Rajan Wilcox
Fuel tank explosions have been a concern since the TWA Flight 800 accident in July 1996, which was caused by the ignition of a flammable fuel/air mixture in the center wing fuel tank. Since then, there has been a significant emphasis on developing solutions to prevent such incidents. One notable 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. Additionally, self-sealing fuel tanks have been employed, particularly in aircraft, to prevent fuel leakage and ignition upon damage. These tanks are designed with layers of rubber and reinforcing fabric that absorb fuel and seal punctures.
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What You'll Learn
Self-sealing fuel tanks
Fuel tanks are protected from explosions on planes through various methods, one of which is the use of self-sealing fuel tanks (SSFTs). These tanks are typically used in aircraft fuel tanks or fuel bladders and prevent fuel leakage and ignition when damaged.
Early attempts at creating self-sealing fuel tanks involved covering metal tanks with materials that expanded when pierced. The Henderson Safety Tank Company provided crash-proof self-sealing fuel and oil tanks, which were standard on the Miles Master trainer aircraft. German aircraft designers used layers of rubber over leather hide with a treated fibre inner surface for the Junkers Ju 88.
During World War II, several companies in the United States, such as Firestone Tire and Rubber Company, Uniroyal, and Goodyear, developed and patented self-sealing fuel tank designs. These tanks were used in aircraft like the Vought F4U Corsair fighters and the Spitfire Mk IX.
While self-sealing fuel tanks have the advantage of preventing fuel leaks and increasing aircraft survivability, they also make the aircraft heavier, resulting in reduced manoeuvrability and operational range. Additionally, the sealing of bullet holes can clog fuel strainers or carburettor jets with dissolved rubber. Newer technologies, such as inert foam-filled tanks, aim to prevent detonation without the weight penalty of self-sealing tanks.
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Fuel tank inerting
An inerting system replaces the air with a gas that cannot support combustion, such as nitrogen. This process decreases the probability of combustion of flammable materials stored in a confined space. Three elements are required to initiate and sustain combustion in the ullage: an ignition source (heat), fuel, and oxygen. By replacing the oxygen with nitrogen, the risk of combustion is reduced.
The use of fuel tank inerting in commercial aircraft has been considered since the crash of TWA Flight 800 in 1996. A report by an FAA committee in 2001 stated that US airlines would need to spend $35 billion to retrofit their existing fleets with inerting systems. However, the weight, resource requirements, and dispatch reliability of military fuel tank inerting systems have indicated that they may not be practical for commercial aircraft.
Despite this, companies like Collins Aerospace have developed fuel tank inerting systems that improve aircraft safety by reducing oxygen in the fuel tank to prevent combustion. These systems use membrane gas separators to reduce the oxygen concentration of the air entering the fuel tank. Precise pressure, temperature, and oxygen concentration controls optimize performance over various flight phases.
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Federal Aviation Administration (FAA) directives
Following the TWA Flight 800 accident in July 1996, the Federal Aviation Administration (FAA) has issued numerous directives and regulations to enhance fuel tank safety and prevent explosions. Here are the key directives:
- Airworthiness Directives: The FAA has issued multiple Airworthiness Directives to address potential ignition sources in fuel tanks. These directives aim to correct and mitigate ignition sources to prevent fuel tank explosions.
- Special Federal Aviation Regulation (SFAR 88): The FAA enacted SFAR, which establishes comprehensive regulations for fuel tank safety. It includes requirements for eliminating or reducing exposure to flammable vapors and conducting research into methods for significant reduction of explosion risks.
- Fuel Tank Protection Task: The FAA has initiated a Fuel Tank Protection Task, which involves two research areas working together to find practical solutions. This task force examines methods to reduce or eliminate fuel tank flammability and develop regulatory text, considering cost and benefit analyses.
- Fuel Flammability and Inerting Research: The FAA is actively engaged in Fuel Flammability Research to understand the effects of various parameters on flammable vapors within fuel tank ullage. Simultaneously, Fuel Tank Inerting Research focuses on validating inerting requirements and designing economical and practical methods to render fuel tanks inert, reducing the risk of explosions.
