Safety First New FAA Rules for Commercial Transport Fuel Tanks

On July 17, 1996, at about 8:30 in the evening, Trans World Airlines Flight 800 (TWA800) crashed into the Atlantic Ocean shortly after taking off from John F. Kennedy International Airport in New York. All 230 people aboard the airplane were killed. The airplane was bound for Charles De Gaulle International Airport, Paris, France. The flight data recorder and cockpit voice recorder ended simultaneously, about 13 minutes after takeoff. Evidence indicates that as the airplane was climbing near 13,800 feet, an in-flight explosion occurred in the center wing fuel tank (CWT). The CWT was nearly empty, and contained slightly more than 600 pounds (about 100 gallons) of fuel. A large portion of the airplane wreckage was recovered from the ocean floor, and sections of the airplane have been reconstructed, including the CWT, the passenger cabin above the CWT, and the air conditioning packs and associated ducting beneath the CWT. The reconstruction shows outward deformation of the CWT walls and deformation of the internal components of the tank that are consistent with an explosion originating within the tank. The essential ingredients for a fire are well known: fuel, oxygen and a source of heat to initiate the process. In the space in and around a fuel tank, the recipe holds true, but with a few unique twists and turns. Fuel tank fires/explosions require an energy source sufficient for ignition (such as an arcing wire) and temperatures within the cell sufficient for the propagation of flames in the air/fuel vapors found in the empty part of the tank (called the ullage). But this vapor is not the only potential source of fuel. There are many other possible suspects, including the actual wire insulation itself, dust and debris which has settled around wire bundles, oil, hydraulic fluid and numerous plastic acoustic insulation materials. FAA regulations at the time of the crash required protection against the ignition of fuel vapor by lightning, components that generate sufficient heat to create an autoignition, or damaged parts that could become sources of ignition. Such conditions are known as single-point failures. But despite the regulations, fuel tank temperatures frequently become elevated, allowing an explosive fuel-air mixture to form in the ullage—especially when airplanes are on the ground between flights during warm weather, or when air conditioning units are located directly beneath the CWT. This is significant because when the temperature of a combustible fuel-air mixture becomes hot enough, any single ignition source can cause an explosion. Clearly such an occurrence is inconsistent with the basic tenet of transport aircraft design—that no single-point failure should prevent continued safe flight and landing, or significantly reduce the capability of the airplane or the ability of the crew to cope with the resulting failure conditions. TWA Flight 800 was painstakingly reconstructed in the most extensive forensic investigations of an airplane accident to ever occur. An explosion in the center wing fuel tank has been identified as the cause of the disaster. Many commercial transport planes have electronic fuel quantity indicating systems (FQIS) that measure the amount of fuel aboard the airplane and display that information in the cockpit. Such systems include electrical probes that are mounted in each fuel tank. The system is typically designed to carry a maximum of 26 volts with low current through insulated wires. The TWA Flight 800 investigation has uncovered several concerns about the wiring servicing the FQIS in the center wing fuel tank: Investigators have learned that (FQIS) wiring was routed in bundles with nearly 400 other wires, some of which carry up to 350 volts. Safety Board investigators have also found that a connector on certain tank probes was designed in a manner that allows its sharp edges to cut through the insulation of the wiring creating a potential spark gap. The Safety Board's investigations of wires, wire bundles and electrical connectors recovered from Flight 800 wreckage revealed wiring with insulation cracks that exposed the conductor. The investigation also found wire bundles contaminated with semiconductive residues—such as metal drill shavings—along the path where the center tank wiring was routed. In an effort to determine if these findings were unique to the Flight 800 airplane, or existed on other transport airplanes, the Board examined wiring on more than 20 other transport category airplanes. They found accumulations of contaminants on wiring that included lint, grease, liquids, paper, and metallic corrosion—inhibiting compounds. Investigators also found wire bundle clamps that cut into the wire when the rubber lining crumbles, and cracks in the insulation of wire deep enough to expose the conductor. These findings have raised the Safety Board's concerns about the safety of electrical systems—especially as airplanes age. It is important to note that the NTSB has not yet identified the specific ignition source that ignited the explosion on Flight 800, but the condition of the fuel-tank wiring is clearly a major source of concern. At a recent U.S. Congressional hearing on the matter, NTSB officials responded to government questions on wire aging, arcing, and contamination. Edited