U.S. Department of Transportation Federal Aviation Administration 

# Advisory Circular

Subject: Built-in Fire Extinguisher/Fire Suppression Systems in Class C and Class F Cargo Compartments Date: DRAFT Initiated By: ANM-112 AC No: 25.851-X

This advisory circular (AC) provides guidance concerning comp liance with the airworthiness standards for transport category airplanes pertaining to Class C and Class F cargo compartments that incorporate built-in fire extinguisher/suppression systems. Th is guidance is applicable to transport category airplanes for which a new, amended, or supplemental type certificate is requested. This guidance applies to Class C cargo compartments with built-in fire extinguisher/suppression systems regardless of their location (i.e., lower lobe, main deck, or upper deck) and to Class F cargo compartments, which must be located on the main deck to meet the intent of the rule and be acces sible in flight to crewmembers.XX/XX/XX D R A F T AC 25.851-X

ii Contents Paragraph Page

1 Purpose. ...................................................................................................................... ................1

2 Applicability. ................................................................................................................ .............1

3 Cancellation. ................................................................................................................. .............1

4 Related Documents. ............................................................................................................ .......2

5 Definitions.................................................................................................................... ..............2

6 Background. ................................................................................................................... ............3

7 Compartment Classification. ................................................................................................... ...4

8 Testing Volumetric Concentration Levels. ................................................................................6

9 Airplane Test Conditions. ..................................................................................................... .....8

10 Evaluation of Alternate Gaseous Extinguishing/ Suppression Systems and Alternate Agents.11

11 Evaluation of Alternate Liquid Agent and Fi re Extinguishing/Suppression Systems. ............13

12 Use of Simulants for Ce rtification Testing. .............................................................................14

13 Establishing Duration for the Suppression System. .................................................................16

14 Airplane Manual Considerations. ............................................................................................17

15 Placards and Markings in Cargo Compartments. ....................................................................18

Appendices

Appendix A. Analytical Methods for Determ ining Halon 1301 Concentration Levels. ............ A-1 Appendix B. References ........................................................................................................ ..... B-1

Figures

Figure 1. Effect of Cargo Load on Halon 1301 Concentration Levels ........................................... 9 Figure 2. Halon Concentrat ion Time History ............................................................................... 10

Tables

Table A-1. Terms and Symbols .................................................................................................. A-1XX/XX/XX D R A F T AC 25.851-X

1 1 PURPOSE. This advisory circular (AC) provides guidance con cerning compliance with the airworthiness standards for transport categ ory airplanes pertaining to Class C and Class F cargo compartments that incorpor ate built-in fire ex tinguisher/suppression systems. This guidance is applicable to tr ansport category airplanes for which a new, amended, or supplemental type certificate is requested. This guidance applies to Class C cargo compartments with built-in fire exti nguisher/suppression sy stems regardless of their location (i.e., lower lobe, main dec k, or upper deck) and to Class F cargo compartments, which must be located on the main deck and be accessible in flight to crewmembers.

2 APPLICABILITY.

2.1 The guidance in this documen t is for airplane manufactur ers, modifiers, foreign regulatory authorities, and Federal Aviati on Administration (FAA) transport airplane type-certification engine ers and their designees.

2.2 The material in this AC is neither manda tory nor regulatory in nature and does not constitute a regulation. It describes accep table means, but not the only means, for demonstrating compliance with the applicable regulations. The FAA will consider other methods of demonstrating compliance that an applicant may elect to present. While these guidelines are not mandatory, they ar e derived from extensive FAA and industry experience in determining compliance with the relevant regulations. If, however, we become aware of circumstances that convi nce us that following this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additi onal substantiation as a basis for finding compliance.

2.3 The material in this AC does not chan ge or create any addi tional regulatory requirements, nor does it authorize changes in, or permit deviations from, existing regulatory requirements.

2.4 Except in the explanations of what the regulations require, the term “must” is used in this AC only in the sense of ensuring app licability of this particular method of compliance when the acceptable method of co mpliance described in this AC is used.

3 CANCELLATION. The following FAA policy is cancelled by this AC:

 Memorandum TAD 95-008, dated August 11, 1995, Guidance on Protection Afforded by Halon Fire Extinguishing Systems in Class C Cargo Compartments

.

 Memorandum 98-112-24, dated March 9, 1998, Halon Concentration Levels

.XX/XX/XX DRAFT AC 25.851-X

2  Memorandum 98-113-143, dated October 29, 1997, Standards for Cargo or Baggage Compartments.

4 RELATED DOCUMENTS.

4.1 Sections of Title 14, Code of Federal Regulations (14 CFR):

 Section 25.851(b), Fire extinguishers.

