(1) The fuel, flammable fluid, and vapor system designs should be reviewed early in the certification process to insure that all fuel or flammable fluid or vapor tanks are properly identified and isolated from engines, engine compartments, and other designated fire zones during both normal and emergency operations such as in-flight engine compartment or other fire zone fires. In some cases fuel or flammable fluid components must be located in an engine compartment or other designated fire zone.
In these cases, an equivalent safety finding (which considers the design, construction, materials, fuel lines, fittings, and controls used in the system, or system segment, contained in the engine compartment or other designated fire zone) should be undertaken as a part of the normal certification process. If the level of safety provided is equivalent to that provided by removing the system or system segment from the engine compartment or designated fire zone, then the design should be accepted. For fuel tanks only, isolation is required by regulation to be achieved by use of either a firewall (reference paragraph AC 29.1191 for Firewall Requirements) or by use of a shroud. A shroud if used should be fireproof (see § 1.1 for definition and the definition of a Severe Fire for further details) and should be drainable (or otherwise inspectable) to insure the fuel tank is not leaking in service. For other flammable fluid or vapor tanks, the regulations allow either the identical treatment previously described for fuel tanks (i.e., firewalls or shrouds) or, alternatively, use of an equivalent safety finding. Regulations require that the equivalent safety finding be based on system design, tank materials, tank supports, and flammable fluid system connectors, lines, and controls. In all cases the flammable fluids, fuels, and vapors should be sufficiently isolated from hazardous heat fluxes during both normal and emergency operations to prevent autoignition.
(2) In addition, the regulations require at least ½-inch of clear airspace between each flammable fluid or vapor tank, and each firewall or shroud that isolates the system, unless equivalent means (such as fireproof insulation) are used to prevent hazardous heat transfer from each engine compartment or other fire zone to the flammable fluid or vapor mass (or its container surface) at the fluid or vapor’s minimum autoignition temperature. If in- service structural deflections are significant, they must be taken into account when certifying the ½-inch minimum clear airspace requirement. For example, if a ½-inch clearance exists on the ground but in some normal and emergency flight conditions (e.g., autorotation) the ½ inch is reduced to ¼ inch at a critical time (in-flight engine fire), then the design (static) configuration should have at least a ½ plus ¼ equals 3/4-inch static clear airspace to insure the regulation’s intent is met.
Alternatively, fireproof insulation or additional stiffeners could be used to insure the regulation’s intent is met (i.e., the thermal equivalent of ½ clearance is maintained at all times). Any material used as insulation on or used adjacent to flammable fluid or vapor tank, should be certified as chemically compatible with the flammable fluid or vapor and to be non-absorbent in case of fuel or vapor leaks. Otherwise, the material should either be treated for compatibility and non-absorbency or not accepted.
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AC 29.1187. § 29.1187 DRAINAGE AND VENTILATION OF FIRE ZONES.
a. Explanation. To insure that any component malfunction which results in fuel, flammable fluid or vapor leaks is safely drained or vented overboard and to insure that a fire hazard is not created during either normal or emergency service, there should be complete, rapid drainage and ventilation capability present for each part of the rotorcraft powerplant installation and any other designated fire zone which utilizes flammable fluid or vapor carrying components. As a minimum, the routing, drainage, and ventilation system should accomplish the following:
(1) It should be effective under normal and emergency operating conditions.
(2) It should be designed and arranged so that no discharged fluid or vapor will create a fire hazard under normal and emergency operating conditions.
(3) It should prevent accumulation of hazardous fluids and vapors in any engine compartments and other designated fire zones.
b. Definitions Procedures . Drip Fence. A physical barrier that interrupts the flow of a liquid on the underside of a surface, such as a fuel tank, and allows any leaked liquid to drip from the surface away from a hazardous locations to a safe external drain.
c. . The design of flammable fluid and gas systems running through engine compartments and other designated fire zones should have a thorough hazard analysis performed early during certification. The analysis should be updated periodically as design changes dictate. The hazard analysis should identify and quantify all normal and emergency service failures that could result in leakage of fuel, flammable fluids and vapors. Once these potential hazards are identified and quantified, appropriate design features, such as drains, drip fences and vents, that minimize or eliminate the hazard should be provided. These means should be analyzed and/or tested, as necessary, to insure that their size, flow capacity, and other design parameters are adequate to rapidly remove hazardous fluids and vapors safely away from the rotorcraft under normal and emergency flight conditions. Typically a venting or draining system should be designed to a 3-to-1 flow capacity margin over the probable worst case leak to which it could be subjected. Adverse effects such as clogging and surface tension flow reduction should be accounted for in design. Testing, including flight testing, using inert fluids or vapors may be necessary for proper design certification. In some instances it may be appropriate to include ventilation and drainage tests when the aircraft is parked.
