Aviation Fuels with Improved Fire Safety

Aviation Fuels
with Improved
Fire Safety

A PROCEEDINGS

Committee on
Aviation Fuels with Improved Fire Safety

National Materials Advisory Board

Commission on Engineering and Technical Systems

Board on Chemical Sciences and Technology

Commission on Physical Sciences, Mathematics, and Applications

National Research Council

NMAB-490
NATIONAL ACADEMY PRESS
Washington, D.C. 1997

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The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competencies and with regard for appropriate balance.

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This study by the National Materials Advisory Board and the Board on Chemical Sciences and Technology was conducted under Federal Aviation Administration Research Grant No. 93-G-040 and Department of Defense Contract No. MDA972-92-C-0028.

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Cover: Schlieren frames of Jet A under different air and fuel flow rate conditions. Source: Thor I.
Eklund and Joseph C. Cox. 1978. Flame Propagation Through Sprays of Antimisting Fuels. NAFEC
Technical Letter Report NA-78-66-LR. Atlantic City, N.J.: National Aviation Facilities Experimental
Center.

Printed in the United States of America.

COMMITTEE ON AVIATION FUELS WITH IMPROVED FIRE SAFETY

Technical Consultant

National Materials Advisory Board Liaison Representative

National Materials Advisory Board Staff

NATIONAL MATERIALS ADVISORY BOARD

BOARD ON CHEMICAL SCIENCES AND TECHNOLOGY

Preface

Fire hazards associated with aircraft fuels have been a major concern of the military services, the Federal Aviation Administration, aircraft manufacturers, and aircraft operators. Current approaches to reduce the likelihood of fuel ignition emphasize design criteria for the routing of electrical wiring and fluid lines, fuel tank venting, engine fire walls, fire detection and suppression systems, and material fire resistance, as well as procedural rules regarding the storage, handling, and dispensing of fuel. However, as described in the 1996 National Research Council publication Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft: "Fuel flammability can overwhelm post-crash fire scenarios." Thus the reduction of the fire hazard of fuel is most critical for improving survivability for impact-survivable accidents.

The purpose of the Workshop on Aviation Fuels with Improved Fire Safety, held on November 19-20, 1996, at the National Research Council's Georgetown Facility, Washington, D.C., was to review the current state of development, technological needs, and promising technology for the future development of aviation fuels that are more resistant to ignition during a crash.

The organizing committee, which included recognized experts in aviation fuels, propulsion systems, combustion and flammability, additive materials, and analysis methods, developed the workshop format and agenda and identified potential participants. The committee felt that the most effective way to address this multidisciplinary topic was to include a series of invited presentations to provide background and perspective for workshop discussions and to introduce information on state-of-the-art technological developments. The invited speakers prepared summary papers, which are included in this proceedings (Parts II-IV). The workshop summary presented in Part I of this proceedings describes the subsequent workshop discussions and present ideas for research and development that workshop participants felt were needed to develop fuels with improved fire safety.

This workshop provided a unique opportunity for participants representing a diverse range of technical backgrounds and experiences to get together to discuss a very difficult and important problem. The importance of the problem was impressed on the participants by a tragic accident that occurred at Baldwin Municipal Airport outside Quincy, Illinois, on the first evening of the meeting (November 19). The accident, as reported by the Chicago Tribune (November 20, 1996), involved a collision, on the ground, of a commuter aircraft with a private aircraft. The subsequent fuel fire, which resulted in 13 fatalities, was so intense that rescue personnel could not intervene.

This proceedings provides a summary of the results of the workshop and represents the views and opinions of the participants. The objective of the workshop was to identify technical issues and develop ideas for investigation and further development in an important area that has been essentially neglected for more than a decade. The committee hopes that the results of this effort catalyze further assessments by government and industrial organizations that will lead to real improvements in aviation fuel fire safety for both military and commercial operations.

Comments and suggestions can be sent via Internet electronic mail to nmab@nas.edu or by FAX to the NMAB (202) 334-3718.

John J. Wise, chair
Committee on Aviation Fuels with
Improved Fire Safety

Acknowledgments

The committee on Aviation Fuels with improved fire safety acknowledges the hard work of the invited speakers, who prepared outstanding presentations that provided important background for workshop discussions and excellent papers for inclusion in this proceedings. The committee would also like to thank all of the workshop participants for contributing their time and energy to the workshop discussions.

The committee would also like to thank Douglas Mearns of the Naval Air Systems Command and Robert Morris of the Naval Research Laboratory for their help in planning the workshop. The committee is particularly grateful to Thor I. Eklund, whose vision helped to initiate this effort and whose perseverance and expertise helped to complete the proceedings.

Finally, the committee gratefully acknowledges the support of Thomas Munns, National Materials Advisory Board (NMAB) senior program officer, Aida C. Neel, NMAB senior project assistant, and Jack Hughes (until August, 1996) and Bonnie Scarborough, NMAB research associates.

Contents

I. SUMMARY OF WORKSHOP

II. PRESENTED PAPERS: FUEL AND ADDITIVE TECHNOLOGIES

4 Potential Surfactant Additives: The Search for the Oxymoron
Paul Becher

5 Fire Safety in Military Aircraft Fuel Systems
Robert Clodfelter

6 Rheology: Tools and Methods
Saad Khan, Joseph R. Royer, and Srinivasa R. Raghavan

7 Jet Fuel Chemistry and Formulation
William F. Taylor

8 Concepts for Safe-Fuel Technology
Bernard R. Wright

III. PRESENTED PAPERS: AIRCRAFT FUEL SYSTEM REQUIREMENTS

9 Engine Fuel System Design Issues
Matthias Eder

10 Applications of Vulnerability Analysis and Test Methods to Aircraft Design
Hugh Griffis

11 Aircraft Fuel System Design Issues
Harendra K. Mehta and A. Thomas Peacock

IV. PRESENTED PAPERS: CHARACTERIZING FUEL FIRES

12 Combustion Fluid Mechanics: Tools and Methods
Gerard M. Faeth

13 Fundamentals of Fuel Ignition and Flammability
William A. Sirignano

14 Post-Crash Fuel Dispersal
Sheldon R. Tieszen

APPENDICES

Figures and Tables

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SUMMARY OF WORKSHOP

The National Research Council (NRC) Workshop on Aviation Fuels with Improved Fire Safety, held on November 19 and 20, 1996, in Washington, D.C., presented a rare opportunity for individuals familiar with different scientific and technical aspects of the fuel fire-safety problem to discuss opportunities for long-term research and development (see Appendix A). Invited papers covered a broad range of relevant topics to provide background for workshop discussions. Workshop discussions were devoted to the history of fuel fire-safety research, issues of fundamental fuel science, fuel system implementation issues, the technical status of analytical tools and materials, and recent advances in technical capabilities.

Part I of this proceedings presents a summary of the discussion that occurred at the workshop, as well as background information and technical and design issues associated with the fuel fire-safety problem prepared by the workshop planning committee and NRC staff. Chapter 1 provides information on the history of fuel fire-safety research and practices, and places the problem in a policy context. Chapter 2 presents workshop discussions on fuel and additive technologies, aircraft fuel system requirements, the characterization of fuel fires, and fuel fire-safety strategies. The most relevant technological advances and the most promising opportunities for research identified by workshop participants are summarized in Chapter 3. Workshop papers are included in Parts II, III, and IV for aircraft fuel system requirements, fuel and additive technology, and fuel fire characterization, respectively.

