Abstract:
A process method utilizing customized, specifically-shaped pieces of reticulated polyurethane foam (RPF) to fill an aircraft fuel tank or tank compartment to provide ignition mitigation and prevent explosion in the tank. The process involves inserting the shaped pieces of RPF through existing access ports into a fuel tank in order to fill the tank, excepting minimal planned void spaces. This process effects ignition mitigation by acting as an ignition blocker, mechanically interfering with the compression wave that precedes the flame front in an explosion, and changing the vaporous mixture above the fuel level (ullage) in the tank. The foam pieces are assembled and fitted together throughout the tank in a pattern that replicates the shape of the tank. After the foam insertion is complete, the fuel tank is filled with purging fluid, drained through a filter until no debris is found, and the new maximum fuel quantity is recalibrated.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The inventive concept disclosed relates generally to methods employed to prevent and/or minimize fuel ignition, fire, and/or explosion in the interior of aircraft fuel tanks. In particular, the inventive concept disclosed is concerned with specific methods of installing shaped, reticulated sheets or blocks, of foam to fill the internal space of fuel tanks of transport category aircraft. 
     2. Background 
     Since 1959, there have been sixteen documented incidents of fuel tank ignition events in aircraft. These fuel tank ignition events have resulted in 542 fatalities, 11 hull losses and 3 incidents causing substantial damage. The causes of the fuel tank ignition events were attributed as follows: 4 were caused by external wing fires, 4 by electrostatics, 3 by faulty fuel pumps or wiring, 2 by lightning, and 3 to unknown causes. 
     On Jul. 17, 1996, TWA Flight 800 sustained an in-flight break-up after taking off from Kennedy International Airport in New York, resulting in 230 fatalities. The National Transportation Safety Board (“NTSB”) conducted a lengthy investigation and determined that ignition of the flammable fuel/air mixture in a center wing fuel tank had occurred, causing an explosion that disintegrated the aircraft in flight. Although the exact ignition source could not be determined, the NTSB concluded that the most likely ignition source was a short circuit outside the center wing fuel tank that allowed excessive voltage to enter the tank through electrical wiring associated with the fuel quantity indication system (FQIS). 
     The NTSB announced their official findings regarding the TWA 800 accident at a public meeting held on Aug. 22 and 23, 2000 in Washington. D.C. Primarily as a consequence of TWA Flight 800, the Federal Aviation Administration (“FAA”) issued numerous airworthiness directives intended to reduce possible ignition sources and thereby the risk of another fuel tank explosion. On May 7, 2001, the FAA promulgated rulemaking to establish several new transport category airplane fuel tank safety requirements (66 Federal Registry 23086, May 7, 2001). The rulemaking, effective Jun. 6, 2001, included Amendment 21-78, Amendment 25-102 and Special Federal Aviation Regulation (“SFAR”) No. 88 entitled “Transport Airplane Fuel Tank System Design Review. Flammability Reduction and Maintenance Requirements.” SFAR No. 88 required that type certificate holders and supplemental type certificate holders conduct a revalidation of the fuel tank system designs on the existing fleet of transport category airplanes capable of carrying thirty (30) or more passengers or a payload of 7,500 pounds or more. 
     Legislation was enacted as 14 CFR §25.981 (Rule 25.981) and FAA Advisory Circulars AC 25.981-1B and 25.981-2 were issued to provide compliance guidance. Compliance with Rule 25.981 required each applicant to develop a failure analysis for the fuel tank installation to substantiate that ignition sources would not be present in the fuel tanks. The requirements of this section are in addition to the more general propulsion failure analyses requirements of 14 CFR 25.901 and 14 CFR 25.1309 that have been applied to propulsion installations. 
     14 CFR §25.981 (a) (3) defines three failure scenarios that must be addressed in order to show compliance with the rule (known as the “three phases” of compliance):
         (a) Each single failure, regardless of the probability of occurrence of the failure, must not cause an ignition source;   (b) Each single failure, regardless of the probability of occurrence, in combination with any latent failure condition not shown to be at least extremely remote (i.e., not shown to be extremely remote or extremely improbable), must not cause an ignition source; and   (c) All combinations of failures not shown to be extremely improbable must not cause an ignition source.       

