US 20030124496 A1
A fire simulator comprises fuel distribution means (12) for fuelling flames in a fire simulation, the fuel distribution means being associated with fuel-heating means (9, 11) disposed generally beneath the fuel distribution means for applying the fuel distribution means heat that emanates from the flames in use, thereby promoting vaporisation of liquid fuel in the fuel distribution means.
1. A fire simulator comprising fuel distribution means for fuelling flames in a fire simulation, the fuel distribution means being associated with fuel-heating means disposed generally beneath the fuel distribution means for applying to the fuel distribution means heat that emanates from the flames in use, thereby promoting vaporisation of liquid fuel in the fuel distribution means.
2. The simulator of
3. The simulator of
4. The simulator of any preceding claim, wherein the fuel distribution means is exposed to direct heat radiation from the flames in use.
5. The simulator of
6. The simulator of
7. The simulator of
8. The simulator of any preceding claim, wherein the fuel distribution means is arranged to eject fuel with a downward component of movement toward the fuel-heating means.
9. The simulator of
10. The simulator of any preceding claim, wherein the fuel-heating means includes a layer of particulate refractory material.
11. The simulator of
12. A fire simulator, substantially as hereinbefore described with reference to or as illustrated in FIGS. 2 and 3 or FIG. 4 of the accompanying drawings.
 This invention relates to fire-fighter training. In particular, the invention relates to fire-fighter training installations such as those used to simulate fires in aviation scenarios, notably those of aircraft crash-landings.
 The invention is not limited to aviation fire-fighting scenarios: it has application in simulators for other fire-fighting scenarios such as road or railway crashes that, like an aircraft crash-landing, can involve a substantial fuel spill. Indeed, preferred aspects of the invention involve simulators that can be adapted for a variety of different fire simulations not necessarily involving fuel spillage, including aircraft, collapsed buildings, road vehicles, trains and multiple-scenario incidents. Such simulators can also be used for ‘joint services’ training, i.e. to train members of other emergency services, notably the police and paramedics, who must co-ordinate their work with fire-fighters from time to time.
 Speed and skill are of the essence to all fire-fighters but fire-fighting in aviation scenarios, such as aircraft crash-landings, requires particularly fast response and skilled teamwork if loss of life is to be minimised. It is generally accepted that unless a burning crash-landed aircraft is accessed and the fire suppressed within two minutes of ignition, there is little hope of survival for those on board who may have survived the landing itself. As there is so little time for mistakes, this places extraordinary demands upon the skill of fire-fighters based at civil airports and military airbases. There are corresponding demands upon the training of those fire-fighters, both as individuals and as a team, and hence upon the quality of the simulators on which those fire-fighters practice.
 All substantial airports and airbases have dedicated fire tenders on standby for substantially immediate high-speed access to any crash site within the airport or airbase perimeter. Such tenders include vehicles known in the art as Major Airport Crashtrucks or MACS. Upon approaching the stricken aircraft, the practice is to drive the tenders close to the aircraft for the purpose of laying down fire-retardant foam and simultaneously gaining access to the fuselage of the aircraft to free its passengers and crew. Indeed, recent practice in civil aviation fire-fighting is to drive a specially-adapted tender right up to the aircraft for the purposes of puncturing its fuselage and injecting foam to protect people who may still be alive within.
 Of course, accidents are characterised by their unpredictability and there is no way of knowing what difficulties fire-fighters will encounter when they reach a crash-landed aircraft. Their fire-fighting strategy must therefore be fully flexible. For example, the orientation of a-burning aircraft with respect to the prevailing wind will have a considerable influence upon how the fire-fighters can approach the aircraft, suppress the fire and access the fuselage. Also, obstructions such as airport vehicles and broken-off engines, undercarriage components, wings or other parts of the aircraft can block access to the fuselage and will, in all likelihood, be on fire themselves. This is all quite apart from the different types of aircraft fire with which fire-fighters must contend: a fire confined to an engine or the undercarriage, for example, will require a quite different strategy to a fire involving spilled fuel.