Background and Historical Perspective

The fear of fire has influenced aviation practices throughout the history of flight. The dangers posed by the combination of flight and uncontrolled fire have been etched into the public's consciousness by film footage of wartime aviation, the Hindenberg conflagration, and tragedies in the Apollo and space shuttle programs. Media coverage of commercial airplane accidents typically includes grim photographs of the aftermath.

Fire can be defined, in these instances, as the uncontrolled release of energy through the combustion of flammable materials, with destructive consequences. However, it is precisely the release of energy through fire, or combustion, that makes flight a viable means of transportation. Significant mechanical force is required to lift an aircraft and payload off the ground and move it through the atmosphere. The energy for this motive force comes from fuel. Jet engines provide for the controlled release of energy from fuel, which is used for aircraft propulsion. Hence, although the controlled release of energy through the combustion of flammable materials makes flight possible, the uncontrolled combustion of the same materials has potentially destructive consequences.

AIRCRAFT FIRE SAFETY

Throughout the history of aviation, preventing and mitigating fire has been a major consideration in all aspects of aviation design. Current designers of commercial and military aircraft incorporate a broad array of fire safety features, such as firewalls, shrouded and break-away fuel lines, flame arrestors, fuel-line isolation, explosion-proof electromechanical equipment, detectors and extinguishing systems, and fire-resistant materials, into their designs. In the last decade, a range of new requirements have imposed stringent fire safety standards on civil aircraft. These requirements include the use of low-heat-release-rate materials for cabin linings, floor-level emergency escape lighting, heat-resistant evacuation slides, fire-resistant liners for cargo compartments, improved hand-held fire extinguishers, and fire-resistant seat cushions. The Companion Report to the Aviation Safety Research Act of 1988 (Public Law 100-591) defined the goal of a "totally non-combustible aircraft cabin." In 1995, the National Research Council (NRC) published a study, commissioned by the Federal Aviation Administration (FAA), that defined research that could lead to the development of materials to meet that goal (NRC, 1995a, 1995b).

Design features alone, however, cannot protect an aircraft against the danger of fire. Although the NRC reports acknowledged the importance of research on fire- and smoke-resistant materials, they also noted that burning fuel has dominated most post-crash fire scenarios and causes even the most fire-resistant materials to burn readily. Therefore, "the reduction of the fire hazard of fuel is critical in improving survivability in post-crash fires" (NRC, 1995b, 41).

To appreciate the contribution of aviation fuel to the total aircraft fire load, consider that fuels comprise a substantial portion of an aircraft's weight. For example, one version of the Boeing 747-400 holds up to 57,285 gallons of fuel; this constitutes approximately 195 tons, or 45 percent of the maximum takeoff weight of the aircraft. Currently there are no regulations requiring that commercial airlines use low volatility fuels (e.g., Jet A). In fact, insurance industry classifications have had the effect of increasing the volatility of the aircraft fuels used because they separate flammable fluids into two categories using a minimum flash point temperature of 100^oF as the divider. In the mid-1970s, the Jet A specification for minimum flash point temperature, which had been set at 110^oF, was lowered to 100^oF to correspond with these classifications.

Several defense agencies have already taken steps to prevent or mitigate the danger of aviation fires, including switching to lower volatility fuels, inerting fuel tanks, and working to develop an antimisting additive for the fuel. In two instances, military aircraft have switched to lower volatility fuels to increase fire safety. For the past 40 years, the U.S. Navy has used a special low volatility aviation kerosene (JP-5) to ensure fire safety on aircraft carriers. Although JP-5 is costly, it has also been shown to reduce the vulnerability of aircraft fuel tanks to ignition by gunfire. The U.S. Air Force has recently completed a 20-year transition from the more easily produced JP-4 fuel to the less volatile JP-8 fuel (comparable to the commercial Jet A fuel). The conversion involved the entire Air Force fleet, as well as U.S. Army aircraft, and was an effort to reduce combat losses from fuel tank ignition caused by gunfire.

The U.S. Air Force has pioneered many safety innovations in fuel systems, including the installation of porous foams in fuel tanks and dry bays, the inerting of fuel tanks with nitrogen and halogenated compounds, and the use of bladder fuel tanks in helicopters. With the exception of bladder fuel tanks, these innovations were designed to improve fire safety in combat, rather than to improve fire protection in the event of release of fuel during an airplane crash.

The Army has devoted substantial resources to improving fire safety for tracked vehicles (e.g., tanks) that use diesel fuel. Antimisting agents, water and fuel emulsions, and halons for inerting fuel tanks have all been evaluated. Many of the Army's fuel safety concepts were reported in the findings of a 1984 ad hoc committee that explored possible research approaches to fuel fire safety (see Appendix B). Similarities between diesel fuel and low-volatility aviation kerosenes have created a synergism between fire safety research on Army tanks and research on aircraft fuel safety. However, military aircraft fuel has different requirements than civil aircraft fuel. In military aircraft, the fuel is used as a heat sink and exposed to higher temperatures (up to 300^oF) for longer periods of time under certain operational conditions.

FEDERAL AVIATION ADMINISTRATION RESEARCH ON FUEL FIRE SAFETY

The transition from piston-powered to turbine-powered aircraft enabled the replacement of highly volatile gasoline with much less volatile aviation kerosenes This transition resulted in a reduction in fuel fire hazards in both combat situations and aircraft crashes. But the need for improvement remained. In the case of aviation kerosenes with flash points higher than ambient temperatures (e.g., Jet A and JP-5), the main post-crash fire hazard was associated with misting of the kerosene as it spilled from ruptured fuel tanks. Kerosene mist is far more susceptible to ignition than kerosene in bulk form. The flammability of mist, as well as its susceptibility to being ingested and ignited by a turbine engine, was documented in the 1950s by Pinkel et al. (1958). Research was undertaken to find fuel additives that could prevent the formation of fine mist.

Gelled Fuel Program

The first additive considered by the Federal Aviation Administration (FAA) for preventing the formation of fine mist was CAB-O-SIL^®, a fumed silica formulation studied in the early 1960s. CAB-O-SIL was suspended in the fuel to increase viscosity and reduce the misting propensity. This early additive was soon followed by a range of gel and emulsion formulations, causing the FAA's fuel safety research program to be dubbed the "gelled fuel" program. Candidate additive packages were screened by shooting plastic bags of modified fuel from a compressed air gun into a coarse screen and ignition flame. Rudimentary testing for fuel system compatibility was also undertaken using engine combustor rigs and fuelpumps. Despite enormous problems with fuel system compatibility, the gelled fuel program proceeded to a full-scale-crash phase involving an RB-66 jet aircraft at the Navy jet track facility in Lakehurst, New Jersey. The program was terminated after the unexpected post-crash fuel fire of a test aircraft in 1972.

Antimisting Kerosene (AMK) Program

Although the major focus of the FAA fuel safety program was on gels and emulsions, a number of long-chain polymers used to improve the viscosity index were also tested. These polymers, in fact, represented the direction that additive manufacturers would take in the future.