 Section 25.855, Cargo or baggage compartments.

 Section 25.857, Cargo compartment classification.

 Section 25.858, Cargo or baggage compartment smo ke or fire detection systems.

 Section 121.314, Cargo and baggage compartments.

4.2 FAA Advisory Circulars.
This list identifies the current version of each AC at the time of publication of this AC. If any AC is revised after publ ication of this AC, you should refer to the latest version for guidance. You can find the latest version of the following ACs at http://www.faa.gov/regulations_po licies/advisory_circulars/

.

 AC 20-42D, Hand Fire Extinguishers for use in Aircraft , issued January 14, 2011.

 AC 25-9A, Smoke Detection, Penetration, and Ev acuation Tests and related Flight Manual Emergency Procedures , issued January 6, 1994.

 AC 25-17A, Transport Airplane Cabin Interiors Crashworthiness Handbook , issued May 18, 2009.

 AC 25-18, Transport Category Airplanes M odified for Cargo Service , issued January 6, 1994.

 AC 25-22, Certification of Transport Ai rplane Mechanical Systems , issued March 14, 2000.

 AC 25.857-X, Class B and F Cargo Compartments , issued ................201X.

 AC 120-42B, Extended Operations (ETOPS and Polar Operations) , issued June 13, 2008.

 AC 135-42, Extended Operations (ETOPS) and Op erations in the North Polar Area , issued June 10, 2008.

4.3 FAA Order 8150.4, Certification of Cargo Containers with Self-Contained Temperature Control Systems (Active ULDs) , dated January 14, 2011.

5 DEFINITIONS. The terms “extinguishing system” and “suppr ession system” are used interchangeably in this AC. Systems that are located in remo te (e.g., inaccessible) areas are not requiredXX/XX/XX DRAFT AC 25.851-X

3 to extinguish a fire in its entirety. These sy stems are intended, instead, to suppress a fire until it can be completely extinguished by ground personnel following a safe landing.

6 BACKGROUND.

6.1 Paragraphs (c) and (f) of § 25.857 provide sta ndards for certification of two classes of cargo compartments, Class C and Class F. A Class C cargo compartment is configured in a manner that incorporates a built-in fi re extinguisher/fire s uppression system to extinguish or control any fire likely to occur in the cargo compartment. Therefore, the means to extinguish a fire applies to bul k-loaded cargo, cargo loaded on pallets, and cargo within unit load devices. A Class F cargo compartment must be accessible in flight. Both Class C and Class F cargo comp artments have fire or smoke detection systems to alert the crew to the presence of the fire anywhere within the cargo compartment (i.e., anywhere car go is designed to be carried).

6.2 A Class F cargo compartment that uses a bui lt-in fire suppression system should meet the same requirements as a Class C cargo compartment. For example, per §§ 25.851, 25.855, and 25.857 a Class F cargo compartment with a built-in fire suppression system must ensure that:

6.2.1 No discharge of the extinguisher can cause structural damage;

6.2.2 The capacity of each required built-in extinguishing system must be adequate for any fire likely to occur in the compartment where used, considering the volume of the compartment and the ventilation rate;

6.2.3 There are means to exclude hazardous quant ities of smoke, flames, or extinguishing agent, from any compartment occupied by the crew or passengers;

6.2.4 There are means to control ventilation and dr afts within the compartment so that the extinguishing agent used can control any fi re that may start within the compartment; and

6.2.5 Flight tests are conducted to ensure compliance with th e provisions in §§ 25.857(f), 25.855(h), 25.858, and 25.1301(a).

6.3 Cargo fire extinguishing systems installed in airplanes today primarily use Halon 1301 as the fire suppression agent. One widely used method to certify Halon 1301 cargo fire suppression systems requires an initial concen tration of 5 percent by volume in order to knock down a cargo fire. Subsequent concentr ation levels should not drop below 3 percent by volume for the remainder of the flight in order to suppress a cargo fire until it can be completely extinguished by ground personnel following a safe landing.

6.3.1 Since Halon 1301 is approximately five times heavier than air, it tends to stratify and settle after it is released into the cargo compartment. Also, due to temperature differences and ventilation patterns in a vent ilated compartment, Halon 1301 starts to stratify shortly after discharg e and the concentration level decays faster in the upperXX/XX/XX DRAFT AC 25.851-X

4 locations of the compartment than in the lower locations. Halon 1301 also has a tendency to move aft due to a ny upward pitch, or forward in any downward pitch of the airplane in flight. For some airplane certifi cation programs, the concentration levels of Halon 1301 have been measured at va rious locations th roughout the cargo compartment, and an arithmetic average of the individual sampling locations has been used to determine an overall concentration level for the cargo compartment. This averaging technique may allow the concen tration level to dr op below 3 percent by volume at individual sampling locations n ear the top of the cargo compartment.

