P w 

U.S. Department 

of Transportation 

Federal Aviation 

Administfation 

Advisory 

Circular 

CONSOLIDATED REPRINT 

I 

This consolidated reprint incorporates 

Change 1 

Subject: QUALITY CONTROL FOR THE 

MANUFACTURE OF COMPOSITE 

STRUCTURES 

Date: 6/26/89 ACNo: 21-26 

Initiated by: AI R- 2 () () Change: 

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 Federal Aviation Regulations (FAR) 

Part 21, Certification Procedures for Products and Parts, regarding quality 

control (QC) systems for the manufacture of composite structures involving 

fiber reinforced materials, e.g., carbon (graphite), boron, aramid (Kevlar), 

and glass reinforced polymeric materials. This AC also provides guidance 

regarding the essential features of QC systems for composites as mentioned in 

AC 20-107, Composite Aircraft Structure. Consideration will be given to any 

other method of compliance the applicant elects to present to the Federal 

Aviation Administration (FM). 

2. RELATED FAR SECTIONS. Sections 21.125, 21.143, 21.303, and 21.605. 

3. RELATED REFERENCE MATERIAL. 

a AC 21-1, Production Certificates. 

b. AC 21-6, Production Under Type Certificate Only. 

c. AC 21-303.1, Certification Procedures for Products and Parts. 

d. AC 20-107, Composite Aircraft Structure. 

4. DEFINITIONS. Appendix 1 contains a list of terms commonly used in the 

manufacture of composite structures. 

5. QUALITY CONTROL SYSTEM. 

fabrication should be basica 

established to meet the requ 

QC system should include pro 

materials, control of in-pro 

to evaluate the end product 

A QC system 

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irements of 

cedures that 

cess manufac 

that is e 

to any 0th 

FAR Part 2 

ensure th 

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for conformi ty to desi 

stablished for compos 

er QC system that has 

1. For example, the 

e quality of incoming 

*hods, and testing per 

gn requirements. The 

te 

been 

ormed 

QC 

system should include standards to be used for nondestructive and destructive 

tests, visual inspection techniques during the fabrication process, and 

product final acceptance. The standards used to determine the acceptance or 

rejection of manufacturing-induced defects and damage should take into account 

the process and inspection capability. These standards should be based on 

approved data developed as a result of proof of structure evaluations 

conducted in accordance with referenced AC 20--107 and applicable FAR 

airworthiness standards. This AC addresses those areas of a QC system that 

may require further expansion to accommodate composite fabrication. 
AC 21-26 6/26/89 

6. MATERIAL AND PROCESS SPECIFICATI,ONS. Unlike parts which use metallic 

materials in the manufacturing process, the material properties of a composite 

structure are manufactured into the structure as part of the fabrication 

process. Therefore, it is essential that material’ and process specifications 

used to produce composite structures, contain sufficient information to ensure 

that critical parameters in the fabrication process are identified to 

facilitate production and final inspection. Typical material and process 

SpeMIications, respectively, should contain the following information: 

a. Basic fiber, matrix and cured component properties, and required 

testing including type, number, and frequency of tests. 

b. Conditions and requirements during the manufacturing process including 

fabrication method, specific tooling, environmental conditions, significant 

parameters during the cure cycle, inspection criteria at each operation, and 

storage and handling conditions throughout the process. 

7. MATERIALS. The FAA requires that all drawings submitted for type design 

approval contain sufficient information or references to material 

specifications or other such data that clearly identify the materials and 

processes that are necessary to ensure production of like articles. When 

materials for a composite structure are produced in accordance with a 

proprietary process, or the composition of the materials is proprietary, the 

production approval holder or applicant may prefer to exclude this proprietary 

data from the type design submitted to the FM. In these instances, the 

applicable type design drawings should refer to a specification which contains 

this proprietary information so that complete traceability to material 

composition is possible. These specifications will then be made available to 

the FAA for review and approval under an arrangement between the FAA and the 

production approval holder or applicant which will protect the proprietary 

nature of the product. Under FAR sections 21.33 and 21.157 (Inspection and 

tests), the FAA is authorized to review such data upon request. The following 

are recommendations regarding QC procedures, chemical characterization, 

testing techniques, and supplier control of purchased materials: 

a. The production approval holder or applicant should have an incoming 

material acceptance plan that ensures conformity and uniformity of purchased 

composite materials to specifications identified in the approved type design. 