excerpts from the testimony of Bernard Loeb, Director, Office of Aviation Safety, National Transportation Safety Board before the Subcommittee on Oversight, Investigations, and Emergency Management Committee on Transportation and Infrastructure help explain the specific concern with aging aircraft wiring: Electrical systems are critical to the safe operation of transport category airplanes, and wiring is used to distribute power and communication signals throughout these systems. Many transport category airplanes contain over 200 miles of wiring. Today, I would like to discuss the Safety Board's efforts to address aircraft wiring issues raised in accident and incident investigations, including the recent investigations of TWA flight 800. As background, let me begin by describing how electrical wiring is constructed, routed through airplanes, and how the malfunctions can occur. Electrical wire consists of a conductor that is encased in a protective layer of insulation. These wires are quite thin and their insulation even thinner, often about the thickness of three human hairs. Wire is routed throughout an airplane in a series of bundles with clamps and connectors. Safe routing practices include measures to prevent wires from wear, abrasion, contamination, and contact with other components; to gently bend and turn wires during installation to prevent cracking of the insulation; and to physically separate wires from systems whose signals may interfere with one another. These wires are subject to deterioration from aging as well as mechanical damage from maintenance activity. These factors have combined to increase the incidence of wire-related aircraft safety problems, including short-circuits, electrical equipment malfunctions and failures, and fire. Guidelines for these safe wire routing practices in aircraft are described in a series of Federal Aviation Administration (FAA) advisory circulars and manufacturer standard practices manuals. The NTSB investigation found wire bundles contaminated with semi-conductive residues—such as the metal drill shavings shown here—along the path where the center wing tank wiring was routed. When the protective layer of insulation on a wire is compromised and the conductor is exposed, the potential exists for a hazardous electrical system malfunction caused by a short circuit or an arc. A short circuit occurs when electricity takes an unintended path. For example, condensation and other conductive materials that are sometimes found on wire bundles can bridge the gap between a wire conductor and adjacent metallic structure. When electrical current follows the unintended path to the metallic structure, a short circuit that could interrupt the function of an electrical system occurs. Short circuits can transfer power to adjacent wires or draw an excessive current from the power source, overheating wires and creating fire hazards. Electrical arcing is a type of short circuit in which high current crosses a gap, emitting sparks. The sparks include molten material from the wire conductor as it is vaporized by the high energy discharge, producing extremely localized heat. The arcing could ignite flammable products in the area and could potentially initiate an explosion. Any time you get a wire insulation that is cracked, down to the bare conductor, and that bare conductor can come in contact with other wire or metal, you have the potential for an arc. Obviously, safeguards must be maintained to preclude arcing, particularly in the vicinity of flammable materials or explosive fuel-air mixtures. We now know that wiring insulation deteriorates with age. Laymen tend to think of aging only in terms of elapsed time. However, aircraft wiring insulation aging rate is also a function of the operating environment and conditions. This aging rate is principally affected by four factors: vibration, moisture, heat, and mechanical stress. These factors vary throughout the aircraft, and certain insulation types are more vulnerable to these factors than others. Thus, the aging rates will vary. Let me now discuss some of the wiring issues raised during the TWA flight 800 investigation. The investigation of this accident has been one of the most extensive investigative efforts in the Safety Board's history. Based on an evaluation of the recovered wreckage and a detailed study of the sequence of break-up events, we have determined that the fuel-air vapor in the center fuel tank ignited, causing an explosion of the tank, and initiating the breakup of the airplane. It should be noted that the primary device for protecting an aircraft from the hazards of electrical malfunctions, the circuit breaker, does not protect against arcing faults. Although circuit breakers do protect against the electrical overheating of wires, they do not protect against arcing faults because circuit breakers are designed to activate based on the heat input. Arcs develop high energy, but in a very short period of time. Safety Board staff have met with industry representatives who are developing devices known as arc-fault interrupters for potential use in aircraft. These devices can recognize the rapid current and/or voltage signatures associated with arcing faults and act to interrupt the circuit. We will continue to monitor the development of this technology and believe this technology is important to the