Katerina Swanson
Fuel tank explosions are a serious safety concern in aircraft, as evidenced by the TWA flight 800 accident in July 1996, which was caused by the explosion of the center wing fuel tank. To prevent such incidents, various methods have been developed to protect fuel tanks from explosions. One approach is the use of self-sealing fuel tanks, which are designed to prevent fuel leaks and ignition after damage. These tanks have layers of rubber and reinforcing fabric that absorb fuel and swell to seal punctures. Another method is fuel tank inerting, commonly used by the military, which involves replacing the flammable gas space above the fuel tank with a non-flammable atmosphere. While this method has its limitations in terms of weight and resource requirements, it offers a practical solution to reduce the risk of fuel tank explosions.
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What You'll Learn
Self-sealing fuel tanks
The use of self-sealing fuel tanks (SSFT) is a method of protecting aircraft fuel tanks from explosions. SSFTs are designed to prevent fuel tanks from leaking and igniting after sustaining damage.
The development of self-sealing fuel tanks began as early as World War I, with companies such as Firestone Tire and Rubber Company, and Goodyear, working on the technology during World War II. Early designs included metal tanks covered inside or outside by an expandable material, or a regular tank covered by a layer of rubber and a layer of doped or painted canvas. However, these early attempts were often ineffective against larger-caliber gunfire.
By the time self-sealing fuel tanks were used in World War II, they had evolved significantly from their World War I counterparts. The U.S. Navy's fuel tanks during the war, for example, could withstand .50-inch (12.7 mm) bullets and, occasionally, 20 mm (0.79-inch) autocannon shells. While the use of self-sealing fuel tanks reduced fuel capacity and made aircraft heavier and less manoeuvrable, it also greatly improved their chances of surviving damage to their fuel tanks.
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Fuel tank inerting
The ullage contains a mixture of evaporated fuel and air, which can ignite under certain conditions. By replacing the air with a gas that cannot support combustion, such as nitrogen, the risk of explosion is significantly reduced. This technique has been commonly used by the military for combat aircraft and was first proposed for use in passenger aircraft in the early 1960s by Cleve Kimmel. However, due to the weight and resource requirements, it was not considered practical for commercial transport aircraft until the crash of TWA Flight 800 in 1996.
Following the TWA Flight 800 accident, the National Transportation Safety Board (NTSB) determined that the probable cause was an explosion of the center wing fuel tank (CWT) due to the ignition of a flammable fuel/air mixture. This led to increased emphasis on fuel tank safety and the development of inerting systems for commercial aircraft. The NTSB identified the "Elimination of Explosive Mixture in Fuel Tanks in Transport Category Aircraft" as the top priority in its Most Wanted List in 1997.
To address this issue, the Federal Aviation Administration (FAA) has worked on developing inerting systems that can reduce the oxygen concentration in the ullage to below the threshold required for combustion. One such system, based on membrane gas separation technology, uses a hollow fiber membrane to separate supplied air into nitrogen-enriched air (NEA) and oxygen-enriched air (OEA). This technology has been successfully tested by Boeing and is used for generating oxygen-enriched air for medical purposes.
In addition to aircraft, inerting systems have also been used in other industries, such as oil tankers, where the empty space above the oil cargo is filled with inert gas to prevent the explosion of hydrocarbon vapors. The use of inerting systems has been required on oil tankers since the SOLAS regulations of 1974, and they are also applied to other types of cargo, such as bulk chemicals, where the inerting gas must be compatible with the chemicals used.
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Federal Aviation Administration (FAA) regulations
The Federal Aviation Administration (FAA) has implemented several regulations and safety measures to protect aircraft fuel tanks from explosions. These regulations are designed to reduce the risk of catastrophic fuel tank explosions and enhance aviation safety. Here are the key components of FAA regulations regarding fuel tank protection:
- Fuel Tank Flammability Reduction: The FAA has issued regulations aimed at reducing the flammability of fuel tanks in transport category airplanes. This includes the establishment of acceptable flammability exposure values and the requirement to install ignition mitigation means in affected fuel tanks. The regulations provide performance-based requirements, allowing manufacturers and operators to choose from a variety of commercially feasible methods to achieve these safety objectives.
- Ignition Source Prevention: The FAA has enacted comprehensive regulations to correct potential ignition sources in fuel tanks. This includes the issuance of Airworthiness Directives and the implementation of SFAR 88, which addresses the elimination or reduction of exposure to flammable vapors in transport airplanes. The regulations also emphasize the importance of considering human error during aircraft design to mitigate potential ignition sources introduced through human actions.
- Fuel Tank Inerting: The FAA has evaluated and considered the implementation of fuel tank inerting systems to reduce flammability. While a final rule has not been established, the Aviation Rulemaking Advisory Committee (ARAC) has studied fuel tank inerting methods and their cost-benefit analysis. Ground-based inerting (GBI) has been suggested as a potentially cost-effective method for fuel tank flammability reduction.
- Fuel System Design and Performance Standards: FAA regulations set forth design and performance standards for fuel systems in transport category airplanes. These standards aim to prevent catastrophic fuel vapor ignition caused by lightning and other ignition sources. The regulations include critical design configuration control limitations (CDCCLs) to protect design features that prevent ignition and fuel tank explosions. The regulations also specify requirements for fuel strainers, vent systems, and vapor vents to mitigate the risk of explosions.