Copies of supplier laboratory test reports showing actual test results should 

accompany each batch of material received for review and approval; however, a 

material supplier’s test report alone should not be considered adequate 

documentation to substantiate that materials satisfy all specification 

requirements. Samples of these materials should be taken on a batch-to-batch 

basis and tested to engineering and manufacturing requirements for conformity 

of physical, chemical, mechanical, and processing properties to verify the 

accuracy of suppliers’ laboratory reports. Sample testing could be 

accomplished by the production approval holder or applicant at its production 

facilities or by an independent laboratory approved for such testing under the 

production approval holder's or applicant’s QC system. Such testing may be 

decreased once confidence in the quality of products from a particular source 

has been established by the production approval holder. 

Par 7 
AC 21-26 CHG 1 

b. Resin matrix systems: The production approval holder or applicant 

should require chemical characterization tests of the matrix material. A 

typical material specification sets upper and lower limits on the amount of 

reactive functional group per unit of material, as well as other design 

requirements such as viscosity, color, and moisture content. The 

characterization tests should identify and measure the amounts of individual 

constituents of the resin system such as the basic epoxide, curing agent, 

accelerator, hardener, etc., and the reacted constituents caused by resin 

mixing, storage, fiber impregnation, etc. 

c. It is recommended that tests such as High Performance Liquid 

Chromatography (HPLC), Fourier Transform Infrared Spectroscopy (FTIS) and 

Gelation Permeation Characterization (GPC) for chemical characterization, and 

Differential Scanning Calorimeter (DSC) and Dynamic Rheological Analysis for 

thermal analysis and viscosity/flow/tack/gelation characteristics or their 

equivalent be conducted, and resulting traces compared to the standards 

specified in the material specification. Experience has shown that one method 

of testing a particular sample may yield the same “fingerprint” as an 

acceptable sample within specification limits, yet the sample is rejectable 

when tested using another method. Therefore, it is likely that more than one 

type of characterization test may be necessary,, since all chemical ingredients 

affecting a given composite’s performance may not be quantifiable by a single 

test method. Accordingly, the production approval holder’s or applicant’s 

material specification must define the combinations of acceptable test 

techniques and test results which adequately demonstrate the material’s 

conformity and process capability. Techniques for chemical characterization 

are not universal but are strongly dependent upon the resin formulation. 

Therefore, test methods adapted to each material should be developed 

considering the sensitivity of the method to detect deviations in formulation. 

d . Reinforcement Fibers: The production approval holder or applicant 

should establish procedures in coordination with its material suppliers to 

control the quality of incoming fibrous materials such as roving, tow, and 

woven fabric. Testing for mechanical properties is the primary method used 

for fiber characterization. This method is also used to qualify or screen 

continuous filaments since tensile strength and modulus are sensitive 

indicators of change in composite material mechanical response. In addition, 

a means should be developed to control the quality of fiber surface treatment, 

sizing, etc. 

e. Preimpregnated Material (Prepreg): In applications where prepreg 

tape, fabric, or roving are primary materials of composite structures, it is 

necessary to test for the chemical, physical, and mechanical fiber and matrix 

dominated properties identified in the applicable prepreg material 

specification. Quality control procedures previously described for resins and 

fibers may be used to control the quality of incoming prepreg materials. 

Chemical characterization should be performed on resin extracted from the 

prepeg material to be used for production, where applicable. 

Par7 
AC 21-26 6/26/89 

f . Adhesives: The QC procedures recommended for structural adhesives are 

similar to those for resin matrix systems; that is, the procedures should 

provide assurance that each incoming batch conforms to the chemical, physical, 

and mechanical properties identified in the material specification. 