protection of critical aircraft systems from the dangers associated with electrical arcing. But today, our principal concerns have to do with aging aircraft and wiring safety including: ensuring that low voltage fuel system wiring is separated from high voltage wiring from other systems; ensuring that no wiring is routed in proximity to flammable oxygen, fuel, and hydraulic lines or critical flight control cables; and preventing the contamination of wiring by fluids, flammable lint, metal shavings, or other debris. In addition to addressing the deficiencies of copper wire, it is critical to begin the process of evaluating and incorporating alternative single-transmission methods, including fiber optics, infrared and frequency modulated radio technologies. Multiple wires and wire types are combined in bundles and routed throughout commercial transport aircraft. Note the accumulation of dust and debris in this NTSB photo. The New FAA Fuel Tank Safety Rules As a direct result of the NTSB investigation into the TWA Flight 800 crash, The U.S. Department of Transportation's Federal Aviation Administration (FAA) has recently issued new Transport Airplane Fuel Tank System Design Review, Flammability Reduction and Maintenance and Inspection Requirements. This final rule from the FAA requires manufacturers of existing airplanes to conduct design reviews of fuel tank systems to ensure failures could not create ignition sources within the tank. The new rule covers three fundamental areas of fuel tank safety: The prevention of ignition sources, fuel flammability and fuel tank inerting. For new type designs, the rule requires demonstrating that ignition sources cannot be present in fuel tanks when failure conditions are considered. The rule is the direct result of the TWA 800 accident investigation, in which the NTSB determined the probable cause of the explosion was ignition of the flammable fuel/air mixture in the center wing fuel tank. As was mentioned earlier, the actual ignition source has yet to be identified with certainty; nevertheless, the NTSB determined that the most likely source was a short circuit outside of the center wing tank that allowed excessive voltage to enter the tank through electrical wiring associated with the fuel quantity indication system (FQIS). The FAA mandated remedy is to design new plane types, and to retrofit existing fleets, with fail-safe systems which ensure catastrophic failure conditions will not exist for the life-time of the fleet or a particular airplane model. During the comment period for the new rule, many engineers argued that the best way to eliminate fuel-tank explosions is to cut out any possible ignition source, such as damaged wires. Others held that since wires can become accidentally damaged during routine maintenance, the solution lies in a combination of using less-flammable fuels (such as the F4 formula used by the military), reducing fuel-tank temperatures and/or designing fuel-tank inerting technology to replace flammable oxygen with nitrogen or another inert gas. A review of the new FAA rule (available on the Internet at http://www.faa.gov/avr/arm/nprm.htm) reveals that the FAA has opted for a comprehensive solution that requires all of these fail safe precautions be considered in new type designs. While the specific technologies which must be employed are far from certain, the effect of the new FAA ruling is not: from this point forward the manufacturers of new type designs, as well as owners of existing transport fleets, must take additional steps to eliminate ignition sources and further reduce fuel-cell flammability. In terms of ignition sources, the new rule calls for "the isolation of systems and components so that the failure of one element will not cause failure of the other." This would, for example, mean that the practice of co-bundling low-power FQIS wires together with higher-powered wires servicing other systems would be eliminated to the greatest extent possible. Wearing and chafing of electrical power wires routed through and around fuel tanks would also be reduced under the new rule with the addition of multiple layers of Teflon sleeves or other types of wire protection coverings. Inspections of wiring to fuel pumps on certain airplanes with over 35,000 flight hours have shown significant wear to the insulation of wires inside conduits that are located in fuel tanks. In nine reported cases, wear resulted in burnthrough of the conduit into the main tank fuel cell. Additional insulation chafing and cracking was found under wire bundle strain-relief clamps. Wiring strands taken from U.S. Navy aircraft show (top) cracks in the wire insulation that expose the copper conductor, and (bottom) faults in insulated wire that were hidden under a clamp; arcing is clearly present. The FAA also ruled that corrosion of wires, shielding, connectors and fuel tank system components is an additional possible source of ignition within fuel tanks. The FAA report notes that corrosion and damage to wire insulation on FQIS probe wiring was found on 6 out of 8 probes removed from one in-service airplane. Corrosion buildup of conductive sulfide deposits, particularly on damaged wire, may result in a location where arcing could occur if high powered electrical energy was transmitted to the FQIS wiring from adjacent wires that power other systems. Yet