Fire safety is an overriding concern in the design and operation of a commercial airliner, primarily because of the potential large loss of life in a single accident. Therefore, the Federal Aviation Administration (FAA) has maintained an extensive R&D program to improve aircraft fire safety. The research is driven by accidents, new airplane designs and new technology.

A previous article in Fire Protection Engineering summarized 20 years of R&D to improve aircraft fire safety and the airliner fire safety improvements derived from this research.1 Research conducted in the last decade has produced additional aircraft fire safety improvements.2 This article summarizes major improvements implemented over the last decade for each research driver, focusing on thermal acoustic insulation, fuel tank explosions, composite aircraft, and lithium-ion batteries.

Figure 1. Hidden In-Flight Fire Incident Involving Thermal Acoustic Insulation


Insulation blankets comprised of fiberglass batting, encapsulated within a plastic film, line the entire aircraft fuselage shell to attenuate noise and provide thermal insulation for passenger comfort. During a hidden, in-flight fire, the fire resistance of the insulation is important because it is often the first item ignited and the predominant hidden material (Figure 1). Tests showed that the FAA-required, vertical, Bunsen burner flammability test produced marginal pass/fail results for the insulation film on the fatal MD-11 inflight fire aircraft, a metalized polyethyleneteraphalate (PET).3

Also, insulation may be a beneficial factor during a post-crash fire if modified to act as a barrier against fire penetration through the fuselage by an external fuel fire. Delaying fuselage fire penetration gives passengers more time to escape. Full-scale tests showed that improved insulation materials and barriers would substantially delay fire penetration.4

Figure 2. FAA Next Generation Burner for Insulation Fuel Fire Burn-through Resistance Evaluation

In-Flight Fire Ignition Resistance
To examine the behavior of different types of insulation blankets, a series of large-scale fire tests was conducted in an open-ended mock-up of the attic area above the passenger cabin ceiling. In a confined attic space, ignition and flame propagation may occur because of radiant heat feedback and containment of melted film near the ignition source. In general, when subjected to a relatively severe ignition source, the PET films were the most flammable, but more fire-resistant films prevented flame propagation, including polyvinyl fluoride (PVF) and polyimide (PI).5

The next step was to develop an improved flammability test method with pass/fail criteria that would identify materials capable of resisting a severe ignition source. It was found that the radiant panel test standard for flooring materials6 gave a good correlation with the large-scale fire test data. The criterion adopted was, essentially, that the specimen did not ignite, which is specified by not allowing any flaming beyond a 2-inch (50 mm) length from the point of flame application, and no continued flaming after removal of the pilot flame.

Electrical arc testing was an important part of the insulation hazard assessment due to the reported incidents involving flame spread on thermal/acoustical insulation blankets caused by electrical arcing. The tests showed that the metallized PET blankets consistently ignited with significant flame spread. In contrast, the polyimide and PVF blankets did not ignite, and the plain PET blankets exhibited minimal flame spread and self-extinguished.7

These findings prompted the FAA to issue Airworthiness Directives (ADs), requiring the replacement of metallized PET insulation in more than 700 aircraft.8,9 The FAA also improved the Federal Aviation Regulations by requiring the radiant panel test method and criteria for insulation, replacing the Bunsen burner test method.10

After Sept. 2, 2005, any large transport aircraft manufactured in the United States was required to be lined with insulation compliant with the radiant panel test criteria. In addition, a radiant panel test methodology was developed to evaluate installation methods found to contribute significantly to insulation flammability.11 Lastly, another AD was adopted to replace insulation blankets made of PET called AN-26 because of its vulnerability to ignition and fire spread from an electrical arc.12