The British Civil Aeronautics Administration (CAA) had been supporting the development of additive packages that were derivatives of the early long-chain polymer candidates. The CAA finally decided that FM-9, an additive package from Imperial Chemical Industries (ICI), was the best candidate for large-scale testing and evaluation. FM-9 consists of long-chain polymers (tertiary butyl styrene) that remain coiled in kerosene until the fuel is subjected to a certain level of shear force. The molecules then extend and interact with one another to inhibit the formation of mist. As long as the polymers remain coiled, the flow behavior of the modified fuel is close to the flow behavior of neat fuel. A British research report states that "FM-9 fuel has no measurable viscoelastic properties under the particular experimental conditions studied" (Coomer et al., 1977). However, problems with the solubility of the additive in kerosene necessitate the addition of surfactants as well.

In 1976, the FAA subjected FM-9 to extensive intermediate-scale fire tests at the Naval Weapons Center at China Lake, California. Based on the promising results of these tests, the FAA entered into negotiations with the CAA to define a joint research program focused on the FM-9 additive package. These negotiations resulted in a Memorandum of Understanding specifying responsibilities for each of the signatories (Churchill, 1982). The research program to develop and evaluate FM-9 came to be known as the antimisting kerosene (AMK) program.

In 1980, antimisting kerosene was considered "the concept that has the greatest potential for reducing post-crash fire risk" by a Special Aviation Fire and Explosion Reduction (SAFER) Advisory Committee (FAA, 1980). The new AMK program was much more broadly based than the earlier gelled fuel program. The AMK program involved aircraft component and engine manufacturers, as well as airframe manufacturers, government and nonprofit laboratories, and universities. Fuel system compatibility studies were undertaken, including flight testing of a Convair 880 jet with one engine operating on fuel with FM-9 added. A special in-line degrader was developed for this test so that the viscosity of the fuel with FM-9 additive could be mechanically reduced prior to pumping it into the engine.

Several defense agencies were involved to various degrees in the AMK program. Both the Navy and the Air Force explored the fire safety effectiveness of long-chain polymer fuel additives in tests with live gunfire. The Navy, however, had reservations about using FM-9 because of its known affinity for water and the Navy's need to keep water out of its fuel supplies.

The AMK program culminated in December 1984 with a joint FAA and National Aeronautics and Space Administration (NASA) "controlled impact demonstration" (CID) The CID involved a remotely piloted flight and "crash" of a surplus FAA B720 aircraft using only FM-9 fuel and equipped with degraders on all four engine cases. As part of the demonstration, undertaken at Edwards Air Force Base, the plane moved through stationary wing cutters on the ground. In an unanticipated event, one engine was damaged by these cutters. The CID resulted in a destructive fire caused by the massive ignition of fuel spray by the disintegrating engine (see Figure 1-1). The role played in this ignition by fuel that had already gone through the degrader (and that was therefore no longer antimisting) has been the subject of discussion. Although the AMK program produced a large number of research and engineering reports, many of which are discussed in a final summary report (Yaffee, 1986), the CID marked the end of substantive fuel safety research by the FAA.

POLICY CONTEXT

In 1980, the SAFER Advisory Committee report stated, "The overall safety record of U.S. scheduled air carrier aircraft shows a continuing reduction in the fire-involved accident rate since the advent of jet transport service in 1958" (FAA, 1980). However, since that time, the FAA administrator, the NASA administrator, and industry spokespersons have all recognized that both the total accident rate and the fire-involved accident rate have stabilized since 1980 and that the stabilized rates, combined with continued growth in air traffic, will mean more frequent accidents and more passenger fatalities in the coming years (Murray, 1995; FAA, 1991). Passenger fatalities can be decreased both by lowering the accident rate and by improving crash survivability. Improved aviation fuel fire safety could prevent or lower the number of fatalities in survivable accidents.

The Aviation Safety Research Act of 1988 required that the FAA undertake research on low-flammability aircraft fuels. In his 1988 congressional testimony on fuel safety, then FAA Administrator T. Allan McArtor characterized the agency as "idea-limited" for fuel fire safety improvements (McArtor, 1988). Although research plans were developed for fuel safety (FAA, 1993), no substantive work in this area was undertaken. The NRC's Workshop on Aviation Fuels with Improved Fire Safety described in this proceedings was convened in November 1996 for the purpose of discussing the current state of development, technological needs, and promising technologies for the development of aviation fuels with improved fire safety.

FIGURE 1-1 Controlled impact demonstration (CID) at Edwards Air Force Base, December 1984.

REFERENCES

Coomer, R. I. Faul, and S.P. Wilford. 1977. Measurement of the Extensional Viscosity of Safety Fuels by Jet-Thrust and Triple Jet Techniques. TR-77157. Farnsborough, U.K.: Royal Aircraft Establishment.

Churchill, A.V. 1982. Aviation Fuels: Future Outlook and Impact on Aircraft Fire Threat. AGARD-LS-123. Neuilly-sur-Seine, France: Advanced Group for Aerospace Research and Development, North Atlantic Treaty Organization.

FAA (Federal Aviation Administration). 1980. Special Aviation Fire and Explosion Reduction. FAA-ASF-AT-4. Office of Aviation Safety. Atlantic City, N.J.: FAA Technical Center.

FAA. 1991. Aircraft Safety Research Plan. Atlantic City, N.J.: FAA Technical Center.

FAA. 1993. Fire Research Plan. Atlantic City, N.J.: FAA Technical Center.

McArtor, T.A. 1988. Congressional testimony before the Subcommittee on Transportation, Aviation and Materials of the House Committee on Science, Space and Technology at the hearing on the Controller Performance Research Act and the Aviation Safety Research Act of 1988, held on June 16, 1988.

Murray, T. 1995. Airplane accidents and fires. Pp. 7-23 in Improved Fire- and Smoke-Resistant Materials for Commercial Aircraft Interiors: A Proceedings. Washington, D.C.: National Academy Press.

NRC (National Research Council). 1995a. Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft. Washington, D.C.: National Academy Press.

NRC. 1995b. Improved Fire- and Smoke-Resistant Materials for Commercial Aircraft Interiors: A Proceedings. Washington, D.C.: National Academy Press.

Pinkel I.I., S. Weiss, G.M. Preston, and G.J. Pesman. 1958. Origin and Presentation of Crash Fires in Turbo Jet Aircraft. NACA 1363. Washington, D.C.: National Advisory Committee on Aeronautics.

U.S. House of Representatives Committee on Science, Space and Technology. 1988. Aviation Safety Research Act of 1988: Report to Accompany H.R. 4686. Washington, D.C.: U.S. Goverment Printing Office.

Yaffee, M. 1986. Antimisting Fuel Research and Development for Commercial Aircraft: Final Summary Report. DOT/FAA/CT-86/7. Atlantic City, N.J.: FAA Technical Center.

Workshop Discussions

The NRC Workshop on Aviation Fuels with Improved Fire Safety was held on November 19 and 20, 1996, in Washington, D.C. Participants included representatives of engine manufacturers, airframe manufacturers, petroleum refiners, government agencies, national laboratories, and academia (see participant list in Appendix A). The objectives of the workshop were to summarize the state of current technical efforts that could lead to the development of fuels with improved fire safety, to discuss performance goals for the development of more fire-safe fuels, to forecast future activities for commercializing fire-safe fuels, and to identify promising research topics and key issues.

