(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 NDI for each part and specify accept/reject criteria. The types of NDI 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
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. NDI Techniques. Several NDI 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 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, 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 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 Page 14 Par 10
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 0.01 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 NDI Techniques. In order for NDI techniques to be effective, repeatable, and reliable, certain controls are necessary. An approved NDI 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.
(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 NDI 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
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. NDI Personnel. NDI personnel should be qualified in accordance with a recognized industry or government standard. A list of qualified NDI 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 NDI 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.
Millon y Milito William J. White Deputy Director, Flight Standards Service Page 16 Par 12
Penalty for Private Use $300 Official Business Washington, D.C. 20591 800 Independence Ave., S.W.
Administration Federal Aviation of Transportation U.S. Department