     Compliance with 14 CFR §25.981 (Amendment 25-125) requires investigation of the airplane fuel tank system using analytical methodology and documentation currently used by the aviation industry to demonstrate compliance with 14 CFR 25.901 and 25.1309 but with consideration of unique requirements included in this amendment of this paragraph. 
     The Federal Aviation Administration (FAA) mandates forced certificate holders to develop and implement all design changes required to demonstrate that their aircraft meet the new ignition prevention requirements and to develop fuel tank maintenance and inspection instructions. Specifically, SFAR No. 88 contains six (6) requirements applicable to transport category aircraft: 1) determine the highest temperature allowed before ignition occurs; 2) demonstrate that this temperature is not achieved anywhere on the aircraft where ignition is possible; 3) demonstrate that ignition could not occur as a result of any single point failure; 4) Establish Critical Design Configuration Control Limitations (“CDCCL”), inspections or other procedures to prevent changes to the aircraft that would result in re-introduction or creation of ignition sources; 5) develop visible means to identify critical features of the aircraft where maintenance, repairs or alterations would affect areas or systems of possible ignition; and 6) design of fuel tanks must contain a means to minimize development of flammable vapors in fuel tanks or a means to mitigate the effects of ignition within fuel tanks. 
     Maintenance of ignition source prevention features is necessary for the continued operational safety of an airplane&#39;s fuel tank system. One of the primary functions of the fuel tank system is to deliver fuel in a safe manner. Preventing ignition sources is as important a function of the fuel system as the delivery and gauging of fuel. The failure of any ignition source prevention feature may not immediately result in an ignition event, but a failure warrants maintenance for continued airworthiness because the failure could eventually have a direct adverse effect on operational safety. 
     There have been various solutions proposed and implemented to comply with the mandated transport category aircraft fuel tank ignition mitigation requirements. Examples of compliance methods implemented include electronic solutions such as the installation of Transient Suppression Devices (“TSD”), Ground Fault Interrupters (“GFI”), and similar current limiting devices. These devices are deficient in that they retain possible failure rates that a “passive.” non-electronic solution could resolve. Clearly, there is a need for a simplified and reliable solution to make implementation of the SFAR No. 88 compliance feasible. A better solution would be a less expensive, passive solution that is applicable to commercial and private transport category aircraft. 
     BRIEF SUMMARY OF THE INVENTION 
     An apparatus and process method in accordance with the principles of the present invention includes the use of conformed, interrelated blocks of reticulated foam shaped to replicate the inside dimensions of an aircraft fuel tank. The foam is used as a passive means to mitigate ignition in aircraft auxiliary fuel tanks. The present inventive concept advantageously satisfies the aforementioned deficiencies by providing a method of accomplishing ignition mitigation in transport category aircraft fuel tanks using molded polyurethane safety foam in coordinated shapes to fill the fuel tanks. Polyurethane safety foam is a reticulated flexible foam composed of a skeletal matrix of lightweight interconnecting strands. For purposes of this disclosure, this particular material is referred to as “Reticulated Polyurethane Foam” (“RPF”). 
     Other advantages of the inventive concept include slosh attenuation, hydrodynamic ram attenuation, and foreign object debris barrier properties of RPF. As a surge or explosion mitigating agent, RPF attenuates the sloshing of fuel and, in some cases, eliminates the need for structural baffles within a tank. RPF further provides for a “smooth” sine wave motion of the fuel and reduces rapid redistribution of mass. 
     Hydrodynamic ram effect within a fuel tank or bladder cell is caused when a projectile impacts the exterior structure of the fuel tank. Ram force can be intensified when the tank is penetrated by a high explosive incendiary (“HEI”) delayed detonating-type projectile. The matrix-type structure of RPF absorbs a portion of the shock wave as a projectile penetrates a fuel tank. Attenuation of hydrodynamic ram minimizes damage to the fuel tank structure by reducing the overpressure of the shock wave and helps to orient the round to prevent tumbling. The reduction of fuel tank structural damage can effectively reduce fuel discharge through the projectile entrance and exit points. 
