U.S. Department 

of Transportation 

Federal Aviation 

Administrution 

Advisory 

Circular 

Subject: REPAIR STATIONS FOR 

COMPOSITE AND BONDED 

AIRCRAFT STRUCTURE 

Date: 11/15/96 

Initiated by: AFs-350 

AC No: 

Change: 

145-6 

1. PURPOSE. This advisory circular (AC) provides information and guidance concerning an 

acceptable means, but not the only means, of demonstrating compliance with the requirements 

of Title 14 of the Code of Federal Regulations (14 CFR) parts 21, 43, 121, 125, 127, 135, and 

145 regarding procedures and facilities for repairs and alterations of structure consisting of 

metal-bonded and fiber-reinforced materials (e. g . , carbon, boron, aramid, and glass-reinforced 

polymeric materials mentioned in AC 20-107, Composite Aircraft Structures). Consideration 

will be given to any other method of compliance the applicant elects to present to the Federal 

Aviation Administration (FAA). Mandatory terms used in this AC, such as “must,” are used 

only in the sense of ensuring the applicability of these particular methods of compliance when 

the acceptable means of compliance described herein is used. This AC does not change 

regulatory requirements and does not authorize changes in, or deviations from, regulatory 

requirements. 

2. DEFINITIONS. As used in this AC, the following definitions apply: 

a. Composite. A combination of two or more materials (reinforcing elements, fillers, and 

composite matrix binder), differing in form or composition on a macro-scale. The constituents 

retain their identities, that is, they do not dissolve or otherwise merge completely into one 

another although they act in concert. Normally, the components can be physically identified 

and exhibit an interface between one another. 

b. Batch. In general, a quantity of material formed during the same process or in one 

continuous process and having identical characteristics throughout. Also referred to as a lot. 

A discrete quantity of material with a total commonality of raw materials and process history. 

c. Lot. A specific amount of material produced at one time using the same process and 

the same conditions of manufacture, and offered for sale as a unit quantity. 

d. Shipment. An order of material received by a purchaser. A shipment of preimpregnated (prepreg) material may include rolls of material from more than one batch and 

more than one lot. A shipment of resins or adhesives may include resins or adhesives from 

one or more batches and more than one lot. 
AC 145-6 1 l/15/96 

e. B-Stage. An intermediate stage in the reaction of certain thermosetting resins in which 

the material softens when heated and is plastic and fusible but may not entirely dissolve or 

fuse. This stage helps facilitate handling and processing. The resin in an uncured prepreg is 

usually in this stage. 

f. Original Equipment Manufacturer (OEM). A manufacturer that is the holder of an 

FAA-approved Type Certificate (TC), Production Certificate (PC), Approved Production 

Inspection System (APIS), Production Manufacturing Approval (PMA), Technical Standards 

Order Approval (TSOA), or Delegated Option Authorization (DOA) who controls the design 

and quality of the product/part thereof. 

NOTE: A supplier to a manufacturer is not an OEM. 

3. RELATED READING MATERIAL. Current editions of the AC’s, listed below, may be 

obtained free of charge from the following address: U.S. Department of Transportation, 

Subsequent Distribution Office, Ardmore East Business Center, 3341 Q 75th Avenue, 

Landover, MD 20785. 

a. AC 20-107, Composite Aircraft Structures. 

b. AC 21-26, Quality Control for the Manufacture of Composite Structures. 

c. AC 145-3, Guide for Developing and Evaluating Repair Station Inspections Procedures 

Manuals. 

4. BACKGROUND. Part 145, subpart B and C, provide for limited ratings for specialized 

services for domestic and foreign repair stations, respectively. The certificated repair station 

operations specifications should contain or reference the material and process specifications 

used in performing repair work on composite material structures, components, and adhesivebonded components listed under that specialized service. All major repairs and alterations 

must be performed using data, including material and process specifications, approved by the 

Administrator. This AC is intended to provide information on the repair and fabrication of 

composite materials and adhesive-bonded components, and on the inspection systems, 

equipment, and facilities that a certificated repair station with the appropriate ratings should 

have to perform repairs or alterations on such materials and components. These guidelines are 

intended to supplementhe procedures in Original Equipment Manufacturers (OEM) structural 

repair manuals. Also, it should be understood that the word “composites” as used in this AC 

does not imply a relationship with a certificated repair station Airframe Class 1 or Class 2 

composite construction rating. The rating used in this context refers to older aircraft that were 

manufactured using wood, fabrics, and metal or any combination of these. It should also be 

understood that all air carriers operating under parts 121, 125, 127, and 135, and all repair 

stations certificated under part 145, must comply with 14 CFR part 43, 6 43.13 (a) and (b). In 

addition, for the purposes of this AC the term Maintenance Organization (MO) will be 

substituted for persons performing maintenance under parts 121, 125, 127, 135, and 145. 