8 MANUFACTURING CONTROLS. The manufacture of acceptable and reliable 

composite structures is dependent on the type of process controls employed 

during the fabrication cycle. If all pertinent process variables are 

adequately controlled, there is added assurance that the parts and structures 

produced will be acceptable. Quality control should, therefore, establish and 

implement a plan which verifies that: (1) the parameters affecting material 

integrity and process capability are operating under controlled conditions; 

and (2) individual items, batches, or lots conform to specified quality 

standards. To ensure that QC objectives have been met, process procedures 

should clearly define specific materials, tooling, equipment, cure cycle 

parameters, quality standards, operator qualifications, storage and handling 

requirements, traceability records, and any other special requirements. 

a. Gen era1 : The following are manufacturing process controls that should 

be inclu .ded in a manuf acturer’s QC system: 

(1) The production approval holder or applicant should develop 

integrated quality and production control procedures for operations that 

define product configuration, selection of materials, tooling and facility 

equipment, calibration, sequence of fabrication operations, critical inprocess parameters and processing tolerances, and conformity to quality 

standards. 

(2) Environmental parameters (temperature, humidity, chemical 

contamination) should be defined and controlled, as required, where composite 

parts and structures will be produced; in particular, cutting, layup, and 

bonding areas. 

(a) Unless otherwise validated for the material system in use, 

the area should be temperature and humidity controlled such that the minimum 

temperature is 65 degrees F with a corresponding relative humidity not greater 

than 63 percent and the maximum temperature is 75 degrees F with a 

corresponding relative humidity not greater than 46 percent. The temperature 

and relative humidity values between the minimum and maximum acceptable values 

listed above should form a straight line relationship. Procedures for QC to 

verify and record temperature and humidity conditions should be established to 

ensure environmental stability. 

(b) Contamination restrictions in environmentally controlled 

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 which may adversely affect the manufacturing 

process, e,g., release agents or material containing uncured silicon should 

not be permitted. Also, conditions under which operators may handle materials 

should be defined. 

4 Par8 
6/26/89 AC 21-26 

( C ) Layup and clean room 

systems should be capable of providing 

air filtration and pressurization 

a slight positive overpressure. 

(3) The production approval holder or applicant should establish a 

program to train and/or qualify operators, as appropriate. This program 

should measure operator performance to production standards and provide for 

requalification as necessary. 

(4) Prior to the start of production, manufacturing processes should 

be qualified by demonstrating that the combination of materials, tooling, 

equipment, procedures, and other controls making up the process will produce 

parts having consistent material properties that conform to design 

requirements. As part of the process qualification, destructive and/or 

nondestructive inspection (NDI) of appropriate tool proofing specimens should 

be conducted to determine conformity to specified design requirements. 

Destructive tests of specimens verify conformity to the specified physical and 

mechanical properties. Nondestructive inspection of specimens verify that 

discrepancies caused by fabrication procedures remain within allowable limits. 

(5) Once the manufacturing process has been established it should not 

be changed unless a comparability study and necessary testing of differences 

has been completed. In addition, processes should be reviewed and requalified 

if necessary, whenever any significant changes are made to the process such as 

sources of material, cure cycle, equipment controls, or autoclave loading 

patterns and tool design changes. Process capability should be demonstrated 

by inspection and testing as necessary to determine conformity to design 

requirements. 

(6) In order to control adhesives and matrix formulation when resin 

is mixed during part manufacture, documented procedures should be employed to 

cover the identity of required constituents, respective quantities, and mixing 

methods and techniques. Tests should be performed on each resin mix to ensure 

conformity to specification requirements and should including sampling methods 

for chemical testing. The mixing process performed under these procedures 

should ensure that the sequence of additions and mixing techniques are 

followed, proper chemical reactions are achieved, aeration of the mix is 

prevented, and the mixing equipment is properly cleaned. 

(7) After initial process qualification, testing (process control 

panels, etc,) for conformity to design requirements should continue on an 

appropriate frequency to ensure that the manufacturing process, materials, and 

associated tooling continue to operate in a state of control and produce 

conforming parts. 

(8) For accepting or rejecting cured structures, NDI equipment and 

procedures should be used to evaluate specified material defects resulting 

from fabrication and assembly operations. The NDI technique used for 

inspection should have the sensitivity to detect maximum allowable discrepancy 

type and size in the part. 