Invited speakers presented papers outlining a broad range of basic, but relevant, technologies. Topics included fuel chemistry and rheology, new fuel additives, airframe and fuel system design, post-crash fuel dispersal, and modeling post-crash scenarios. Papers provided background and perspective for the workshop discussions and presented information on state-of-the-art technological developments. The papers (Parts II, III and IV of this report) are divided into three general categories: aircraft fuel system requirements, fuel and additive technologies, and fuel fire characterization. Part II includes five workshop papers dealing with fuel and additive technologies by Becher, Clodfelter, Khan et al., Taylor, and Wright. Part III includes the three workshop papers related to aircraft fuel system requirements by Eder, Griffis, and Mehta and Peacock. Part IV includes three papers covering the characterization of fuel fires by Faeth, Sirignano, and Tieszen.

The development of a fuel with improved fire-safety is generally considered to be a challenging, high risk research objective. For one thing, the concept of a "fire-safe" fuel appears to be an oxymoron (see Becher)¹ because the goal is to prevent the burning of a material that is developed specifically to be burnt for its combustion energy. Second, the burning of fuel must be prevented under conditions involving the dissipation of enormous amounts of mechanical energy (crashes and impacting or exploding weapons), situations that provide ample opportunity for ignition. Finally, the fuel requirements that must be met to ensure the reliability and safety of normal aircraft operations severely limit changes that can be made in fuel characteristics.

The workshop presented a rare opportunity for individuals familiar with different aspects of the fuel fire-safety problem to combine their expertise in an effort to clarify the obstacles to and possibilities for progress in this field. Portions of the workshop were devoted to outlining issues of fundamental science, describing aircraft design and performance requirements for fuel systems, identifying the current technical status of analytic tools and materials, and describing recent advances in technical capabilities. Workshop discussions are summarized in this chapter according to topic—fuel and additive technologies, requirements for aircraft fuel systems, the characterization of fuel fires, and fire inerting and suppression technologies.

FUEL AND ADDITIVE TECHNOLOGIES

Aviation kerosene consists mostly of hydrocarbon molecules with eight to sixteen carbon atoms arranged in various molecular configurations. It is produced primarily through the straight-run distillation of crude oil. Depending on the composition of the raw crude and other petroleum stock used for production, aviation kerosene typically undergoes further processing to reduce the sulfur and acid content either by chemical treatment or by more severe processing. More severe processing methods include hydrotreating and hydrocracking. These processes can remove heteroatoms, enhance fuel hydrogen content, and change distillation characteristics (see Taylor).

The actual composition of aviation kerosene can be affected by overall market demands for petroleum. For example, refineries often produce a single product suitable for either Jet A fuel or for blending into home heating oil. Because the flash point temperature for home heating oil must be at least 120^oF to meet local building safety codes (Blake, 1982), the flash point temperature for aviation kerosene is usually between 125^oF and 135^oF, even though the specification minimum is 100^oF. The portion of crude available for aviation kerosene represents a trade-off against the demand for other petroleum products, such as gasoline and diesel fuel. Other circumstances, such as wartime contingencies, political instability, or petroleum embargoes, can also necessitate the use of off-specification fuel. Because less than 10 percent of the refineries in the U.S. produce virtually all of the aviation kerosene (Karlick, 1980), disruptions from accidents and natural disasters could also impact fuel characteristics.

The past 15 years have seen extensive modernization and automation of U.S. refineries, as well as a shake-out of unprofitable plant capacity. New processing techniques have become available to fuel refiners, and refinery methodologies have become more widely known (see Taylor). New technologies and more flexibility in processing have enhanced refiners' ability to produce fuels from a wider range of raw stock.

Approaches to reducing fuel flammability, including the FAA's gelled fuel and antimisting kerosene (AMK) programs, generally involve the use of additives to change fuel characteristics and decrease the likelihood of unwanted ignition. Workshop participants discussed barriers to implementing additive programs in general, as well as relevant advancements in the development of polymers and surfactants and improvements rheology, measuring equipment, and analytical methods.

Fuel hydrocarbons, by themselves, tend to be resistant to reactions; but the dissolved oxygen and additives inevitably present in fuel are more reactive. Efforts are made, therefore, during refining, transportation, and storage, to minimize the presence of chemicals and impurities that could react adversely with these materials, and also to remove any particulate and water contamination. Several workshop participants pointed out that the need to minimize contaminants directly affects the viability of safety fuels. Fire-safe fuel additives must not be adversely reactive either with existing aircraft fuels or with additives already in use, such as antistatic compounds, antioxidants, corrosion inhibitors, and metal deactivators. A viable fire safety additive has to be an integral chemical component of the kerosene formulation and must have predictable, measurable, and consistent characteristics.

Polymers and surfactants may be useful for reducing fuel flammability, according to several workshop speakers and participants. Because of the emphasis on evaluating FM-9 during the AMK program, many other polymers in existence at the time were never investigated for potential antimisting capabilities. Today, even more polymers and surfactants are available (see Becher; Hall, 1996). In addition, more powerful computational capabilities offer possibilities for molecular modeling and polymer design that were not available at the time of the FM-9 program. With molecular design, in principle, a polymer fuel additive could be developed that would self-degrade thermally prior to injection into the combustor. This would obviate the need for a mechanical degrader to restore acceptable fuel flow characteristics, as was necessary with the FM-9 additive (Hall, 1996).

A parallel situation exists with surface active agents, or surfactants. Thousands of surfactant molecules are available for use in safety fuel formulations (see Becher). The U.S.Army has extensively tested fire-safe candidate fuels that employ surfactants to make emulsions of water and fuel. Participants noted that the FM-9 formulation required surfactants to enable the styrene-based polymer to swell enough to prevent mist formation. A number of workshop participants felt that future fire-safe fuel additive packages might include both high molecular weight polymers and surfactants. It was suggested that recent developments in microemulsion technology could be of interest in developing such formulations.

In addition to the increased technical sophistication of the petroleum industry and the broad array of polymers and surfactants available, fire-safe fuel research is now facilitated by advances in the rheology of polymer solvent systems. With dynamic rheological measurements, polymer-solvent interactions can now be characterized at the microstructural level. For example, with steady-flow rheology, changes in microstructure caused by fluid flow can now be quantified in simple fluid systems. In addition, with advanced techniques for measuring extensional viscosity, composition effects for very dilute polymer systems and for solutions of associative polymers (typically water-soluble polymers that also possess hydrophobic groups) can now be identified. Rheological methods to characterize non-Newtonian flow behavior have also become more firmly established (see Khan et al.). These advances in rheological tools have enhanced the capacity for designing effective fuel additives.

The availability of large numbers of polymer compounds and surfactants coupled with the complexity of kerosene composition, means an unmanageable array of candidate additive packages is available for investigation. However, workshop participants felt that progress in analytical methods, including molecular computation and measurement technology, could alleviate this problem. The past 15 years have yielded increasingly automated, technically sophisticated equipment for process monitoring and quality assurance testing. With chromatography, mass spectrometry, and infrared spectrometry, more detailed and quicker chemical analyses than were available during previous programs can be done.

Given the advances in materials and tools described above, participants with expertise in the area of chemical additives felt that an effective fire-safe fuel additive package could probably either be selected or designed, provided that fuel performance requirements were identified in advance. Participants also felt that modernization and new technology have provided today's refineries with more flexibility to compensate for changes in fuel properties caused by additives.