AC 29.1189. § 29.1189 (Amendment 29-26) SHUTOFF MEANS.
a. Explanation.
(1) This section establishes the requirements for controlling hazardous quantities of flammable fluids which flow into, within, or through designated fire zones.
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(2) When any shutoff valve is operated, any equipment, including a remaining engine, which is essential for continued flight, cannot be affected.
b. Procedures.
(1) Combustible fluid supply lines which pass into, within, or through a firewall into the fire zone must incorporate shutoff valves. This requirement does not apply to lines, fittings, and components which were certified with and are part of the engine.
These requirements do not apply to oil systems for Category B rotorcraft with reciprocating engines with less than 500 cubic inches displacement or to any other installation where all components, including the oil tanks, are fireproof or are located in an area that will not be affected by an engine fire.
(2) Eight fluid ounces or less of a combustible fluid is not considered hazardous and no more than this amount should be present after activating the shutoff valve.
(3) Engine isolation is to be maintained when incorporating shutoff valves into engine fuel and lubrication lines. The design must insure that when one engine is shut down or fails and the fuel and lubrication fluid shutoff valves are activated, the remaining good engine is not affected in any way, and the rotorcraft can continue safe flight to a landing. This should be demonstrated by test.
(4) Each shutoff valve located in a fire zone should be fireproof. If the shutoff valve is located outside of the fire zone, then it should be at least fire resistant or protected so that it will function under a worst case fire condition within a fire zone. This should be demonstrated by test.
(5) Except for ground-use-only auxiliary power unit installations, the flammable fluid shutoff to all engine installations must be protected from inadvertent operation.
Where electrical shutoffs are used, the switches must be guarded or require double actions. If the shutoffs are mechanically activated, the design of the knob and the location of the lever must be such that inadvertent actuation cannot occur. It must be possible to reopen the shutoff valve in flight after it has been closed and this should be demonstrated by test.
AC 29.1191. § 29.1191 (Amendment 29-3) FIREWALLS .
a. Explanation. This section states the certification requirements for the proper certification of fireproof protective devices such as firewalls, shrouds, or equivalent.
These devices are necessary to isolate each engine (including combustor, turbine, and tailpipe sections of turbine engines and auxiliary propulsion units (APU); each APU;
each combustion heater; each unit of combustion equipment; or each high temperature device (or source) from personnel compartments and critical components (not already protected under § 29.1191). The isolation of these fire zones is necessary to prevent the spread of fire, prevent or minimize thermal injuries and fatalities, and prevent Page E - 187
damage to critical components that are essential to a controlled landing. Even though § 29.1191(b) implicitly excludes APU’s, combustion heaters, and other combustion equipment that are not used in flight; they should be protected by fireproof enclosures, because of § 29.901(d) and the requirements of the relevant parts of §§ 29.1183 through 29.1203. This is because, even if the device is rendered inoperative in flight, it typically contains residual heat, fuel, fumes and potential ignition sources (i.e., “potential hazards”). Each fireproof protective device must, by regulation, meet the following criteria:
(1) Its design and location must take into account the probable fire path from each fire zone or source considering factors such as internal airflow, external air flow, and gravity.
(2) It must be constructed so that no hazardous quantity of air, fumes, fluids, or flame can propagate through it to unprotected parts of the rotorcraft.
(3) Its openings (e.g., shaftholes, lineholes, etc.) must be sealed with close fitting fireproof grommets, bushings, bearings, firewall fittings, or equivalent that prevent burn through and leakage of hazardous fumes or fluids from the fire zone.
(4) It must be fireproof (see definition).
(5) It must be either corrosion resistant or otherwise safely protected from corrosion.
b. Definitions.
(1) Fireproof Protective Device. A fireproof protective device is a device such as a firewall, shroud, enclosure, or equivalent used to isolate a heat or potential fire source (severe fire) from personnel compartments and from critical aircraft components which are essential for a controlled landing.
(2) Fireproof. Fireproof is defined in § 1.1 “General Definitions.”
