Aviation Fuels with Improved Fire Safety A Proceedings (1997) / Chapter Skim
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III. Presented Papers: Aircraft Fuel System Requirements
III. Presented Papers: Aircraft Fuel System Requirements
Pages 59-78
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From page 59... ...
Ill PRESENTED PAPERS AIRCRAFT FUEL SYSTEM REQUIREMENTS
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Fuel has traditionally been used for fuel valve actuation, inlet variable geometry actuation, and start bleed and cooling valve actuation. Recent military engine designs that employ thrust vectoring exhaust nozzles have expanded the use of fueldraulics to high power (100 H.P., 3500 psi)
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From page 62... ...
In most cases, dynamic seals on sliding shafts incorporate dual shaft seals for increased reliability. Whenever possible, a minimum level of fuel flow in fuel lines and components is preferred in order to provide cooling and prevent fuel deposit formation due to high ambient temperatures.
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From page 63... ...
The pumping elements are made of high strength tool steel or stainless 63 hardenable steel; some vane pumps and piston pumps utilize tungsten carbide on wear surfaces to minimize wear. Fuel controls perform a variety of functions, from simple fuel flow metering to complex fuel flow staging and sequencing and a host of other engine controlling functions.
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From page 64... ...
Viscosity affects low temperature pumpability, servosystem damping, and hydrodynamic fluid film lubrication. AVIATION FUELS WITH IMPROVED FIRE SAFETY: A PROCEEDINGS The low temperature limit of 12 centistokes is the current limit, and a significant increase in viscosity will adversely affect fuel pumpability at the engine inlet.
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From page 65... ...
The critical areas involved in reducing vulnerability to fires and explosions are listed below: fire and explosion elements damage modes and effects hardening approaches modeling and testing FIRE AND EXPLOSION ELEMENTS Fires and explosions require three critical elements, an ignition source, a flammable material, and oxygen. Remove any one of the three elements, and the potential for fire and explosion is eliminated.
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From page 66... ...
Oxygen concentrations are altered by environmental factors, such as temperatures of the fuel and ullage, atmospheric pressure, and internal air flow. These environmental factors TABLE 10-1 Ignition Sources from Ballistic Threats Class of Threat Specific Threat Guns/Projectiles Armor piercing incendiary Guns/Projectiles High explosive incendiary Missiles Contact-fuzed missile Missiles Proximity-fuzed missile Bombs High explosive materials AVIATION FUELS WITH IMPROVED FIRE SAFETY: A PROCEEDINGS TABLE 10-2 Ignition Sources from Mechanical Failures Specific Threat Ignition Sources Electrical Electrical Electrical Leaks Mechanical Damaged wiring Static discharge Lighting Spray of flammable material Engine part penetrates case Sparking and arcing Sparking and arcing Sparking and arcing Hot surface ignition Flame and hot air outside of engine case alter the fueVair concentration ratio.
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From page 67... ...
; adding a system that extinguishes fires; rapidly shutting off sources of flammable material; adding fire walls; and separating flammable materials from ignition sources. These hardening approaches are applicable to several classes of dry bays and equipment.
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From page 68... ...
The air gap tables and numerous rules are used to predict the probability of fire. The WINFIRE model is a physics/chemistry-based model that assesses the probability of a sustained fire in a dry bay Trade-off studies Hardening ~ feature No / Design adequate?
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From page 69... ...
Fuel in engine inlet can cause a rapid explosion or cascading fire. Separated critical redundant equipment Separated critical redundant equipment Fire resistance/ablative material added to contain fire Critical equipment and lines removed from internal weapons bay Weapons bay sealed to eliminate flame propagation to other dry bays Fluid path restricted by use of double walls Valves shut at fire walls Fuel lines routed within fuel tanks In-line shut-off valve and sensor for each redundant branch Reservoir sensor with pump shut-off switch Power packs to suppress fire Bladders to seal small holes Foams to fill the dry bay Active or passive fire suppression system Inerted dry bay air Pilot-activated shut-off of engine fuel supply and pilot-activated fire suppression system Fuel removal from inlet before going into combat Bladders to seal small holes and limit fuel into engine inlet 69
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This problem makes it difficult to compare model and test data. SUMMARY This paper has used a systems engineering process to break down a complex technical problem into its basic elements: flammable material; ignition source; and oxygen.
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An effective vulnerability reduction program requires a robust and comprehensive systems engineering process based on tests and analyses. Early analyses of vulnerability and numerous critical tests using components, simulators, and replicas are needed to support design decisions.
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A typical engine feed system consists of electrically driven pumps, fuel lines, valves, and fittings (shown in Figure 1 1-2~. An independent fuel feed is also provided for the auxiliary power unit.
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In areas where ignition sources cannot be completely avoided, attempts have been made to minimize inadvertent leaks of flammable fluids and to provide ventilation to prevent the accumulation of vapors. In addition, structural designs have been made crashworthy to reduce the fire risk following a crash.
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From page 75... ...
Fuel lines are adequately separated from electrical wiring; as a further precaution, fuel lines are routed below the wiring runs to ensure that any fuel leakage will not contact potential ignition sources, such as wires and electrical components. Fuel Carrying Components Components and lines carrying fuel are sometimes located in or near fire zones where leaking fuel poses a potential risk.
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From page 76... ...
FIGURE 11-3 A shrouded fuel line in a pressurized compartment. Considerations for Safety Fuels Performance and Uses In view of the focus of this workshop on fuels with improved fire safety, it will be helpful to review the secondary uses of fuel in an aircraft fuel system.
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From page 77... ...
CompatibilitJ, Safety fuels must also demonstrate compatibility with existing engine and airframe fuel systems and materials (small modifications may be acceptable) , current fuels and additives, existing airport fuel systems and ground supply equipment, and be capable of accepting different fuels (switch loading)
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From page 78... ...
AVIATION FUELS WITH IMPROVED FIRE SAFETY: A PROCEEDINGS In any concept that involves modifying fuels to improve safety, higher vapor pressure fuels and foreign fuels should be included. A user panel should assist in evaluating different concepts and detailed R&D activities.
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