 The demands of fire-fighter training have led to the emergence of fire-fighting simulators in which fluid-fuelled flames are controlled to respond realistically to efforts by trainees to suppress them, in so-called ‘hot-fire’ training. Aviation fire simulators are typically sited at an airfield or airbase, close to the fire-fighters' base at that facility. Flame generators can extend across the ground to simulate a fuel spill and can also be associated with mock-ups of above-ground structures associated with a fire scenario, such as a metal tube representing a section of aircraft fuselage which may have structures representing wings and engines to one or both sides, or a metal box representing an airport vehicle. In an analogy apt for acting-out scenarios, these mock-ups are referred to in fire-fighter training as ‘props’. That term will be used hereafter in this specification when referring to such mock-ups.
 In early days, the fuel used in aviation fire simulators was liquid fuel such as oil or jet fuel but whilst their flames are realistic in appearance, those fuels give rise to levels of pollution that would be unacceptable today in frequently-used simulators that are often situated near urban settlements. Consequently, there has been a move toward gas-fuelled simulators and here the challenge is to maintain realism and controllability.
 An example of a gas-fuelled fire-fighting simulator is disclosed in U.S. Pat. No. 5,055,050 to Symtron Systems, Inc., which comprises a diffuser such as a pan filled with a bed of dispersive medium such as water or gravel in which a burner system comprising a network of perforated pipes is submerged or buried. The pipes carry pressurised liquefied petroleum gas (LPG)—preferably propane—which is initially in its liquid phase but, with reducing pressure, flashes into the vapour phase within the pipes as it approaches the holes in the pipes. Thus, the pipes contain a mix of vaporising liquid propane and propane vapour. The gas issuing from the pipes diffuses as it rises through the dispersive medium and then burns on the surface of the dispersive medium. Two or more pans can be used side-by-side.
 Whilst such use is not specifically disclosed in U.S. Pat. No. 5,055,050, it is well known in the art that the flames can be controlled to respond appropriately to the trainee fire-fighters' actions. For example, the fuel flow rate in different parts of the network of pipes or in different pans can be varied under central control via remote valves. It is also known that the pans can be used beside a prop such as a mock aircraft fuselage to lend further realism to training scenarios.
 The simulator arrangement of U.S. Pat. No. 5,055,050 enjoys certain benefits such as low cost and is suitable for many training requirements, but suffers some drawbacks and compromises that the present invention seeks to avoid. For example, its flames are not optimally realistic in their appearance, behaviour and responsiveness. Furthermore, as will be explained below, the exposed bed of the dispersive medium causes several problems.
 Dealing firstly with the flame quality of the simulator of U.S. Pat. No. 5,055,050, the aim of any fire simulator is to mimic the behaviour of a flame as it develops from ignition to eventual extinction. Spilled liquid fuel burns in a similar manner to the same fuel in an open-topped tank. Upon ignition, the height of the flames is initially quite small. However, the flames progressively grow larger and spread quickly across the full area of the spillage, eventually reaching a limiting height determined by the burning velocity of the flame. The flame grows during this phase because its radiant heat promotes the evaporation of liquid fuel. The increased rate of evaporation causes the flame to grow and this applies additional radiant heat to the remaining liquid fuel, increasing the rate of evaporation still further until the burning velocity of the flame prevents further flame growth.
 Reference is made at this point to FIG. 1, whose source is Drysdale, D. An Introduction to Fire Dynamics, 2nd edition, p. 12, published in 1998 by John Wiley & Sons. This is a schematic representation of a burning surface showing the heat and mass transfer processes involved in combustion. Importantly, it shows that in all fire occurrences, heat flux supplied by the flame (QF″) transfers to the fuel surface. This heat transfer then increases the volatility of the fuel, hence adding to the conflagration.
 Clearly, therefore, a key aspect of simulating a liquid fuel spill fire is to transmit radiant heat to liquid fuel so as to promote the evaporation of that liquid fuel. Here, the simulator of U.S. Pat. No. 5,055,050 fails because the dispersive medium blocks the transmission of radiant heat from the flame burning above that medium to the liquid fuel situated within that medium. Consequently, it takes an inordinate length of time for evaporation to take effect, meaning that a flame of realistic character is slow to establish and that its development from ignition is inherently unrealistic. Indeed, the fuel in the pipes can still contain a significant fraction of liquid fuel when it emerges from the pipes. Moreover, in practice, fuel can soak into a particulate dispersive medium to the extent that the flames become unrealistically difficult to extinguish.
