Composite Aircraft Materials: Standardization

Sept. 1, 2003

Composite Aircraft Materials

More and more maintenance technicians are now maintaining composite aircraft. I spoke at the IA renewal seminars in Spokane and Tacoma, Washington, this year about composite aircraft repair. I find that IAs are asking more and more questions about composite aircraft. There is an ever-increasing interest in composite aircraft repair. This article will highlight some recent developments toward standardization of composite materials.
Historical perspectives
Composite materials have not achieved the same level of standardization as metallic materials. This includes a general lack of industry material and process specifications and shared databases for basic properties, as well as test standards that continue to evolve. In recent years, NASA, the Federal Aviation Administration (FAA), and industry have worked together to develop a cost-effective method of qualifying composite material systems and sharing a central material qualification database. Material qualification is a process of establishing the database to control an "original" material system. Comments were due June 12, 2003, for the Draft AC 23-16a "Acceptance Guidance on Material Procurement and Process Specifications for Polymer Matrix Composite Systems." It provides information and guidance concerning an acceptable means, but not the only means, of compliance with Title 14 of the Code of Federal Regulations (14 CFR) Part 23. It is applicable to the material and process specifications, or other documents, used to ensure sufficient control of composite prepreg materials in normal, utility, acrobatic, and commuter category airplanes. A prepreg batch is a prepreg containing reinforcement material from one batch impregnated with one batch of resin in one continuous operation. It must be stored at a below zero temperature to prevent the resin from starting the curing process. This AC was developed with the help of the industry to ensure adequate composite material control and to promote standardization of material and process specifications. Using the AC's own words, there are implications for maintenance as well as for manufacturing. The following two FAA ACs present recommendations for showing compliance with FAA regulations associated with composite materials: AC 20-107A Composite Aircraft Structure and AC 21-26 Quality Control for the Manufacture of Composite Structures. AC 20-107A sets forth an acceptable means, but not the only means, of showing compliance with the provisions of 14 CFR parts 23, 25, 27, and 29 regarding airworthiness type certification requirements for composite aircraft structures. Paragraph 5 of AC 21-26 recognizes that the material properties of a composite structure are manufactured into the structure as part of the fabrication process. Material and process specifications used to produce composite structures must contain sufficient information to ensure that critical parameters in the fabrication process are controlled in production. It also provides information on elements that are to be contained in a typical material and process specification. Two other ACs that may be helpful in maintaining aircraft are AC 43-14 Maintenance of Weather Radar Radomes and AC 145-6 Repair Stations for Composite and Bonder Aircraft Structure. Both of these ACs provide information on storage and handling of metal-bonded and fiber-reinforced materials.Industry standards MIL-HDBK-17 has been developed to provide guidance for developing base material properties. Consistent and repeatable industry engineering practices would help assure compliance with the requirements for material control. The guidance in this document pertains to prepreg materials, but many of the basic concepts are generic for composite material control. Future efforts to continue development of reliable industry standards for the control and testing of composite materials are needed. Consistent and repeatable industry engineering practices would benefit from the compliance with requirements for material control. Shared composite databases and specifications throughout the industry would enhance the efficiency of material suppliers, end users, and regulators. It will assist us in making repairs that are compatible with the original structure.Structural substantiation Analysis and base material data alone is generally not adequate for substantiation of composite structural designs. The material qualification testing provides a means for quantitative assessment of the variability of key base material properties, leading to various statistics that are used to establish material acceptance, equivalence, quality control, and design basis. Structural certification is used to ensure acceptable materials and repeatable manufacturing processes are used for all parts that are integrated into production. Material and process specifications, interwoven throughout the certification process, define the material's attributes and define the qualification tests. Aircraft manufacturers use the material specification to ensure that the delivered aircraft consists of materials that are of the same quality and performance standards achieved in the certification process.
Material specifications
Initial sections of a material specification typically provide general information. The remaining sections contain the engineering criteria and guidelines. The particular order and names for sections in a given material specification may differ from industry standards; however, similar content should be included in material or other specifications and related documents used to control materials. The material specification includes definitions and limitations for storage life, handling life, and out life. These requirements are based on either test data or experience, or both. The supplier documents the storage/out life of a material from the date of manufacture until it arrives at the purchaser. This tracking includes resin intermediates, mixed resin, and film before prepreg process steps. Product certification. The specification defines the material acceptance testing to be performed by the material supplier and the end user (purchaser). The material supplier performs the material acceptance testing that is identified in the material specification. Conformance reports are prepared for each prepreg batch to document that the uncured and cured prepreg requirements have been met. The supplier quality department will review these documents before shipment to a purchaser along with the material. All records for each prepreg batch and the original baseline database are kept on permanent file. Records of quality assurance and full prepreg batch traceability are kept for a minimum of 10 years or as required under the applicable production rules. Material identification provides the batch number and roll number on two labels, one inside the core, and the other outside the shipping wrapper. These labels also include the material designation, name of manufacturer, specification number, and date of manufacture.
Process specification
The material and process specifications are interwoven. The material specification is used to ensure that the delivered aircraft contains materials that meet the same quality and performance standards as those achieved in the certification process. The process specification controls the processes used to convert materials into structural components. The specific processes used for fabrication directly determine the performance properties of composite laminates.
Fabrication requirements
Technicians are "qualified" for each of the major fabrication processes (layup, bagging, cure, nondestructive inspection (NDI), testing, and machining) that they perform. All procedures required for fabricating quality laminates need to be adequately detailed in the process specification to ensure consistency. The process specification will have fabrication details for tool preparation, material handling, laminate layup, cure cycle, panel identification, inspection, and machining. The process specification details the steps needed for tool and raw material preparation. The former includes inspections that verify the tools were properly cleaned and in good working condition. It should also include procedures for mold release application. Material preparation process steps will start with identification of the specific materials to be used and a review of their associated records. To prevent moisture condensation, specify the procedure to warm frozen prepreg before opening storage containers. To avoid ply damage and contamination, detail prepreg cutting and handling procedures. The process specification includes details for fabricating a panel to the applicable cure cycle. Before cure, perform a leak check on the vacuum bag and take corrective action as needed. The cure cycle should define heat-up rate, temperature range, time, vacuum, pressure, and cool down rate. Develop procedures to monitor cure time and temperature. Keep records of temperature, vacuum, and pressure for the complete cure cycle. The process specification provides details on machining cured panels. After machining, specimens are inspected for damaged edges and measured for dimensional accuracy per requirements. In the process specification, quality assurance procedures are included to monitor the fabrication processes as well as the necessary materials, equipment, facilities, and tooling. Each examination, inspection, or testing task performed for quality assurance is defined with a direct link to the fabrication requirements. Summary This AC establishes the guidance for developing composite material and process specifications that ensures adequate material controls. Structural certification ensures that acceptable materials and repeatable manufacturing processes are used for all parts that are integrated into the production. Material and process controls are essential prerequisites for a repeatable and safe product. The AC does not address how this new standardization may impact the repair of these composite aircraft using prepreg materials. Fred Workley is the president of Workley Aircraft and Maintenance Inc. in Alexandria, VA, Benton City, WA, and Indianapolis, IN. He holds an A&P certificate with an Inspection Authorization, general radio telephone license, a technician plus license, ATP, FE, CFI-I, and advance and instrument ground instructor licenses.