11 Figure 2. Halon Concentration Time History
8.4 This simulation of cargo loading assumes that the Halon 1301 concentration is homogeneous throughout the compartment, and that the volume taken up by the loaded cargo is uniformly distributed throughout the compartment. Both of these assumptions are not true in an actual loaded compartment, so caution should be exercised to relate the measurements taken to an actual loaded compartment in flight. Figure 2 shows a time history of the volumetric concentration of a fire extinguishing agent (Halon 1301) in a cargo compartment with various percentages of cargo loaded. Historically, the time that it takes for the airplane ventilation system to completely replace the volume of air in a cabin or cargo compartment has been called the air exchange rate. The effective time used in figure 2 of this AC refers to the time that it would take to completely remove the entire quantity of fire suppression agent once it is discharged. This depends on the specific design of the cargo compartment (e.g., volume of the compartment, the amount of cargo present, sources of air entry, and sources of air and suppression agent leakage).
8.5 Analysis should be provided to ensure that the suppression agent will not fall below the minimum concentration levels with a cargo load factor as follows:
8.5.1 For cargo compartments using only standard cargo containers, the maximum possible volume occupied by containerized cargo should be determined, and this value should be used as the cargo load factor. This maximum volume becomes an airplane operating limitation.
8.5.2 For all other configurations, a minimum cargo load factor of 75 percent by volume should be used in the analysis.
8.6 Appendix A to this AC provides guidance on analyzing Halon 1301 concentration levels.
8.7 The suppression system certification test should be conducted, at a minimum, during steady-state cruise with a maximum cabin-to-ambient pressure differential. The ventilation system should be configured according to the airplane flight manual (AFM) procedures for a cargo compartment fire. The system should also be demonstrated acceptable for unpressurized flight conditions unless there is a limitation on unpressurized flight for the airplane.
8.8 Cargo compartment leakage rates will vary between airplanes. This is especially significant for changes introduced by a supplemental type certificate modifying airplanes that have been in service. Some preliminary testing should be done to determine the maximum leakage rates seen to date and expected in service. For new type designs, the issue of wear and tear on the compartment (e.g., cargo compartment door seals, fire resistant tape used on adjacent liner material, etc.) should also be addressed when establishing the decay rate in a brand new airplane at the factory. For example, cargo compartment door seals present during the flight test should reflect the worn state prior to replacement according to instructions for continued airworthiness.
Prior to flight testing and during the conformance inspection of the airplane, it should be confirmed that the cargo compartment door seals represent a worn state per maintenance instructions provided by the airplane manufacturer for seal replacement.
Note: Due to EPA restrictions, an FAA-accepted halon simulant, such as HFC-125, should be used for testing (see section 11 of this AC).
9 EVALUATION OF ALTERNATIVE GASEOUS EXTINGUISHING/SUPPRESSION SYSTEMS AND ALTERNATIVE AGENTS.
9.1 The Montreal Protocol, in existence since 1987, is an international agreement to phase out production of ozone-depleting substances, including halogenated hydrocarbons, also known as halon. The Montreal Protocol prohibits the manufacture or import of new halon in all developed countries as of January 1, 1994, and will extend this prohibition to developing countries in the future. The U.S. EPA has subsequently released a regulation banning the intentional release of halons during repair, testing, and disposal of equipment containing halons and during technician training. However, the EPA has provided the aviation industry an exemption from their ban on the intentional release of
13 halons in determining compliance with airworthiness standards. A European Union (EU) regulation1 also contains provisions that allow exemptions for critical uses of halon, including fire extinguishing in aviation. The EPA/EU exemption is predicated on the basis that there is currently no suitable alternative agent or system available for use on commercial transport category airplanes. It is the understanding of the FAA and the European Aviation Safety Agency that once a suitable replacement extinguishing agent or system has been found, the EPA/EU will remove the exemption.
9.2 To date, the FAA Technical Center testing of alternative gaseous extinguishing/suppression agents has not yielded any acceptable alternative halon replacement agents for use in cargo compartments. For example, testing at the Technical Center using HFC-125 demonstrated the need for large concentrations of this agent, which would carry weight penalty and toxicity concerns. In addition, HFC-125 may exhibit significant over-pressure events when sub-inerting concentrations of the agent are introduced into a fire under certain environmental conditions (e.g., small amount of agent, intense fire, low ventilation rate, etc.). The Technical Center will continue to pursue this line of research to identify alternative gaseous, liquid, and other fire extinguishing/suppression agent systems. Acceptable means of compliance for these immature systems are beyond the scope of this AC. We will revise this AC when suitable standards are developed for these systems.