Page 2 Par 2 
11/15/96 AC 145-6 

5. ENGINEERING DATA AND PROCEDURES. 

a. Source of Data. 

(1) Maintenance organizations must accomplish bonding of aircraft structure and 

composite material repairs/alterations in accordance with parts 43, 145, and 14 CFR parts 

121, 127, and 135, $6 121.379, 127.140, and 135.437. It is incumbent upon the repair 

facility to work closely withthe manufacturer of the particular product and the FAA, when 

necessary, to determine what specific data may be necessary to support the repair. Much of 

the information needed for maintenance, repairs, and alterations is contained in the 

manufacturer’s maintenance manual or structural repair manual (SRM). 

(2) The maintenance organization must use data approved by the FAA Administrator 

(approved data) when performing a major repair or alteration. In the event that the 

manufacturer’s repair specifications are deficient with regard to specific information, 

technique, or repair method, or if the damage being repaired exceeds that covered by the 

manufacturer’s repair specifications, the maintenance organization must obtain FAA-approved 

data prior to accomplishing the repair. The maintenance organization can request the TC or 

supplemental type certificate (STC) holder to develop repair data and to obtain FAA approval, 

or the maintenance organization can develop the data and submit it to the FAA for approval. 

Other sources of FAA-approved data include an FAA Aircraft Certification Office (ACO), an 

appropriately rated FAA Designated Engineering Representative (DER), a maintenance 

organization having SFAR 36 authorization to approve repair data, a holder of a Delegation 

Option Authorization (DOA), and an FAA-approved Designated Alteration Station (DAS). 

Maintenance organizations are cautioned that simple OEM concurrence for a major repair does 

not constitute FAA approval unless specifically stated and documented (e.g., FAA, DOA, or 

DER signature). In addition, utilization of various FAA-approved data and documents do not 

constitute approved data for other than the repair specified in the data. Therefore, the 

maintenance organization is at risk in making a judgmental evaluation to use various FAAapproved data that are not specifically approved for the particular repair. 

b. Data Requirements and Compliance. Each maintenance organization must have and 

maintain the approved or acceptable data necessary to formulate and support the repairs or 

alterations it makes. Data should include the drawings and specifications necessary to define 

the repair or alteration including dimensions, materials, and. processes necessary to define the 

structural strength. In addition, maintenance organizations should perform and document the 

inspections and tests necessary to determine compliance with the applicable airworthiness 

requirements and the approved repair or alteration data. 

c. Data Types. For repairs or alterations that are not covered by the manufacturer’s 

manuals, the following design data containing the information indicated should be available 

and utilized by the maintenance organizations when purchasing and receiving materials, 

fabricating repairs or alterations, and installing the repairs or alterations. 

Par 5 Page 3 
AC 145-6 11/15/96 

(1) Drawings. Engineering drawings should at least describe the following: 

(a) Materials (called out by material specification). 

(b) Attachment to existing structure. 

(c) Fabrication 

specification). 

and processing procedures (called out by fabrication and process 

(d) 1 l Number of plies and stacrsmg orientation/sequence. 

(e) Shape. 

(f) Dimensions and tolerances. 

(g) Protective coating and sealant. 

(2) Reports. Engineering reports, analyses,, and checklists should show that the 

repaired structure would continue to comply with applicable type certification regulations. 

(3) Material Specifications. Materials used for repair or alteration must meet the 

qualification requirements established by the OEM’s approved type design or the maintenance 

organization s FAA-approved data. Material specifications (adhesives, resins, prepreg tape 

and broadgoods, core materials, etc.) should completely describe the material required and 

should normally include the following sections and data: 

(a) Material Classifications. Identifies the material specified and acceptable 

types, classes, grades, and styles of that material. 

(b) Material Properties. Specifies the required material’s physical, 

processability, chemical, and mechanical (laminate, sandwich, and bond strength tests, etc.) 

properties for a material that meets the specification. 

(c) Material 

to the specifications o

Qualification. Specifies tests and procedures to qualify a material 

(d) Supplier Quality Control. Specifies supplier quality control tests and 

procedures, and specifies compliance certifications and data required with each material batch. 

(e) Purchaser Quality Control. Specifies purchaser acceptance tests and 

procedures required for each batch per shipment received. 

(f) Material Test Methods. Specifies the types of tests, types of test specimens, 

and test procedures for testing the material’s physical, processability, mechanical, and 

chemical properties. 

Page 4 Par 5 
I l/15/96 AC 145-6 

(g) Packaging and Marking Requirements. Specifies requirements for 

suppliers to follow when packaging and shipping an order of material to a purchaser. 

storage. 