     The foreign object debris barrier capability of RPF materials is an inherent beneficial effect rather than a product specification requirement. The RPF used in this inventive concept is a natural filter, due to its inherent structure resembling a fibrous network. The finer the porosity of a material used as a fuel filter, the greater the entrapment of foreign objects and loose debris. The RFP material entraps loose debris within a fuel tank and minimizes the amount of debris entering an adjacent tank, the tank fuel lines, or the engine fuel system. 
     Explosion within a fuel tank containing kerosene-type fuels occurs as a result of the existence of flammable mixture in the ullage in combination with an ignition source. Examples of possible ignition sources include incendiary ammunition penetrating the fuel tank, static discharges, lightning strikes, switch refueling, and electrical shorts. Reticulated polyurethane foam (RPF) is in effect a three-dimensional fire screen, which minimizes the possibility of gasoline and kerosene-type (such as jet aircraft fuel) explosions under one or a combination of the following theories: the foam acts as a heat sink, (i.e., it removes energy from the combustion process by absorbing heat); it mechanically interferes with the compression wave that precedes the flame front in an explosion; and, the high surface-to-volume of reticulated polyurethane foam enables the strands to collect or coalesce the droplets of fuel, thus changing the vaporous mixture in the empty space above the fuel level (ullage), in the tank. Coalescing causes the vaporous mixture to become lean, which minimizes possible explosion. 
     The present inventive concept advantageously allows for greatly increased effectiveness in preventing the hazardous ignition of fuel within aircraft auxiliary tanks (satisfying all three phase requirements of 14 CFR §25.981 compliance), is passive and therefore far less likely to experience a system failure, and available at a cost of implementation far less than that of alternative electronic solutions. 
     Experience in the aviation industry has shown that any fuel tank can be filled to maximum of about 85% capacity (not including a fuel swell of 12%) with reticulated polyurethane foam blocks, not including any planned voids. The foam blocks should be kept clear of tank components such as the fuel inlets, fuel sensors, float switches, and tank vents. 
     Other embodiments of an apparatus and method in accordance with the above principles of the inventive concept may include alternative or optional additional aspects. One such aspect would be use of foam of sufficient porosity and ignition mitigation properties composed of a synthetic or naturally occurring material other than polyurethane. 
     Another aspect of the present invention is the use of block foam material in an interrelated, geometrical shape different from those shapes depicted in  FIG. 4 . 
     Another aspect of the present invention is to use alternate access ports of the aircraft auxiliary fuel tank to insert the foam blocks and position them in a distinct pattern to minimize any voids along tank walls. 
     Another aspect of the present inventive concept is the utilization of a “fully packed” design concept. A fully packed system is defined as one where all potential fuel tank ullage is filled with reticulated polyurethane foam with cutouts for components only. This system is most desirable where minimal or no tank over-pressure can be tolerated. 
     Another aspect of the present inventive concept is the utilization of a grossly voided design concept. A grossly voided system is defined as one where the fuel tank contains strategically positioned reticulated foam for explosion suppression. This system provides for minimal weight penalty and fuel retention, and is best suited for a fuel system that can withstand substantial overpressures. 
     These and various other advantages and features of novelty which characterize the invention are pointed out in the accompanying descriptive matter and drawings which form a further part hereof. For a better understanding of the invention, its advantages and the objects obtained by its use reference should be made to the drawings in which are illustrated and described specific examples of an apparatus and method in accordance with the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE VIEWS OF THE DRAWINGS 
         FIG. 1  illustrates a general overall fuselage-cutaway view of a Boeing® 737 jet aircraft, further showing the location of the center wing fuel tank. 
         FIG. 2  shows the location of the center wing tank of the Boeing® 737 aircraft, as would be seen looking through the bottom of the fuselage, and further, a tank access panel utilized by maintenance personnel for ground servicing. 
         FIG. 3  depicts a close-up view of the access panel to the center wing tank shown in  FIG. 2 . 