9.3 If the proposed design will use an alternative agent or alternative gaseous fire extinguishing system in lieu of a Halon 1301 system, then the recommended approach would be to (1) perform testing that meets the minimum performance standards for that application as developed by the International Halon Replacement Working Group, and
(2) address installation issues. The International Halon Replacement Working Group was established in October 1993. This group was tasked to work towards the development of minimum performance standards and test methodologies for non-halon aircraft fire suppression agents/systems in cargo compartments, engine nacelles, hand fire extinguishers, and lavatory trash receptacles. The International Halon Replacement Working Group has been expanded to include all system fire protection research and development for aircraft and now carries the name International Aircraft Systems Fire Protection Working Group. Completion of the testing according to the minimum performance standards does not mean that an agent is approved for use on an airplane.
Concentration testing should be conducted to ensure the installation provides acceptable concentration levels anywhere within the cargo compartment where cargo can be carried. Additional testing, including full scale fire extinguishing testing within an actual cargo compartment or high-fidelity model simulator, may be necessary. An applicant proposing to use a non-halon fire extinguishing agent should contact the FAA as soon as possible to enable the FAA to consider their design, determine what testing is required, and to propose any additional testing to ensure the applicant’s design is acceptable.
1 Regulation (EC) No. 2037/2000 of the European Parliament and of the Council of 29 June 2000 on substances that deplete the ozone layer.
9.4 To ensure compliance when using the guidance in this AC, the following should be provided for gaseous agent systems:
9.4.1 The test data and gaseous agent distribution profiles that meet the certification criteria and in the minimum performance standards as developed by the FAA Technical Center as part of the International Halon Replacement program (see paragraph 4.4 of this AC);
9.4.2 A system description document that includes a description of the distribution of the gaseous agent under the test conditions in the cargo compartment;
9.4.3 A detailed test plan; and
9.4.4 Chemical data that describes the agent and any toxicity data.
9.5 Pre-Test Considerations.
9.5.1 An analyzer (e.g., Halonizer, Statham-derivative analyzer) capable of measuring the agent distribution profile in the form of volumetric concentration is required.
9.5.2 An analyzer (e.g., Halonizer, Statham-derivative analyzer) and associated hardware should be configured for the particular application.
9.5.3 The fire suppression system should be completely conformed prior to the test.
9.5.4 The fire extinguisher bottle(s) should be serviced and prepared for the prescribed test(s).
9.6 Test Procedures.
9.6.1 Perform the prescribed distribution test in accordance with the test plan. See section 7 of this AC for guidance on probe placement.
9.6.2 An analyzer (e.g., Halonizer, Statham-derivative analyzer) should record the distribution profile as volumetric concentration for the agent.
9.7 Test Result Evaluation.
9.7.1 Produce the data from the analyzer (e.g., Halonizer, Statham-derivative analyzer) in graphical format. This format should be the volumetric concentration of the agent versus time. A specific percent volumetric initial concentration and a specific percent volumetric metered concentration for the length of the test duration, as determined by previous testing conducted according to the established minimum performance standards, is required for airworthiness approval of cargo compartment systems.
9.7.2 Using the appropriate minimum performance standards evaluation criteria, evaluate the distribution profile of the agent for acceptable performance. The acceptability of the test data would be dependent on the distribution profile and duration exhibited by each probe according to section 7 and paragraph 9.7.1 of this AC for cargo compartment fire extinguishing systems.
15 10 EVALUATION OF ALTERNATIVE LIQUID AGENT AND FIRE EXTINGUISHING/SUPPRESSION SYSTEMS.
10.1 The FAA Technical Center released a technical note (see paragraph 4.4.4 of this AC) that represents the latest minimum performance standards for a water spray system.
However, as mentioned within the body of the report, additional developmental testing may be needed for such a system. Additional testing would be required to demonstrate compliance with an aerosol spray-can fire threat. The Technical Center continues to perform research towards identifying alternative liquid and other fire extinguishing/suppression systems. Acceptable means of compliance for these immature systems are beyond the scope of this AC. We will revise this AC when suitable standards are developed for these systems.
10.2 If the proposed design will use a liquid fire extinguishing agent or system, or any non-halon agent, the applicant should contact the FAA. The FAA may initiate an issue paper addressing the use of an alternative fire extinguishing agent or system.