Storage Conditions. Specifies environmental conditions and time scale for 

0 i Recertification Procedures. Specifies the requirements for retest prior to 

use of over-aged material (e.-g., material whose out-time or shelf life has been exceeded). 

l 

6) Qualified Products List Section. Specifies a list of supplier products that 

are currently qualified to the requirements of the specification. Qualified materials must be 

listed on an approved Qualified Products/Parts List (QPL) and purchased from qualified 

suppliers. Substitutions to the QPL should be made in accordance with procedures approved 

by the Administrator for the maintenance organization. 

(4) Fabrication and Process Specifications. Fabrication and process specifications 

should: 

0 a 

0) 

process tests and 

0 C 

(a 

Completely describe fabrication and process procedures requirements. 

List each major step in a fabrication and process procedure, including ininspections. 

A 

Include 

Specify 

processing parameters. 

0 e 

properties. 

(f) 

(8) 

(5) ND1 

0 a 

00) 

0 C 

00 

accept/reject Nondestructive Inspection (NDI) test standards. 

appropriate type and number of witness test coupons to verify 

tilve limits for adhesive bond, laminate, and sandwich panel strength 

Identify facility, equipment, and storage requirements. 

Specify material cure cycle requirements. 

Specifications. ND1 specifications should: 

Identify method type. 

Completely describe ND1 methods and procedures. 

Specify equipment and calibration frequency. 

Outline test procedures. 

Par 5 Page 5 
AC 145-6 llll5f96 

(6) Defect Acceptance/Rejection Criteria. Standards for accept/reject criteria for 

cured or bonded structure should be established from the design data. Usually these are 

identified in the OEM’s structural repair manual or applicable fabrication snecifications. 

Inspection procedures and equipment used to detect flaws 

standards or other FAA-approved data. 

should be consistent with these 

6. RAW MATERIAL INSPECTION PROCEDURES, 

a. General. The processability and resulting strength-related properties of composite 

materials used in structural repair depend upon the composition of the fiber/resin prepreg 

materials from which they are manufactured. In general, prepregs consist of surface-treated 

glass, aramid, or carbon fibers impregnated with a reactive and chemically complex thermoset 

resin formulation or an amorphous or semicrystalline thermoplastic resin. Thermosetting 

resins are often “staged” or partially reacted during the prepreging process and may undergo 

compositional changes during transport, handling, and storage. Inadvertent or minor changes 

in resin composition may cause problems in processing and may have adverse effects on the 

performance and long-term properties of many composite repairs. This also applies to 

adhesive systems used in structural bonded repairs (e.g., film adhesives, primers). The 

capability of a repair to satisfactorily maintain its structural integrity for the remaining life of 

the part or aircraft is dependent, in part, on the maintenance organization’s knowledge of the 

physical, chemical, mechanical, and processability properties of the incoming materials (e.g., 

prepreg materials, adhesives, etc.) used in that repair. The physical, chemical, mechanical, 

and processability test results are; therefore, vital to the continued airworthiness of the 

repaired structure. 

b. Material Acceptance. 

(1) General. The maintenance organization should have an incoming material 

acceptance plan (see paragraph 7 for material handling and storage procedures) addressing the 

issues in paragraph 6a, above, that ensures conformity of purchased material (i.e., prepreg 

material, film and paste structural adhesives, and wet lay-up resins, etc.) to OEM or other 

FAA-approved material specifications. Copies of the original material manufacturer and 

supplier laboratory test reports showing actual test results, if applicable, should accompany 

each batch of material received for review and approval. When material is purchased from an 

approved supplier in accordance with the OEM or other FAA-approved material specification, 

such test reports are usually adequate documentation of the material condition and conformity 

to the material specification. When material is purchased from a non-approved source, 

verification testing should be performed. Specific material called out in the SRM or 

maintenance manuals without material specifications should be controlled as indicated in the 

material manufacturer’s recommendations covering the material designation, storage, and 

handling requirements. 

(2) Verification Testing. Unless the maintenance organization can provide evidence 

that the approved material from a given source is of an acceptable and consistent quality, 

verification testing should be performed by the maintenance organization or by an independent 

Page 6 Par 5 
IL l/W96 AC 145-6 

laboratory, which has been approved for such testing by the maintenance organization as part 

of its incoming material acceptance plan. The type and frequency of the testing should be 

consistent with the material specification (reference paragraph 5c(3)). Once confidence in the 

quality of materials received from a particular source has been established, the maintenance 

organization may reduce the level of verification testing. 

c. Supplier Quality Control Tests. The following discussions identify quality control 

tests of physical, chemical, mechanical, and processability properties typically performed by 

the supplier. In most cases the required tests were performed as specified in material quality 

control sections of OEM or other FAA-approved material specifications. 

(1) Physical Tests. Physical tests typically conducted may include wet resin content 

and fiber area weight, volatile content, tack, flow, and gel time. 

(2) Chemical Characterization Tests, High performance liquid chromatography 

(HPLC) and infrared spectroscopy (IR) are two chemical characterization tests that provide the 

capability for rapid screening and quality control fingerprinting of the resin’s constituents. 