 Returning to the problems suffered by an exposed bed of dispersive medium, one of the major problems is that the dispersive medium lacks structural integrity and can bear no significant load. This means that props cannot be supported on the bed and that vehicles cannot drive over the bed without risking fracture of the pipes underneath the surface and so possibly causing a genuine conflagration. It follows that areas of the simulator are artificially off-limits to fire tenders and, for safety reasons, have to be delineated as such with markers or barriers that extend beyond the forbidden area.
 Given the reliance upon close approach of fire tenders to aircraft in aviation fire scenarios, it is hugely unrealistic to prevent tenders, in training, accessing areas of the simulator installation that, in an analogous real fire, correspond to areas around an aircraft upon which the tender would advantageously be driven. This problem is particularly acute given that tenders must be driven artificially gently and slowly during training to avoid accidentally driving onto the forbidden areas: in real life, their drivers will approach an accident site at the highest possible speed and brake as hard and late as they can. It is similarly unrealistic to have to place props beside rather on top of the bed, where the simulated fire is raging.
 Another disadvantage of the exposed bed of dispersive medium is that props cannot be dragged across the bed if it is desired to rearrange their position: they can only be lifted into place by a crane. This limits the adaptability of the simulator by increasing the cost and timescale of any changes in the orientation or layout of the props, such as may be necessary to track changes in wind direction, if indeed such changes are possible within the confines imposed by the extent of the beds surrounding the location of the prop. Aside from developing fire-fighting skills applicable to different situations, the ability to vary training scenarios is important to maintain the trainees' interest and focus.
 There is also the problem that fire-fighter trainees cannot walk safely on the bed of dispersive medium as they fight the simulated fire: even a shallow pan of water is self-evidently unsuitable for access on foot, and the alternative medium of gravel or other particulate refractory material presents a trip hazard that could cause a trainee to stumble into the flames. This drawback further deprives the simulator of realism, because, in real life, fire-fighters will expect to advance on foot as they fight back the flames whereas, when using the simulator, their advance will be limited by the margins of the bed.
 Yet another drawback of the exposed bed of dispersive medium is that the medium can be disturbed by the flow of water used by trainee fire-fighters to simulate foam. That flow typically reaches 11,000 litres per minute from each nozzle used to fight the fire. Where the dispersive medium is a particulate medium such as gravel, for example, such a powerful jet of liquid can wash the gravel about within the pan, removing gravel from some parts of the pan and piling it up elsewhere in the pan. At best, this varies the depth of the bed of gravel to the detriment of optimal dispersion and combustion of the fuel rising from the perforated pipes. The behaviour of the simulator may therefore vary unpredictably from one training exercise to the next, unless the gravel is raked back into a level layer between those exercises. At worst, sections of the pipes can be exposed, depriving the out-flowing fuel of any dispersive effect and exposing the pipes to the full radiant heat of combustion.
 The present invention seeks to solve these problems and therefore to extend the use of gas-fuelled simulators into other parts of the simulator market, providing a simulator in which the realism of training is as great as can be allowed by the safety of those who operate and train on it.
 The invention resides in a fire simulator comprising fuel distribution means for fuelling flames in a fire simulation, the fuel distribution means being associated with fuel-heating means disposed generally beneath the fuel distribution means for applying to the fuel distribution means heat that emanates from the flames in use, thereby promoting vaporisation of liquid fuel in the fuel distribution means. This added vaporisation improves the quality of the flames in terms of their realism and responsiveness.
 The fuel-heating means suitably includes a layer of particulate refractory material such as gravel, and a foraminous sheet or mesh can be interposed between the fuel distribution means and that layer of particulate refractory material.
 Preferably, the fuel-heating means absorbs radiant heat emanating from the flames and radiates to the fuel distribution means some of the heat thus absorbed. Nevertheless, the fuel-heating means can also reflect to the fuel distribution means some of the radiant heat emanating from the flames.
 A grating is preferably disposed above the fuel distribution means to define a working surface on which a fire-fighter using the simulator can walk. More preferably, the working surface can be driven upon by a fire-fighting vehicle such as a fire tender or a Major Airport Crashtruck without damaging the fuel distribution means. Fuel emanating from the fuel distribution means rises through the grating in use to create flames extending above the grating. The grating is therefore interposed between the fuel distribution means and the bulk of the flames in use, but it permits the return of radiant heat from the flames to impinge upon the fuel-heating means. Advantageously, however, the fuel distribution means is also exposed to direct heat radiation from the flames in use.