11 USE OF SIMULANTS FOR CERTIFICATION TESTING.
11.1 The aviation industry may continue to use halon in cargo fire suppression applications as long as acceptable alternatives have not been identified and shown to provide an equivalent level of safety. The EPA/EU is allowing the aviation industry to use halon to demonstrate system functionality as long as a simulant or alternative extinguishing agent or alternative fire extinguishing system cannot be used in place of the halon during system or equipment testing for technical reasons. However, certain U.S. states continue to ban the release of halon for testing. The FAA Technical Center and the International Aircraft Systems Fire Protection Working Group are concentrating efforts on evaluating alternative fire extinguishing agents and the use of simulants during certification testing. An effective simulant (HFC-125) has been used in place of Halon 1301 during certification tests of engine nacelle fire extinguishing systems to predict actual Halon 1301 volumetric concentration levels. The FAA recommends that all applicants consider use of a simulant in lieu of discharging Halon 1301.
11.2 Testing at the FAA Technical Center successfully demonstrated that there is a suitable simulant (HFC-125) for cargo compartment and engine nacelle gaseous fire extinguishing systems (see paragraphs 4.4.5 and 4.4.6 of this AC). It appears that HFC-125 can be used as a simulant for Halon 1301 provided that suitable conversion equations are used to correlate the concentrations of Halon 1301 and HFC-125. To ensure acceptable, successful means of compliance, the same information as outlined in section 7 of this AC should be provided.
11.3 A simulant is defined in this AC as a chemical agent that adequately imitates the discharge and distribution characteristics of a given extinguishing agent. It need not be an actual fire suppressant. For certain cases due to, for example, cost of the extinguishing agent, problems with supply of the extinguishing agent, impact to the environment from discharge of the agent, etc., it may be more appropriate to use a
16 simulant in the application. If an applicant proposes a simulant, they should complete adequate analysis and testing to establish the validity of the simulant across the operating environment of the airplane application. As a minimum, corroborating information would need to be provided as to the detailed chemical analysis of the simulant and evaluation testing of the fire extinguishing system operated with the simulant that demonstrates the equivalent behavior. To ensure compliance when using the guidance in this AC, the following should be provided to the FAA to corroborate the applicant’s findings:
11.3.1 Test data and distribution profiles using the simulant that meet the approved certification criteria and meet the minimum performance standards as developed by FAA Technical Center as part of the International Aircraft Systems Fire Protection Working Group (see paragraph 4.4 of this AC);
11.3.2 A system description document that includes a description of the distribution of the simulant under the test conditions in the cargo compartment;
11.3.3 A detailed test plan; and
11.3.4 Chemical data that describes the simulant and any toxicity data.
Note: Any unique observances during development testing or minimum performance standards testing will need to be addressed appropriately prior to the start of airplane certification testing.
11.4 The distribution of the simulant in the application should be compared with Halon 1301 under the following conditions:
11.4.1 Given the same filling conditions, the simulant is loaded into the fire extinguisher bottle based on an equivalent liquid fraction to the Halon 1301 charge weight required. This is an equivalent statement to the mass of the simulant being a specific percentage of the Halon 1301 charge weight required.
11.4.2 The fire extinguisher bottle containing the simulant is pressurized with nitrogen in an identical manner required by the Halon 1301 charge weight.
11.4.3 The simulant is discharged into the test environment (i.e., cargo compartment).
11.5 Pre-Test Considerations.
11.5.1 An analyzer (e.g., Halonizer, Statham-derivative analyzer) capable of measuring the simulant distribution profile in the form of volumetric concentration is required.
11.5.2 An analyzer (e.g., Halonizer, Statham-derivative analyzer) and associated hardware should be configured for the particular application.
11.5.3 The fire suppression system should be completely conformed for Halon 1301.
11.5.4 The fire extinguisher bottle(s) should be serviced and prepared for the prescribed test(s).
11.6 Test Procedures.
11.6.1 Perform the prescribed distribution test in accordance with the test plan. See section 7 of this AC for guidance on probe placement.
11.6.2 An analyzer (e.g., Halonizer, Statham-derivative analyzer) should record the distribution profile as volumetric concentration for the simulant.
