been properly executed.
Discovery of bond adhesion failures in service justify immediate
directed inspections and repair. Thorough production or repair records
are important to tracing the probable cause of such a problem.

Some experts believe that service monitoring of bonded structures or
repairs, including selected NDI, and teardown inspections for retired
aircraft, provides data to correlate accelerated test results from
qualification and in-process control with real‑time service exposure.
The FAA plans to continue to work with industry in this area of life
assessment.

**4.0 Certification Considerations**

This section covers certification considerations for the type design,
production, and maintenance of aircraft products that use bonding in
joints, attachments and repairs. Section 3.2 emphasizes the importance
of an IPT approach for design and substantiation of bonded structures.
This approach ensures the bonded structural details are producible and
subjected to quality control procedures. This also ensures key
characteristics of the substantiated design are met, and accessibility
of bonded structure for inspection or other maintenance functions is
incorporated into the design. Efforts in structural substantiation,
which occur during certification of the type design, typically have a
bearing on subsequent production and maintenance. Either design data or
validated analyses, or both, are needed to disposition manufacturing
defects and service damage. Rework and repair procedures and repair
designs need to be substantiated for both factory and field
implementation.

4.1 Design and Construction

Drawings and specifications are needed to define the product type
design. This includes information on the dimensions, materials, and
processes necessary to define the structural strength of the product.
Design and process details known to affect the performance of bonded
structures are included in the type design data and controlled in
production. As stated in § 23.601, tests are used to demonstrate the
suitability of each questionable design detail or part having an
important bearing on safety in operations.

All new materials and fabrication methods require testing to qualify
their use in design and construction of aircraft structure. This
includes taking into account the effects of environmental conditions,
such as temperature and humidity. Use qualification test results to help
derive material and process controls. Material procurement
specifications control adhesives, substrates, and ancillary materials
used for bonding processes. Each specific combination of adhesive,
substrate materials, and bond process procedures is qualified. Material
qualification of an adhesive used with different substrates and bonding
processes may be established separately to support subsequent material
control; however, each unique bond process still requires qualification
and supporting data.

Fabrication methods are required to "produce consistently sound
structure" per § 23.605. Use of the word \"sound\" in the context of
bonded structure means structure with adequate strength and durability.
Section 3.1 has some discussion on strength and durability tests for
bonded joints. Accelerated test methods that have been successfully
applied for the latter use cleavage forces to expose the bonded surfaces
to extreme environmental conditions. Such tests help detect a weak
interface, which is associated with poor bonding processes. Any
significant amounts of adhesion failures noted in qualification tests
are an indication of either incompatible materials or inadequate bonding
processes, or both. In § 23.605, there is also a specific reference to
fabrication methods such as "gluing, spot welding, or heat-treating" as
processes that require an approved process specification. Adhesive
bonding is such a process.

Relationships with material and process controls help establish the
minimum number of batches, process runs, and test repetitions needed in
generating qualification data. If the qualification tests performed were
insufficient to properly characterize bonding materials and processes,
subsequent controls will be difficult to meet.

As outlined in PS-ACE100-2002-006, experience has shown that material
qualification is based on a minimum of two process runs and three
replicates per run for each of three material batches. This approach
remains feasible for adhesive material qualification or a coupled
adhesive material and bond process qualification. The recommendation for
a separate bond process qualification is based on a minimum of six bond
process runs, each with a unique batch combination of qualified adhesive
and substrate materials. Material strength properties and design values
derived from qualification or other tests will not contain any data with
significant evidence of adhesion failure. Equivalency testing is used to
validate minor changes in bond materials or processes. A major change
requires a complete requalification.

4.2 Structural Substantiation

Policy Statement PS‑ACE100‑2001‑006, for static strength substantiation
of composite airplane structure, also applies to bonded structures. A
key point from this policy is the need for large-scale tests to validate
new structural design and manufacturing construction per §§ 23.305 and
23.307. This policy also covers other aspects of static strength
substantiation including building block approach, structural analysis,
environmental effects, manufacturing anomalies, impact damage, and
overload test factors. In general, building block analyses and tests can
be used to reduce the amount of large‑scale testing and eliminate some
overload factors used in final proof of structure for new designs. Such
an approach may also be applied for final proof of structure in lieu of
large-scale tests only if the design and manufacturing details conform
to those which experience has shown the method to be reliable (for
example, derivative aircraft).