6.3.2 Testing data from the FAA William J. H ughes Technical Center (see Appendix B for applicable references) and other data from standardized fire extinguishing evaluation tests indicate that the use of averaging techniques may not substantiate that there are adequate concentration levels of fire exti nguishing agent throughout the compartment to effectively suppress a cargo fire. If a car go fire occurred and was subsequently suppressed by Halon 1301, the core of the fire could remain hot for a period of time. If the local concentration of Halon 1301 in the vi cinity of the fire core dropped below 3 percent by volume, and sufficient oxygen is available, re-ignition could occur. FAA tests have shown that when the Halon 1301 concentration level drops below 3 percent by volume and the cargo fire reignites, the c onvective stirring cause d by the heat of the fire may be insufficient to raise the local c oncentration of Halon in the vicinity of the fire. Therefore, compliance testing should i nvolve the use of point concentration data from each probe. Additionally, the probes cl osest to the cargo compartment ceiling should be at least at the highe st level that cargo and baggage can be loaded as specified by the manufacturer and certified by the appropriate airworthiness authority. In addition, certification test da ta acquisition should include analysis and/or data taken after landing at a time increment which represents the completion of an evacuation. Due to the ozone depleting characteristics of halon, the U.S. Environmental Note: Protection Agency (EPA) has established re strictions on using halon for testing. Applicants should instead use a halon simula nt, such as pentafluoroethane (HFC-125), for tests. Applicants will find useful info rmation and technical reports on HFC-125 at the FAA William J. Hughes Technical Center Fire Safety Branch website, http://www.fire.tc.faa.gov.

7 COMPARTMENT CLASSIFICATION. All cargo compartments must be properly classified in accordance with § 25.857 and meet the requirements of §§ 25.851, 25.855, 25.857 and 25.858. In order to establish appropriate requirements for fi re protection, a system for classification of cargo or baggage compartments was developed and a dopted for transport category airplanes. Classes A, B, and C were initially establishe d; Classes D, E, and F were added later. Class D was later eliminated from the regulations. The classification is based on the means by which a fire can be detected and the means available to control the fire.

7.1 A Class A compartment (see § 25.857(a)) is one th at is located so close to the station of a crewmember that the crewmember would di scover the presence of a fire immediately. In addition, each part of the compartment is easily accessible so that the crewmemberXX/XX/XX DRAFT AC 25.851-X

5 could quickly extinguish a fire with a portable fire extingu isher. A Class A compartment is not requ ired to have a liner.

7.2 A Class B compartment (see § 25.857(b)) is one that is accessible in flight but more remote than a Class A compartment and must , therefore, incorporate a fire or smoke detection system to give warning at the pilo t or flight engineer station. Because a fire would not be detected and extinguished as quickly as in a Class A compartment, a Class B compartment must have a liner in accordance with § 25.855(b). In flight, a crewmember must have sufficient access to a Class B compartment to effectively fight a fire with a hand fire extinguish er in any part of the comp artment when standing at any one access point, without stepping into the compartment. There must be means to ensure that, while the access provisions ar e being used, no hazardous quantity of smoke, flames, or extinguishing agent will enter areas occupied by the crew or passengers.

7.3 A Class C compartment (see § 25.857(c)) differs from a Class B compartment in that it is not required to be accessible in flight and must, therefore, have a built-in fire extinguishing or suppression system to suppress or control any fire. A Class C compartment must have a liner and a fire or smoke detection system in accordance with §§ 25.855(b) and 25.857(c)(1). There must al so be means to exclude hazardous quantities of extinguishant and products of combustion from occupied areas (see § 25.857(c)(3)).

7.4 A Class D compartment was one in which a fi re would be completely contained without endangering the safety of the airplane or the occupants. A Class D compartment was not accessible to a crewmember in flight, and inst ead of providing smoke or fire detection and extinguishment, Class D compartments we re designed to contro l a fire by severely restricting the supply of avai lable oxygen. Due to several un controllable fires in Class D compartments, they were eliminated by Amendment 25-93, effective March 19, 1998. As explained in Amendment 25-93, Class D cargo compartment standards pre-dating Amendment 25-93 are inadequate. Therefore, the FAA does not approve new Class D cargo compartments even in airplanes w ith a certification basis pre-dating Amendment 25-93.