(3) Controlled Landing. A landing which is survivable (i.e., does not fatally injure all occupants) but may produce an unairworthy, partially salvageable, or unsalvageable rotorcraft.
(4) Severe Fire. See definition in paragraph AC 29.859.
c. Procedures. Fireproof protective devices are typically certified by analysis, tests, or a combination conducted during the certification process, including flight tests or simulated flight tests, as follows:
(1) Fireproof protective devices should be provided wherever a hazard exists which requires isolation from a severe fire (see definition) to avoid fires in personnel Page E - 188
compartments and to avoid thermal damage to critical components (such as structural elements, controls, rotor mechanisms, and system components) that are necessary for a controlled landing. A thorough hazard analysis should be conducted during certification to identify, define and quantify in order of severity (i.e., maximum temperature, hot exposed area, etc.) all thermal hazards or zones that require fireproof protection in a given design. Engines (including the combustor, turbine, and tailpipe sections of turbine engines), APU’s, combustion heaters, and combustion devices are required by regulation to be isolated. Other high temperature devices may also require isolation because of local hot spots (which occur during normal operations or from failure modes) that can thermally injure occupants or cause spontaneous combustion of surroundings. A hazard analysis should identify these potential problems and provide proper certification solutions.
(2) Fireproof protective devices should be able to withstand at least 2000 ± 150° F for at least 15 minutes (reference AC 20-135). The fireproof protective device should allow the protected parts, subsystems or systems to perform their intended function for the duration of a severe fire (see definition). For firewalls, examples of flat, geometry materials undergoing uniform heat fluxes with material gauges that automatically meet the certification requirements are given in figure AC 29.1191–1. If firewalls are utilized that involve other materials, significant geometric changes, or significantly non-uniform heat fluxes, then automatic compliance may not be assured. In such cases the fireproof protective devices should be analyzed and, in some cases, tested in accordance with AC 23–2 to ensure proper certification.
For example, a curved protective surface may absorb a uniform incident heat flux unevenly and create a local hot spot that exceeds 2050° F that burns through in less than 15 minutes; whereas, a flat surface of equal thickness would not exceed 2050° F and would not burn through in less than 15 minutes. It should be noted that composite materials are not generally used for protective devices because of their inability to withstand high temperatures (i.e., exceedance of the glass transition temperature);
however, some specially formulated composites have been previously certified as engine cowlings. Titanium is an acceptable material for fireproof protective devices such as firewalls. However, use of titanium should always be carefully considered and reviewed, because it can lose all structural ability and burn severely (self combust) above 1,050° F, under certain thermodynamic environments, and contribute to the fire instead of providing the intended fire protection. AC 33 –4, “Design Considerations Concerning the Use of Titanium in Aircraft Turbine Engines” and MIL-HDBK-5D contain more detailed information on the unique thermal properties of titanium.
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FIGURE AC 29.1191-1
TABLE OF MATERIALS AND GAGES ACCEPTABLE
FOR FIREPROOF PROTECTIVE DEVICES WITH FLAT SURFACE GEOMETRIES (1) MATERIAL(2) MINIMUM THICKNESS(3) Titanium Sheet .016 in Stainless Steel .015 in Mild Carbon Steel .018 in Terne Plate .018 in Monel Metal .018 in Firewall Fittings (Steel or Copper Base) .018 in(4) NOTES:
(1) Assumes essentially flat vertical or horizontal surfaces undergoing a uniform heat flux. Any significant variation in either geometry or heat flux distribution should be examined in detail for adequate gauge thicknesses on a case-by-case basis.
(2) Must have corrosion protection if not inherent in the material itself.
(3) The minimum thickness is for thermal containment only. Structural integrity considerations may require thickness increases. MIL- HDBK-5D contains material allowable versus temperature data for common metallic materials.
(4) This is the minimum wall thickness measured at the smallest dimension (e.g., thread root or other location) of the part.
(5) Distortion of thin sheet materials and the subsequent gapping at lap joints or between rivets is difficult to predict; therefore, testing of the simulated installation is necessary to prove the integrity of the design. However, rivet pitches of 2 inches or less on non load-carrying titanium firewalls of .020 inch or steel firewalls of .018 inch are acceptable without further testing.
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(3) The probable path of a fire (as affected by internal and external air flow during normal flight and autorotation, gravity, flame propagation paths, or other considerations) should be taken into account when performing the hazard analysis of item (1). Such a review will insure that fi reproof protective devices are placed in the proper location for intercepting, blocking or containing a severe fire before occupants are injured and a controlled landing is prevented. If the probable path cannot be readily determined by inspection or analysis, testing using simulated airflows, rotorcraft attitudes, and dyed inert fluids or vapors can be used to aid in this determination.