(9) Specifications used by QC should define allowable limits for each 

discrepancy such as: adhesive voids, porosity, delaminations, damaged core, 

core node bond separations, potting cracks, short core, lack of adhesive, etc. 

Par 8 5 
AC 21-26 6/26/89 

b. Fabrication of Parts: The following are manufacturing controls unique 

to laminate and wet layup, filament winding, and pultrusion that a QC system 

should include: 

(1) Laminate Layup. 

(a) Standards and methods should be established to ensure proper 

orientation, stacking, and nesting of the plies during layup operations. The 

programs that control tape head and table motions, and tape feed and tape 

stacking, should be approved by QC. Standards should be established for such 

in-process variables as tape orientation, gaps, and overlap. 

(b) For automatic and hand layup methods, controls should be 

established for in-process variables that affect the cured laminate quality: 

e.g. p resin content, ply compaction, laminate density, porosity, which may 

result from debulking, pre-bleeding, and bagging operations. 

I 

(2) Wet Layup. 

(a) Procedures should be established to ensure correct material 

selection, orientation and stacking, or nesting of the plies during wet layup 

operations. 

(b) Procedures should be established to control in-process 

variables such as resin content, aeration and air pockets. 

(3) Filament Winding. 

(a) Preimpregnated filaments for dry winding should be stored 

and handled according to procedures similar to those for other prepreg 

materials to ensure that the material’s original properties have been 

maintained, especially material flow and tack to ensure proper bonding for 

subsequent winding operations. 

(b) Procedures should be established to ensure that the working 

life of the resin system exceeds the anticipated winding time and to ensure 

that gelation does not take place before completion of winding. 

(c) Standards and methods should be established to control 

machine-dominant parameters such as feed rate, feeder arm and mandrel motions, 

mandrel dwell angle, number of circuits per pattern, total number of circuits 

for complete coverage, number of plies per layer, winding angle, fiber tension 

and alignment, band-width, and fiber/resin ratio. 

(d) Procedures should be established to control process 

variables during the winding operation such as resin viscosity, fiber wetting, 

fiber tension, fiber band-width and alignment, air entrapment, degree of 

compaction, and fiber damage. 

Par 8 
6/26/89 AC 21-26 

(4) Pultrusion. 

(a) Procedures should be established to control pultrusion 

start-up, steady state, and shutdown operations including disposition of 

material produced during start-up and shutdown operations. 

(b) Process procedures should define limits for important 

process parameters that determine product quality such as line speed, die 

temperature profile for the particular operating conditions and resin system, 

clamping pull-through lead, resin temperature, die input temperature, material 

orientation for preform operation, and material tension. 

c. Assembly of Components: The following are manufacturing controls 

unique to the assembly of fabricated composite parts and structures that a 

manufacture’s QC system should include: 

(1) Sandwich Construction. 

(a) Core material should be inspected for cell damage, crush, 

correct length and thickness, and should be properly cleaned and protected 

against contamination. 

(b) Cured laminates should be inspected for dimensional 

conformity. Faying surfaces should be properly cleaned and protected from 

contamination before subsequent assembly and bonding operations. 

(c) Uncured laminates used for subsequent co-curing operations 

should be properly protected and stored to prevent contamination and to retard 

chemical advancement. 

(d) Procedures should be established to control those critical 

parameters of the assembly process that affect the integrity and uniformity of 

the end item such as: bondline thickness, bondline pressure and temperature 

distribution, and part “floating” within the tooling when the resin or 

adhesive is in the liquid Hate during cure cycle (bagging and tooling 

requirements). 

(e) The performance of the sandwich construction process should 

be evaluated for each autoclave or oven load by testing specimens of the 

sandwich construction. Test specimens should be made from specially prepared 

panels that have been fabricated from the same materials, and using the same 

production methods, environmental working conditions, and cure cycle as the 

production parts they represent. When co-curing of laminates is required to 

fabricate an assembly, a solid laminate process control panel should also be 

prepared for specimen testing. Cutouts or cutoffs from production parts may 

also be used providing they are representative of the product.ion part 

configuration. 

(2) Curing Process. 