 In preferred arrangements, the fuel distribution means ejects fuel with a downward component of movement toward, and optionally onto, the fuel-heating means. This improves vaporisation still further.
 This International patent application claims priority from the Applicant's United Kingdom Patent Application Nos. 0005012.0, 0014311.5 and 0102569.1, the contents of which are incorporated herein by reference. Those applications are not continuing in their own right as they refer to prototype development but copies of them are available on the public file of this application, from the date on which this application is published. The discussion of flame characteristics and their testing and analysis set out particularly in Application Nos. 0005012.0 and 0014311.5 may be of background interest to readers of this specification.
 In order that this invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:
FIG. 1, which has already been described, is a diagram of a burning surface;
FIG. 2 is a schematic sectional side view of a fuel spill simulator in accordance with a first embodiment of the invention;
FIG. 3 is a perspective view of a serpentine array of fuel distribution pipes being part of the first embodiment of the invention;
FIG. 4 is a schematic sectional side view of a fuel spill simulator in accordance with a second embodiment of the invention;
FIG. 5 is a schematic sectional side view of a fuel spill simulator that is not in accordance with the invention but includes a grating that can be adapted for use advantageously with the simulators of the invention;
FIG. 6 is a perspective view of an array of support frames laid over serpentine arrays of fuel distribution pipes, as part of the simulator of FIG. 5;
FIG. 7 is a perspective view corresponding to FIG. 6 but showing gravel laid over the fuel distribution pipes within all of the support frames and grating bars laid on some of those support frames over the gravel;
FIG. 8 is an enlarged perspective view of one of the support frames of FIG. 7, with the grating bars partially cut away to show gravel within the frame and that gravel being partially removed to show a fuel distribution pipe normally buried by the gravel;
FIG. 9 is a perspective part-sectioned view of part of the array of support frames bordering the central trench of FIG. 5, showing their drainage provisions;
FIG. 10 is a schematic perspective view of a substantially complete simulator corresponding to FIG. 5; and
 FIGS. 11(a) and 11(b) are schematic plan views of a simulator corresponding to that shown in FIGS. 5 and 10, showing how a prop such as a mock-up aircraft can be positioned and re-positioned on the working surface.
 Referring firstly to FIG. 2 of the drawings, in a first embodiment of the invention, a fuel spill simulator 1 comprises a steel pan 2 set into concrete foundations 3 that support the pan 2. The pan 2 may, for example, be circular or rectangular in plan, and is bordered by service trenches 4 that contain control equipment 5 and services such as fuel supply pipework and power or control cabling (not shown). The trenches 4 shown in FIG. 2 may, of course, represent opposed sections of one continuous trench 4 that surrounds the pan 2.
 The pan 2 and the trenches 4 are surmounted by a grating 6 that defines a flat, level working surface on which a trainee fire-fighter can walk and upon which a fire-fighting vehicle can preferably drive. Full details of the grating 6 will be given later. In the embodiment illustrated, the working surface defined by the grating 6 extends beyond the trenches 4 into neighbouring or surrounding areas 7 on the other side of the trenches 4 from the pan 2, which areas may surmount neighbouring pans of similar design. In any event, the grating 6 should be flush with the neighbouring or surrounding areas 7 to minimise trip hazards and will eventually extend to a contiguous concrete apron or blockwork surface (not shown) with which it preferably defines a continuous substantially level surface.
 The base of the pan 2 is dished slightly to promote drainage of fire-fighting water W or precipitation through a central drain 8, from which the water is preferably filtered and recycled. The pan 2 supports a layer of gravel 9 of substantially uniform thickness and a plurality of vertical grating supports 10 that support the grating 6 at intervals across its width over the pan 2. The supports 10 extend from the grating 6 to the pan 2 and so extend through a mesh 11 over the gravel 9 such that their base portions are surrounded by gravel 9. It will be evident that in view of the dished shape of the pan 2, the supports 10 are of various lengths to suit their position with respect to the centre of the pan 2, while keeping the grating 6 level.