Some additional guidance on the static strength substantiation of bonded
structure relates to specific design detail, sizing approaches, and test
considerations. Any adhesion failures noted during building block or
final proof of structure testing are identified and linked to a specific
cause (for example, manufacturing mistake) and corrected for subsequent
certification. The use of coupons to derive overload factors to cover
all aspects of environmental effects and an average shear stress
analysis should be viewed with caution. For example, such practice often
does not cover real-time degradation mechanisms such as the poor creep
resistance of bonded joints with insufficient overlap length or the
effects of secondary load paths that may exist in real structure. Data
needed to substantiate a simple sizing approach includes sufficient
testing with enough structural detail to account for foreseeable
property variability, while ensuring the structure meets realistic
environmental and loading conditions.

Large-scale tests are also needed to validate the fatigue and damage
tolerance of bonded structure with new design and manufacturing
construction, per § 23.573. Building block analyses and tests provide
support similar to static strength substantiation. Defect and damage
threat assessments provide the starting point. Static strength and
fatigue evaluations include manufacturing anomalies and accidental
damage that may not be detected by factory quality control or field
inspection methods. Damage tolerance evaluations include accidental or
environmental damages that are detectable in the field, ranging from
small sizes that require directed inspection to more severe damage that
would be readily detected within a few flights. Within the scope of
static strength, fatigue and damage tolerance rules, it is intended that
circumstances leading to lost ultimate load capability should be rare.

Part 23 regulations have language specific to bonded structure in
§ 23.573(a)(5), which has three options to substantiate limit load
capability (see Section 2.1 for a quote). These options do not supersede
the need for a well-qualified bonding process and rigorous quality
controls for bonded structures. For example, fail safety implied by the
first option is not intended to provide adequate safety for the
systematic problem of a bad bonding process applied to a fleet of
aircraft structures. Instead, it gives fail safety against bonding
problems that may occasionally occur over local areas (for example,
insufficient cure or contamination). Static proof tests to limit load,
which are posed as the second option, may not catch weak bonds that
require time and environmental exposure to degrade the strength of a
bonded joint. Finally, the third option is open for future advancement
and validation of NDI technology to detect weak bonds, which degrade
over time and lead to adhesion failures. As discussed in Section 3, such
technology has not been reliably demonstrated at a production scale to
date.

It is impractical to directly evaluate the long-term durability of
bonded structures, which includes real-time environmental exposure, in
large-scale tests before certification. As discussed in Sections 3.1 and
3.2, aggressive environments and extreme loading (for example, cleavage
forces) are used in smaller scale tests to expose bonded interfaces to
conditions, which are known to accelerate degradation mechanisms for
weak or contaminated bonds. Although this approach helps ensure good
bonding processes, the long-term durability of bonded production
aircraft structure is validated by service experience. As a result,
close ties between the service and production departments of a
manufacturer are essential.

4.3 Production

Quality management is essential for production of bonded structure. The
facilities control environment and cleanliness of bonding processes to a
level validated by qualification and proof of structure testing.
Adhesives and substrate materials are controlled to specification
requirements that are consistent with material and bond process
qualifications. This includes requirements for storage, handling, and
material characteristics or process parameters needed to achieve
consistent chemical, physical, and mechanical properties. Expendable
materials used in bonding processes are also controlled and disposed per
requirements to insure the integrity of bonded structure. Document plans
for regular maintenance of production facilities.

Process steps, equipment, and tooling crucial to achieving structural
bond integrity are fully defined and controlled within tolerances given
in specifications or equivalent documents. The production tolerances
meet standards validated in qualification, design data development, and
proof of structure tests. Some key bonding process steps requiring such
control include: (1) surface preparation, (2) mating part dimensional
tolerance control, (3) adhesive mixing and application, (4) bondline
thickness, and (5) adhesive cure. Document plans for regular maintenance
of production equipment and tooling.