another ignition source identified by the FAA is fuel leakage and subsequent fuel fire outside of the fuel tank caused by both corrosion and bad material choices in connectors servicing the pump motor. Both conditions can lead to electrical arcing through the connector housing. According to Armin M. Bruning, President of Lectromechanical Co. in a recent Flight Safety Foundation Special Report, Age-Related Failures of Aircraft Wiring Remain Difficult to Detect, aging can be as big a concern as physical damage in wire insulators. Bruning describes two common forms of wire failure in aircraft: traumatic failure and gradual aging failure. Traumatic failure brings itself to our immediate attention by faulty aircraft operation and is typically located by visual inspection. Many types of physical and mechanical factors are applied either continually or intermittently to aircraft wire insulation and sometimes result in traumatic failure. Among the most common of forces are bending, flexing, torsion, vibration, chafing (abrasion) cutting, impact and crushing. The ability of aircraft wire insulation to resist these forces decreases as the wire ages. Such gradual-aging failure occurs when wire insulation degrades to the point that microfractures occur. Because insulation microfractures cannot readily be detected by visual inspection of wire, gradual-aging failures are seldom reported. Therefore the extent of gradual aging failure of aircraft wire is not known. Although traumatic and age-related failure of aircraft wire appear identical, the preventative measures are very different. Bruning states that tests have shown substantial changes in the aging rates of aircraft wiring from variations in exposure to two environmental factors: moisture and temperature. The Case for High-Temperature Overmolded Cable Assemblies The FAA has adopted a lessons-learned approach in its recommendations on preventing fuel-cell explosions. And the greatest source of hard data on fuel-cell inerting and other techniques to prevent fuel-tanks from exploding has come from the military. The military has used liquid nitrogen, foam and halon gas to eliminate flammable air/fuel vapors and to lower temperatures in jet wing fuel-cells. The elimination of ignition hazards through the use of specialized cable protection materials also has a long history throughout the military services. The F-16 Fighting Falcon was one of the earliest applications of explosion-proofing in military aircraft. Glenair high-temperature overmolded cable assemblies are specially designed to protect fuel-cell wiring from the effects of moisture, heat, caustic chemicals and mechanical stress conditions. In the F-16 Falcon, every available cubic inch of internal wing and body space is used to carry either fuel, ordinance or a critical airborne system. Usable space is at such a premium, that the cable harnesses used to transmit fuel quantity data, power the fuel pump, interconnect the fly-by-wire flight control system, and provide data-links between critical avionic sensors and cockpit devices must travel directly through the jet's internal fuel cells. Such "Total Immersion Fuel Tank Air-Borne Cables" (TIFTAC) must exhibit extremely high integrity while fully immersed in jet fuel, lubricating oil, hydraulic fluid and other liquid chemicals. The cables must be able to withstand penetration and degradation from these caustic fluids for their entire lifespan, as any failure could potentially introduce an ignition source into the fuel cell and result in a dangerous flight safety condition. This is especially true since the F-16, for reasons of efficiency, performance and resistance to battle damage, carries no mechanical backup to its fly-by-wire flight control system. That the F-16 TIFTAC cables must provide such faultless performance under extreme conditions of strain, vibration, shock, and sudden changes of pressure and temperature only adds to the challenge of engineering and building these mission-critical cable assemblies. The F-16's Fly-by-Wire control system has evolved over the years from a four channel analog system to the quadraplex digital FBW system used in today's F-16/C and /D designs. But regardless of the nature of the electrical signal, a specially fabricated overmolded assembly is used to house and protect the electrical wires as they intersect, and are routed through, the F-16's internal fuel cells. The overmolded cable assembly used on the F-16 features a unique high-temperature cable jacketing material which is inert to corrosive fluids and extremely rugged in terms of mechanical and environmental stress generation mechanisms. The special overmolded cables are manufactured by Glenair using both one of-a-kind transfer molds as well as high volume injection molding equipment. Glenair has been manufacturing fuel cell cable assemblies since 1984 when it first received qualification to build these high-reliability cables for the F-16 fighter from the original manufacturer, General Dynamics—Fort Worth. Glenair's overmolded design replaced a shrink-booted conduit assembly which failed to perform both environmentally and electrically. The introduction of overmolded cables into the F-16 fuel-cell was part of a larger program to prevent fuel-cell explosions which included fuel-tank inerting (the use of non-flammable Halon gas to eliminate