13 moisture-tight bags that are sealed to prevent moisture and dust contamination and labeled with the information in paragraph 7.3.4.4, to maintain traceability.
8. Specifying that the MO must allow the raw material to thaw within the container or bag until the condensation on the exterior of the container has dissipated. This is to prevent atmospheric moisture from condensing on the material.
9. Requiring the MO to place remaining raw materials on the roll back in their moisture-tight container or bag and placed back in the freezer after recording the out time on the attached record.
10. Requiring the use of appropriate clothing and gloves for handling adhesives, prepreg composite materials, and prepared bond surfaces (e.g., protective glasses and breathing mask). Carbon/graphite materials are considered hazardous materials and require special handling. The MO must follow regulations and consult the local authorities on the proper procedures for waste disposal.
11. Requiring the MO to retain the batch number, lot number, roll number, shelf life, and out time records for a specified time (recommended period is a minimum of 2 years from the time an MO depletes or disposes of the material).
12. Describing how the MO should position/orient the roll of material in the freezer to prevent fiber breakage or resin migration, as required in the applicable material specification.
13. Requiring that the MO scrap or recertify materials that have exceeded their shelf life or out time limit in accordance with the applicable material specification or other FAA-approved procedures.
8 HANGARS, FACILITIES, AND EQUIPMENT. Repairs or alterations performed on the aircraft in a hangar must have substantiating data that show it meets the requirements appropriate for a given structure. This would need to be demonstrated for the repair or alteration design and processes to be applied with the structure attached to the aircraft.
The facilities and equipment used for repair or alteration of composite or metal-bonded structures must be capable of meeting the repair or alteration process requirements established by the DAH or other FAA-approved or acceptable repair or alteration processes. The MO must maintain and calibrate its equipment in accordance with its applicable procedures acceptable to the Administrator. This paragraph gives examples of facilities and equipment used in repair or alteration of composite or adhesively-bonded aircraft structure, and guidelines for controlling part quality.
8.1 Autoclave. An autoclave capable of providing positive pressures and temperatures consistent with the repair or alteration process. The autoclave should have integral vacuum and thermocouple capability for each part.
8.2 Oven. An air-circulating oven capable of providing the required cure or drying temperature and equipped with vacuum capability, when required.
8.3 Other Methods. Other methods used for temperature and pressure control include vacuum bagging, Double Vacuum Bag Debulk (DVD), thermal blanket, mechanical pressure, heat lamp, and heat gun.
8.4 Layup and Clean Rooms. Unless otherwise validated for the material system in use, an MO must environmentally and operationally control layup and clean rooms in the following areas:
1. Temperature and humidity, in accordance with the repair or alteration process acceptable to the Administrator.
2. Air filtration and pressurization capable of providing slight, positive over-pressure.
3. Room designed to minimize dirt traps (e.g., recessed lights, sealed floors, and no ledges) with a routine cleaning schedule established.
4. Clean and filtered compressed air when used in the clean room.
5. Contamination restrictions in cutting, layup, and bonding areas must 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 or alteration process (e.g., release agents or materials containing uncured silicone must not be permitted). Also, an MO must prohibit eating and smoking in these areas and ensure that other work by technicians does not introduce contaminants into the repair or alteration.
8.5 Freezers. Freezers must be capable of storing adhesives, prepregs, etc., in accordance with the applicable material specification.
8.6 Cleaning. The MO must provide equipment to accomplish the various cleaning processes
(e.g., alkaline or acid etching bath, and abrasive blasters).
8.7 Spray 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.
8.8 Core Cutting and Core Foam Cutting Tools. Capability should be available to cut, rout, and profile the honeycomb in either the compressed or expanded form. The MO should keep hand routing to a minimum.
8.9 Part Drying Capability. For bonded repairs or alterations where the MO reuses some composite parts, a moisture dry-out capability should be available to adequately dry the entire part before layup and bonding of the repair or alteration. 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.
8.10 Laboratory Test Facilities. Laboratory test facilities must 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
15 mechanical testing of specimens, such as the tests listed in paragraph 6.2.7.4, must also be available, if required.
8.11 NDI. X ray, ultrasonic, or other types of NDT equipment must be available for inspection of parts, as required in the repair or alteration documentation.
8.12 Repair Staging Area. A dedicated area for staging of kitted detail parts prior to bond helps to reduce contamination of detail parts.