 Exposed fuel distribution pipework 12 constituting a burner extends over the gravel layer 9 and the mesh 11 and around the supports 10 in a sinuous, serpentine array. The pipes 12 of the array are preferably of maintenance-free stainless steel. As can be seen in FIG. 3 which shows an array of pipes 12 over the pan 2 but omits the intermediate gravel layer 9 for clarity, the pipes 12 are perforated to define downwardly-facing orifices, holes or nozzles for the egress of propane supplied from a supply pipe 13 leading from control equipment 5 within the trench 4 beyond the outer edge of the pan 2. The propane is in the liquid phase under pressure before it enters the pipes 12, but flashes into the vapour phase as it flows through the pipes 12 before its emergence from the orifices, holes or nozzles in the pipes 12, whereupon the gas streams downwardly to approach the gravel layer 9.
 During its journey through the pipes 12, a mix of propane vapour and swiftly-vaporising liquid propane is warmed by the radiant heat to which the pipes 12 are exposed. This promotes the evaporation of the remaining liquid fraction and the flammability of the fuel as a whole, which beneficially simulates the behaviour of a real fuel spill. The radiant heat radiates downwardly from the flames above the grating 6 and upwardly from the gravel layer 9, this latter radiation being due to reflection of radiant heat that originated from the flames, and heating of the gravel layer 9 itself by that heat. The openings of the grating 6 are large enough to permit substantial radiant heat flux to pass through the grating 6, but not so large as to present a trip hazard for fire-fighters walking on the working surface defined by the grating 6.
 As can be seen in the enlarged detail view included in FIG. 2, an array of parallel or intersecting rods 14 sandwiched between the gravel 9 and the pan 2 act as groynes to resist movement of the gravel 9 with respect to the pan 2, especially down the slope of the dished pan base 2. Where the rods 14 intersect, they are preferably interlaced in woven manner to define openings for water drainage down the dished shape of the pan base 2. Retention of gravel 9 is further assured by the aforementioned wire mesh 11 that lies on top of the layer of gravel 9 under the fuel distribution pipework 12. Once heated in use, that mesh 11 can further contribute to the upwardly-radiating heat that warms the fuel distribution pipes 12 and the propane streams emanating from those pipes 12.
 The enlarged detail view included in FIG. 2 also makes plain that the gravel 9 comprises various particle sizes. To be specific, the stone specification is of igneous rocks selected from the following group of classifications, namely: fine-grained granite; diabase; gabbro; basalt; and rhyolite. The stone is crushed and provided as sized aggregate conforming to ASTM-C33, grade 2 (or equivalent), as follows:
 As can be seen in FIGS. 2 and 3, each trench 4 beside the pan 2 contains a fuel supply control unit for regulating the supply of fuel to the fuel distribution pipes 12 and a pilot control unit for lighting the fuel ejected from the pipes 12, which units are shown together as control equipment 5 hung on a side wall of the trench 4. The trench 4 is closed in use by a porous lid 15 under the grating 6 (omitted from FIG. 3), which lid 15 serves to protect the control equipment 5 from radiant heat but can be opened to afford access to the control equipment 5 when required. The trench 4 also contains an air pipe 16 whose purpose is to purge the trench 4 of flammable and potentially explosive gases that may build up in use, when the trench 4 is closed by the lid 15. The air pipe 16 does this by introducing air to pressurise the trench 4: this helps to prevent dangerous contaminants entering the trench 4 and forces excess air together with any contaminants out of the trench 4 through the porous lid 15.
 The embodiment of FIG. 4 is broadly analogous to that of FIGS. 2 and 3 in that it provides for fill vaporisation of fuel by downward projection above gravel 9, so like numerals are used for like parts. The key differences are that, in FIG. 4:
 the pan 2 is cambered so that water runs outwardly from the centre and drains into the trench(es) 4;
 the supply pipes 13 that supply the fuel distribution pipework 12 are centrally located with respect to the pan 2, inboard of the fuel distribution pipework 12, rather than being at the outer edge of the pan 2;
 the trenches 4 lack lids and so are open in the sense that they vent freely to atmosphere through vented covers 17; and
 the control equipment 5 is recessed into cavities in the trench wall for protection from heat and water.