explosive fuel/air in fuel cells). To date, Glenair has manufactured over 40,000 of these overmolded cable assemblies with zero real-time failures. Glenair overmolded fuel-cell cable assemblies were first used on the F-16 Fighting Falcon. The cables replaced a shrink-booted conduit assembly which failed to meet environmental, electrical and mechanical requirements. Integrating fuel-cell wiring into factory built harness assemblies solves a number of critical problems in the F-16, including the ability to efficiently upgrade or replace wiring systems and the convenience of laying cables directly into the fuel-cell; thus eliminating the cost and maintenance of conduit raceways. The procedures developed by Glenair to meet the high performance standards specified by General Dynamics-Fort Worth have been applied successfully by the company to similar overmolded assemblies for the F 18, the F-22, the V-22 and the F-2. The fuel cell cable for the F-18 is a unique hybrid design—the first of its kind to integrate fiber optic and copper media into an overmolded harness. Each successive design has validated the utility and reliability of overmolding as a solution to the unique challenges of routing electrical wires into corrosive, and explosive, fuel-cell environments. In regards to the FAA's new rule on commercial transports, the overmolded fuel-cell cable directly solves two of the most critical problems: (1) it effectively eliminates the potential for arcing caused by cracks and breaks in wire insulation, by encasing individual wire conductors within a high-temperature material which is inert to caustic fluids, and (2) it effectively isolates the wires servicing fuel-cell electronic equipment from other, higher-voltage wires servicing other equipment. But overmolding technology has other advantages as well. As noted above, one of the original design requirements which led to overmolding of fuel-cell wires is their inertness in caustic fluids. This capability enables engineers to lay the cable directly into the jet fuel, thus avoiding the high costs associated with the installation and maintenance of heavy, metal conduit raceways. Overmolding also provides additional mechanical protection of wire conductors from machine tool shavings and other metal contaminants present in aircraft. Overmolded cables also protect wires from abrasion damage which can result from strain-relief clamps, wire-ties or from vibration and shock stress mechanisms. As the overmolding materials used by Glenair have much better resistance to heat than conventional insulated wire, it can survive for many more years in high temperature applications. Finally, overmolding prevents environmental damage of mating connectors, by effectively sealing the mating interface from the fuels and gasses present in the tank. Because of these many advantages, overmolding has won a permanent place on a broad range of fighter jets. The special processes and materials developed by Glenair—which insure proper adhesion of molding materials to the interconnect sub-assemblies and the elimination of leaks and chafing where cables penetrate bulkheads—have positioned Glenair as the go-to manufacturer and supplier of these increased-safety cable assemblies. Glenair's unique capabilities in fuel cell cable manufacture include: Three tightly controlled, 100% inspection and test manufacturing plants, complete with value-added engineering services and an abundance of manufacturing and assembly capacity. Complete cable overmolding and jacketing capabilities using a broad range of U.S. FAA and European JAR approved harsh environment coverings as well as NBC (nuclear, biological, chemical) materials. Custom fiber optic and electrical cable extru- sion capabilities and one of the industry's broadest selections of metal and fabric shielding, sleeving and conduit materials. Proven shielded and unshielded designs for virtually any environmental or electromagnetic application. ISO 9001, BSENISO9000, MIL-I-45208A and MIL-Q-9858A quality systems. The Case for Fiber Optic Interconnect Cabling Glenair is well positioned to supply fuel-cell cable assemblies for both military and commercial aircraft. The company uses both U.S. FAA and European JAR approved overmolding materials, and has assembly facilities in the United States and England. In 1976 the U.S. Air Force replaced a wiring harness of an A-7 aircraft with an all optical data link in its airborne light technology program (ALOFT): 302 electrical cables, over 1,200 meters in length and weighing over 40,000 grams were replaced with 12 fibers, 76 meters in length, weighing less than 1,700 grams. In the ALOFT program, the principal design requirement was the reduction of size and weight. But one of the major ancillary benefits was the development of a safer aircraft. Compared to copper, optical fiber is relatively smaller in size and significantly lighter in weight—a major advantage in interconnect systems servicing airborne avionics. As a practical matter, fiber is also easier to install—especially in retrofit programs—because the smaller cable diameters can fit comfortably within the footprint or layout of existing electrical conduits and harnesses. Smaller size and weight also make it possible to run multiple backup cables for each electronic system or device. The ability to provide complete redundancy for all