(4 pr 

parameters of the pr 

aced ures 

s, na 

shou Id be devel oped to control the critical 

.mely chemical r eact ions of the resin and 

Par 8 7 
AC 21-26 6/26/89 

consolidation of the plies to achieve manufacturing consistency and quality 

parts which are of uniform consolidation, within void tolerances, and of 

correct fiber volume. 

(b) Procedures should identify the equipment and materials which 

will be used to bag the uncured laminate or sandwich construction. This will 

ensure the proper control of resin flow, volatile content, and ply 

consolidation during the cure cycle since such materials (e.g., peel plies) 

may alter properties of the structure. 

(c) Procedures should define the relationship of the variables 

(time, temperature, pressure) in the cure cycle (and post-cure cycles) which 

control compaction and consolidation and cure reaction. These controls should 

include the acceptable limits of variables during the cure cycle and action to 

be taken when such limits are exceeded; e.g., submit cured parts to QC (MRB) 

for evaluation and disposition. 

(d) Procedures should be established to ensure that, when the 

autoclave or oven is loaded with uncured parts having different 

configurations, materials, bagging configurations, and tooling heat-up 

characteristics, each part is properly cured. 

(e) Procedures should describe actions to be taken when 

heating, cooling, vacuum, or pressure interruptions are experienced during 

autoclave/oven cure; e.g., submit to QC (MRB) for disposition. 

(f) The performance of the curing process should be evaluated 

for each autoclave or oven load by testing specimens representative of the 

composite structure. Test specimens should be fabricated from the same 

materials, and using the same production methods, environmental working 

conditions, and cure cycle as the production parts they represent. Cutouts or 

cutoffs from production parts may also be used provided that they are 

representative of the production part configuration. The use of validated 

direct-cure monitoring techniques may also be used if found directly 

applicable. 

(3) Secondary Bonding. 

(a) Cleaning procedures for surfaces to be bonded together 

should specify the chemicals and abrasive to be used on faying surfaces. 

Standards for determining properly cleaned surfaces and methods for protecting 

clean surfaces from contamination (including peel plies) should be available 

at the place of fabrication. 

(b) Quality control standards should establish adhesive bondline 

thickness and pressure to be applied along the bondline during the cure cycle. 

(c) Tool proofing procedures should demonstrate the capability 

of the tooling to maintain correct bondline thickness, equal pressure 

distribution along the bondline, and properly cured adhesive. 
6/26/89 AC 21-26 

(d) Before the application of adhesives, the production parts 

should be preassembled and inspected in the tooling to determine that the 

expected bondline thickness and pressure distribution on the bondline will be 

within design limits. Shims required to fill bonding gaps should only be used 

in accordance with an approved QC procedure. 

(f) Methods for applying adhesive films should preclude the 

formation of defects such as voids due to air entrapment, bridging, and gaps 

between adjacent films. 

(g) Standards for the application of adhesives (e.g., pastes) 

should require a complete and even coating of adhesive with wetting of 

surfaces and exclusion of air in the adhesive. 

(h) Controls for high temperature curing adhesives should be 

recorded and should include heat-up and cool-down rates, cure time and 

temperature, vacuum, and pressure parameters. 

. (1) Controls for room temperature curing of adhesives should 

specify: 

mating, 

1. Minimum and maximum time between mixing of adhesive and 

clamping, or tooling of parts into final configuration. 

2. - Minimum time at particular temperatures to achieve 

adequate handling strength for subsequent processing, and the conditions (time 

and temperature) for achieving full cure. 

. (J) The conditions under which bonded s tructures may be 

subjected to a su bseq uent curing operation should spe cify: 

1. - 

2. - 

Maximum allowable temperature. 

Maximum number of subsequent curing cycles. 

3. 

delaminations, etc,, 

Procedures to prevent bond line separation, blistering, 

of the structure when subsequent curing temperatures 

approach that of the previous cure temperature. 

(4) Other Holding Methods. I? or techniques s uch as resin injection, 

compression, and transfer molding proc esses, specific ations should define the 

limits for all critical process parame ters that deter mine product quality. 