- Performance-Based Requirements: The FAA has amended regulations to require operators and manufacturers of transport category airplanes to implement measures that reduce the chances of fuel tank explosions. These requirements set acceptable flammability exposure values and mandate the installation of ignition mitigation measures.
- Safety Review and Standards: The FAA has established clear standards and expectations through Special Federal Aviation Regulations (SFAR). These regulations require design approval holders to conduct comprehensive safety reviews of fuel tank systems, demonstrating that fuel tank explosions will not occur in approved designs. The reviews must comply with new and existing standards outlined by the FAA.
These directives and regulations demonstrate the FAA's commitment to enhancing fuel tank safety and reducing the risk of explosions in commercial aircraft. By implementing these measures, the FAA aims to protect passengers, crew, and the aviation industry from the catastrophic consequences of fuel tank explosions.
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Ground-based inerting (GBI)
Fuel tank explosions have been a significant concern since the TWA Flight 800 accident in July 1996, which was caused by the ignition of a flammable fuel/air mixture in the center wing fuel tank (CWT). To address this issue, the Federal Aviation Administration (FAA) has been working on various solutions, including ground-based inerting (GBI).
The Aviation Rulemaking Advisory Committee (ARAC) formed a Fuel Tank Inerting Working Group to evaluate the effectiveness of GBI in reducing the flammability of CWTs. They conducted a series of ground and flight tests, including on a Boeing 737-800, to measure the oxygen concentration in the fuel tank under different conditions. The results showed that GBI could keep the oxygen concentration in the CWT below 10- to 12-percent, which is the range for inert conditions, for extended periods under quiescent conditions. However, certain wind and flight conditions could lead to cross-venting, resulting in significant increases in oxygen levels.
To address this challenge, modifications to the vent system were made, enhancing the benefits of GBI even at low to moderate fuel loads. Further testing on a Boeing 747 SP test article validated the effectiveness of GBI in inerting large and geometrically complex spaces. This involved instrumenting the CWT with thermocouples and sample gas probes to measure inerting and flammability parameters. The tests also identified potential issues with NEA mixing in the GBI process and proposed solutions.
Overall, GBI shows promise as a cost-effective method for reducing the flammability of fuel tanks in commercial transport airplanes. However, additional research and development are needed to address the challenges associated with maintaining inert conditions during various flight and ground conditions.
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Flexible fuel bladders
The use of flexible fuel bladders offers several advantages. Firstly, they are customizable and can be designed to fit the specific shape and size requirements of an aircraft. Secondly, they are durable and made from materials that provide mechanical strength and compatibility with the intended fuel contents. Thirdly, they are cost-effective, as they are typically less expensive than traditional metal fuel tanks.
It is important to note that all fuel bladders should be housed in secondary containment (bunding) to minimize the risk of leakage and contain a potential spill. The EPA has set clear guidelines for the use of secondary containment and imposes fines for the discharge of fuel into the environment.
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Frequently asked questions
Fuel tank explosions occur due to the ignition of flammable fuel/air vapors in the tank. A spark is all it takes to set off an explosion.
After the TWA flight 800 accident in July 1996, the Federal Aviation Administration (FAA) has issued directives and regulations to address potential ignition sources. This includes fuel tank inerting, which involves replacing flammable gas above the fuel tank with a non-flammable atmosphere. Additionally, self-sealing fuel tanks are used to prevent fuel leaks and ignition after tank damage.
Self-sealing fuel tanks have layers of rubber and reinforcing fabric, including vulcanized rubber and untreated natural rubber. When the tank is punctured, the untreated layer swells as it absorbs fuel, sealing the puncture.
Yes, some challenges include the weight and resource requirements of certain safety systems, which can impact the plane's performance and practicality. For example, military fuel tank inerting systems may not be practical for commercial transport planes due to these factors.