8.13 Cutting Tools. Tooling materials and configurations depend on the materials being machined. Diamond or carbide saws and router bits are suitable for cutting and trimming of carbon composites. Hole-drilling operations in carbon composites should also use carbide drills and countersinks, or other means recommended by the DAH. It is important to have backside support when drilling to prevent damage. Generally, composite machining requires a vacuum system incorporated in the cutting head.
8.14 Layup Tooling. Composite materials are laid up on a tool or in a mold, and are generally cured under a vacuum, with the temperature controlled by an autoclave, oven, or by other means, such as a heat blanket. Tooling may assist in obtaining the proper part geometry and layup orientation. In the case of a repair or alteration, it may not be necessary to use separate tooling.
8.14.1 Initial Qualification.
8.14.1.1 The tooling used in the repair or alteration 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:
1. Include a thermal profile of the tool to assess the temperature distribution during cure and to select thermocouple locations.
2. Encompass the use of the tooling, raw materials, expendable items
(e.g., release films and bagging materials), layup and bagging techniques, and cure cycle used in fabrication.
3. Include inspection of the cured part to ensure conformance to design requirements prior to release of the tooling for repair or alteration operations.
8.14.1.2 Any significant change in the tooling, materials, and processing methods after initial approval requires reapproval.
8.14.2 Requalification. The tooling must 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.
8.14.3 Routine Checks. Before each use, an MO must visually check the tool for damage, such as scratches, distortion, etc.
16 9 FABRICATION AND PROCESSES PROCEDURES. The MO must use detailed processing specifications and procedures for repair or alteration of composite or adhesive-bonded structure. The specifications and procedures must be consistent with FAA-approved or accepted maintenance instructions and with available facilities and equipment. If not already incorporated in the applicable SRM, the specifications and procedures developed must incorporate and address the following points:
9.1 Damage Assessment and Teardown. If the MO needs to accomplish localized repair or alteration on a damaged area, it must fully and clearly define the extent of the damage.
Since teardown generates dust and dirt, the MO should accomplish teardown in a separate area, segregated from the fabrication area. The FAA recommends that the MO accomplish additional NDI after teardown to identify any additional damage that may have occurred or become apparent as a result of the teardown. Due to the tendency of composites to rebound and mask any impact damage, the MO should supplement a visual inspection or coin tap test for determination of the extent of any composite damage with additional NDI techniques (e.g., ultrasonic or other approved NDI methods). This is especially true for those instances when the sandwich face sheets are thicker than three to four plies or for any solid laminate construction. The SRM should provide guidance. For information on in-hangar repairs or alterations, see paragraph 8.
9.2 Repair Classification. It is the MO’s responsibility to classify repairs as major or minor.
Major repairs and alterations must be performed in accordance with technical data approved by the FAA. The proposed repair/alteration should be reviewed in context with part 43 appendix A; AC 20-107; and AC 120-77, paragraph 12 to determine the classification. Exclusive reliance on the part 43 appendix A list of major repairs and alterations to make the major/minor classification might result in the misclassification of repairs and alterations to composite bonded critical structure. Part 43 appendix A does not address evolving airplane design, construction, and repair techniques, such as composite structures, advanced materials, and bonding processes used on new generation aircraft.
9.3 Fabrication and Processing of Repair and Alteration Parts. The MO must fabricate detailed parts using materials approved by the DAH or qualified to the applicable FAA-approved or accepted specifications. The configuration of the details must be traceable to the applicable drawings, tooling, repair or alteration configurations identified in the SRMs, or other FAA-approved drawings and/or documents. Key steps in repair or alteration part fabrication and processing are:
9.3.1 Evaluation of Repair Size Limits. Bonded repairs require significant substantiating data.
Even when meeting all relevant airworthiness regulations, repair size limits are required to reduce the risk of catastrophic structural failure due to potentially undetectable disbond and weak bond conditions in a bonded repair. Bonded repairs must meet the appropriate airworthiness requirements for the structure being repaired, which will define a maximum repair size. Additionally, bonded repairs to critical structure should have a second size limit based on residual strength capability, as specified in policy statement PS-AIR-20-130-01.
9.3.2 Prefit. The MO should prefit detail parts of adhesively-bonded and composite material structure using tooling intended for repair, alteration, or manufacture. Detail parts must be controlled from the point of bonding prefit until they become an integral part of the assembly.
9.3.3 Preparation of Metallic Details. Proper cleaning and surface preparation are critical to achieving a good bond. Care must be taken to prevent contamination of the bondline.