AIRCRAFT FUEL SYSTEM REQUIREMENTS

The overall issues of concern regarding the properties of aviation fuels have not changed much since they were originally documented in the 1950s (Barnett and Hibbard, 1956; Sharp, 1951). Requirements for aircraft fuel systems are continually evolving in response to experience, scientific advances, practicality, and best judgment. Fuel system requirements include fuel properties for safe and efficient operation along with characteristics of aircraft systems that enable the use of fuel with specified properties. Many of the barriers that must be overcome to implement a fire-safe fuel involve fuel performance and requirements for fuel properties. Altering fuels to add fire safety features for the case of an aircraft crash may very well cause problems with fuel performance during normal operation. Workshop participants discussed barriers to the implementation of fire-safe fuels and additives presented by interrelations among fuel specifications, fuel system requirements, and the operational requirements of existing aircraft.

Fuel properties are specified to ensure safe and efficient aircraft operation. Performance specifications for aviation fuel for the commercial aviation kerosene Jet A are identified in ASTM specification D1655; for JP-8 they are identified in military specification MIL-T-83133A; and for JP-5, they are identified in military specification MIL-T-5624L. These specifications ensure that fuels meets requirements in the following areas:

• Combustion performance. Requirements are set to ensure that aviation fuels have adequate altitude relight capability, that radiative heat transfer to the combustor liner not be excessive, that the level of emissions is acceptable, and that temperature patterns at the combustor exit are acceptable.

• Fuel thermal behavior. Specifications for commercial transport are primarily associated with gum deposits that can form on hot surfaces traversed by the fuel as it is pumped to the combustor nozzles. Fuel thermal stability is more crucial in military aircraft because of the high operating temperatures in the fuel system and the thermal effects on fuel circulated for cooling purposes. Fuel temperatures at the engine inlet can run as high as 250^oF, and the fuel is then heated as it passes through as many as three additional pumping stages and is used as a fluid to power thrust control actuators (see Eder). A new additive package has recently been demonstrated to enhance the thermal heat sink capabilities of both Jet A and JP-8 (Heneghan et al., 1996).

• Flow characteristics. Requirements specify allowable pressure drops across small orifices in the engine fuel controls, as well as allowable low temperature viscosities, freeze point temperatures, and performance criteria in fuel tank boost pumps. Fuel flow requirements for military aircraft are extremely complex because fuel is increasingly used by the military as a lubricant for accessories and as a surrogate hydraulic fluid (see Eder).

• Materials compatibility. Problems associated with wear and tear on components caused by flowing fuel, with the integrity of seals and sealants, and with the sensitivity of thermal barrier coatings on turbine blades to fuel constituents have been identified.

If fuel properties are changed to improve fire safety, other performance specifications might not be met. For example, decreasing the volatility of a fuel by raising the flash point temperature might interfere with engine relight capability.

Aircraft systems design criteria are established for the safe and effective utilization of specified fuels. For commercial aircraft, these criteria are established in FAA advisory circulars, certification requirements, and industry design practices. For military aircraft, design considerations have been codified in formal specifications, such as MIL-F-87168 for the Air Force (see Clodfelter). There are also specifications for aircraft materials that are exposed to fuels. These materials are selected on the basis of minimal adverse effects on performance and minimal aging as a result of exposure to fuels, and for not, themselves, degrading fuel quality. Satisfactory metals for pumps, lines, and accessories are aluminum, certain stainless steel compositions, some nickel-steel alloys, Monel, carbon molybdenum steel, and chrome-molybdenum steel. Bronze, nickel, copper, zinc, cadmium, and brass do not have satisfactory properties. Similarly, depending on the temperature, polymeric materials suitable for seals and gaskets include nylon polyethylene, fluoropolymer resins (e.g., Teflon^®), and fluoroelastomers (e.g., Fluorel^®and Viton^®) (CRC, 1983). Several workshop participants pointed out that fuels altered to improve fire safety could require major changes in design and materials specifications for many aircraft system components.

Workshop participants pointed out that operators of both military and commercial aircraft will continue to depend on existing and derivative aircraft in both the nearterm (up to 5 years) and the midterm (up to 10 years) future. The U.S. military fleet contains a large number of older aircraft (20 to 35 or more years), with minimal replacements (e.g., C-17 transport, F-22 fighter, and Joint Strike Fighter) planned in the next 25 years (NRC, 1997). Also, no new civil aircraft designs are anticipated in the near- to mid-term because the industry's focus has been on derivative aircraft models (NRC, 1996a). Hence, fuels with improved fire safety must be evaluated for current airplane models.

If current aircraft have to be retrofitted, fuel performance in existing engines could be a significant barrier to implementation. Engine designs have been developed based on the availability of a known, specified fuel. These designs have been tested to demonstrate that they meet safety and performance criteria and have been certified based on using that fuel (Blake, 1982). Because the effects of changing fuel properties on engine performance are not known, extensive testing and requalification would be required. Participants knowledgeable about engine design and manufacturing stated that this would significantly affect manufacturers of engines and components who provide warranties on their products, because the off-specification fuels might degrade safety and performance.

Past experience provides some information on the time and cost that would be involved in the development of a fuel modification and subsequent retrofitting. Participants pointed out that the qualification of the Stadis 450^® antistatic additive used in Jet A-1 fuel took seven years; the Air Force transition from JP-4 to JP-8 took 20 years. The Air Force transition could also provide a basis for estimating the costs of changing fuel specifications for a broad spectrum of aircraft and aircraft systems (see Clodfelter). Problems likely to be caused by the tremendous variation in age and system characteristics of aircraft in both the civil and military fleets have not been estimated. There could be major differences in the compatibility of the new fuel with various aircraft.

Finally, workshop participants discussed the wide variety of existing aircraft design features that already minimize the likelihood of fuel ignition. Most of these design features are compatible with any type of fluid and, therefore, do not present barriers to the implementation of a fire-safe fuel. A number of features are designed specifically to prevent the ignition of fuel vapors by electrical sources (see Mehta):

• Engine accessories are electrically bonded to the engine case, which in turn is bonded to the airframe, to prevent discharges of static electricity, as well as discharges induced by circuitry and electromagnetic fields.

• Explosion-proof electromechanical equipment is used wherever there may be contact with fuel liquids or vapors.

• Fuel tank capacity and sensor circuits are designed with electrical power requirements an order of magnitude lower than the minimum spark ignition energy of 0.2 mJ.

• Fuel vent ports from aircraft surge tanks are located inboard of the lightning strike zone, and tank structure is designed to prevent internal sparking in case of a lightning strike.

Additional fire safety design features include the strategic placement of fuel drain masts, shrouded fuel lines within the aircraft pressure hull, flame arrestors in vent lines, fireproof hose assemblies in engine nacelles, the separation of fuel lines from electrical wiring, and the ventilation of compartments and bays where fuel vapor can accumulate. Fuel leaks are prevented by measures such as fuel line stretch flexibility, fuel shutoff capability outside the engine fire zone, and duplicate wiring to the shutoff valve actuator. Finally, aircraft powerplant technology now has on-board computers capable of controlling fuel utilization and monitoring performance discrepancies.