         FIG. 3(A)  is an illustration of the manner in which the access panel is removed from the exterior of the center wing tank 
         FIG. 4  shows a cutaway view of the structural members of the center wing tank, in accordance with section line  4 - 4  of  FIG. 1 . 
         FIG. 5  illustrates the profile of a single, specifically-shaped RPF block which has been precisely cut to fit, in a vertical orientation, the exact contour of one section of the center wing tank. 
         FIG. 5(A)  presents a side view of the RPF block of  FIG. 5 . 
         FIG. 6  presents a diagram of the aggregate of a plurality of sequentially-arranged, pre-cut RPF blocks to be installed adjacent to one another, thereby conforming to the contour of the forward compartment of the center wing tank. 
         FIG. 7  is a stylized rendering of the center wing tank, further showing the packing arrangement of a plurality of RPF blocks properly installed in the forward compartment of the center wing tank. 
         FIG. 8  depicts the initiation of installation of RPF blocks on the right side of the aft compartment. 
         FIG. 8A  shows the initiation of installation of RPF blocks on the left side of the aft compartment, the right side of the compartment having been completed 
         FIG. 8B  presents depicts the beginning of installation of RPF blocks in the middle section of the aft compartment. 
         FIG. 8C  depicts the completion of installation of RPF blocks in the aft compartment. 
         FIG. 9  illustrates a diagram of the resultant installation of all RPF blocks in the forward, center, and aft compartments of the center wing tank of a typical Boeing® 737 aircraft. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present inventive concept includes the use of shaped, interrelated, sequential blocks of reticulated polyurethane foam (RPF) blocks to fill specific fuel tank(s) of a transport category aircraft. The objects, features, and advantages of the concept presented in this application are more readily understood when referring to the accompanying drawings. The drawings, totaling nine figures, show the basic components and functions of embodiments and/or the proper method steps. In the several figures, like reference numbers are used in each figure to correspond to the same component as may be depicted in other figures. 
     For illustrative purposes only, and not by way of limitation, the methods and systems described in this disclosure apply primarily to Boeing® 737 series aircraft. This detailed description section is merely exemplary in nature and is not intended to limit the methods and uses shown in this inventive concept. There is no intention for the applicant to be bound or constrained by any expressed or implied theory(ies) set forth in the relevant technical fields, background, brief summary, or the present detailed description of the inventive concept as it relates to Boeing® 737 aircraft. Further, there is no intent to confine the inventive method disclosed to one particular make, model, or series of aircraft, or particular configurations of aircraft fuel tanks. 
     By utilizing the disclosed method of installing RPF blocks  50  to fill one or more fuel tanks of an aircraft, the aircraft operator prevents or minimizes the potentially damaging or catastrophic effects of fuel ignition, fire, and/or explosion. For ease of explanation and for illustrative purposes, the disclosed method is described with regard to installation of the RPF blocks into the center wing tank  112  of a Boeing® 737 aircraft  110 . 
     The discussion of the present inventive concept will be initiated with  FIG. 1 , which illustrates an overall view of a Boeing® 737 jet aircraft  110 .  FIG. 1  also depicts the fuselage  111 , and the center wing tank  112 , which is encircled. As in most similar jet aircraft, the predominance of the fuel load is generally carried in fuel tanks constructed within the left wing  104  and right wing  105 , with equal quantities in each wing  104 ,  105 . The wing-loaded fuel serves to add a counter-balancing weight to offset the upward wing structural bend due to the aerodynamic lift force generated by the wings  104 ,  105  when in flight. However, for substantially increased range, the aircraft  110  may be loaded with additional fuel in the center wing tank  112  and other internal tanks, if available. 
       FIG. 2  is a stylized rendering looking upward at the undersurface of the Boeing® 737 fuselage  111 , further revealing the location of the center wing tank  112 . Also shown in  FIG. 2  is a center wing tank access panel  20  utilized by maintenance personnel during ground servicing of this particular model and series of the Boeing® 737. 