9.3.3.1 Cleaning and Drying. For metal details, the cleaning processes should remove oxides and contaminants (e.g., oils, waxes, greases, and cutting oils) from the part surface. The MO generally uses emulsifiers and solvents for contaminant removal and as a preliminary cleaning process. For subsequent cleaning operations to remove oxides and light surface scratches, the MO generally uses acid and alkaline etching processes. The MO may also use abrasive cleaning methods for removal of corrosion and paint. The MO must be careful when removing excess cleaning fluids before further processing.
The MO must sufficiently dry the surface in accordance with the FAA-approved repair or alteration process. Phosphoric etch is typically used for metal bonds.
9.3.3.2 Surface Preparation. After successful prefit of the bonded metallic structure, the MO must prepare the detail parts and the bond surface using the procedure recommended by the DAH or an alternate procedure approved by or acceptable to the FAA. The MO must control and maintain surface preparation solutions at the proper concentration in accordance with FAA-approved procedures. After addition of makeup material or complete change of solution, the MO must retest the cleaning solutions and find the solutions acceptable before production use.
9.3.3.3 Adhesive Primer Application. After surface preparation, the MO must spray-prime detail metallic parts without delay, using the procedure recommended by the DAH or an alternate procedure approved by or acceptable to the FAA. The MO must exercise special care to avoid physical contact or contamination of the prepared surface prior to and after priming.
Any compressed air used must be filtered and free of contaminants.
9.3.4 Preparation of Cured Composites and Nonmetallic Honeycomb Details. Proper surface preparation and drying are critical to achieving a good bond.
9.3.4.1 Paint Removal. The MO must remove paint from all surfaces for bonding since it prevents good adhesion. Mechanical abrasion or hand sanding is acceptable for local paint removal on composites. If power sanding, the MO must take particular care not to damage the laminate. Before use, the MO must carefully evaluate and approve alternative methods, such as chemical stripping or blast removal processes. These processes may cause damage to the laminate. Inspect sanded area to ensure plies have not been compromised during the abrasion/sanding process.
18 Note: Sanding composite fiber may pose a health risk. Wear protective clothing during this operation.
9.3.4.2 Cleaning. For composites, the MO must clean the surfaces with compatible chemicals approved by the SRM, material specification, or other FAA-approved or accepted data, in order to remove contaminants and avoid damage to the composite resin system. Care must be taken to remove excess cleaning fluids before further processing.
9.3.4.3 Drying. The structure must be completely dried in accordance with the FAA-approved or accepted repair or alteration process. Drying the damaged composite for a period of time (below the temperature of boiling water) suppresses the tendency for delaminations and face sheet separation in the repair or alteration cycle. The length of time for drying is dependent upon the number of plies and the exposure to moisture the part has experienced.
Note: The FAA has found that moisture absorbed in composite and honeycomb materials has a significant effect on the performance of repairs or alterations and the manner in which the MO should complete them. Moisture in composites can frequently cause delaminations away from the repair or alteration location during the repair or alteration cure cycle. In honeycomb structure, it can result in blown face sheets (face sheets separated from the core).
9.3.5 Composite Ply Cutting. The MO must cut composite plies to orientation and stacking sequence, as required. If returning the cut plies to the freezer before use, the MO should kit and collate them as to stacking sequence and ply orientation and store them in a protective bag. If templates are developed for ply cutting, they should be identified as to ply orientation and sequence. Plies should not be cut directly on the tool, in order to avoid damage to the tool surface. If returning cut plies to the freezer, store plies flat or rolled around a supporting tube. No folding or creasing of the plies is allowed.
9.3.6 Layup. In laying up composites, the MO must take special care to ensure proper ply orientation (and that repair or alteration plies are within Original Equipment Manufacturer (OEM) SRM alignment tolerance). The exact method of layup is dependent on part, tooling, resin/adhesive flow, surface prep, etc. It must be consistent with the DAH’s process specification or other FAA-approved/accepted specifications.
9.3.6.1 The MO should accomplish layup in accordance with environmental conditions required by the process specification. The MO should be careful to accomplish the layup or bonding within the remaining allowable out time and/or work life of the material. The MO’s procedure should specify a maximum time for accomplishment.
9.3.6.2 Adhesives or primed faying surfaces to be bonded should not be handled with bare hands, nor should the MO allow any contamination to result from contact with supporting fixtures or mechanical handling equipment. Appropriate
19 gloves should be worn by all persons handling cleaned parts, cleaned and primed parts, or adhesive films.