They are fundamentally different from one another and provide direct, but different, 

information about a resin’s composition HPLC and IR tests are conducted by the supplier to 

ensure that the resin type, purity, and concentration, and the distribution of resin components 

are consistent from batch to batch and per the material specification. These tests are 

conducted on resin samples extracted from a number of rolls in each batch of prepreg material, 

because B-staging alters the chemical composition of the resin increasing the variety of 

chemical components and because the resin may chemically advance as a function of time in 

the prepreg. 

(3) Mechanical Tests. For laminates, the mechanical property tests normally 

performed by the supplier for each batch of prepreg material are longitudinal tension (room 

temperature, dry) and compression (room temperature and elevated temperature, both dry). 

The room temperature dry tensile modulus of elasticity and ultimate tensile strain are obtained 

from the load-deflection curve of the longitudinal tension test. Mechanical testing relevant to 

the use of the material should be chosen (e.g., lap shear, climbing drum, and T-peel are most 

appropriate for adhesive bonding). 

(4) Processability Tests. Laminate processability tests may include laminate ply 

thickness measurements, resin content, void content, and density tests conducted on laminates 

fabricated from at least one roll per batch of prepreg material. 

7. MATERIAL HANDLING AND STORAGE PROCEDURES. 

a. General. Improper storage and handling of repair materials such as prepreg tape and 

broadgoods, adhesives, potting compounds, and resins can adversely affect material 

properties. This makes it important that a maintenance organization’s approved system for 

bonding of aircraft structure and for composite materials repair includes specified procedures 

for storage and handling of raw materials used for repairs. A maintenance organization’s 

Par 6 Page 7 
AC 145-6 1 l/15/96 

approved procedures should require repair materials to be stored and handled in ways that 

assure they will be acceptable for use and will provide expected finished properties when 

processed. Material specifications normally specify shelf life when the material is stored at the 

specified temperatures. Instructions regarding handling and storage should be carefully 

followed. 

b. Shelf Life and Temperature Considerations. Polymer matrix prepreg tape and 

broadgoods, adhesives, potting compounds, and resins should be handled and stored in 

accordance with approved specifications to ensure retention of material properties. Thermoset 

matrix composites and structural adhesives in current use should be stored in sealed containers 

or bags in accordance with the requirements specified in the appropriate material or process 

specification, often near O°F (-18°C). This retards the “aging, ” or partial curing, of the 

polymer and extends the shelf life. The sealed containers or bags prevent moisture 

condensation on the cold material and moisture absorption. When the material is removed 

from the freezer for processing, it is allowed to thaw within the sealed container or bag until 

the condensation on the exterior has dissipated. Thin rolls of prepreg and adhesives do not 

need a long thaw period. The period of time the material is outside the freezer should be 

documented in an “out time” ledger. In order to keep track of “shelf life” and “out-time,” a 

maintenance organization’s approved system should include record keeping procedures for 

these items. 

c. Storage and Handling Recommendations. Polymer matrix prepreg materials and film 

adhesives normally have a backing sheet that improves their handling characteristics and 

protects the material from handling-related damage. Non-woven unidirectional tapes, for 

example, can otherwise easily split between fibers. Clean, white, lint--free cotton gloves are 

recommended when handling adhesives and surfaces prepared for bonding to prevent skin oils 

from being transferred to the material. Clean hands are mandatory when working with these 

materials. Many of today’s composite material fibers can be damaged if they are bent or 

folded with small radii. For this reason handling procedures should prohibit sharp bending, 

creasing, gouging, or other actions that could break or damage fibers. These materials should 

be handled by properly trained personnel. It is recommended that a maintenance 

organization’s procedures for storage and handling of these materials include the following: 

(1) Frost-free cold storage equipment that opens from the top is recommended. Walkin freezers can be used for large volume storage. 

(2) Determining which raw materials (e.g., prepreg tapes and broadgoods, film 

adhesives, paste adhesives, etc.) must be stored in a freezer. 

(3) Placing a tag or record on each roll or container showing the batch number, roll 

number, shelf life ending date, and the total allowable out of freezer time (out-time). The tag 

or record should include a place to record cumulative out time. 

(4) Specifying the highest allowable freezer storage temperature. 

Page 8 Par 7 
1 P/15/96 AC 145-6 

(5) Regularly monitoring freezer storage temperature. 

(6) Requiring that materials be stored in moisture-tight bags or containers in the 

freezer to prevent moisture absorption. If kitted parts are returned to the freezer for storage 

they should also be placed in moisture-tight bags labeled to maintain traceability. 

(‘7) Specifying that the material should be allowed to thaw within the container or bag 

until the condensation on the exterior of the container has dissipated. This is to prevent 

atmospheric moisture condensation on the material. 