(4) Each opening in a protective device should be sealed with close fitting sealing devices such as fireproof grommets, bushings, firewall fittings, rotating seals or equivalent that are at least as effective as the fireproof protective device itself. This is necessary to insure that no local breakdowns in protection occur. For materials not listed as acceptable in item (1), FAA/AUTHORITY standards and analysis and testing should be required in accordance with the definition of a severe fire for proper substantiation.
(5) Each protective device should be fireproof in order to withstand a severe fire (see definition). Unless designs and materials have been previously FAA/AUTHORITY approved (e.g., see Item 1), the protective device’s design and material selection should be tested to insure its fireproof thermal and structural integrity. A full-scale test of a structurally loaded article or a representative sample should be conducted to insure proper compliance is achieved. Also, the continued sealing ability of the protective device in its deformed state due to a hard controlled landing should be considered during certification (e.g., use of ductile materials). The corrosion environment should be defined and appropriate protection provided. P hased inspections should be specified, if necessary, to insure continued corrosion integrity. Certification tests for adequacy of corrosion protection should be conducted, using sample plates or by other equivalent means, as required.
AC 29.1193. § 29.1193 (Amendment 29-13) COWLING AND ENGINE COMPARTMENT COVERING.
a. Explanation.
(1) Section 29.1193(a) requires the cowling and engine compartment coverings to withstand structural loads experienced in flight.
(2) In order to prevent pooling of flammable fluids, § 29.1193(b) requires ventilation and complete drainage from the cowling and engine compartment as specified in § 29.1187.
(3) In § 29.1193(c), (d), and (e), clarification of fireproof requirements is provided along with interaction between the requirements of § 29.1191 for firewalls.
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AC 29-2C, Chg 4 5/1/2014
b. Procedures.
(1) Compliance with § 29.1193(a) can be shown by analyzing the cowling and engine compartment covering and determining that no structural degradation will occur under the highest loads experienced on the ground or in flight.
(2) Compliance with § 29.1193(b) can be accomplished by ensuring that the drain will discharge positively with no traps and is a minimum of 0.25 inches in diameter.
No drain may discharge where it might cause a fire hazard. This can be demonstrated by colored liquid flowing through the drain system while in flight. The dye should not impinge on any ignition source during any approved flight regime.
(3) Compliance with the fireproof requirements of § 29.1193(c), (d), and (e) can be accomplished by demonstrating that the material will withstand a 2,000° F ± 150°F flame for 15 minutes while still fulfilling its design purpose. This testing should accurately simulate, as near as practicable, the likely fire environment to prove the materials and components will provide the necessary fire containment when exposed to a fire situation in service. In addition to the fireproof requirement s, the requirements of § 29.1191 must also be met. The primary objectives are:
(i) To contain and isolate a fire and prevent other sources of fuel and/or oxygen from feeding the existing fire; and
(ii) To ensure that components of the engine control system will function effectively to permit a safe landing and/or shutdown of the engine.
AC 29.1193A. § 29.1193 (Amendment 29-26) COWLING AND ENGINE COMPARTMENT COVERING.
a. Explanation. Amendment 29-26 adds a new § 29.1193(f) that requires redundant retention means for each panel, cowling, engine, or rotor drive system covering that can be opened or readily removed. Conventional fasteners for these devices are subject to frequent operation by maintenance personnel and have deteriorated, failed from wear or vibration, or been left unsecured after preflight inspections. Such a failure could be hazardous if a loose panel, cowling, or covering strikes, or is struck by, the rotors or by critical controls.
b. Procedures.
(1) Compliance with § 29.1193(f) can be accomplished by simulating, or actually failing, one or more of the retention devices or by structural analysis. If a failure of a single retention device can contribute to multiple failures, these multiple failures should be considered. It should be shown that the cowling or cover will not open, strike, or be struck by the rotor or other critical component.
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(2) Consideration should be given to minimize the possibility of latches being improperly closed that could result in a cowl coming open in flight.
(3) The failure of one latching device should not cause the failure of another latching device.
(4) The consequences of “forgetting” to latch a cowl should be considered.
(5) The use of safety straps should be considered to minimize the impact of a latching device failure.