9.3.7 Bagging. Bagging must be sufficient to remove all trapped air and produce the desired resin/adhesive flow. If necessary, the MO must incorporate an intermediate debulk cycle.
The MO should use only noncontaminating films, tapes, etc. for flow control purposes.
When using vacuum-bagging techniques, the MO must vacuum check the vacuum-bagged repair or alteration for leaks to ensure air does not contaminate the repair or alteration during the cure cycle. A vacuum bag reduces the pressure to about
0.5 atmospheres (atm) to apply pressure to consolidate the composite. It will contain some air. The MO must ensure the bagging process includes the correct bleeder cloth and pressure plates.
9.3.8 Curing. The MO must accomplish curing by controlling the appropriate parameters, such as vacuum, pressure, temperature, viscosity, etc., with respect to time in accordance with the approved process specification or other FAA-approved procedures to establish the correct state of cure. The MO must pay attention to both the lowest temperature thermocouple and the maximum temperature rise to avoid undercure and overheating, respectively. The MO must use a sufficient number of thermocouples in contact with the part to monitor the temperature of the curing material. It should give particular attention to bondlines and heatsinks.
9.3.9 QC. The MO must establish and implement a plan that critical process parameters (such as temperature and humidity) shall be verified. Test coupons may be necessary to verify the repair or alteration process.
9.3.10 Inspection. The fabrication specification must specify methods of NDI for each part and specify accept/reject criteria. The types of NDI specified must be consistent with the types of flaws expected and the type of construction. The MO may leak-check small honeycomb parts for porous bondlines or edges and improperly sealed fasteners by submersion. When an entire part or a large area of a part is subject to an elevated temperature cure cycle to cure a repair or alteration to an area of the part, the MO should conduct an NDI of the entire area of the heated part after curing. This is necessary because moisture absorbed in laminate or core areas of the part that the MO is not repairing or altering can damage the part during the elevated temperature curing cycle of the repair or alteration.
9.3.11 Documentation. A detailed shop record must accompany every part from prefit or assembly of details through final acceptance. The record must address each critical step required during the fabrication process and must include in-process QC checks that the MO must complete before moving to the next process step. The FAA recommends that the shop record identify the part number, process and material specification numbers, adhesive/prepreg roll number, ply number and orientation, cure parameters, date, and the results of specimen testing.
9.4 Process Changes. The MO must review and requalify procedures, if necessary, whenever there are any significant changes to items, such as type or source of material,
20 cure cycle, equipment controls, or tool design. Process capability must be demonstrated by inspection and testing, as necessary, to determine conformity to repair or alteration data requirements. Any changes to FAA-approved procedures, processes, or data will require approval by the FAA.
10 NDI PROCEDURES.
10.1 NDI Techniques. The MO may use several NDI techniques to detect discrepancies in aircraft composite structures and components; however, the most commonly used techniques are visual, audio sonic (tap testing), radiography, ultrasonic, acoustical impedance, infrared thermography, and shearography.
10.1.1 Visual. Visual inspection is the most widely used NDI method. Discrepancies that the MO can usually observe 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. Transmitted light helps to reveal discrepancies within the specimen in cases where both surfaces are accessible and the material is translucent.
10.1.2 Audio Sonic. Sonic testing makes use of frequencies in the audible range from about 10 hertz (Hz) to 20 Hz. The “tap test” is a common technique/procedure used for detecting voids or delaminations in thin face sheet honeycomb construction. When tapping any area, the MO may use a small tap hammer, coin, or other suitable object.
When using this technique, 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 this test. The tap test may not be reliable for composite layups over three plies or thicker metal skins without a calibration standard.
10.1.3 Radiography. The MO may occasionally use radiography in composite fabrication and repairs or alterations. In addition, the MO may also use radiography 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 layup, 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. An MO usually performs radiography through the thickness of the product for detection of anomalies.
10.1.4 Ultrasonic. In this technique, the MO uses the attenuation of sound wave energy 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. A B-scan displays a long, cross-sectional view of the part under test and shows discovered discrepancies.
A C-scan displays a plan view of the signal amplitude across the part. C-scans provide
21 detailed images and location of defects and are capable of detecting defects in the magnitude of 0.01 square inch. That said, the practical limits are typically on the order of
0.5 inch x 0.5 inch defect detectability. C-scans are used to satisfy the inspection records requirements. A-scans and B-scans are used for real-time defect detection.