Post-crash Fire Burn-through Resistance
A new test method was developed to measure the penetration or burn-through resistance of thermal acoustic insulation during a post-crash fuel fire.13 Tests indicated that a variety of materials could provide the needed four minutes of burn-through protection, demonstrating the feasibility of this criterion. The four-minute value was based on an analysis of past accidents, which showed that evacuation times varied considerably—depending on many factors—but rarely exceeded five minutes, and accounting for the time to melt the aluminum fuselage. A replacement burner dubbed the "NexGen Burner” was also developed; it was made from readily available materials because the previous burner specified by FAA was no longer manufactured (Figure 2).14

FAA adopted a final rule that contained a new requirement for burn-through resistance insulation installed in commercial transport aircraft. Newly manufactured aircraft were required to have burn-through resistant insulation after Sept. 2, 2009.15,16 In addition, an advisory circular was developed and published that provides guidance on the "installation details and techniques that have been found to be acceptable to realize the full potential of materials having satisfactor y fire-resistant properties.”17


A major research program was conducted by the FAA to protect against fuel tank explosions. It was largely driven by TWA 800 and two other fatal center wing tank explosions; viz., 737, Manila, 1990 and 737, Bangkok, 2001. The three accidents had striking similarities, but in all three cases the ignition source that triggered the explosion could not be determined.

Figure 3. Schematic of On-Board Inert Gas Generation System (OBIGGS)

Fuel Tank Inerting
FAA developed a practical and effective fuel tank inerting system, or On-Board Inert Gas Generating System (OBIGGS), capable of providing protection throughout the entire aircraft flight and ground profile. Unlike the heavier and less reliable military OBIGGS in the C-5A and C-17, the FAA-developed system is simple, lightweight and practical, utilizing available engine bleed air to continuously provide nitrogenenriched air (NEA) to inert the center wing fuel tank (Figure 3).

The NEA is generated by the air separation modules, which contain hollow fiber membranes capable of separating nitrogen from oxygen in air. However, the critical feature of the FAA OBIGGS was a dual-flow capability – low flow rate/high NEA purity during ground, ascent and cruise conditions and high-flow rate/low NEA purity compatible at the required inerting concentration during descent. With a simple design, and few moving parts, the FAA OBIGGS was reliable, relatively lightweight (about 160 pounds [72 kg]) and inexpensive ($150,000-$200,000) for a 747.

Aircraft flight tests were needed to corroborate the predicted performance of the OBIGGS and demonstrate its operational capability. The OBIGGS was initially tested in an Airbus A32018 and later in the NASA B747 shuttle carrying airplane (SCA).19 A unique instrument developed by FAA, called an on-board oxygen analysis system (OBOAS), measured the oxygen concentration at eight CWT locations.20

The results from one A320 test are shown in Figure 4. During ground, ascent and cruise, at the low-flow NEA setting, the oxygen concentration continuously decreased. At the onset of descent, the NEA flow rate was set at the high setting. The oxygen concentration increased as air rushed into the CWT during descent; however, the higher NEA flow rate prevented the oxygen concentration from exceeding the limiting oxygen concentration (LOC) of 12%.

Figure 4. A320 Oxygen Concentration Histories in Center Wing

Limiting Oxygen Concentration
The Limiting Oxygen Concentration (LOC) is the minimum concentration of oxygen in air that will allow fuel vapor combustion. FAA tests in a simulated fuel tank determined that the LOC was 12% at sea level to 10,000 feet (3 km), and increased approximately linearly thereafter to 14.5% at 40,000 feet (12 km).21,22 The 12% value is fairly consistent with LOC values in the literature for the hydrocarbon constituents of jet fuel. It was an enabling factor in the development of a simple and cost-effective OBIGGS for commercial transport aircraft.

FAA Regulation to Prevent Fuel Tank Explosions
On July 21, 2008, FAA issued a final rule titled "Reduction of Fuel Tank Flammability in Transport Category Airplanes,”23 which was made possible by the FAA OBIGGS development. It was estimated that the final rule would prevent one or two catastrophic fuel tank explosions over a 35-year period. Five thousand aircraft in the U.S. fleet would be impacted by the rule at a compliance cost of more than $1 billion.