CHARACTERIZING FUEL FIRES

The literature applicable to post-crash aircraft fires is abundant. However, most past studies have focused on ignitability, based on small quantities of fuel, and the burning of large fuel pools and have been intended to provide information either for fire-fighting tests or for the effects of fire exposure on materials and structures. The overall development of a fire during an airplane crash is still not well understood.

To disrupt the processes of fire development, it is necessary to have a clear understanding of them. For aircraft crashes where spilled fuel burns, it is important to understand how the fuel is released, how it is dispersed, how it is ignited, and how the fire is spread. The process is different for each crash scenario, and the changes from one scenario to another must be ascertained. The effectiveness of fire safety measures must be gauged for the full range of possible accident scenarios. Advances in analyzing risk and vulnerability have enabled far more comprehensive and reliable systems analyses of proposed fuel candidates than was possible prior to the CID crash in 1984 (see Griffis).

Workshop speakers and participants discussed recent developments in analytical techniques that could be used to analyze fire development processes in crash scenarios. Important developments have occurred in computational approaches to the prediction of fuel dispersal and in the modeling of fluid dynamic and combustion phenomena.

A recent study of post-crash fuel dispersal has demonstrated the complexity of the fuel release, atomization, and dispersal processes (see Tieszen). This study recommends using a computational approach involving smooth particle hydrodynamics to investigate fuel release on impact in certain very high energy crash modes. Detailed modeling of the atomization of fuel after release is currently computationally impossible partly because there is a four-orders-of-magnitude difference in scale between fuel droplet size and wing cross section. However, the post-atomization dispersal process becomes more tractable as the overall concentration of liquid fuel in air decreases.

Workshop participants pointed out complexities associated with the aerodynamic flow field that develops around a crashing aircraft that is moving in a direction other than along the fuselage center line. The flowfields undoubtedly include regions of strong vorticity, recirculation, and local acceleration.

The modeling of fluid dynamic and combustion phenomena relevant to post-crash fires has improved dramatically in the last 10 years, and some impressive tools are now available for analyzing turbulent combustion, spray formation, and spray burning (see Faeth). Notable advances include the use of universal functions to analyze turbulent diffusion flames, the development of stochastic separated flow (SSF) models for prediction of droplet life histories in fuel sprays, and the clarification of the role of vorticity in the breakup of liquid jets. Although the most extensive efforts at computational and experimental verification have been directed toward axisym-metric jets and wall jets, the same methods could have a significant impact on the characterization of post-crash fires if applied to the release of aviation fuel during a crash.

These computational tools were presented to the workshop as a cohesive engineering framework based on laminar diffusion flamelet concepts and classical turbulence models. Universal state relationships can be used to correlate various fluid properties with the degree of mixing predicted by turbulence models. Laminar flamelet concepts can then be used in conjunction with turbulence models, and their effectiveness is a key feature of the success achieved in modeling turbulent flames.

The locally-homogeneous flow approximations and state relationships used for gaseous fuels have been used to analyze turbulent burning sprays. Although these analyses are not accurate in predicting the changes undergone by the spray over time, they have revealed an extremely important physical fact about burning sprays, that combustion occurs only in regions of relatively low mixture fraction. Once this is understood, much of the process of spray formation can be analyzed without including combustion effects.

With new analytical methods, a more accurate method for predicting flame radiation is now available. Stochastic techniques allow turbulent fluctuations in temperature to be factored into the radiation transfer equations, thereby rectifying the underestimation of radiation flux, which was characteristic when averaged thermal properties were used. However, major inaccuracies in the prediction of overall fire development remain, primarily because of the difficulty of characterizing the size, distribution, and evolution of soot in turbulent fires.

With newly developed SSF methods, handling phenomena with droplet turbulence interaction is now computationally tractable. SSF methods can successfully predict droplet life histories in a burning spray. The accuracy of SSF methods suffers most from uncertainties concerning initial spray characteristics. Although recent studies have clarified the role of vorticity and turbulence in the primary breakup of liquid jets and sheets and have demonstrated that the breakup leads to universal normal distributions of droplet size, the rate of breakup processes has not been predicted by any generalized method. Droplet sizes in primary breakup are limited by surface tension; however, viscosity affects droplet sizes in secondary breakup. According to workshop participants, methods for predicting secondary breakup accurately will also require further investigation of the rate processes.

Workshop participants pointed out that, to be applicable to safety fuels, existing computational (or modeling) techniques for spray combustion would have to be modified to include rheological effects. In particular, the analysis of the primary and secondary breakup of fuel jets would have to include the effects of shear-rate-dependent viscosity, normal stresses, and elongational viscosity. The time dependence of changes in the elastic and viscous fluid properties would most likely also be important. Direct application of current turbulent spray combustion models to the analysis of incendiary ignition of fuel in tanks appears to be more difficult. However, if adequate descriptions of the turbulent flow variables can be developed, some concepts, such as the universal state relationships, may be useful.

FIRE INERTING AND SUPPRESSION TECHNOLOGIES

Strategies discussed thus far have been aimed at disrupting or eliminating aspects of the fire development process. Fuel additives are primarily aimed at preventing fuel spray ignition, while aircraft design features enhance the fire resistance of the aircraft structure. Workshop participants drew a distinction between fire-safe fuels, which they classified as a preventive strategy, and other ameliorative strategies. Fuelfire inerting and suppression emerged as an area of interest in both the workshop discussions and invited papers (see Clodfelter; see Wright).

Strategies were discussed for increasing the chances of survival and rescue of passengers via suppression of fire around crashed aircraft. Approaches to fire suppression depend on the aircraft itself deploying countermeasures and on the means of deployment remaining effective after the crash. Halon fire extinguishing agents have been the agents of choice for aircraft engine and cargo compartment fires, but the production of halon agents, as well as certain other chlorofluorocarbons, has been terminated because they contribute to the depletion of stratospheric ozone. The search for alternatives has driven researchers toward other technologies, such as pyrotechnically generated aerosols, water mist systems, and low volatility agents (see Wright). The four concepts proposed by workshop participants for fire suppression are:

• Surface enhancement of halons. A surfactant or other substance would be used to concentrate halon or halon replacement agents at the surface of a burning pool.

• Low volatility technology. A chemical compound would be deployed that would thermally decompose into one or more extinguishing agents when passing through a fire plume.

• Propellant generated solid aerosols. Solid generators would burn to release agents, such as potassium compounds, that could extinguish a fire.

• Micro-encapsulation. Capsules would degrade to a dry chemical fire suppressant, releasing their contents, which would be another liquid or gaseous extinguishing agent.

Advancements since the 1980s relevant for deployment mechanisms include the development of air bag gas generator technologies by which a variety of suppression agents could be generated in situ from various reacting solids.

Exclusive focus on suppression around crashed aircraft may prevent fire safety approaches from being applied to other fire scenarios, such as fuel tank ignition by incendiary projectiles. If the focus is on ignition and flame spread, then processes like fire spread on liquids, hot projectile boundary-layer ignition, and spray combustion become the phenomena of interest (see Sirignano). Concepts arising from this perspective in workshop discussions were:

• Pressure sensitive fuel. This fuel would burn well in the engine combustor but very poorly upon release into the atmosphere.

• Two fuel combination. This fuel system would comprise two separate components that would burn poorly when unmixed but would burn efficiently when blended and injected into the engine.