Such parameters are, for example, resi n mix, feed-rat e and temperature, mold 

temperature, and back pressure or vacu urn. In additio n, molding process 

specifications should identify the tim ing and sequent e of automatic 

operations. These critical parameters should be defi ned in the process 

specification and identified in the as sociated operat ion sheets. 

9. FINAL ACCEPTANCE. 

a. Final acceptance procedures and quality control standards should 

provide added assurance that the completed structure meets its functional and 

design requirements. 

Par 8 9 
AC 21-26 6/26/89 

b. Final acceptance records should provide evidence that the following 

significant production and QC activities, specifically designed to assure the 

quality of composite structures, have been completed: 

(2) 

(3) 

(4) 

Incoming material acceptance. 

In-process fabrication and assembly controls. 

Maintenance of tooling and facility equipment. 

Calibration of inspection and laboratory test equipment. 

(5) Inspection acceptance of functional characteristics at detail and 

assembly levels. 

(6) Nondestructive inspection acceptance. 

(7) Configuration control. 

(8) Material Review Board disposition. 

c J . Nondestructive Inspection (NDI). There are several ND1 techniques 

used in detecting discrepancies in composite structures; however, the most 

commonly used techniques are visual, audio sonic (coin tapping), radiography, 

ultrasonics, and mechanical impedance testing. 

(1) Visual. Visual inspection is the most widely used NDI 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, resin rich and resin poor 

areas, and surface wrinkles. Reflected light is used for observing surface 

irregularities and other defects, while transmitted light (assuming both 

surfaces are accessible and the material is translucent) helps to reveal 

discrepancies within the specimen. 

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

this test. 

(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 defects, mislocated or 

misdrilled holes, poor fit-up, thick bonds, fiber discontinuities, poor tape 

lwup, or lack of adhesive fillets. In the case of carbon/epoxy, 

10 Par 9 
6/26/89 AC 21-26 

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-scans display a long cross-sectional view of the part under test and show 

discovered discrepancies. C-scans display the effect in a plane 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 0.01 square inch. Ultrasonic techniques can 

generally be used to detect porosity, laminar inclusions, delaminations, and 

fastener hole flaws. The disadvantage of the ultrasonic method is the 

inability of the method to distinguish between voids and small delaminations 

since the attenuation characteristics of the two flaws are identical. 

(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. 

e. Controlling Nondestructive Inspection Techniques. In order for 

nondestructive inspection techniques to be effective, repeatable, and 

reliable, certain controls are required. These controls should be approved by 

the production approval holder or applicant. The following controls are 

recommended: 

(1) A QC approved ND1 specification and procedure to be used. 

(2) Periodic qualification of personnel conducting the inspection 

technique. This would normally include regularly scheduled vision 

examinations and inspection of a standard with a known defect. 

(3) Establishment of realistic acceptance standards for use by 

in-process and final inspection personnel. 

(4) Calibration of equipment used in the inspection technique 

including any QC standards with known defects that may be used. The 

calibration system should provide for periodic requalification of any such 

equipment at specific time intervals. 

(5) An internal audit program for validating the effectiveness of the 

ND1 program. 

Pa- 9 11 
AC 21-26 6/26/89 

10. STORAGE AND HANDLING. Composite materials and adhesives may be subject 

to deterioration if not stored under proper environmental conditions. Storing 

these materials at low temperatures (e.g., 0 degrees) in freezers retards 

partial curing of polymer materials and extends their shelf life. Therefore, 

it is essential that composite material handling and storage procedures are 

established, followed, and subjected to periodic independent auditing to 

ensure continued conformity to the materials’ chemical, physical, and 

specified mechanical properties. 

a. Uncured Material Storage. Because composite structures are made from 

polymeric materials (resins, prepregs, adhesives), procedures which ensure 

that these materials are acceptable at the time of use in the manufacturing 

area should be established. 

(1) Material suppliers should provide documents with each shipment 

that identify the environmental conditions under which the purchased materia! 

was previously stored and shipped, i.e., temperature and humidity conditions 

and accumulated time in and out of refrigeration. 

(2) Mat erial should be stored under envir onmentally controlled 

conditions and m onitored for compliance with speci fication requirements. 

Historical records for all stored material should be maintained, i.e., 

periodic recording of storage conditions, time in and out of storage, etc. 