Production certificate holders show that a quality control system has
been established and maintained for bonded structures. As discussed in
Section 4.1, methods of fabrication reliably produce structure with
adequate strength and durability. Process control and production records
document the key characteristics (KC), key process parameters (KPP), or
equivalent data, which indicate the bonding processes were properly
applied. Changes in the quality control system are subject to review by
the Administrator.

A disposition process is needed for all manufacturing defects and
nonconformities detected by the quality control system. Either design
data or analyses, or both, validated by previous tests are used for
acceptance, rework or repair. Any adhesion failures discovered during
production require immediate actions to isolate the problem and
determine the cause.

4.4 Continued Airworthiness

Bonded structures and repairs need to meet continued airworthiness
requirements. Maintenance inspection and repair work is needed for
damage due to accidental events (for example, foreign object impact),
environmental effects or other causes. The initial detection of damage
in composite or metal bonded structures is followed by more extensive
inspection (for example, ultrasonic methods) to determine the full
extent of damage requiring repair.

Inspection and repair process steps documented in maintenance manuals
that are based on approved data (procedures validated by qualification
and structural substantiation) provide a number of advantages. If damage
is beyond the scope of that previously approved, the manufacturer or
other approving authority will be involved in the disposition, and
additional data may be required for substantiation. Any de‑bonding,
which includes significant adhesion failures, found in service are
reported to the manufacturer. Such findings require immediate action to
identify and correct the cause of de-bonding (for example, manufacturing
error). Records are kept for maintenance activities, including repair,
inspection, and rework.

Quality management is essential for bonded repairs to airframe
structures, which are performed in a shop or on the aircraft. The
environment and cleanliness of repair bonding processes are controlled
to a level validated by qualification and proof of structure testing.
Adhesive and substrate repair materials are qualified and controlled to
approved specifications. This includes requirements for storage,
handling, and material characteristics or process parameters needed to
achieve consistent chemical, physical, and mechanical properties.
Expendable materials used in bonding processes are also controlled and
disposed per requirements that ensure the integrity of a bonded repair.
Document plans for regular maintenance of repair shop facilities,
equipment, and tooling.

The bonded repair of airframe structures in service follows the same
basic principles as bonding performed in the factory. However, technical
issues related to the damaged condition of the structure need to be
addressed, and the associated additional process steps are documented in
approved repair procedures. The procedures also recognize and address
the field difficulties of applying process steps to a specific repair.

Repair starts with a determination of the full extent of structural
damage, including necessary inspections, and an engineering disposition.
Damaged material, paint, and other surface coatings are removed per
approved processes. Removal of aviation fluids, cleaning, and drying of
the structure also follows approved procedures. Process steps for
surface preparation, adhesive mixing, application of repair materials,
and bond cure are defined, applicable, and complete for the specific
repair to be performed. The necessary equipment, tooling, and ancillary
materials needed to perform the bonded repair are available.

Perform and record process control data and quality inspections, which
are defined in approved procedures for bonded repair. Quality controls
address the field difficulties of preparing a chemically active bond
surface and applying the proper heat-up rate, cure temperatures, and
dwell times throughout the repair. Records for a successful bonded
repair indicate the process was correctly performed to produce sound
structure with a high degree of reliability.

A disposition process is needed for nonconformities (for example,
insufficient cure temperature, overheating) and defects (for example,
porosity, de-bonding) found by inspection. Procedures based on approved
data are used for acceptance and rework.

4.5 Other Elements

Industry‑accepted engineering databases, specifications, guidelines, and
test standards continue to evolve with the help of organizations such as
MIL-HDBK-17, SAE AMS-P17, SAE CACRC, and ASTM. Use of such information
is encouraged in the development, certification, production, and
maintenance of bonded structures. The specific application of standards
to a particular aircraft product is approved in the substantiation
process. The use of some data requires a demonstration of material and
process control. For example, an equivalency test sampling process is
used for new users of material data that depend on manufacturer
processing (see ACE-100 Policy Statement \"PS-ACE100-2002-006\").