Workshop discussions also identified potential technologies for fuselage hardening involving intumescent coatings and fire-resistant carbon fiber or polymer foam aircraft insulation. Hardening technologies can increase escape times by enhancing the resistance of the aircraft fuselage to burn-through from external fuel fire.

COST CONSIDERATIONS

Workshop participants discussed significant changes in the technical landscape since the CID crash in 1984. Changes in the corporate culture, driven by international competition, have significantly reduced private sector investment in long-term scientific and engineering research. The government's debt-driven budgetary pressures have resulted in an overall reduction in federal research funding of approximately 30 percent (NRC, 1996b). The end of the Cold War has resulted in a downsized aerospace industry, and the longer-term effects of deregulation have resulted in a more cost conscious and conservative outlook by airlines and commercial aircraft manufacturers.

Some participants felt that the type of fuel safety research done in the past could not be contemplated in today's climate because the private sector would be unlikely to invest resources in a long-term fuel safety research program without government support, and the government would be unlikely to commit large resources for an engineering demonstration of an industry sample product without a substantial systems analysis of the costs, benefits, and likelihood of success. Hence, for economic reasons, as well as for technical reasons, a future fuel safety research program will have to be grounded in fundamental research and subscribed to by all stakeholders. Participants felt, however, that technical difficulties transcend issues of cost and that the technical difficulties place safety fuel development squarely in the realm of long-range research.

REFERENCES

Barnett, H.C., and R.R. Hibbard. 1956. Properties of Aircraft Fuels. TN-3276. Washington, D.C.: National Advisory Committee for Aeronautics.

Blake, C.L. 1982. The Impact of Petroleum, Synthetic, and Cryogenic Fuels on Civil Aviation. DOT/FAA/EM-82/29. Atlantic City, N.J.: Federal Aviation Administration Technical Center.

Coordinating Research Council (CRC). 1983. Handbook of Aviation Fuel Properties. Report Number 530. Atlanta, Ga.: CRC.

Hall, Richard. 1996. New Materials Technology: Polymer Additives. Presentation to the Workshop on Aviation Fuels with Improved Fire Safety, National Research Council, Washington, D.C., November 19, 1996.

Heneghan, S.P. S. Zabarnick, D.R. Ballal, and W.E. Harrison III. 1996. JP-8+100: The development of high-thermal-stability fuel. Transactions of the ASME 118(September): 170-179.

Karlick, M.A. 1980. A petroleum primer. Part I. Refining the petroleum. Airline Pilot 49(3): 12-19.

NRC (National Research Council). 1996a. New Materials for Next-Generation Commercial Transports. Washington, D.C.: National Academy Press.

NRC. 1996b. Driving Innovation Through Materials Research: Proceedings of the 1996 Solid State Sciences Committee Forum. Washington, D.C.: National Academy Press.

NRC. 1997. Aging of U.S. Air Force Aircraft: Interim Report. Washington, D.C.: National Academy Press.

Sharp, J.G. 1951. Fuels for gas-turbine aero-engines. Aircraft Engineering 23:2-6.

Summary of Progress and Opportunities

Developing a safer fuel has always been considered an unusually challenging, or high risk, research objective. First, the concept of preventing the burning of a substance used specifically for its combustion energy appears to be an oxymoron. Second, the conditions under which fire must be prevented are precisely the conditions that involve the dissipation of enormous amounts of mechanical energy, such as crashes and impacting or exploding weapons. Third, present day requirements for fuel performance severely limit the kinds of changes in fuel characteristics that can be made.

In this chapter, the ideas and observations of individual workshop participants are summarized. The industries and technologies represented by workshop participants rarely come together in a common endeavor. Consequently, a portion of the workshop was devoted to outlining fundamental aircraft design and fuel performance requirements, identifying the current technical status of relevant technologies, and describing advances in technical capabilities. Opportunities are identified for each major area—fuels and additives, fuel systems, and the characterization of fuel fires.

FUEL AND ADDITIVE TECHNOLOGIES

Aviation kerosene is produced primarily from the straight-run distillation of crude oil. The portion of crude oil available for aviation kerosene represents a trade-off against the demand for other petroleum products, such as gasoline and diesel fuel. Two advances in fuel production that could have a positive effect on the development of improved fuel formulations were discussed. First, extensive modernization and automation, with new technologies and more flexible processing, has increased the ability of U.S. refineries to produce fuels from a wider range of raw stock and to tailor the properties of fuels. Second, with the development of automated and technically sophisticated measurement equipment and techniques (e.g., chromatography, mass spectrometry, and infrared spectrometry), more rapid and detailed chemical analyses of fuel formulations can be made.

Many of the workshop participants believe that fuel additive packages are likely to include both high molecular weight polymers and surfactants. There are literally thousands of potential polymer and surfactant additives, many of which either did not exist fifteen years ago or have not been investigated as fuel additives. Along with improved fire performance, fire-safe fuel additive packages will have to be nonreactive with the base kerosene as well as with additives presently used during fuel handling, storage, and service in aircraft tanks and components.

Advances in the understanding of polymer rheology complement the technical advances in the petroleum industry described above. Workshop participants discussed several advances that could help in the development of fuel additives, including:

• molecular modeling techniques that allow for designing and tailoring polymer and surfactant systems with specified characteristics at the microstructural level

• advances in measuring extensional viscosity that can identify composition effects for very dilute polymer systems and for solutions of association polymers

• techniques for making dynamic rheological measurements that can characterize polymer-solvent interaction at the microstructural level

• rheological modeling methods that provide means for relating non-Newtonian flow behavior to the microstructure of polymer systems

In principle, the capability exists of employing rheological methods and molecular design techniques to develop polymer and surfactant additives that would thermally self-degrade prior to injection into the combustor rather than requiring a mechanical degrader to restore acceptable fuel flow characteristics. The availability of large numbers of polymer compounds and surfactants coupled with the complexity of kerosene composition, make for a huge number of potential fire-safe fuel packages. New analytical methods, molecular computation, and measurement technology have made it feasible to select likely candidate packages for investigation, although many of these techniques would need to be specifically tailored to the complex rheology expected to be present in antimisting kerosene. Several workshop participants suggested that, once fuel performance requirements have been identified, an additive package could be either selected or designed. However, the full power of state-of-the-art molecular design and analysis capabilities cannot be effectively used until safety fuel performance requirements have been identified and appropriate small scale flammability screening tests to evaluate additive packages have been developed.

AIRCRAFT FUEL SYSTEM REQUIREMENTS

Aircraft fuel system requirements are based on experience, science, practicality, and engineering judgment. Fuel system requirements include the fuel properties needed for safe and efficient operation of the aircraft, along with the characteristics of aircraft Systems that enable the use of fuel with specified properties. Fuel performance specifications have evolved in the areas of combustion performance, fuel thermal behavior, flow characteristics, and materials compatibility.

