Lightning and the Composite, Electronic Airplane

FAA Federal Air Regulation (FAR) 23.867 governs protection of aircraft against lightning and static electricity, reflecting the influence of decades of NASA lightning research, particularly the NF-106B program. FAR 23.867 directs that an airplane "must be protected against catastrophic effects from lightning,” by bonding metal components to the airframe or, in the case of both metal and nonmetal components, designing them so that if they are struck, the effects on the aircraft will not be catastrophic. Additionally, for nonmetallic components, FAR 23.867 directs that air­craft must have "acceptable means of diverting the resulting electrical current so as not to endanger the airplane.”[166]

Among the more effective means of limiting lightning damage to aircraft is using a material that resists or minimizes the powerful pulse of an electromagnetic strike. Late in the 20th century, the aerospace industry realized the excellent potential of composite materials for that purpose. Aside from older bonded-wood-and-resin aircraft of the inter­war era, the modern all-composite aircraft may be said to date from the 1960s, with the private-venture Windecker Eagle, anticipating later air­craft as diverse as the Cirrus SR-20 lightplane, the Glasair III LP (the first composite homebuilt aircraft to meet the requirements of FAR 23), and the Boeing 787. The 787 is composed of 50-percent carbon lami­nate, including the fuselage and wings; a carbon sandwich material in the engine nacelles, control surfaces, and wingtips; and other compos­ites in the wings and vertical fin. Much smaller portions are made of aluminum and titanium. In contrast, indicative of the rising prevalence of composites, the 777 involved just 12-percent composites.

An even newer composite testbed design is the Advanced Composite Cargo Aircraft (ACCA). The modified twin-engine Dornier 328Jet’s rear fuse­lage and vertical stabilizer are composed of advanced composite materials produced by out-of-autoclave curing. First flown in June 2009, the ACCA is the product of a 10-year project by the Air Force Research Laboratory.[167]

NASA research on lightning protection for conventional aircraft structures translated into use for composite airframes as well. Because experience proved that lightning could strike almost any spot on an airplane’s surface—not merely (as previously believed) extremities such as wings and propeller tips—researchers found a lesson for designers using new materials. They concluded, "That finding is of great impor­tance to designers employing composite materials, which are less con­ductive, hence more vulnerable to lightning damage than the aluminum allows they replace.”[168] The advantages of fiberglass and other compos­ites have been readily recognized: besides resistance to lightning strikes, composites offer exceptional strength for light weight and are resistant to corrosion. Therefore, it was inevitable that aircraft designers would increasingly rely upon the new materials.[169]

But the composite revolution was not just the province of established manufacturers. As composites grew in popularity, they increasingly were employed by manufacturers of kit planes. The homebuilt aircraft market, a feature of American aeronautics since the time of the Wrights, expanded greatly over the 1980s and afterward. NASA’s heavy investment in light­ning research carried over to the kit-plane market, and Langley released a Small Business Innovation Research (SBIR) contract to Stoddard- Hamilton Aircraft, Inc., and Lightning Technologies, Inc., for develop­ment of a low-cost lightning protection system for kit-built composite aircraft. As a result, Stoddard-Hamilton’s composite-structure Glasair III LP became the first homebuilt aircraft to meet the standards of FAR 23.[170]

One of the benefits of composite/fiberglass airframe materials is inherent resistance to structural damage. Typically, composites are produced by laying spaced bands of high-strength fibers in an angu­lar pattern of perhaps 45 degrees from one another. Selectively wind­ing the material in alternating directions produces a "basket weave” effect that enhances strength. The fibers often are set in a thermo­plastic resin four or more layers thick, which, when cured, produces extremely high strength and low weight. Furthermore, the weave pat­tern affords excellent resistance to peeling and delamination, even when struck by lightning. Among the earliest aviation uses of composites were engine cowlings, but eventually, structural components and then entire composite airframes were envisioned. Composites can provide addi­tional electromagnetic resistance by winding conductive filaments in a spiral pattern over the structure before curing the resin. The filaments help dissipate high-voltage energy across a large area and rapidly divert the impulses before they can inflict significant harm.[171]

It is helpful to compare the effects of lightning on aluminum aircraft to better understand the advantage of fiberglass structures. Aluminum readily conducts electromagnetic energy through the airframe, requir­ing designers to channel the energy away from vulnerable areas, espe­cially fuel systems and avionics. The aircraft’s outer skin usually offers the path of least resistance, so the energy can be "vented” overboard. Fiberglass is a proven insulator against electromagnetic charges. Though composites conduct electricity, they do so less readily than do alumi­num and other metals. Consequently, though it may seem counterintu­itive, composites’ resistance to EMP strokes can be enhanced by adding small metallic mesh to the external surfaces, focusing unwanted currents away from the interior. The most common mesh materials are alumi­num and copper impressed into the carbon fiber. Repairs of lightning – damaged composites must take into account the mesh in the affected area and the basic material and attendant structure. Composites miti­gate the effect of a lightning strike not only by resisting the immediate area of impact, but also by spreading the effects over a wider area. Thus, by reducing the energy for a given surface area (expressed in amps per square inch), a potentially damaging strike can be rendered harmless.

Because technology is still emerging for detection and diagno­sis of lightning damage, NASA is exploring methods of in-flight and postflight analysis. Obviously, the most critical is in-flight, with aircraft sensors measuring the intensity and location of a lightning strike’s cur­rent, employing laboratory simulations to establish baseline data for a specific material. Thus, the voltage/current test measurements can be compared with statistical data to estimate the extent of damage likely upon the composite. Aircrews thereby can evaluate the safety of flight risks after a specific strike and determine whether to continue or to land.

NASA’s research interests in addressing composite aircraft are threefold:

• Deploying onboard sensors to measure lightning-strike strength, location, and current flow.

• Obtaining conductive paint or other coatings to facili­tate current flow, mitigating airframe structural dam­age, and eliminating requirements for additional internal shielding of electronics and avionics.

• Compiling physics-based models of complex compos­ites that can be adapted to simulate lightning strikes to quantify electrical, mechanical, and thermal parameters to provide real-time damage information.

As testing continues, NASA will provide modeling data to manufac­turers of composite aircraft as a design tool. Similar benefits can accrue to developers of wind turbines, which increasingly are likely to use com­posite blades. Other nonaerospace applications can include the electric power industry, which experiences high-voltage situations.[172]