(8) Requiring remaining material on the roll to be placed back in its moisture-tight 

container or bag and placed back in the freezer after recording the out-time on the attached 

record. 

(9) Requiring the use of appropriate clothing and gloves for handling adhesives, 

prepregs, and prepared bond surfaces. 

(10) Requiring that batch number, lot number, roll number, shelf life, and out-time 

records be retained for a specified time (suggest a minimum of 2 years from the time material 

is depleted or disposed). 

(11) Describing how the roll material is to be located/oriented in the freezer to prevent 

fiber breakage or resin migration. 

(12) Requiring that materials that have 

scrapped or recertified in accordance with the 

procedures. 

$. FACILITIES AND EQUIPMENT. The facilities and equipment used for repair or 

exceeded their shelf life or out-time limit be 

material specification or other FAA-approved 

alteration of composite structure must be capable of meeting the repair process requirements 

established by the OEM or other FAA-approved repair process. The following guidelines are 

offered as examples of facilities and equipment used to control part quality when 

accomplishing repair or alteration of composite or adhesively bonded metallic structure of 

aircraft: 

a. Autoclave. An autoclave capable of providing positive pressures and temperatures 

consistent with the repair process. The autoclave should have integral vacuum and 

thermocouple capability for each part. 

b. Oven. An air-circulating oven capable of providing the required cure or drying 

temperature and equipped with vacuum capability when required. 

c. Temperature and Pressure Controller. A method of temperature and pressure 

control (e .g . , vacuum bag, thermal blanket, heat lamp, mechanical pressure, etc.). 

Par 7 Page 9 
AC 145-6 11/15/96 

d. Lay-up and Clean Rooms. Unless otherwise validated for the material system in use, 

lay-up and clean rooms should be environmentally and operationally controlled in the 

following areas: 

(1) Temperature and 

repair process. 

(2) Air filtration and pressurization capable of providing slight positive over-pressure. 

humidity in accordance with the OEM or other FAA-approved 

(3) Room designed to minimize dirt traps (e.g., recessed lights, sealed floors, no 

ledges, etc.) with a routine cleaning schedule established. 

(4) Contamination restrictions in cutting, lay-up, and bonding areas should prohibit 

the use of uncontrolled sprays, exposure to dust, handling contamination, fumes, oily vapors, 

and the presence of other particulate or chemical matter that may adversely affect the repair 

process (e.g., release agents or material containing uncured silicone should not be permitted). 

Also, eating and smoking should be prohibited in these areas. 

e. Freezers. Freezers capable of storing adhesives, prepregs, etc., at manufacturer’s 

recommended conditions should be used. 

f. Cleaning. Equipment must be provided to accomplish the various cleaning processes. 

g. Spfay Room. For adhesively bonded metallic structures, a dedicated spray capability 

should be available for bonding primer application. Spraying the primer produces greater 

uniformity and consistency than other methods of application. 

h. Honeycomb Core Cutters. Capability should be available to cut, rout, and profile the 

honeycomb in either the compressed or expanded form. Hand routing should be kept to a 

minimum. 

i. Part Drying Capability. For composite material repairs where some composite parts 

are re-used, a moisture dry-out capability should be available to adequately dry the entire part 

before lay-up and bonding of the repair. The dry-out capability should provide uniform heat 

and a flow of dry air or gas. One method is a large oven with integral vacuum capability and 

dry air input into the vacuum bag of the part. 

j. Laboratory Test Facilities. Laboratory test facilities should be available if required to 

perform routine testing of etch solutions, finished laminates for resin content, chemical 

composition of materials, etc. In addition, laboratory equipment and facilities for mechanical 

testing of specimens, such as tension, compression, lap shear, climbing drum peel, short beam 

shear, T-peel, and wedge should also be available if required. 

Page 10 Par 8 
11115196 AC 145-6 

k. NDI. X-ray, ultrasonic, or other types of acoustic test equipment should be available 

for inspection of parts. The use of tanks for submersion leak testing of parts has been shown 

to be a good way to verify proper adhesive/resin flow and sealing of honeycomb sandwich 

parts. 

1. Kit Storage Area. A dedicated area for storage of kitted detail parts prior to bond 

helps to reduce contamination of detail parts. 

m. Cutting Tools. Diamond or carbide saws and router bits are suitable for cutting and 

trimming of composites. Carbide drills and countersinks for hole drilling operations in 

composites, or other means recommended by the OEM, should also be used. 

9. TOOLING. 

a. Initial Qualification. The tooling used in the repair or fabrication of the part should 

undergo an approval procedure that demonstrates its capability to consistently produce parts of 

proper fit, form, and function. This procedure should include inspection of the cured part to 

ensure conformance to design requirements prior to release of the tooling for repair 

operations. The procedure may encompass the use of the tooling (e.g., molds, dies), raw 

material, expendable items (e.g., release films, bagging materials), lay-up and bagging 

techniques, and cure cycle used in fabrication. Any significant changes in the tooling, 

materials, and processing methods after initial approval would require reapproval. 

b. Requalif’ication. The tooling should be requalified periodically by inspection of a 

cured part produced with the tool for proper fit, form, and function to ensure continued 

conformance to established requirements. 