       FIG. 3  depicts an expanded view of the access panel  20  shown in  FIG. 2 .  FIG. 3(A)  is an illustration of the manner in which the access panel  20  is removed from the exterior of the center wing tank  112 , further showing a clamp ring  21 , one of a plurality of washers  22 , and one of a plurality of bolts  23 , the washers  22  and bolts  23  utilized for insertion through apertures  24  for fastening the clamp ring  21  onto a lower center wing panel  26 . 
       FIG. 4  illustrates a stand-alone sectional view of the center wing tank  112 , in accordance with section line  4 - 4  of  FIG. 1 . The structural integrity of the center wing tank  112 , as positioned within the fuselage  111 , substantially determines the internal contour of the center wing tank  112 . The aircraft  110  structural members shown in  FIG. 4  include the front spar  120 , the rear spar  121 , the floor beams  122 , and the tank ceiling  127  abutting the aircraft floor beams  122 . As shown in  FIG. 4 , the center wing tank  112  comprises a forward compartment  131 , a center compartment  132 , and an aft compartment  133 . The three compartments  131 ,  132 ,  133  are separated by spanwise beam #2  124  and spanwise beam #1  125 . In each of the three compartments  131 ,  132 ,  133 , the fuel tank floor  128  comprises lower stiffeners  128  and the fuel tank ceiling  127  comprises upper stiffeners  129 , which give additional rigidity to the tank compartments  131 ,  132 ,  133 . 
     Before continuing with the immediate discussion of installation of the RPF aggregate blocks  50 , explanation will be given of the methodology and process of measuring, sizing, and cutting the RPF blocks  50 . Shown in  FIG. 5  there is illustrated, by way of example, a forward compartment RPF block  51 . 
       FIG. 5  shows a profile view of a forward compartment RPF block  51 . This forward compartment RPF block  51  is designated as such due to the fact that the contour of its outer perimeter corresponds to the location of a specific, vertically-oriented cross-section of the forward compartment  131  of the center wing tank  112 . In examining  FIG. 5 , the topmost section shows four upper cutouts  52  which provide clearance for the upper stiffeners  129  of the forward compartment  131  as shown in  FIG. 4 . The left edge  55  of RPF block  51  is fabricated so as to correspond to the forward tank wall  123 , and further shows an arcuate cutout  53  which provides clearance for a tank fuel pump, or other tank component. The bottommost section of the forward compartment RPF block  51  illustrates three cutouts  54  which allow clearance for the three lower stiffeners  130  of the forward compartment  131 , as shown in  FIG. 4 . 
     The right edge  56  of the forward compartment RPF block  51  corresponds to spanwise beam #2  124 , as is depicted in  FIG. 4 .  FIG. 5(A)  is a view looking directly at the right edge  56  of the forward compartment RPF block  51 , and further showing the nominal width  57  of the forward compartment RPF block  51 . The width of the aggregate RPF blocks  50  are fabricated in the range of 2.0 inches to 4.0 inches, depending on the size and contour of the fuel tank in which RPF blocks  50  are to be installed. A directional UP arrow and a FORWARD arrow are printed on the surface of the RPF block  51  to further ensure the installer places the block  51  in the correct orientation. 
     The illustrated forward compartment RPF block  51  of  FIG. 5  is further given a part number (P/N) to indicate its exact location in the forward compartment  131  of the center wing tank  112 . The part number also defines the order of its loading in the installation sequence of all RPF blocks  50 . The general manner of construction of the contour of the previously-described forward compartment RPF block  51  is typical of the parameters to be met for each of the aggregate RPF blocks  50  to be inserted in the forward compartment  131  of the center wing tank  112  of a Boeing® 737 series 400 aircraft as well as those RPF blocks to be installed in the mid compartment and aft compartment of the center wing tank  112 . Similarly, the general manner of construction of the contour of any other RPF block described above is typical of the parameters to be met for any RPF block to be installed in any of a variety of aircraft fuel tanks. 
     The contours of the aggregate of all RPF blocks  50  are a culmination of determinations made of the internal dimensions, profile, connections, attachments, and integral components of the inner surface of the fuel tank at the specific measured increment along one length of the forward, center, and aft compartments  131 ,  132 ,  133  of the center wing tank  112 . This methodology is applicable to the determination of the size and contour of any RPF block that may be fabricated for insertion into any of an unlimited variety of aircraft fuel tanks. 
