There is an increasing need for aircraft fuel tank flammability reduction solutions, primarily for commercial aircraft to comply with fuel tank flammability reduction regulations. These regulations describe requirements to make commercial aircraft fuel tank flammability exposure equivalent to unheated conventional aluminium wing tanks, on a fleet-wide average basis.
Contemporary commercial aircraft incorporate fuel tank inerting systems or in-tank oxygen reduction methods to reduce the flammability exposure of their fuel tanks on a fleet-wide average basis. This resulted from regulations developed by the FAA, EASA and other regulatory authorities around the world to reduce fuel tank-related fires and explosions. The regulations generally require that aircraft fuel tanks have a flammability exposure that is equivalent to that of a conventional [unheated] metal wing fuel tank. Commercial aircraft are being produced with fuel tank flammability reduction methods, primarily featuring replacing the air above the fuel with inert gas produced using air separation technology on-board inerting systems. Although effective, such inerting systems are often larger than desired, use on-board resources such as compressed air, and are costly to operate over their life cycle. Such systems are also difficult to monitor their effectiveness, to assure an inert atmosphere in the space above the fuel. Initial regulations focused on reducing the flammability of fuselage-mounted fuel tanks that have adjacent heat sources that may heat the fuel and cause it to be flammable for more time than is acceptable for protection against fuel tank explosions. It is expected that many future aircraft platforms will have composite wing fuel tanks which reduces cooling of the fuel in the tanks while in flight compared to conventional metal tanks, due to the fact that the composite material generally has higher insulation properties. This often results in the requirement for all of the tanks on such future aircraft platforms [including some aircraft programs currently in development] requiring inerting of all fuel tanks and not merely the fuselage mounted fuel tanks. Such ‘all-tanks’ inerting systems are often large, have many air separation modules (ASMs), and use a lot of compressed air which results in energy being lost to the compressed air supply. The overall cost of operation of such systems is therefore often unacceptably high.
A typical flammability exposure of a conventional metal wing tank (green line) is shown in FIG. 1. This shows that the conventional metal wing fuel tank becomes flammable during the climb portion of the flight at about 10,000 feet, by entering the ‘Jet-A’ fuel type, flammability envelope. The wing tank then remains in the flammability zone until the fuel in the tank cools to approximately 70 degrees F. In comparison, the heated Center Wing Tank (CWT) heats up while the aircraft is operating on the ground, pre-flight (red line). It enters the Jet-A flammability zone at about 100 degrees F., while still on the ground. The CWT then remains in the flammability zone for most of the flight, except for the time spent to the right of the Upper Flammability Limit (UFL), which constitutes a larger percentage of the flight than the exposure of the unheated wing tank profile. Although FIG. 1 shows just one of the flight profiles depicted in typical regulatory rules, it shows that during such high ambient temperature flight profiles, a CWT has a higher flammability exposure than the unheated wing tank. Fuel tanks with reduced in-flight cooling properties (such as composite skin wing tanks) may also exhibit flammability exposure that is above that for a conventional metal wing tank, and can therefore similarly require a solution to reduce their flammability exposure.
On board inerting systems overcome the problem of the CWT having a higher flammability exposure than that of an unheated wing tank by making the otherwise flammable air/vapor space above the fuel inert, and therefore not flammable. Such inerting system solutions add significant weight and cost to the aircraft platform, and may significantly add to the operational costs of the aircraft due to reduced reliability and increased maintainability. Most of the regulations allow some time when the tank is not flammable, by design or by being inoperative. Aircraft fuel tanks other than conventional unheated metallic wing tanks must reduce the flammability exposure on a fleet-wide average basis to below the levels prescribed in these regulations.