10. FABRICATION AND PROCESSES PROCEDURES. A maintenance organization 

should use detailed processing specifications and procedures for repair or alteration of 

composite or adhesive-bonded structure. The specifications and procedures should be 

consistent with FAA-approved maintenance instructions and with available repair facilities and 

equipment. The specifications and procedures developed should incorporate and address the 

following points, if they are not already incorporated in the Structural Repair Manual. 

a. Damage Assessment and Teardown. If localized repair of a damaged area needs to 

be accomplished, the extent of the damage should be fully and clearly defined. Due to the 

tendency of composites to rebound and mask any impact damage, and the strong likelihood of 

non-visible microcracking, a visual inspection or coin tap test for determination of the extent 

of any composite damage should be supplemented with additional ND1 techniques (i.e., 

ultrasonic or other approved ND1 methods). Because of the dust and dirt generated, teardown 

should be accomplished in a separate area segregated from the fabrication area. It is 

recommended that additional ND1 be accomplished after teardown to identify any additional 

damage that may have occurred, or become apparent, as a result of the teardown. Absorbed 

moisture in composite and honeycomb materials has been found to have a significant effect on 

the performance of repairs and the manner in which the repairs should be made. 

Par 8 Page 11 
AC 145-6 llfl5l96 

Moisture in composites can frequently cause delaminations away from the repair location 

during the repair cure cycle, and in honeycomb structure it can result in blown skins (face skin 

separated from the core). To suppress this tendency for blown skins in sandwich structure, 

drying of the damaged moisture-contaminated part is recommended. To suppress the tendency 

for interlaminar delaminations and face sheet separation in the repair cycle, the damaged 

composite should be dried for a period of time (below the temperature of boiling water). The 

length of time for drying is dependent upon the number of plies and the exposure to moisture 

the part has experienced. Paint should be removed from all surfaces to be bonded since it 

prevents good adhesion. Mechanical abrasion or hand sanding is acceptable for local paint 

removal on composites. Particular care should be taken not to damage the laminate if power 

sanding is used. Alternative methods, like chemical stripping or blast removal processes, 

should be carefully evaluated and approved before use. These processes may cause damage to 

the laminate. 

b. Fabrication and Processing of Repair Parts. Detail parts should be fabricated using 

OEM-approved materials or using materials qualified to the applicable FAA-approved 

specifications eThe configuration of the details should be traceable to the applicable drawings, 

tooling, repair configurations identified in the Structural Repair Manuals, or other FAAapproved drawings and/or tooling. Key steps in repair part fabrication and processing are: 

(1) Prefit. Detail parts of adhesively bonded metallic and composite material 

structure should be prefitted, using tooling intended for repair or manufacture. Detail parts 

should be controlled from the point of bonding prefit until they become an integral part of the 

assembly. 

(2) Cleaning and Drying. For metal details, the cleaning processes should remove 

contaminants (e.g., oils, waxes, greases,, cutting oils) and oxides from the part surface. 

Emulsifiers and solvents are generally used for contaminant removal and as a preliminary 

cleaning process. For subsequent cleaning operations to remove oxides and light surface 

scratches, acid and alkaline etching processes are generally used. For composites, the surfaces 

should be cleaned with compatible chemicals in order to remove contaminants and avoid 

damage to the composite resin system. Abrasive cleaning methods may also be used for metal 

and composite parts for removal of corrosion and paint. Care must be taken to remove excess 

cleaning fluids before further processing. The structure should be sufficiently dried in 

accordance with the FAA-approved repair process. 

(3) Metallic Surface Preparation. After successful prefit of the bonded metallic 

structure, the detail parts and the bond surface should be prepared using the OEMrecommended procedure or alternate procedure approved by the FAA. Surface preparation 

solutions should be controlled and maintained at the proper concentration in accordance with 

FAA-approved procedures. After addition of make-up material or complete change of 

solution, cleaning solutions should be retested and found acceptable before production use. 

Page 12 Par 10 
11/15/96 AC 145-6 

(4) Adhesive Primer Application. After surface preparation, detail metallic parts and 

the bond surface should be expeditiously spray-primed consistent with the approved OEM or 

other FAA-approved procedures. Special care should be exercised to avoid physical contact or 

contamination of the prepared surface prior to and after priming. Any compressed air used 

should be filtered and free of contaminates. 

(5) Composite Ply Cutting. Composite plies are cut to orientation and stacking 

sequence, as required by-the approved repair data. If cut plies are to be returned to the freezer 

before use, they should be kitted and collated as to stacking sequence and ply orientation, and 

stored in a protective bag. If templates are developed for ply cutting, they should be identified 

as to ply orientation and sequence. 