The new Boeing 787 is constructed of composite fuselage and wings in order to gain significant operational cost savings from lower weight, corrosion resistance and less maintenance due to increased fatigue strength. The composite material is comprised of multiple, alternately directed layers of epoxy-impregnated, continuous graphite fibers. Fire safety was a concern because epoxy resins are flammable.

During FAA certification of the B787, Boeing was required to demonstrate that the level of fire safety in the B787 was equivalent to a conventional (aluminum) aircraft. FAA conducted research and testing to characterize and understand the fire behavior of this type of composite structure and to support the certification process.

When heated, the epoxy resin vaporizes and burns, leaving behind an inert insulation layer of graphite fibers. This causes a reduction in internal heating as each subsequent ply of epoxy-graphite burns, and a reduction in the burning rate with time. Overall, the composite displayed superior fire burn-through resistance and relatively good fire resistance.24

Boeing proposed that the burn-through resistance of the B787 composite fuselage provided an equivalent level of safety with the insulation burn-through resistance regulation. To evaluate this proposal, which would negate the need for burn-through-resistant insulation in the B787, FAA developed a small-scale test to expose composite materials to a simulated post-crash fire and analyze gas emissions that could migrate into the cabin and impact survivability.25 Fullscale, post-crash fire tests helped develop scaling factors to use in conjunction with the small-scale test to predict cabin gas concentration levels.26 The full-scale fire tests again exhibited the superior burn-through resistance and low gas emissions of the carbon fiber composite when subjected to a severe jet fuel fire (Figure 5).

Figure 5. Burn-through Resistance of Composite Fuselage Skin Subjected to a Jet Fuel Fire for 5 Minutes

Boeing also had to demonstrate that the B787 provided protection against a hidden in-flight fire. Intermediate scale tests were required similar to those performed by FAA during development of the improved fire test method for thermal acoustic insulation. To obviate the need for these intermediate-scale tests in future certification programs, FAA developed a small-scale fire test method to measure the in-flight fire resistance of composite fuselage structure.

The flammability of fuel vapor inside a composite wing fuel tank was examined by FAA and compared with aluminum tanks.27 Fuel vapor concentration was measured in wing tanks made of both materials, under conditions simulating heating on the ground from the sun and in-flight air flow cooling in a wind tunnel. It was shown that composite wing fuel tanks are more flammable than their aluminum counterparts during solar heating, and that painted surfaces greatly impacted the heat-up for both types of tanks. However, rapid cooling and reduction in flammable vapors was observed in both tanks under simulated flight conditions.


Due to their high energy density and design, malfunctioning lithium batteries can experience thermal runaway, causing high surface temperatures, fire and even explosive-like hazards.

The incident that highlighted the dangers of lithium battery fires in aircraft occurred at Los Angeles International Airport in 1999. Two off-loaded pallets of lithium batteries from an incoming flight caught fire. It took airport firefighters about 25 minutes to extinguish the difficult fire. Since 1991, more than 44 air-transport-related battery fire incidents have occurred, mostly involving freighter aircraft.28

FAA conducted tests on the two main types of lithium batteries: primary or metal (non-rechargeable)29 and ion (rechargeable).30 With either type of battery, thermal runaway of a single battery in a typical cardboard shipping box resulted in thermal runaway and ignition of the remaining batteries in the box (Figure 6).

Figure 6. Thermal Runaway Propagation in Bulk-loaded Lithium Batteries.

However, the metal batteries were found to be far more hazardous. A metal battery fire involves burning lithium, which can be ejected in a molten state. It produces heavy smoke and overpressures, which would breach the cargo compartment liner, raising the likelihood of fire and smoke spreading to the cabin and cockpit.