 The relative simplicity of the FIG. 4 embodiment will be evident upon comparing the drawings, which reduces its cost in comparison with the FIG. 2 embodiment but without sacrificing performance. Specifically, the trenches 4 perform the dual function of housing and providing access to the control equipment 5 and also draining water from the pan 2. This obviates the central dedicated drain 8 of FIG. 2. Furthermore, the open trench design provides inherent explosion relief without the need for the purging air pipes 16 of FIG. 2. Being recessed into the trench wall, the control equipment 5 no longer needs the protection of the porous lid 15 from radiant heat, but it will need to be positioned above the maximum water level that is predicted to be in the trench under the maximum flow rate of incoming water W in use. It will also be apparent that the inboard supply pipes 13 that supply the fuel distribution pipework 12 can be shorter and simpler than the outboard supply pipes 13 of FIG. 2.
 FIGS. 5 et seq show a fuel spill simulator that is not in accordance with the invention, being more akin to the aforementioned Symtron prior art in that the fuel distribution pipes 12 are buried in dispersive gravel 9. These Figures and the description that follows have been included because they describe a simulator having many features that can be adapted and applied with advantage to simulators within the inventive concept. They also give further details of the grating 6 mentioned in the preceding embodiments of the invention.
 The simulator of FIG. 5 shares some features with the embodiments of FIGS. 2 and 4 and so again, like numerals are used for like parts. Unlike the embodiments of FIGS. 2 and 4, there is no pan; instead, a steel-edged recess is simply formed in a concrete slab foundation 3 to contain a layer of gravel 9. A typical depth for this recess would be up to 500 mm but this depends on the drainage requirements and what the total finished area of the simulator might be.
 The gravel 9 is surmounted by a grating 6, preferably lying flush with the surrounding concrete or blockwork apron 18, that stands on vertical supports 10 extending upwardly from the base of the recess. In this simulator, a trench 4 extends centrally along the recess and, as shown in the enlarged detail view included in FIG. 5, the fuel distribution pipework 12 lies on the base of the recess and so is disposed below the gravel layer 9. Again, the pipework 12 is perforated to define a series of holes, apertures or nozzles to eject fuel in use, but unlike the embodiments of FIGS. 2 and 4 which eject fuel downwardly for maximum evaporative effect, the fuel of the FIG. 5 simulator can be ejected in any direction as it is intended to be dispersed by the gravel 9 in any event.
 As in FIG. 4, the trench 4 of the FIG. 5 simulator is closed by a vented cover 17 so as to vent explosive gases to atmosphere and the control equipment 5 is recessed into cavities in the trench walls. Also, whilst no camber or dish is evident from FIG. 5, the base of the recess is very gently inclined, sloped or dished toward the trench to promote drainage of water from the gravel layer 9. It is advantageous that water does not drain away too quickly, so as to allow enough time for the flare-off of unburned gas; otherwise, that unburned gas may be entrained in a fast-moving stream of water and swept away to cause dangerous gas accumulations downstream.
 To describe the grating 6 and its supports 10 in detail, the description of the FIG. 5 simulator will now continue with reference to the remaining drawings. It will be evident to the skilled reader how the grating 6 and supports 10 shown in those drawings can be adapted to suit the embodiments of FIGS. 2 and 4 in which, unlike FIG. 5, the fuel distribution pipework 12 is exposed above the gravel layer 9. In particular, it will be readily apparent how most if not all of the grating features of the FIG. 5 simulator can be applied to the preceding embodiments if a suitably adapted support is used.
 Referring then to FIGS. 6 to 9 of the drawings, the abovementioned grating supports 10 are defined by the upstanding walls 10A, 10B of fabricated square support frames 20 that are open to their top and bottom and that lie upon and are fixed to the base of the recess of FIG. 5. As best shown in FIGS. 6 and 7, the support frames 20 fit together in rectilinear arrays in mutually-abutting modular fashion, so that each support frame 20 helps to support its neighbours against side loadings in use. The walls of the various support frames 20 thus lie in orthogonally-intersecting vertical planes.