11.7 Test Result Evaluation.
11.7.1 Produce the data from the analyzer (e.g., Halonizer, Statham-derivative analyzer) in graphical format. This format should be the volumetric concentration of the simulant versus time. A specific percent volumetric initial concentration and a specific percent volumetric metered concentration for the length of the test duration as determined by previous testing conducted according to the established minimum performance standards should be included.
11.7.2 Using the Halon 1301 certification criteria, evaluate the distribution profile of the simulant for acceptable performance. The acceptability of the test data would be dependent on the distribution profile and duration exhibited by each probe.
12 ESTABLISHING DURATION FOR THE SUPPRESSION SYSTEM.
12.1 The adequacy of the capacity of the “built-in system” means that there is sufficient quantity of agent to extinguish or suppress the fire anywhere baggage and cargo is placed within the cargo compartment for the duration required to land and evacuate the airplane. Current built-in cargo fire extinguishing systems use Halon 1301 as the fire extinguishing agent where protection is afforded as long as the minimum concentration levels in the cargo compartment do not drop below 3 percent by volume. The time for which a suppression system will maintain the minimum required concentration levels should be identified as a certificate limitation.
12.2 The applicant should work with the airplane owner/operator and the civil aviation authority providing operational approval to ensure that the cargo compartment fire extinguishing system is sized adequately to provide the required protection for the specific route structure. Protection time should account for some holding time to allow for weather and other possible delays, and may specify the speeds and altitudes used to calculate airplane diversion times based on one-engine-out considerations.
12.3 Operational approval for the airplane should include the maximum allowable time, following the discovery of a fire or other emergency situation, required to divert the airplane to an alternative landing site. The maximum allowable time should include a 15-minute allowance for holding and/or approach and landing in addition to the actual time required to reach the alternative landing site and account for the completion of an evacuation of all occupants.
12.4 AC 120-42B and AC 135-42 provide guidance on an acceptable means for obtaining approval under applicable operational rules for two-engine airplanes operating over a route that contains a point farther than one hour’s flying time at the normal one-engine inoperative cruise speed (in still air) from an adequate airport. The ACs include specific criteria for deviations of 75 minutes, 120 minutes, 180 minutes, and other authorized operations from an adequate airport plus an allowance for 15-minute holding and/or approach and landing.
12.5 Flight-test data, supplemented by analysis for cargo load factors and additional metering system bottles as applicable, determines the maximum protection time provided by the cargo fire extinguishing system. This maximum protection time may not be the same as the maximum allowable time required to divert the airplane. The certificate limitation for total time, including the 15-minute allowance for holding and/or approach and landing as applicable, should never be greater than the maximum protection time provided by the cargo fire extinguishing system.
12.6 The following examples illustrate these issues:
12.6.1 Example 1:
Maximum Protection Time Provided By cargo fire extinguishing system = 127 minutes Maximum planned diversion time = 112 minutes + 15 minutes Certificate limitation for total time = 127 minutes Note: This example includes an allowance of 15 minutes for holding and/or approach and landing.
12.6.2 Example 2:
Maximum Protection Time Provided By cargo fire extinguishing system = 68 minutes Maximum planned diversion time = 60 minutes + 15 minutes Certificate limitation for total time = 68 minutes (less than the maximum planned diversion time) Note: In this example, the system would not support the planned route. The FAA could approve the route if additional suppression agent was provided that would extend the suppression time to 75 minutes.
19 13 AIRPLANE MANUAL CONSIDERATIONS.
To ensure fire extinguishing/suppression system effectiveness and continued safe flight and landing, the applicable airplane manuals should contain appropriate directives, for example:
13.1 Any procedures related to fighting a cargo compartment fire should be clearly defined in the AFM.
13.2 AFMs should contain instructions to land at the nearest adequate airport (or suitable airport for ETOPS) following detection of a cargo fire.
13.3 Cargo loading restrictions (certified type of loading according to compartment, limits for loading heights and width, etc.) should be clearly described in the weight and balance manual or any other appropriate airplane manual.
13.4 Where the use of airplane manuals is considered to be impractical during cargo loading activities, all necessary information may be introduced in crew operating manuals or part of dedicated instructions for cargo loading personnel.
14 PLACARDS AND MARKINGS IN CARGO COMPARTMENTS.
14.1 Experience has shown that under certain circumstances and despite clear instructions in the applicable airplane documentation, cargo loading personnel may not follow loading restrictions. Pallets have been loaded higher than certified, or bulk cargo may be stowed up to the ceiling, adversely affecting smoke detection and fire protection/fire suppression system effectiveness. To visually indicate the applicable loading restrictions to each person responsible for cargo-loading activities in a compartment, placards and markings indicating the maximum loading height and widths for the approved configuration should be installed in that compartment. The airplane’s weight and balance manual should describe these loading restrictions.