     In preparing for installation of RPF blocks in the center wing tank  112 , a plurality of planar sheets of reticulated polyurethane foam (RPF) material is manufactured. Each of said sheets, in the preferred embodiment, generally comprises a thickness  57  of 2.0 inches. However, in general applicability, the thickness is dependent upon the size and type of aircraft fuel tank in which the RPF blocks are to be inserted. During the manufacturing process, the planar sheets of reticulated polyurethane foam may be dyed a purple color. The purple color facilitates the trouble-shooting of fuel contamination or irregularities associated with the engine fuel feed or tank. 
     By way of example only, in the case of the center wing tank  112  of a −300 series Boeing® 737 aircraft, engineering drawings are executed in a sequential series of scaled renderings of the profile of the forward compartment  131  of the center wing tank  112 . The profile of the compartment is measured and scaled at regularly-spaced increments (2.0 inches in the preferred embodiment) along an essentially horizontal line extending from the right side to the left side of the interior of the forward compartment  131 . In the same manner, engineering drawings are rendered for the incremental profile of the center compartment  132  and aft compartment  133  of the center wing tank  112 . 
     A plurality of cutouts of RPF blocks  50  is made from the manufactured sheets of RPF, each block cutout excised according to the previously-described scaled renderings and further, each RPF block cutout is progressively identified with a part number and an orientating UP arrow and FORWARD arrow is printed thereon. Cutting and shaping of the individual RPF block cutouts from the RPF sheets is accomplished by use of several optional means: a mechanical blade type cutting, specially designated/manufactured smooth blade type cutting tools, an extremely fine-toothed band saw-type blade, or a hot-wire type cutting tool per SAE AIR4170A. 
     Once the entirety of the aggregate RPF blocks  50  required for the center wing tank  112  have been excised, each cutout is painted with a part number (P/N) and numerical sequencing corresponding to the sequential placement of each RPF block cutout along the line of measured increments within each of the forward, center, or aft compartments  131 ,  132 ,  133 . The RPF blocks  50  are individually packaged and arranged in a stack or stacks which correspond to the orderly, sequential installation of each RPF block  50  along the horizontal line of increments previously measured. Further, detailed written instructions regarding the installation are drafted and organized in a manual for the guidance of technicians who will install said RPF blocks  50 . 
     To install the shaped block reticulated polyurethane foam in the center wing tank  112 , the aircraft  110  must be positioned on a level surface and at a convenient height for access to the center wing tank  112 . The access opening cover is then removed and the RPF blocks  50  are inserted through the access openings with the exercise of care to avoid tearing or abrading the RPF blocks on the lip of the opening.  FIG. 2 ,  FIG. 3  and  FIG. 3(A)  illustrate the relative position of the access panel  20  of the center wing tank  112 . 
     As stated previously, a very specific order of insertion of the RPF blocks  50  must be followed so that all spaces that are intended to be filled in the center wing tank  112  are indeed filled. Empty spaces in the tank can only be those left by design, which is referred to as “planned voiding.” The center wing tank  112  under discussion here should be filled with the RPF blocks  50  fitted according to the patterns specified in the specific engineering drawings and installation instructions of each designed block contour. 
     By way of further illustration,  FIG. 6  depicts a diagram of a plurality of sequentially-arranged, pre-cut RPF blocks to be installed adjacent to one another, thereby conforming to the interior contour of the forward compartment  131  of the center wing tank  112  of a Boeing® 737 aircraft. Each of the RPF blocks  50  has a shape and an order of installation which ultimately conforms to a specific contour of the forward compartment  131  at a precise point in the forward compartment  131 .  FIG. 7  is a stylized rendering of the center wing tank  112 , further showing the packing arrangement of a plurality of RPF blocks  50  in the process of being installed in the forward compartment  131  of the center wing tank  112 . 