- Lightning Protection: The FAA has amended airworthiness regulations regarding lightning protection of fuel systems. The amendments establish a single performance-based standard for both fuel tank structure and fuel tank systems, allowing applicants to choose how to achieve the required level of safety. These changes align with the current understanding of lightning-related risks and fuel tank flammability exposure, addressing issues of inconsistency and impracticality in previous regulations.
- Safety Philosophy and Accident Prevention: The FAA's safety philosophy emphasizes addressing aviation safety threats whenever practicable solutions are found. The regulations aim to reduce the chances of fuel tank explosions, which pose catastrophic risks to aviation safety. The FAA also considers the potential impact of in-flight fuel tank explosions on the public perception of terrorism, which could have substantial adverse effects on the aviation industry.
These Federal Aviation Administration (FAA) regulations outline a comprehensive approach to protecting aircraft fuel tanks from explosions. By implementing safety measures, performance standards, and ignition source prevention, the FAA aims to mitigate the risk of fuel tank explosions and enhance the overall safety of air transportation.
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Fuel tank design
Fuel Tank Inerting:
This process involves replacing the potentially flammable gas space above the fuel tank (known as ullage) with a non-flammable atmosphere. Military aircraft commonly use fuel tank inerting, but it has been deemed impractical for commercial transport airplanes due to weight, resource requirements, and dispatch reliability concerns. However, a variation called "nitrogen inerting" increases the critical temperature of an explosive mixture, preventing spontaneous explosions when sparked.
Fuel Tank Liners:
Liners are designed to prevent fuel leaks from punctured tanks and maintain tank integrity during low-altitude crashes or mishaps. The effectiveness of a liner is tested through a "drop test," where the liner is filled with fuel and dropped from a height of about 50 feet.
Ground-Based Inerting (GBI):
A cost-effective method suggested by the Aviation Rulemaking Advisory Committee (ARAC) is ground-based inerting. This method aims to reduce the flammability of fuel tanks in commercial transport airplanes.
Fuel Tank Protection Task:
The Federal Aviation Administration (FAA) has initiated a Fuel Tank Protection Task with two research areas working together to find practical solutions. This includes examining fuel tank inerting methods and developing regulatory text to determine the cost and benefit of proposed rule changes.
Fuel Flammability Research:
This research area examines the effects of various parameters on flammable vapors within a fuel tank's ullage. It aims to define these effects and contribute to the development of practical methods to reduce flammability.
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Aviation Rulemaking Advisory Committee (ARAC)
The Aviation Rulemaking Advisory Committee (ARAC) was formed in response to the TWA Flight 800 accident in July 1996, caused by the ignition of a flammable fuel/air mixture in the center wing fuel tank (CWT). The purpose of the ARAC was to determine methods to mitigate the hazards posed by flammable vapors in fuel tanks and evaluate proposed rules to reduce the flammability of commercial transport fuel tanks.
The 1998 ARAC report concluded that fuel tank inerting, a process of replacing flammable gas space above the fuel tank with a non-flammable atmosphere, was not cost-effective. However, the FAA convened a second ARAC committee to further explore ways to inert fuel tanks. This committee acknowledged the challenges of applying military fuel tank inerting systems to transport airplanes due to weight, resource requirements, and dispatch reliability.
The ARAC Fuel Tank Inerting Harmonization Working Group initially assumed a 75% effectiveness rate in reducing fuel tank flammability but later amended this range to 25-75% due to uncertainty in predicting effectiveness. Despite some disagreements on the effectiveness of proposed solutions, the ARAC's efforts reflect a commitment to enhancing fuel tank safety and mitigating the risks associated with fuel tank explosions.
The work of the Aviation Rulemaking Advisory Committee (ARAC) is a testament to the aviation industry's proactive approach to safety. By convening experts and conducting thorough research, the ARAC plays a crucial role in identifying solutions to mitigate the hazards associated with fuel tank explosions. Their efforts have led to a better understanding of fuel tank flammability and the development of regulatory measures to enhance the safety of air transportation for passengers, crew, and the industry as a whole.
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Frequently asked questions
There are a few methods to protect a fuel tank from exploding in a plane, including:
- Self-sealing fuel tanks, which prevent fuel leaks and ignition after the tank has been damaged.
- Fuel tank inerting, a process of replacing potentially flammable gas space above the fuel tank with a non-flammable atmosphere.
A self-sealing fuel tank is a type of fuel tank that is designed to prevent fuel leaks and ignition after the tank has been damaged. It typically has layers of rubber and reinforcing fabric, including one layer of vulcanized rubber and another of untreated natural rubber, which can absorb fuel when it comes into contact with it.
When a self-sealing fuel tank is punctured, the fuel seeps into the layers of rubber, causing the untreated layer to swell and seal the puncture. This prevents fuel from leaking out and reduces the risk of ignition.
No, self-sealing fuel tanks tend to have lower capacity than non-sealed tanks, and they make the aircraft heavier and less maneuverable. However, aircraft with self-sealing fuel tanks can withstand much more damage than those with conventional fuel tanks. Most jet fighters and all U.S. military rotary wing aircraft use some type of self-sealing tank.
Besides self-sealing fuel tanks and fuel tank inerting, another method to prevent fuel tank explosions is to eliminate or reduce potential ignition sources. This can include correcting potential ignition sources through Airworthiness Directives and comprehensive regulations.