Halon 1301, the fire extinguishing agent in passenger aircraft cargo compartment fire suppression systems, has no observable effect on a metal battery fire.29 Conversely, when an ion battery overheats, the flammable electrolyte vents and ignites in the presence of an ignition source. However, Halon 1301 extinguishes the electrolyte fire and prevents re-ignition at a concentration of 3%, which is the minimum concentration required to be maintained by a cargo compartment fire suppression system.30 Because of the inability of a halon fire suppression system to control a metal battery fire, an Interim Final Rule was issued that prohibits the bulk shipment of metal lithium batteries on passengercarrying aircraft.31

FAA has also conducted tests with shipping containers to ascertain their capabilities for withstanding a lithium battery fire.32 Typical cardboard shipping boxes will burn and be consumed by a shipment of either type of lithium batteries experiencing thermal runaway.

Available robust shipping containers, such as metal pails and drums recommended by the International Civil Aviation Organization (ICAO), were ineffective against metal battery fires because of the build-up of pressure, which caused the sealed lid to fail and expel the burning batteries. However, burning ion batteries were contained in a cardboard box designed to safely ship oxygen generators.33

A preliminary performance standard for a shipping container for lithium-ion batteries was developed, which was partly based on the oxygen shipment standard. The documented findings32 were the primary source of information contained in the FAA Safety Alert for Operators (SAFO) titled, "Risks in Transporting Lithium Batteries in Cargo by Aircraft.”34 FAA research strives to better understand and safeguard against the fire hazards of lithium battery cargo shipments.

Constantine Sarkos is with the Federal Aviation Administration.