(6) Lay-up and Bagging. The exact method of lay-up is dependent on part, tooling, 

resin/adhesive flow, surface prep, etc., and should be consistent with the OEM’s process 

specification, or other FAA-approved specifications. Lay-up should be accomplished in 

accordance with environmental conditions approved by the OEM. Care should be taken to 

accomplish the repair within the remaining allowable out-time and/or pot life of the material. 

The maintenance organization’s procedure should specify a maximum time for 

accomplishment. Adhesives or primed faying surfaces to be bonded should not be handled 

with bare hands, nor should any contamination be allowed to result from contact with 

supporting fixtures or mechanical handling equipment. Clean, white, lint-free gloves should 

be worn by all persons handling cleaned parts, cleaned and primed parts, or adhesive films. In 

laying up composites, special care should be taken to ensure proper ply orientation and 

stacking sequence. Bagging should be sufficient to remove all trapped air and produce the 

desired resin/adhesive flow. If necessary an intermediate debulk cycle should be incorporated. 

Only non-contaminating films, tapes, etc., should be used for flow control purposes. When 

vacuum bagging techniques are used the vacuum bagged repair should be vacuum checked for 

leaks to insure air does not contaminate the repair during the cure cycle. A sufficient number 

of thermocouples in contact with the part should be used to monitor the temperature of the 

curing material and particular attention should be given to bondlines and heatsinks. 

(7) Curing. Curing should be accomplished by controlling the appropriate parameters 

such as vacuum, pressure, temperature, viscosity, etc., with respect to time in accordance with 

OEM or other FAA-approved procedures to establish the correct state of cure. Attention 

should be paid both to the lowest temperature thermocouple and the maximum temperature rise 

to avoid undercure and overheating respectively. 

(8) Quality Control. The maintenance organization should establish and implement a 

plan that verifies that the parameters affecting material integrity and process capability are 

operating under controlled conditions. For metal bonded structure, test specimen coupons 

may be necessary to verify the repair process. 

(9) NDI. The fabrication specification should specify methods of ND1 for each part 

and specify accept/reject criteria. The types of ND1 specified should be consistent with the 

types of flaws expected and the type of construction. It is recommended that small honeycomb 

Par 10 Page 13 
AC 145-6 11/15/96 

parts be leak-checked, by submersion, for porous bond lines or edges and improperly sealed 

fasteners. When an entire part, or a large area of a part, is subjected to an elevated 

temperature cure cycle to cure a repair to an area of the part, the entire area of the part that 

was heated should be non-destructively inspected after repair curing. This is necessary 

because moisture absorbed in laminate or core areas of the part not being repaired can damage 

the part during the elevated temperature curing cycle of the repair. 

(10) Documentation. A detailed shop record should accompany every part from 

prefit or assembly of details through final acceptance. The record should address each critical 

step required during the fabrication process and should include in-process quality control 

checks to be completed before moving to the next process step. It is recommended that the 

shop record identify the part number, process and material specification numbers, 

adhesive/prepreg roll number, cure parameters, date, and the results of specimen testing. 

c. Process Change. Procedures should be reviewed and requalified if necessary, 

whenever any significant changes are made to items such as type or source of material, cure 

cycle, equipment controls, or autoclave loading patterns and tool design. Process capability 

should be demonstrated by inspection and testing as necessary to determine conformity to 

repair data requirements. Any changes to FAA-approved procedures, processes, or data will 

require approval by the FAA. 

11. NONDESTRUCTIVE INSPECTION (NDI) PROCEDURES. 

a. ND1 Techniques. Several ND1 techniques may be used to detect discrepancies in 

composite structures; however, the most commonly used techniques are visual, audio sonic 

(coin tapping), radiography, ultrasonic, and mechanical impedance testing. 

(1) Visual. Visual inspection is the most widely used ND1 method. Discrepancies 

that can usually be observed include: Discoloration (due to overheating), foreign matter, 

crazing, cracks, scratches, blisters, dents, orange peeling, pitting, air bubbles, porosity, resinrich and resin-poor areas, and surface wrinkles. Reflected light is used for observing surface 

irregularities and other defects. Transmitted light (assuming both surfaces are accessible and 

the material is translucent) helps to reveal discrepancies within the specimen. 

(2) Audio Sonic. Sonic testing makes use of frequencies in the audible range from 

about 10 Hz. to 20 Hz. “Coin tapping” is a common technique used for detecting voids or 

delaminations. When tapping any area, a coin or other suitable object may be used. When 

this technique is used, a clear, sharp, ringing sound is indicative of a well-bonded solid 

structure, while a dull sound or thud indicates a void or delamination. Automated sonic 

devices that produce a consistent tapping rate and force are available and can be used for 

test. 