critical cabling is a major motivating factor in the introduction of fiber in avionic systems. Optical fiber is also particularly useful in airborne applications due to its electromagnetic immunity. Since fiber optics use light to transmit signals, it is not subject to electromagnetic interference, radio frequency interference, or voltage surges. The total electrical isolation of fiber clearly makes it a safer, spark-free media for use in hazardous environments, such as aircraft fuel cells. Glenair has worked closely with engineers on a broad range of military programs—from the F-22 to the RAH-66 Helicopter—to analyze system requirements and to design high-reliability fiber optic solutions that meet both short and long term cost requirements. The layout and configuration of a fiber optic system can vary widely based on the application environment. Commercial telecommunications systems, for example, typically feature extremely long backbone cables, spliced fiber interstices, and inexpensive ST type connectors at the many termination points in the system. The connectors used in such applications are typically commodity Fiber optic interconnect technologies are ideally suited for use in systems which require the increased safety which comes from complete electrical isolation. Glenair fiber optic connectors, termini, and accessory backshells are specifically designed for use in airborne applications. solutions geared to the low to moderate performance and reliability requirements of that industry. At the other end of the spectrum, fiber optics deployed in military avionics take the form of engineered interconnect harnesses and/or multi branch conduit systems. The connectors used in such applications accommodate multiple fiber optic cables and typically utilize precision contacts, or termini, as the primary mechanism for aligning and connecting the optical fibers. In many such aerospace applications, fiber optics are being employed as replacements or upgrades to existing copper conductor cable harnesses servicing existing black box flight deck equipment, weapon systems, surveillance cameras, sensors, and so on. In all applications of this caliber, the new fiber optic system must adhere to the same rigorous qualification standards and performance requirements that applied to the legacy electrical systems. For this reason, the design, configuration and packaging of fiber optic interconnects has closely mirrored existing Mil-Spec standards, such as those covering interconnect mateability, accessory interface dimensions, material finishes, and so on. The design of fiber optic termini, special purpose backshells and other accessories is similarly controlled by existing packaging requirements and aerospace industry dimensional standards. The D38999 connector is currently the most commonly specified multi-pin cylindrical interconnect in both fiber and copper conductor aerospace applications. When used to connect multiple strands of fiber simultaneously, the D38999 connector functions as a container or shell for the precision termini which perform the actual marriage of the fiber strands. Glenair's M29504/4 & /5 approved fiber optic termini in conjunction with Glenair's new Advanced Fiber Optic Composite D38999-style connector offers a complete fiber optic interconnect solution. Glenair also offers a full range of fiber optic connector accessories, cables and termination tooling. Today, the use of fiber optic systems to carry digitized video, voice and data is universal. In business and industry, fiber optics have become the standard for terrestrial transmission of telecommunication information. Although still in its infancy, fly-by-light flight control systems may someday replace fly-by-wire systems with cabling which is both lighter, smaller and safer. The use of fiber optics in the media servicing fuel quantity sensors, would appear to be a timely and practical way to eliminate both electrical ignition sources as well as inadvertent pathways for voltage surges from adjacent wiring. Fiber optic interconnect cabling may be deployed in limited, shortrun applications—such as in wiring systems for fuel-cell instruments; or in broader applications—such as the fly-by-wire flight control systems in today’s front-line fighters. Glenair's World of Interconnect Solutions The prevention of equipment and instrument malfunctions, the elimination of ignition sources and the reduction of maintenance cycles are all possible in commercial transports with the adoption of cable overmolding and fiber optic technologies from Glenair. Both technologies are easily adaptable to the unique requirements of commercial transport aviation. Overmolded fuel cell cables, either freely-routed within the fuel cell or enclosed in smooth metal conduit, represent a cost-effective solution to the new FAA rule on fuel-tank safety. Fiber optic interconnect systems, with their complete EMI immunity and electrical isolation are ideally suited for use in fuel-cells and other hazardous applications, and can be deployed in both short-run applications as well as in complete fly-by-light systems. Both technologies directly answer the new FAA requirements for increased safety of fuel-cell systems in commercial transport aircraft. For more information, visit our website at www.glenair.com

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