  1. Sarkos, C., "An Overview of 20 Years of R&D to Improve Aircraft Fire Safety,” Fire Protection Engineering, Issue No. 5, Winter 2000, pp. 4-16.
  2. Sarkos, C.P., "Improvements in Aircraft Fire Safety Derived From FAA Research Over the Last Decade,” Report DOT/FAA/AR-TN 11/8, Federal Aviation Administration, Washington, 2011.
  3. Cahill, P., "Evaluation of Fire Test Methods for Thermal Acoustic Insulation,” Report DOT/FAA/AR-97/58, Federal Aviation Administration, Washington, 1997.
  4. Marker, T., "Full-Scale Test Evaluation of Aircraft Fuel Fire Burnthrough- Resistance Improvements,” Report DOT/FAA/AR-98/52, Federal Aviation Administration, Washington, 1999.
  5. Hill, R., Marker, T., Cahill, P., and Sarkos, C., "Development and Implementation of Improved Fire Test Criteria for Aircraft Thermal Acoustic Insulation in Civil Transport Aircraft,” NATO Applied Vehicle Technology Panel, Publication RTO-MP-103 on Fire Safety and Survivability, Brussels, 2002.
  6. ASTM E648, Standard Test Method for Critical Radiant Flux of Floor- Covering Systems Using a Radiant Heat Energy Source, American Society for Testing and Materials, West Conshohocken, PA, 1997.
  7. Cahill, P., "Flammability of Aircraft Insulation Blankets Subjected to Electrical Arc Ignition Sources,” Report DOT/FAA/AR-TN00/20, Federal Aviation Administration, Washington, 2000.
  8. "Airworthiness Directives; McDonnell Douglas Model DC-9-80 and MD- 90-30 Series Airplanes and Model MD-88 Airplanes; Final Rule,” Federal Register, Vol. 65, No. 103, May 26, 2000, pp. 34321-34341.
  9. "Airworthiness Directives; McDonnell Douglas Model DC-10-10F, DC-10-15, DC-10-30, DC-10-30F, and DC-10-40 Series Airplanes and Model MD-11 and 11F Series Airplanes; Final Rule,” Federal Register, Vol. 65, No. 103, May 26, 2000, pp. 34341-34360.
  10. "Improved Flammability Standards for Thermal/Acoustic Insulation Materials Used in Transport Category Airplanes,” Final Rule, DOT/FAA, Federal Register, Vol. 68, No. 147, July 31, 2003, p. 45046.
  11. Thermal/Acoustic Insulation Flame Propagation Test Method Details, Advisory Circular, AC No. 25.856-1, Federal Aviation Administration, Washington, June 24, 2005.
  12. "Airworthiness Directives; Boeing Model 727-200 and 727-200F Series Airplanes; 737-200, 737-200C, 737-300, and 737-400 Series Airplanes; 747-100, 747-100B, 747-100B SUD, 747-200B, 747-200C, 747-200F, 747-300, 747-400, 747SR, and 747SP Series Airplanes; 757-200, 757- 200CB, and 757-200PF Series Airplanes; and 767-200 and 767-300 Series Airplanes,” Final Rule, Federal Register, Volume 73, No. 218, Nov. 10, 2008, pp. 66497-66512.
  13. Marker, T., "Development of Improved Flammability Criteria for Aircraft Thermal Acoustic Insulation,” Report DOT/FAA/AR-99/44, Federal Aviation Administration, Washington, 2000.
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  16. Fire Penetration Resistance of Thermal/Acoustic Insulation Installed on Transport Category Airplanes, Final Rule, DOT/FAA, Federal Register, Vol. 72, No. 8, Jan. 12, 2007, p. 1438.
  17. Installation of Thermal/Acoustic Insulation for Burnthrough Protection, Advisory Circular, AC No. 25.856-2A, Federal Aviation Administration, Washington, July 29, 2008.
  18. Burns, M., Cavage, W., Hill, R., and Morrison, R., "Flight-Testing of the FAA Onboard Inert Gas Generating System on an Airbus A320,” Report DOT/ FAA/AR-03/58, Federal Aviation Administration, Washington, 2004.
  19. Burns, M., Cavage, W., Morrison, R., and Summer, S., "Evaluation of Fuel Tank Flammability and the FAA Inerting System on the NASA 747 SCA,” Report DOT/FAA/AR-04/41, Federal Aviation Administration, Washington, December 2004.
  20. Burns, M., and Cavage, W.M., "A Description and Analysis of the FAA Onboard Oxygen Analysis System,” Report DOT/FAA/AR-TN03/52, Federal Aviation Administration, Washington, 2003.
  21. Summer, S., "Limiting Oxygen Concentration Required to Inert Jet Fuel Vapors Existing at Reduced Fuel Tank Pressures,” Report DOT/FAA/AR-TN02/79, Federal Aviation Administration, Washington, 2003.
  22. Summer, S., "Limiting Oxygen Concentration Required to Inert Jet Fuel Vapors Existing at Reduced Fuel Tank Pressures – Final Phase,” Report DOT/FAA/ AR-04/8, Federal Aviation Administration, Washington, 2004.
  23. Reduction of Fuel Tank Flammability in Transport Category Airplanes, Final Rule, DOT/FAA, Federal Register, Vol. 73, No. 140, July 21, 2008, p. 42444.
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  26. Marker, T., and Speitel, L., "Evaluating the Decomposition Products Generated Inside an Intact Fuselage During a Simulated Postcrash Fuel Fire,” Report DOT/FAA/AR-09/58, Federal Aviation Administration, Washington, 2011.
  27. Summer, S. and Cavage, W.M., "A Comparison of Flammability Characteristics of Composite and Aluminum Fuel Tanks,” Report DOT/FAA/ AR-11/6, Federal Aviation Administration, Washington, 2011.
  28. Hazardous Materials: Transportation of Lithium Batteries, Notice of Proposed Rulemaking, DOT/PHMSA, Federal Register, Vol. 75, No. 6, Jan. 11, 2010, p. 1302.
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  33. Hazardous Material: Chemical Oxygen Generators, Final Rule, DOT/ PHMSA, Federal Register, Vol. 74, No. 237, Dec. 11, 2009, p. 65696.
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