The Anatomy of an Aircraft
ASTM International's newest technical committee, F44 on General Aviation Aircraft, was established after the U.S. Federal Aviation Administration spent a number of years reviewing standards for the design and certification of small airplanes (Code of Federal Regulations, Title 14: Aeronautics and Space, Part 23).1 This article isn't about the history and development of this new committee (see the F44 article for more background) but rather one of the points made during the FAA advisory committee discussions - the idea that a consensus standard might be used in the design and certification of an aviation product.
Individual aircraft are hundreds (if not thousands) of consensus standards all flying in close formation. In fact, aircraft are even powered by consensus standards. Once you understand the anatomy of an aircraft, not just the physical components but also the standards that support those components, you can recognize each of the individual puzzle pieces that come together to paint the picture of a completed aircraft.
The changes that are taking place within Part 23 of the Federal Register are not revolutionary but rather evolutionary - simply the next step in the use of aviation consensus standards. There are three terms that are important to help understand these changes and the anatomy of an aircraft: standards, consensus standards and voluntary consensus standards.
The White House Office of Management and Budget, in Circular A-119, Federal Participation in the Development and Use of Voluntary Consensus Standards and in Conformity Assessment Activities, defines a standard as: "(1) Common and repeated use of rules, conditions, guidelines or characteristics for products or related processes and production methods, and related management systems practices. (2) The definition of terms\; classification of components\; delineation of procedures\; specification of dimensions, materials, performance, designs or operations\; measurement of quality and quantity in describing materials, products, systems, services or practices\; test methods and sampling procedures\; or descriptions of fit and measurements of size or strength."2
Generally, the design and certification standards for airplanes would include the Federal Aviation Regulations and the Advisory Circulars, representing an acceptable means of compliance to the regulations and published policies. The safety standards are contained in the Federal Aviation Regulations while, in general, the certification standards are contained in the Advisory Circulars.
The Merriam-Webster dictionary defines consensus as a "general agreement." Therefore a consensus standard would be a standard developed by general agreement. The FAA follows the Administrative Procedures Act for all rulemaking and loosely follows the APA for all guidance materials, submitting proposed regulations and/or guidance for public review and comment, and then responding to and resolving those comments. It could be argued that regulations and advisory materials published by the FAA are consensus standards.
OMB A-119 defines voluntary consensus standards as "standards developed or adopted by voluntary consensus standards bodies, both domestic and international. These standards include provisions requiring that owners of relevant intellectual property have agreed to make that intellectual property available on a non-discriminatory, royalty-free or reasonable royalty basis to all interested parties."3
The National Technology Transfer and Advancement Act (Public Law 104-113) directs federal agencies with respect to their use of private sector standards. The act's objective is for federal agencies to adopt private sector standards, wherever possible, in lieu of creating proprietary, non-consensus standards in an effort to decrease dependence on costly in-house standards. Some argue that the migration of many of the Part 23 in-house standards to voluntary consensus standards is a radical, revolutionary change. But for those of us in aviation, this really is a progressive, evolutionary change since the vast majority of materials, hardware and components are already governed by voluntary consensus standards.
As we apply the various types of standards to the anatomy of the aircraft, we can start with the fuselage materials. There are more than 4,000 standards that define the design, composition and testing of aerospace structural materials. When I started working in aviation in the early 1970s, nearly all of the hardware we used was governed by Air Force, navy and military standards. Many, if not all, of these standards have been migrated to voluntary consensus standards since 1996 following the enactment of the NTTAA. The National Aerospace Standards are best known for standardizing high strength, precision fasteners and also for all types of screws, nuts and rivets, high pressure hose, electrical connectors, splices and terminations, rod end bearings, and many other types of hardware and components used in aerospace. What isn't well known is that the National Aerospace Standards are voluntary consensus standards.
Today, nearly all of the wiring in aircraft conforms to a voluntary consensus standard with the endorsement of the FAA. FAA Advisory Circular 43.13-1B - Acceptable Methods, Techniques and Practices - Aircraft Inspection and Repair, identifies wires that have been deemed acceptable for aircraft use. In Chapter 11, the FAA states that "All wires in Tables 11-11 and 11-12 have been determined to meet the flammability requirements of Title 14 of the Code of Federal Regulation (14 CFR) Part 25, Section 25.869(a)(4) and the applicable portion of Part 1 of Appendix F of Part 25."4 For more than 15 years the wire standards cited by the FAA as an acceptable means of compliance in Tables 11-11 and 11-12 have been managed as voluntary consensus standards.