The workshop participants discussed several fire-safe design features already used in aircraft systems:

• preventing the ignition of fuel vapor by electrical sources using several methods including electrical bonding of components to the airframe to prevent discharges of static electricity and discharges induced by electromagnetic-fields, the use of explosion-proof electromechanical equipment, the use of low power fuel tank capacity and sensor circuits, locating fuel vent ports from aircraft surge tanks inboard of the lightning strike zone, and designing the tank structure to prevent internal sparking during a lightning strike

• the strategic placement of fuel drain masts

• shrouded fuel lines within the aircraft pressure hull

• flame arrestors in vent lines

• fireproof hose assemblies in engine nacelles

• separating fuel lines from electrical wiring

• ventilating compartments and bays where fuel vapor can accumulate

• preventing fuel leaks by fuel line stretch flexibility, fuel shutoff capability outside the engine fire zone, and duplicate wiring to the shutoff valve actuator

Military and civil aircraft vary widely in terms of age and technologies. Each engine model has been subjected to certification testing to demonstrate safety and performance with a particular fuel. The effects of changing individual fuel properties are largely unknown. However, because engines are developed, validated, and warranted based on the performance of a specified fuel and because changing fuel specifications requires extensive validation tests, several workshop participants pointed out that the impetus is to maintain the status quo. For example, the transition from JP-4 to JP-8 fuel by the Air Force took 20 years. Many of the workshop participants felt that neither producers nor users of aircraft would be likely to accept fuel modifications that require substantial changes to existing fuel systems without a total fleet systems analysis. Safety fuel additive packages would have to be shown to be nonreactive with the base kerosene, as well as additives presently used during fuel handling, storage, and service in aircraft tanks and components.

CHARACTERIZING FUEL FIRES

Although a great deal of research has been done on aircraft post-crash fires and ballistic ignition of aircraft dry bays and fuel tanks, understanding of the overall development of aircraft fuel fires is still conjectural. How the fuel is released, how it is dispersed, how it ignites, and how the fire spreads are still open questions. Finding ways to disrupt fire development requires a clear understanding of these processes. Workshop participants identified promising technical approaches to analyzing fire processes, but these approaches have not been validated for aircraft fuel fires.

In last 10 years, some impressive new tools for analyzing turbulent combustion, spray formation, and spray burning have been developed. Some workshop participants suggested that the application of these tools to aviation fuel release during a crash could have a significant impact on the characterization of post-crash fires. Recent advances that could lead to a cohesive engineering framework based on laminar diffusion flamelet concepts and classical turbulence models include:

• using universal state relationships to correlate fluid properties with the degree of mixing predicted by turbulence models

• the development of stochastic techniques for factoring turbulent fluctuations in temperature into the radiation transfer equations, rectifying any underestimation of radiation flux caused by the use of average thermal properties

• using locally-homogeneous flow approximations and state relationships to analyze salient features of turbulent burning sprays

• using stochastic separated flow methods to compute droplet life history in burning sprays

Barriers to development of numerical representations to model post-crash fire processes include:

• computational complexity resulting from the need to consider size scales over four orders of magnitude—from drop size to wing cross-section size—in modeling fuel atomization after fuel release

• incomplete understanding of fuel release and primary breakup processes

• incomplete understanding of soot characteristics, including size, distribution, and evolution in a crash fire

• incomplete characterization of the initial ignition mechanism

Workshop participants discussed a number of opportunities for advancing the fundamental understanding of fuel fires. First, the complexities associated with the aerodynamic flow field around a crashing aircraft moving in a direction other than along the fuselage center-line have not been investigated. Strong vorticity recirculating flows, and locally accelerated flows can significantly influence the breakup of fuel streams and subsequent fire behavior. Although recent studies have clarified the role of vorticity and turbulence in the primary breakup of liquid jets and sheets, the rate of these breakup processes cannot be predicted by any generalized method. Second, although it is known that surface tension limits the size of drops resulting from primary breakup processes and that viscosity limits the size of drops in secondary breakup processes, methods for making accurate predictions of secondary breakup depend on the rate processes involved, which are still not well understood. Third, to be applicable to fire-safe fuels, methods for analyzing turbulent combusting sprays need to be modified to accommodate non-Newtonian fluid properties. In particular, an analysis of fuel jet primary and secondary breakup would have to include the effects of time- and shear-rate-dependent viscosity, normal stresses, and elongational viscosity. Finally, applying methods for analyzing turbulent spray combustion, including some concepts like universal state relationships, directly to the analysis of the incendiary ignition of fuel in tanks could improve the understanding of these complex processes.

GENERAL CONCEPTS

The subject of fuel fire inerting and suppression emerged as an area of interest in both the invited papers and in workshop discussions. The workshop participants discussed five concepts of fire suppression as alternatives to fuel modification. The assumption was that controlling post-crash fires around the aircraft to increase the chance of the survival and rescue of passengers could be as effective as modifying the fuel. Fire inerting and fire suppression are predicated on sensing fire and effectively deploying countermeasures (and that the means of deployment remain effective after a crash). The concepts discussed included:

• Surface enhancement of halons. A surfactant or other substance would concentrate halon, or halon replacement, agents at the surface of a burning pool.

• Low volatility technology. A chemical compound would thermally decompose into one or more extinguishing agents when passing through a fire plume.

• Propellant generated solid aerosols. Solid generators would be burned to release agents, such as potassium compounds, that could extinguish the fire.

• Suppression gas generation. Gas generator technology used in automotive air bags could be used to generate gaseous suppression agents from reacting solids.

• Micro-encapsulation. Microcapsules, which would de grade in a fire, would release a liquid or gaseous extinguishing agent.

Some workshop participants warned that focusing exclusively on suppression in the area around a crashed aircraft could keep fire safety approaches from being applied to other fire scenarios, such as fuel tank ignition by incendiary projectiles. Others noted that focusing on ignition and flame spread would lead to emphasizing processes like fire spread on liquids, hot projectile boundary-layer ignition, and spray combustion. The concepts that would arise from this perspective include:

• Pressure sensitive fuel. This fuel would burn well in the engine combustor but very poorly when released into the atmosphere.

• Two component fuel. This fuel would have two separate components that burn poorly individually but that burn efficiently when blended and injected into the engine.

In addition, fuel tank inerting technologies offer ways to protect fuel tanks and dry bays from ignition by electrical sources and incendiary devices.

One of the concepts discussed at the workshop that could improve the chances of survival in the event of an external fuel fire is aircraft fire-hardening. Intumescent coatings and fire resistant aircraft insulation (e.g., carbon fiber and new technology foam insulation) can delay aircraft fuselage burn-through as a result of external fuel fires.

RESEARCH OPPORTUNITIES

Workshop participants discussed six primary areas of opportunity for long-term research to establish performance goals, evaluate potential fuel additives and modifications, and develop fundamental capabilities to model the complex phenomena that need to be considered in the development of fuels with improved fire safety. The primary areas include:

• evaluating the effectiveness of various strategies for improving fire safety (e.g., fuel safety additives, inerting, fire suppression) in various scenarios (e.g., incendiary devices, post-crash fires, electrical ignition)

• estimating costs and benefits of proposed fuel safety additive packages on fleet safety and operations

• characterizing the aerodynamics, fuel release and dispersion, ignition, and burning phenomena for a range of aircraft crash scenarios

• investigating the rheology and interfacial phenomena of polymer and surfactant systems in aviation kerosene solvents, as distinguished from the single component solvents commonly used in academic research

• investigating the operational effects of changes in fuel specification parameters individually and in combination

• extending methods for analyzing the atomization and combustion of fuel sprays to fuel systems with non- Newtonian flow characteristics

NOTES

¹Workshop papers included in Parts II, III, and IV will be cited by author only, e.g., (see Becher), to distinguish them from citations in the reference list, which are listed by author and year, e.g., (Blake, 1982).

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