AC 29.1194. § 29.1194 (Amendment 29-3) OTHER SURFACES .
a. Explanation. This section states the fire resistance requirements for material surfaces near engine compartments and designated fire zones (other than tail surfaces not subject to heat, flames, or sparks emanating from a designated fire zone or engine compartment).
b. Definition.
(1) Other Surface. Any airframe, system, or powerplant component aft of and near an engine compartment, a designated fire zone, or another heat source which would receive a heat flux as a result of a fire in the engine compartment or fire zone that would require the component to be fire resistant.
(2) Fire Resistant. In accordance with § 1.1, is defined as follows:
(i) Sheet metal or structural members with the capacity to withstand the heat associated with the fire at least as well as aluminum alloy in dimensions appropriate for the purpose for which they are used.
(ii) Fluid carrying lines, fluid system parts, wiring, air ducts, fittings and powerplant controls with the capacity to perform their intended functions under the heat and other conditions resulting from a fire.
(3) Fire. A fire in either an engine compartment or a designated fire zone is assumed to occur that produces a heat flux on a system, airframe or powerplant component aft of or near the fire. The effect of each such fire on other surfaces must be considered on a case-by-case basis to determine the critical case. Unless a more rationale definition is furnished and approved during certification, the fire in any engine compartment or designated fire zone should be assumed, for purposes of analysis, to be a severe fire (see definition in paragraph AC 29.859).
c. Procedures.
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(1) Other surfaces should be identified during certification by a design review and by a conservative, thorough hazard analysis based on an analytical estimate of the total heat flux (i.e., conduction, convection, and radiation in combination, as applicable) using the definition of a severe fire and of the resultant “other surface” temperature based on a single fire occurring in each engine compartment and designated fire zone, on a case-by-case basis. Once the other surfaces are identified and their severe fire induced maximum temperatures determined, their configuration and material selection should be reviewed on a case-by-case basis to determine either that they are fire resistant, that they can be made fire resistant (within the limits of practicability), or that it is impracticable to make them fire resistant. If the non- fire resistant other surfaces can be readily made fire resistant they should be. If it is impracticable to make them fire resistant, then they should be relocated, insulated, or a combination in order to reduce the total incident heat flux (and, thus, lower their surface temperature) so that they no longer need to be fire resistant. If insulation is used to shield a surface that is subjected to a significant temperature, it must be fire resistant.
(2) A partial validation of analytical heat flux models using the definition of a severe fire can sometimes be achieved during certification tests by using thermocouples or heat-sensitive stickers to measure in-flight temperature ranges and distributions on other surfaces from known thermal environments in engine compartments or other designated fire zones.
AC 29.1195. § 29.1195 (Amendment 29-17) FIR E EXTINGUISHING SYSTEMS.
a. Explanation. This section specifies the types of rotorcraft which must have fire extinguishing systems and the number of discharges. The types of tests and airflow conditions are also specified for demonstration of compliance.
b. Procedures.
(1) The requirements are applicable to each turbine engine powered rotorcraft, Category A reciprocating engine powered rotorcraft, and each Category B reciprocating engine powered rotorcraft with an engine of more than 1,500 cubic inches. There must be a fire extinguishing system for the designated fire zones defined in § 29.1181.
(2) A fire extinguishing system should dilute all of the atmosphere within and entering a compartment with sufficient inert agent that it will not support combustion, and continue the process long enough to extinguish the existing flame and either dissipate the vapors or eliminate the ignition sources. Conventional systems utilize perforated tubing or discharge nozzles to distribute a specific quantity of agent in approximately 2 seconds. HRD (high rate of discharge) systems utilize open end tubes to deliver a given quantity of agent within 1.35 seconds for CO2 and 1 second for all other agents. The HRD systems are recommended for use in compartments having high airflow where the required discharge rates can be more effectively provided by a HRD rather than a perforated tubing system. Tests indicate that unrestricted release through such an open end tube distribution system can be relied on for adequate Page E - 194
distribution, provided the outlets are located properly. Although the discharge times given above are considered satisfactory, any reduction in discharge time below that specified would improve system effectiveness. However, consideration should be given to the time requirements for draining accumulated combustibles, dissipating combustible vapors and cooling or eliminating ignition sources to assure that the minimum agent concentration is maintained for a duration sufficient to prevent reignition of the combustibles.
(3) The possible variety of tankage and plumbing configurations to accomplish the result should be examined for each specific aircraft in order to achieve the optimum.