10.1.4.1 Generally, the MO can use ultrasonic techniques to detect porosity, laminar inclusions, delaminations, and fastener hole flaws.
10.1.4.2 An advantage to ultrasound is the ability to detect the depth of the defect and not just the profile of the defect.
10.1.4.3 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.
10.1.4.4 Another disadvantage is the requirement for representative calibration standards.
10.1.5 Acoustical Impedance. These are methods by which the MO measures structural response to strain excitation to detect delaminated areas or disbonds. The sensitivity of this method decreases with increasing structural flexibility or discrepancy depth.
10.1.6 Infrared Thermography. This technique produces an image of invisible (to our eyes) infrared light emitted by the object due to its thermal condition. The MO may use thermography to detect corrosion, subsurface damages, and degradation of skins, structural members, and joints. A disadvantage of the infrared method is that it is harder to detect temperature differences on the outside surface during windy weather. It is generally used for inspections performed in a hangar or other maintenance facility because they benefit from reduced air movement.
10.1.7 Shearography. This technique is a noncontact, full-field inspection method that uses coherent laser light and interferometry. The instrumentation captures an image of the structure in the unloaded state and compares it with images of the deformed shape as a small load is applied. The MO uses this technique to detect damage or defects, such as disbonds or delaminations, which will locally affect the deformed shape of the structure.
10.2 Controlling NDI Techniques. Certain controls are necessary for NDI techniques to be effective, repeatable, and reliable. An approved NDI specification and procedure must include:
• Periodic qualification of personnel conducting the inspection technique.
• Establishment of realistic acceptance standards consistent with FAA-approved repair or alteration requirements for use by in-process and final inspection personnel.
• 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.
• An internal audit program for validating the effectiveness of the NDI program.
11 PERSONNEL QUALIFICATION.
11.1 MO. Personnel who are performing repairs or alterations to composite or metal-bonded structures must be fully qualified in the preparation and handling of the materials used and in the procedures for layup and curing of a composite or metal-bonded structure, as appropriate. Personnel must be qualified under a program that assures that the MO’s performance is in accordance with established, acceptable performance standards for repair or alteration practices and product quality. The program must include provisions for revalidating MO qualifications on a predetermined basis. The MO is responsible to determine the abilities of its employees performing maintenance functions based on training, knowledge, or practical tests.
11.2 NDI Personnel. NDI personnel should be qualified in accordance with a recognized industry or government standard. The MO should develop and maintain a list of qualified NDI inspectors, indicating each person’s qualifications, level of proficiency, and their requalification date. Refer to AC 65-31 for additional information.
12 INSPECTION RECORDS PROCEDURES. The MO must maintain inspection records for a repaired, altered, and/or fabricated composite and metal-bonded structure that demonstrate compliance with the following:
12.1 Part Identification. The inspection record must identify the part name, part number, and the FAA-approved or accepted data with which the part complies after repair or alteration.
12.2 Material Traceability. The inspection record must identify the structural raw materials
(e.g., adhesives, prepregs, and resins) by material specification, manufacturer, and batch/lot number that MO personnel used in fabrication.
12.3 Conformance to Fabrication Requirements. The inspection record must identify and show compliance with the fabrication process specifications, destructive inspection and NDI specifications, and other specifications defining product acceptance/rejection standards that MO personnel have used for fabrication and QC acceptance purposes. It must also show evidence of QC acceptance of the structural raw materials
(e.g., adhesives, prepregs, and resins) used in fabrication. If MO personnel use salvaged parts in the fabrication process, the inspection record must also identify the specifications and/or procedures used in their repair or alteration. Each completed operation necessary to repair, alter, or fabricate the complete part must show evidence of acceptance by authorized and qualified personnel.
12.4 Product Conformity and Airworthiness. The appropriate MO personnel and authorized inspectors must annotate the inspection record as evidence of compliance with
23 FAA-approved repair or alteration procedures. The MO must complete maintenance record entries (approval for return to service) in accordance with part 43.
12.5 Record Retention. Inspection records are to be retained for the length of time required by parts 91, 121, 125, 129, 133, 135, 137, and 145, as appropriate.
ORIGINAL SIGNED by /s/ John Barbagallo Deputy Director, Flight Standards Service
Advisory Circular Feedback Form If you find an error in this AC, have recommendations for improving it, or have suggestions for new items/subjects to be added, you may let us know by contacting the Flight Standards Directives Management Officer at 9-AWA-AFB-120-Directives@faa.gov.
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