     Generally, the installation of the RPF blocks  50  is accomplished by technicians entering through existing fuel tank access bays and openings, along with the uploading of the RPF blocks  50  into the tank  112 . In the preferred embodiment, as disclosed in this inventive concept, and for illustrative purposes only, the center wing tank  112  of a typical Boeing® 737 aircraft is depicted as undergoing installation of the RPF blocks. On aircraft other than the Boeing® 737, existing fuel tank access openings that may be used for insertion and installation include, but are not limited to, maintenance access holes, wet and dry access bays found on non-cylindrical auxiliary fuel tanks, inspection holes in belly tanks, and the like. A very specific order of insertion of the RPF blocks  50  must be followed so that all spaces that are intended to be filled in the tank are indeed filled. 
     The shaped blocks  50  of reticulated polyurethane foam (RPF) are inserted through one or more of the access bays, ports, etc., and fitted together in accordance with the present invention. Care is necessarily exercised to avoid tearing or abrading the foam on the lip of the access bays. Shaped block foam pieces are inserted and positioned in a manner to ensure that the required internal tank void is filled and maximum ignition source prevention is achieved. The foam material used is considered to be a “memory foam” type, the kind that returns to its original shape after compression down to 40% of its original volume. These “memory foam” RPF blocks  50  may be compressed for passage through the access ports. A key feature of the present inventive concept is that the design of the shaped block foam pieces to allow them to fit through the existing access bays or ports of various aircraft fuel tanks. Previously, installation of similar material had to be accomplished by removing some portions of the aircraft wing skin in some areas. 
     The majority of Boeing® 737 series 300 and series 400 aircraft are characterized by a first internal access port  18  between the aft compartment  133  and the center compartment  132  of the aircraft and a second internal access port  19  between the center compartment  132  and the forward compartment  131 . These access ports are depicted in  FIGS. 8, 8A, 8B, and 8C . In the installation of RPF blocks  50  in the center wing tank  112  of a Boeing® 737 aircraft, the procedure begins with the right side of the aft compartment  133  of the center wing tank  112 . The installer(s) must gain access to the aft compartment  133  first, through the lower access panel  20  located on the underside of the fuselage  111  of the aircraft, as shown in  FIG. 2  and  FIG. 3 . Access is sequentially accomplished through the second internal access port  19 , and the first internal access port  18 . 
       FIG. 8  depicts a quantity of RPF blocks  50  having been installed on the right side of the aft compartment  133 . Next, the installer(s) work the left side of the aft compartment  133 .  FIG. 8A  showing the completion of installation of RPF blocks on the left side of the aft compartment  133 , the right side of the compartment  133  having been completed. As the installer(s) begins exiting the aft compartment  133  through the first access port  18 , he/she installs RPF blocks  50  in the middle section of the aft compartment  133 , as shown in  FIG. 8B ,  FIG. 8C  depicts the completion of installation of RPF blocks  50  in the aft compartment  133 . 
       FIG. 9  illustrates a diagram of the resultant installation of all RPF blocks in the forward, mid, and aft compartments of the center wing tank  112  of a typical Boeing® 737 aircraft. The diagram depicts the aft compartment  133 , the mid 
     The fitted size, shape and installation of the aggregate of RPF blocks  50  should be such that no internal tank voids longer than 2.5 feet exist (with the internal tank fuel probes installed). The RPF blocks  50  must be kept clear of the tank components such as the fuel ports, fuel probes, float switches, and tank vents. The “planned” voiding areas around these structures should not exceed a volume of 10% of the total fuel tank volume and there should be no connecting voids between any of the planned void spaces. The minimum space of foam filled area required between the planned void areas is three (3.0) inches if maximum void size is used. 
     While the present invention has been described above in terms of specific embodiments, it is understood that the invention is not limited to these disclosed embodiments. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications, variations, and other embodiments of the invention will come to mind of those skilled in the art to which this invention pertains, and which are intended to be and are covered by both this disclosure and hereafter submitted claims. It is indeed intended that the scope of the invention should be determined by proper interpretation and construction of the hereafter submitted claims and their legal equivalents, as understood by those of skill in the art relying upon the disclosure in this specification and the attached drawings. It is intended that the scope of the invention is not limited to any particular aircraft.