13) Materials are generally stored in sealed plastic bags or 

containers to prevent moisture from condensing on the cold material and 

migrating into the polymer when it is removed from the freezer and allowed to 

warm to ambient temperature. The time interval from when the material is 

removed from the freezer to the time when the material bag or container may be 

opened is generally empirically determined. Physical characteristics such as 

material roll, stacking height thickness, or material type (e.g., tape vs 

broadgoods) must be considered when determining this time interval. 

Therefore, the production approval holder or applicant should establish the 

minimum time interval between removal of material from its freezer and its use 

to prevent premature removal of materials from storage bags or containers 

before material temperature stabilization occurs. 

b. Handling of Cured Parts. Composite materials and structures require 

specific handling procedures to protect them from damage during production and 

storage. Accordingly, procedures for handling and storing composite materials 

and structures should be established and followed. These procedures should be 

an integral part of the manufactures’ QC system and should provide protection 

during receiving inspection, material. storage, material handling, 

manufacturing process, cure cycle, final inspection, and final product 

storage. 

William J. Sullivan, Acting 

Director, Aircraft Certification 

Service 

12 Par 10 
6/26/89 

APPENDIX 1. DEFINITIONS 

AC 21-26 

Appendix 1 

Adhesive. A substance capable of holding two materials together by surface 

attachment. Structural adhesives produce attachments capable of transmitting 

significant structural loads. 

Chemical Characterization. A series of chemical examinations to identify 

and/or quantify the critical constituents of a material and thereby establish 

a fingerprint of the material which can be used as a basis for future 

comparison. 

Composite. A material containing two or more distinct materials (fillers, 

reinforcing materials, and compatible plastic resin) designed to exhibit 

specific performance properties. 

Cure. To change the physical properties of a material by chemical reaction, 

by the action of heat and catalysts, alone or in combination, with or without 

pressure. 

Delamination. The separation of layers in a laminate through failure of the 

matrix. 

Fiber. A single homogeneous strand of material used as a principal 

constituent in advanced composite materials because of its high axial strength 

and modulus. 

Filament. -- Fiber of extreme length, used in yarns and other compositions. 

a rotating mandrel 

Process in which resin impregnated strands are applied over 

to produce high strength, reinforced cylindrical shapes. 

Laminate. A product obtained by bonding together two or more laminae of the 

same material or of different materials. 

Layup e A fabrication process involving the placement of successive layers of 

materials. Also, the arranged set of laminae. 

Gelation. The formation of a gel, which is a jelly like semi solid material 

state formed by a network of solid aggregates retaining a liquid phase. 

Hand Layup. Method of positioning successive layers of reinforcement material 

(which may or may not be preimpregnated with resin) on a mold by hand. Resin 

is used to impre gnat e or coat t he r ei nfor 

cured to permane ntly fix the fo rmed S hape 

cement material. The resin is 

Matrix. The essentially homogeneous material in which the fibers or filaments 

of a composite material are imbedded. 

Polymer. A high molecular weight organic material (natural or synthetic) 

formed by linking together a large number of repeating chemical units 

(monomers). When two or more monomers are involved, the product is called a 

copolymer. 
AC 21-26 Appendix 1 

6/26/89 

Preiapregnated Material (Prepregl. A term generally used to describe fibrous 

material which has been preimpregnated with a liquid resin and cured to the Bstage. 

Pultrusjon. A process by which continuous lengths of fiber are pulled through 

a pre-shaper of the desired cross-section and on through a heating source from 

which they emerge fully cured. 

Resin. Any of the various solid or semi-solid amorphous natural or synthetic 

organic substances with indefinite and usually high molecular weights and no 

sharp melting point. 

Reinforcement Fibers. Materials such as roving, tow, and woven fabric used to 

produce a structural material superior to the base resin. 

Sandwich Construction. A structural panel concept consisting in its simplest 

form of two relatively thin, parallel sheets (face sheets) of structural 

material bonded to, and separated by, a relatively thick, lightweight core. 

Wet layup. A process in which dry fabric is laid into a mold and impregnated 

with liquid resin as part of the layup process.