14.2 For the design (shape, size, color, and brightness) of these indications, illumination conditions in the compartment should be considered. Markings and placards should not be easily erased, removed, disfigured, or obscured. Further guidance may be derived from compliance demonstrations for part 25 paragraphs regulating other internal markings and placards, for example, in the cockpit or passenger compartment.
Appendix A A-1 Appendix A. Analytical Methods for Determining Halon 1301 Concentration Levels A.1 PURPOSE.
This appendix contains analytical methods for determining Halon 1301 fire extinguishing agent concentration levels in empty or loaded cargo compartments as a function of time.
A.2 EXPLANATION OF TERMS AND SYMBOLS.
Table A-1. Terms and Symbols
SYMBOL DESCRIPTION UNITS CONSISTENT WITH EQUATIONS C(t) Halon 1301 concentration by volume at time (t).
= 𝑉𝐻𝑎𝑙𝑜𝑛 1301 𝑉 Dimensionless VHalon 1301 Volume of Halon 1301 in cargo compartment.
Cubic feet (ft3) Cubic meters (m3) V Cargo compartment free volume
(i.e., volume not occupied by cargo).
= − ( 𝑉𝑐𝑎𝑟𝑔𝑜 𝑉𝑒𝑚𝑝𝑡𝑦 ) Cubic feet (ft3) Cubic meters (m3) Vcargo Cargo volume. Cubic feet (ft3) Cubic meters (m3) Vempty Empty cargo compartment volume. Cubic feet (ft3) Cubic meters (m3) T Time. Minutes (min) E Cargo compartment leakage rate. Cubic feet per minute (ft3/min) Cubic meter per minute (m3/min) S Specific volume of Halon 1301. Cubic feet per pounds (mass) (ft3/lbm) Cubic meters per kilogram (m3/kg) R Halon 1301 flow rate. Pounds (mass) per minute (lbm/min) Kilogram per minute (kg/min)
Appendix A A-2 A.3 HALON 1301 CONCENTRATION LEVEL MODEL.
A.3.1 Cargo compartment fire extinguishing systems generally use a combination of one or two types of Halon 1301 discharge methods. One type rapidly releases all of the fire extinguishing agent from one or more pressurized bottles into the cargo compartment.
This type of discharge method is commonly known as a high-rate discharge or ‘dump’ system.
A.3.2 The second type of Halon 1301 discharge method slowly releases the fire extinguishing agent from one or more pressurized bottles into the cargo compartment. This type of discharge method is commonly known as a metering system.
A.3.3 The following list provides some examples, not all-inclusive, of different combinations of these Halon 1301 discharge methods:
A.3.3.1 One high-rate discharge.
A.3.3.2 One high-rate discharge followed by a second high-rate discharge at a specified later time.
A.3.3.3 One high-rate discharge followed by a metered discharge at a specified later time.
A.3.3.4 Simultaneous high-rate and metered discharges.
A.3.4 The Halon 1301 fire extinguishing system that incorporates both high-rate and metered discharges, as in paragraph A.3.3.4, is illustrated in figure A-1 of this AC.
Appendix A A-3 Figure A-1. Example – Halon 1301 Model
I II III Time Concentration FIGURE 3-1 EXAMPLE – HALON 1301 MODEL
A.3.5 Prior to Phase I: Initial High-Rate Discharge of Halon 1301.
This portion of the extinguishing process illustrates the high-rate discharge method of releasing all of the fire extinguishing agent from one or more pressurized bottles into the cargo compartment.
A.3.6 Phase I: Exponential “Decay” of Halon 1301.
A.3.6.1 The beginning of Phase I represents the initial concentration of Halon 1301 used to knock down a cargo fire. Since no more Halon 1301 is introduced into the cargo compartment during Phase I, the concentration of Halon 1301 undergoes an exponential “decay” versus time.
A.3.6.2 The governing equation for exponential “decay” during Phase I is the following:
∁(𝑡) = ∁(0)𝑒−𝐸𝑡𝑉 Note: “C(0)” is the initial concentration of Halon 1301 used to knock down a cargo fire at the beginning of Phase I, and “t” is the time elapsed since the beginning of Phase I.