. 

tlus 

(3) Radiography. Radiography is generally used in composite fabrication for the 

detection of bondline defects. In addition, radiography may also be used to inspect for the 

presence of foreign material, adhesive voids, location of internal parts, honeycomb core 

Page 14 Par 10 
11/15/96 AC 145-6 

defects, mislocated or misdrilled holes, poor fit-up, thick bonds, fiber discontinuities, poor 

tape lay-up, or lack of adhesive fillets and for water ingress in honeycomb panels. In the case 

of carbon/epoxy, glass/epoxy, and aramid (kevlar)/epoxy, the resolution differences are low 

and defect detection becomes difficult due to the low contrast in the film. Radiography is 

usually performed through the thickness of the product for detection of anomalies. 

(4) Ultrasonic. In this technique, the attenuation of sound wave energy is used for 

flaw detection. Two methods of ultrasonic techniques are available: through-transmission, 

and pulse echo. Three methods of recording and display generally used are: A-scan, B-scan, 

and C-scan. An A-scan is an amplitude versus time display for a specific point on the part 

inspected. It generally lends itself to the contact pulse-echo methods. B-scan displays a long 

cross-sectional view of the part under test and show discovered discrepancies. C-scan displays 

the effect in a plan view usually on a paper that is rolled across the printer bar. C-scans 

provide no defect depth or orientation information; however, they are capable of detecting 

defects in the magnitude of 8.01 square inch. Ultrasonic techniques can generally be used to 

detect porosity, laminar inclusions, delaminations, and fastener hole flaws. A disadvantage of 

the ultrasonic method is the inability to distinguish between voids and small delaminations 

since the attenuation characteristics of the two flaws are identical. Another disadvantage is 

the requirement for representative calibration standards. 

(5) Mechanical Impedance. These are methods in which structural response to strain 

excitation is measured to detect delaminated areas or disbonds. The sensitivity of this method 

decreases with increasing structural flexibility or discrepancy depth. 

b. Controlling ND1 Techniques. In order for ND1 techniques to be effective, repeatable, 

and reliable, certain controls are necessary. An approved ND1 specification and procedure 

should include: 

(1) Periodic qualification of personnel conducting the inspection technique. 

(2) Establishment of realistic acceptance standards consistent with FAA-approved 

repair requirements for use by in-process and final inspection personnel. 8 

(3) Calibration of equipment used in the inspection technique, including any quality 

control standards with known defects that may be used. The calibration system should provide 

for periodic requalification of any such equipment at specific time intervals. 

(4) An internal audit program for validating the effectiveness of the ND1 program. 

12. PERSONNEL QUALIFICATION AND TRAINING. 

a. Maintenance Organization. Repair personnel should be fully qualified in the 

preparation and handling of repair materials and in the repair of bonded honeycomb sandwich 

and composite laminate construction. Personnel should be qualified under a program which 

Par 11 Page 15 
AC 145-6 llll5l96 

assures the maintenance organization’s performance is in accordance with established 

acceptable performance standards for repair practices and product quality. The program 

should include provisions for revalidating maintenance organization qualifications on a 

predetermined basis. 

b. ND1 Personnel. ND1 personnel should be qualified in accordance with a recognized 

industry or government standard. A list of qualified ND1 inspectors should be developed and 

maintained indicating each person’s qualifications and level of proficiency, and their 

requalification date. 

13. INSPECTION RECORDS PROCEDURES. The repair facility should have inspection 

records for repaired and/or fabricated composite items, which demonstrate compliance with 

the following: 

a. Part Identification. The inspection record should identify the part name, part number, 

and the approved repair data with which the part complies after repair. 

b. Material Traceability. The inspection record should identify the structural raw 

materials (e.g., adhesives, prepregs, resins) by manufacturer and batch/lot number that were 

used in fabrication. 

c. Conformance to Fabrication Requirements. The inspection record should identify 

and show compliance with the fabrication process specifications, destructive and ND1 

specifications, and other specifications defining product acceptance/rejection standards that 

have been used for fabrication and quality control acceptance purposes. It should also show 

evidence of quality control acceptance of the structural raw materials (e.g., adhesives, 

prepregs, resins) used in fabrication. If salvaged parts are used in the fabrication process, the 

repair specifications and/or procedures should also be identified on the inspection record. 

Each completed operation necessary to repair or fabricate the complete part should show 

evidence of acceptance by authorized and qualified personnel. 

d. Product Conformity and Airworthiness. The inspection record should be annotated 

by the appropriate maintenance organizations and authorized inspectors as evidence of 

compliance with approved repair procedures. Maintenance record entries (approval for return 

to service) should be made in accordance with part 43. 

e. Record Retention. Inspection records are to be retained for the length of time 

required by parts 121, 125, 127, 135, and 145, as appropriate. 

William J. White 

Deputy Director, Flight Standards Service 

Page 16 Par 12 

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