In 2006, the FAA recognized ASTM International's standard F2490, Guide for Aircraft Electrical Load and Power Source Capacity Analysis, as an acceptable means of compliance to 14 CFR Part 23, 23.1351(a)(2). And again in 2010 the FAA accepted ASTM's F2639, Practice for Design, Alteration and Certification of Airplane Electrical Wiring Systems, as an acceptable means of compliance to 14 CFR Part 23 sections concerning electrical wiring systems. Additionally in 2010, the FAA accepted ASTM F2696, Practice for Inspection of Airplane Electrical Wiring Systems, and F2799, Practice for Maintenance of Airplane Electrical Wiring Systems.
Many of the standards familiar in aviation are Technical Standard Orders, which are thought to have been developed and managed solely by the FAA. Again, the FAA follows the APA and submits the proposed TSO, or proposed changes to an existing TSO, to the public for review and comment prior to finalizing any standard. Once consensus is achieved the FAA will publish a final TSO. There are some TSOs, such as TSO C62e, Aircraft Tires, which are complete standards, including both the safety standard as well as the method of compliance. On the other hand, the majority of TSOs referencing cockpit technologies are reference-only type standards: They simply codify voluntary consensus standards into a regulatory standard with, when necessary, supplemental information not contained in the voluntary standard. Nearly all of the classic mechanic gauges used in aircraft conform to a TSO that incorporates a voluntary consensus standard by reference, and most of the modern instrumentation incorporates multiple voluntary consensus standards.
Anyone involved in modern aircraft electronics is familiar with DO-160, Environmental Conditions and Test Procedures for Airborne Equipment and, for any software controlled equipment, DO-178C, Software Considerations in Airborne Systems and Equipment Certification, standards. Both of these, as well as the 120 pages of additional aircraft standards, are voluntary consensus standards managed by the Radio Technical Commission for Aeronautics - a familiar name in the aircraft avionics and electronics field. From RTCA's List of Available Documents, "RTCA Inc. is a not-for-profit corporation formed to advance the art and science of aviation and aviation electronic systems for the benefit of the public. The organization functions as a Federal advisory committee and develops consensus-based recommendations on contemporary aviation issues."5 This adds another few hundred voluntary aviation consensus standards.
From rivets to rubber, fabric to fuels, titanium to thermal plastics, every element of an aircraft conforms to some type of standard. Some are company-specific, others are government-unique, but the majority of them are public consensus standards. This isn't a new concept\; aircraft have been built this way for more than half a century. But the standards don't stop at the factory. Once assembled, aircraft are fueled with either aviation gasoline or aviation jet fuel: ASTM D910, Specification for Aviation Gasolines, and ASTM D6615, Specification for Jet B Wide Cut Aviation Turbine Fuel - again, voluntary consensus standards. My introduction to ASTM International came with my use of ASTM's Manual 5, Aviation Fuel Quality Control Procedures, while managing aviation fuel storage and distribution facilities.
So, while most people consider an airplane a fuselage, wings, engine, cockpit and empennage, when you look at the anatomy of an airplane and the standards to which each and every part is produced, you quickly realize that an airplane is simply the marriage of a thousand standards, from a dozen different organizations, all flying together in close formation.
1. Code of Federal Regulations, Title 14 – Aeronautics and Space, Part 23 - Airworthiness Standards: Normal, Utility, Acrobatic and Commuter Category Airplanes.
2. Office of Management and Budget, Circular A-119, Federal Participation in the Development and Use of Voluntary Consensus Standards and in Conformity Assessment Activities.
3. Office of Management and Budget, Circular A-119, Federal Participation in the Development and Use of Voluntary Consensus Standards and in Conformity Assessment Activities.
4. Table 11-11. Open Wiring\; Table 11-12. Protected Wiring\; FAA Advisory Circular 43.13-1B - Acceptable Methods, Techniques and Practices - Aircraft Inspection and Repair, .
5. Radio Technical Commission for Aeronautics, "List of Available Documents."Richard A. Peri is vice president of government and industry affairs at the Aircraft Electronics Association, Washington, D.C. He is a member of the ASTM International board of directors and of ASTM International Committees F37 on Light Sport Aircraft, F39 on Aircraft Systems and F44 on General Aviation Aircraft.