Systems can vary from tankage in a central location, which is directed through complex distribution systems to various hazards, to agent which is tanked adjacent to each hazard. Terminology generally accepted to define various arrangements is as follows:
(i) Central System: A single supply of agent, centrally located, with valves to direct the agent to any protected zone or zones.
(ii) Individual System: A separate supply of agent for each protected zone or zones.
(4) The selection of the distribution system should be made with full cognizance of the hazards to be covered. The distributor system (i.e., discharge nozzles fed individually by lines from a central manifold) is the most efficient. The complexity of such a system, however, may show it in a less favorable light than the loop or ring system (i.e., orifices drilled in a distribution line, the loop being fed from one end, and the ring being fed from a point on a continuous circle) as far as weight, complexity of manufacture and types of hazard to be covered are concerned. For HRD systems, open feed lines are recommended. In high air flow zones, outlets should be located as far upstream as possible with the discharge directed across the air stream and slightly downstream such that a helical spray pattern is produced. In zones of low or negligible air flow, the outlet location is not critical but a location at the top-center of the zone with the agent directed downward is suggested.
(5) In a conventional CO2 system, all lines upstream of direction valves should be 4,000 PSI (27,600 kPa) burst and lines which are open should be 2,000 PSI (13,800 kPa) burst. Care should be taken to insure that all valving and/or equipment in the distribution line has an appropriate flow rate. Expansion of fittings, tee, etc., should be checked to insure that not over 150 percent of the inflow area exists downstream.
130 percent is accepted as the safest target value. If overexpansion occurs, snow will form and plug the lines. Because of high storage pressure, the orifice areas of a conventional CO2 system seem to act as the flow control with system flow losses as a minor effect. Because of this, distribution systems of 50 ft. (15.24m) or less can be satisfactorily computed by the following factor:
(i) Line Area = .10 sq. in./lb CO2/sec (142.2 mm2/kgCO2/sec).
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(ii) Orifice Area = .072 sq. in./lb CO2/sec (102.4 mm2 2/kgCO /sec) (72 per cent of equivalent line area).
(iii) Min. Orifice Size = 1/16 in. (1.6 mm) diameter.
3Br, line and fitting losses become a
(6) I n low pressure systems such as “CB” and CH g reater effect in the discharge rates and distribution than was true with CO2 2 .
Consideration should be given to the small I.D. of an AN line fitting with respect to the I.D. of the mating tube sizes. This may be done by extra pressure drop allowances, by enlarging these fittings, or by making special fittings. Within reasonable line lengths, however, area factors can be used with fair accuracy. (It is generally conceded that a system designed to these factors, especially a complex layout, should be carefully tested or analyzed for time of discharge and distribution.) These areas are as follows:
(i) Line Area = .07 sq. in./lb agent/sec (99.6 mm2/kg agent/sec).
(ii) Orifice Area = .05 sq. in./lb agent/sec (71.1 mm /kg agent/sec) (72 perc ent of equivalent line area).
(iii) Min. Orifice Area = 1/32 in. (.8 mm) diameter.
(7) For HRD systems of all types, feed line cross-sectional area is dependent upon the rate desired and upon system volume considerations. The minimum diameter of the feed line is established by the required rate; the maximum diameter of the feed line, and by the need for keeping the system volume at a minimum. Specifically, with the propelling gas in a system pressurized to 400 PSI (2760 kPa), the “volumetric efficiency” should be at least 0.50; that is, the original volume of the propelling gas in the system should be at least ½ the volume of the entire system, including that of the agent container. It is recommended that for HRD systems the feed lines be open. No nozzles or series of perforations are required. It is believed that the unrestricted release of the more volatile liquid agents, as well as carbon dioxide, can be relied upon for adequate distribution, provided the outlets are properly located. It is important that any such system be carefully tested for time of discharge, distribution, and minimum concentrations.
(8) From the basic definition, the system should be effective if the distribution of the agent floods the various portions of a compartment simultaneously and dilutes the incoming air. It is noted that the typical high flow compartment requires a greater proportion of its total agent discharged at the air inlet than does the conventional low air flow zone. All parts of the fire extinguisher system directed to any one powerplant installation should be discharged simultaneously. The theory behind the HRD type system is that with rapid discharge of the agent, the concentration necessary for extinguishment is reached more rapidly with correspondingly less time for dissipation or dilution of the agent by incoming air. The duration of this critical concentration necessary for extinguishment is believed to remain the same as for conventional systems.
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