 Looking at any one of the support frames 20 as shown in FIG. 8, it will be noted that each of its four walls 10A, 10B is a flat elongate plate that is preferably of mild steel. Each plate is welded at each of its opposed ends to a respective orthogonally-disposed neighbouring plate, the welded junctions between the plates thus defining the comers of the square between the walls. Additionally, each plate has a cut-out 21 extending along one of its long edges, namely the lower edge that is disposed generally horizontally and facing downwardly in use. The ends of the cut-outs 21 are defined by feet 22 that have a square fixing plate 23 welded to them at the lower comers of the support frame 20. Each fixing plate 23 is therefore arranged to lie flat against the base of the recess and it is pierced by a through-hole (not shown) that enables the support frame 20 to be bolted or otherwise fixed to the base. Whilst not essential, it is preferred that the support frames 20 are fixed down in this way so as to prevent excessive sideways movement or ‘shuffling’ of the support frames as vehicles drive over the working surface of the simulator.
 The cut-outs 21 in the walls of the support frames 20 align with those of neighbouring support frames 20 in use, and have the dual function of accommodating the serpentine arrays of fuel distribution pipes 12 previously fixed at appropriate locations to the base of the recess, and of permitting water W to drain across the base of the recess toward the central trench 4 of FIG. 5. Specific reference is made to FIG. 9 in this respect.
 The plates defining two opposed walls 10B of each support frame are further provided with castellated upper edges defined by a row of upstanding oblong teeth 24 alternating with, and delineated by, oblong slots 25. As will be most apparent from FIGS. 7 and 8, the purpose of the castellations is to hold a set of oblong-section steel grating bars 26 bridging the open top of the support frame 20 in a parallel spaced array that defines a substantially flat, if locally slightly inclined, working surface level with the upper edges of the walls 10A, 10B and the teeth 24. Thus, the castellations hold the grating bars 26 at a suitable height above the fuel distribution pipes 12, and keep those bars 26 in the correct position during use of the simulator.
 To this end, each grating bar 26 is held at one end in a slot 25 of one castellated wall 10B and at the other end by the corresponding slot 25 of the opposite castellated wall 10B. It will also be apparent from the drawings that the major cross-sectional axis of each grating bar 26 is oriented vertically to maximise its load-bearing ability against loads moving over the grating 6.
 In practice, the grating bars 26 are fitted into the slots 25 only after the aforementioned layer of gravel 9 in the form of igneous stone chippings or other particulate dispersive medium has been poured into the open support frames 20 around the fuel distribution pipes 12, burying them to a depth of say 120 mm. The layer of gravel 9 substantially fills the space around the fuel distribution pipes 12 between the grating bars 26 and the base of the recess. It will be apparent that the gravel 9 has little room to move when so positioned and that any tendency it might have to shift sideways across the recess is limited by the baffle effect of the walls 10A, 10B that effectively partition the gravel bed 9.
 It will also be noted, with particular reference to FIGS. 6, 7 and 10, that neighbouring support frames 20 in rows or columns of the array within the recess are turned through 90° with respect to each other so that their castellated walls 10B never abut one another. Thus, as best shown in FIG. 10, the grating bars 26 define cells 27 in rows or columns corresponding to the support frames 20 and the grating bars 26 of adjacent cells are mutually orthogonal. This alternating arrangement can be appreciated in the check pattern extending over the working surface of the simulator.
 The functional significance of the alternating arrangement of the grating bars 26 is twofold. Firstly, the grating bars 26 are free to slide longitudinally within their slots 25 for the purposes of thermal expansion without distortion but once they have slid to a limited extent (a maximum of 10 mm in the preferred embodiment), they will bear against the non-castellated wall 10A of a neighbouring support frame 20 and so can slide no further. This is important under the dynamic sideways loads likely to be imparted by a swerving or braking fire tender or other emergency vehicle. Secondly, a major benefit of the grating 6 is its ability to dissipate the flow of incoming jets of water or other fire-fighting agents and so to prevent the dispersive medium being disturbed by those jets being played directly on the working surface of the simulator. As the dissipating effect of a straight grating of wholly aligned elements might conceivably be overcome if the incoming jet is aligned with the elements, the alternating arrangement of grating bars 26 has the benefit that it will reliably disrupt jets of water striking the working surface from any angle. In any event, any water that does get through the working surface while retaining damaging momentum will be dissipated by the baffle effect of the walls 10A, 10B between the support frames 20, under the working surface.