The importance of communication and coordinated work between design,
production, and service groups involved in bonding has been covered in
this document. Such an interface is essential when a potential bonding
problem has direct implications to safety and continued airworthiness.
Adhesion failures found during qualification or structural
substantiation indicate either incompatible materials or unacceptable
bond processes, or both. Adhesion failures found in production require
immediate actions to identify the specific cause and isolate all
affected parts and assemblies for disposition. Adhesion failures
discovered in service require immediate actions to determine the cause,
to isolate the affected aircraft, and to conduct directed inspection and
repair. Such cases are immediately reported to regulatory authorities
and, depending on the suspected severity of the bonding problem, the
regulatory authority may ground the aircraft.

Engineers, technicians, and inspectors involved in related design,
production, and maintenance activities need a basic understanding of the
critical technical issues and processing procedures used for bonded
structures and repairs. Factory work force training is needed to ensure
proper execution of specific bonded structure processing steps and
quality inspections. Maintenance work force training is needed to ensure
proper execution of specific bonded repair processing steps and quality
inspections.

**5.0 Summary**

Under CS&CI, the FAA has worked with industry, government agencies, and
academia in support of focused research for the bonded structures. In
2004, the FAA conducted an industry survey and two bonded structure
workshops. The objective is to benchmark industry practices and collect
information on the critical safety issues and certification
considerations for bonded aircraft structures and repairs.

Building on the data collected through the survey and the workshops, the
Small Airplane Directorate (ACE-100) developed this policy statement to
provide guidance from regulatory perspectives. This policy statement
applies to 14 CFR part 23 airplanes.

The bonded structures may include composite-to-composite,
composite-to-metal, and metal-to-metal. These structural bonding
applications have been used for both manufacturing and maintenance
(repair) operations.

Considering continued operational safety, the scope of this policy
statement covers (1) material and process qualification and control,
(2) design development and structural substantiation, (3) manufacturing
implementation, and (4) maintenance implementation.

**6.0 Future Efforts and Standardization**

This policy statement serves as initial guidance for certifying bonded
structures for 14 CFR part 23 airplanes. Efforts in bonded structures
will continue with plans for future updates to the policy and guidance.

The related research on critical bonding issues will remain active. The
FAA will also continue to support industry and working groups in
developing engineering standards for structural bonding.

**Effect of Policy**

The general policy stated in this document does not constitute a new
regulation or create what the courts refer to as a \"binding norm.\" The
office that implements policy should follow this policy when applicable
to the specific project. Whenever an applicant\'s proposed method of
compliance is outside this established policy, it must be coordinated
with the policy issuing office, for example, through the issue paper
process or equivalent. Similarly, if the implementing office becomes
aware of reasons that an applicant\'s proposal that meets this policy
should not be approved, the office must coordinate its response with the
Small Airplane Directorate.

Applicants should expect that the certificating officials would consider
this information when making findings of compliance relevant to new
certificate actions. Also, as with all advisory material, this policy
statement identifies one means, but not the only means, of compliance.

If you have any questions or comments, please contact Mr. Lester Cheng,
Regulations and Policy Branch, at 316-946-4111.

s/ David Showers for

Kim Smith

Distribution:

Manager, Aircraft Certification Division, AIR-100

Manager, Rotorcraft Directorate, ASW-100

Manager, Engine and Propeller Directorate, ANE-100

Manager, Transport Airplane Directorate, ANM-100

Manager, Flight Standards Division, ACE-200

Regional Attorney, ACE-7

Manager, Atlanta Aircraft Certification Office, ACE-115A

Manager, Wichita Aircraft Certification Office, ACE-115W

Manager, Chicago Aircraft Certification Office, ACE-115C

Manager, Alaska Aircraft Certification Office, ACE-115N

Manager, Brussels Aircraft Certification Staff, AEU-100

Manager, Seattle Aircraft Certification Office, ANM-100S

Manager, Los Angeles Aircraft Certification Office, ANM-100L

Manager, Aircraft Evaluation Group, MKC-AEG

Manager, Airplane Certification Office, ASW-150

Manager, Special Certification Office, ASW-190

Manager, Boston Aircraft Certification Office, ANE-150

Manager, New York Aircraft Certification Office, ANE-170