 To help visualise the size of each frame 20, and strictly by way of example only, their pitch or spacing between centres is nominally 1 metre and so the overall width of each frame is 990 mm square to leave a thermal expansion gap of 10 mm all round. The walls 10A, 10B of each frame are 25 mm thick and stand a total of 200 mm above the base of the recess. Each grating bar 26 is of 80 mm×30 mm black bar and the slots 25 that receive the grating bars 26 are of corresponding dimensions. About 170 mm is therefore available under the grating bars 26 and above the base of the recess to accommodate the fuel distribution pipes 12 and the surrounding layer of gravel 9. The spacing between neighbouring grating bars 26 of a given support frame 20 is no greater than 33 mm so as to present no trip hazard to trainee fire-fighters walking on the working surface. The pitch or spacing between centres of the grating bars 26 is therefore nominally 66 mm and there is provision for thirteen of such bars 26 on each support frame 20.
 A grating specified as above can withstand the maximum wheel load of a Major Airport Crashtruck (MAC). Performing structural analysis according to the requirements of BS5950:Part1:1985 using ANSYS 5.0A, and assuming a mass of the tender of 501.1 kN and a maximum axle load of 130 kN, the grating can comfortably withstand braking from 20 kph.
 Moreover, the considerable mass of the grating bars 26 (in the order of 250 kg/m2) imparts thermal inertia that makes them slow to attain damaging temperatures. During typically short bursts of use from cold (anything longer than three minutes of practice fire-fighting is rare in view of the need for extreme speed in real-life aviation fire-fighting), their temperature keeps well within the parameters appropriate to ordinary personal protection equipment (PPE) routinely worn by fire-fighters. Fire-fighter protective footwear and other PPE is rated to withstand temperatures up to 200 Celsius; tests show that the mass of the grating bars keeps their temperature to about 180 Celsius even after exposure to the radiated heat flux of a fire with flame temperatures between 700 and 1100 Celsius.
 A beneficial side-effect of the considerable girth of the grating bars 26 is that corrosion will not significantly reduce their cross-section and hence load-bearing strength during their projected working life. Consequently, the working surface of the simulator needs no expensive or fragile corrosion treatments, and is essentially maintenance-free.
 The load-bearing ability of the working surface is heightened by the elegant design of the fabricated support frames 20, in which downward loads are transferred directly to the foundations through the vertical walls 10A, 10B without putting the aforementioned welds under damaging tensile or bending loads.
 As already mentioned, the simulator shown in FIGS. 5 et seq is modular in nature. Specifically, it is envisaged that a standard module comprises a serpentine fuel distribution pipe 12, an associated fuel supply control unit and nine support frames 20 in a 3×3 array and hence, with the above dimensions, gives a working surface that covers 9 m2. Several such modules can be used together to construct a simulator having a working surface of any required size, such as the one shown in FIG. 10 which comprises eight modules on each side of the central trench, giving a total working area of 144 m2 excluding the area of the trench itself In practice, the working area of a simulator will generally be substantially greater so that large props can be placed on the working surface and correspondingly wide-ranging fuel spills can be simulated.
 The central trench 4 featured in FIGS. 5, 9 and 10 is covered by a removable vented cover 17 as shown in FIGS. 5 and 10, which can be lifted when it is necessary to gain access to the control equipment 5 and ancillary equipment, such as valve trains and service pipework, within the trench 4.
 FIGS. 11(a) and 11(b) show how a prop 28, in this case a mock-up of a military jet, can be placed freely on the working surface of a simulator akin to that of FIG. 10. In both drawings, the prop 28 is aligned with the prevailing wind shown by the arrows as this is the direction in which a crash-landed aircraft is most likely to lie, although other angles to the prevailing wind can obviously be simulated for wide-ranging practice. In FIG. 11(a), the prevailing wind is offset by about 30° with respect to the central trench 4 of the simulator and the central longitudinal axis of the prop 28 is similarly aligned. However in FIG. 11(b), the prevailing wind is aligned with the trench 4 and the prop 28 has been re-aligned accordingly and also advanced across the working surface. Highly advantageously, the prop 28 can simply be dragged across the working surface from one orientation to the other, with no need of a crane to lift the prop 28.
 Many variations are possible within the inventive concept. Consequently, reference should be made to the appended claims and to other conceptual statements herein rather than to the foregoing specific description in determining the scope of the invention.