7.5 A Class E compartment (see § 25.857(e)) is f ound on an all-cargo airplane. Typically, a Class E compartment is the entire cabin of an all-cargo airplane; however, other compartments (e.g., lower lobe inaccessible ca rgo compartments) of such airplanes may be classified as Class E compartments. Shutting off the ventilating airflow to or within the compartment controls a fire in a Class E compartment. A Class E compartment must have a liner and a fire or smoke detecti on system installed in accordance with § 25.857(e)(2). It is not re quired to have a built-in fire suppression system.

7.6 A Class F compartment (see § 25.857(f)) is one that has a means to control or extinguish a fire without re quiring a crewmember to enter the compartment. Class F compartments that include a built-in fire extinguisher/suppre ssion system or require the use of other means acceptable to the FAA w ould meet these requirements. The Class F compartment must have a fire or smoke dete ction system installe d in accordance with § 25.857(f)(1). Unless there are other means of containing the fire and protectingXX/XX/XX DRAFT AC 25.851-X

6 critical systems and structure, a Class F compartment must have a liner meeting the requirements of part III of appendix F to pa rt 25, or other approve d equivalent methods (see § 25.855(b)). The Class F cargo compartm ent was introduced as a practicable and safe alternative to the previous practice of providing large Class B cargo compartments. Class B and Class F cargo compartments are limited to the main deck for accessibility reasons. Lower deck cargo compartments should not be approved as Class F cargo compartments. All lower deck or remote cargo compartments in airplanes carrying passengers need to comply with the Class C cargo compartment requirements of § 25.857(c). There must also be means to ex clude hazardous quantities of extinguishant and products of combustion from occupied areas (see § 25.857(f)(3)).

8 TESTING VOLUMETRIC CO NCENTRATION LEVELS.

8.1 The applicant should demonstrate that th e cargo fire extinguishing system provides adequate concentration levels of extinguishi ng agent to combat a fire anywhere baggage and cargo is placed within the cargo compartm ent for the time duration required to land and evacuate the airplane. A combination of f light testing and analysis may be used. If Halon 1301 is used, an initial minimum c oncentration of 5 percent by volume is required to knock down a cargo fire. Subseque nt gaseous extinguishing agent should, if required for the duration of the flight, be in troduced via a metering or other appropriate system to ensure that point concentration levels do not drop below 3 percent by volume for the remainder of the flight. The duration of agent application should be determined from route analysis (i.e., the time to travel from the farthest distance expected en route to the nearest adequate airport for landing according to applicable operational rules. For extended operation with two-engine airp lanes (ETOPS), appendix K to part 25, AC 120-42B, and AC-135-42 provide regulatory requirements and guidance. These references specify that an analysis or te sts should be conducted to show, considering approved maximum diversion in still air (i ncluding an allowance for 15-minute holding and/or approach and landing), that the abil ity of the system to suppress or extinguish fires is adequate to ensure sa fe flight and landing at a suitabl e airport (e.g., a flight test simulated landing may be used in lieu of an actual landing). The minimum extinguishing agent concentration levels shoul d be maintained for the required duration throughout the cargo compartment where cargo will be carried, including side to side, end to end, and top to bottom. However, f light-test measurements do not have to be made in compartment areas that are desi gnated empty and will not contain cargo.

8.2 The fire extinguishing agent concentration levels should be measured at sufficient vertical, horizontal, and longitudinal locations to ensure that sufficient resolution exists to define the variations in fire extinguishing agent con centration levels throughout the cargo compartment in these planes. Averag ing techniques should not be used in compliance demonstrations for § 25.851(b)(2). An exception to this would be in the event of a probe failure, wher e interpolation of probe concentration data from other nearby probes to yield an estimate of missi ng agent concentration data may be allowed by the FAA. In the event such interpolation is necessary, then a lin ear interpolation of the data will provide an acceptable mean s of approximating the missing data. The applicant should coordinate such s ituations closely with the FAA.XX/XX/XX DRAFT AC 25.851-X

7 8.3 Sampling locations should also be placed as close as practical to potential leakage or ventilation flow areas (e.g., door seals, ve nts, etc.), which can disrupt the local concentration levels. The concentration le vels should not be less than the minimum established for that fire extinguishing agen t at any point within the compartment. Arithmetic averaging of individual sampling locations to determine the concentration levels is not acceptable. The use of averag ed concentration data will no longer be accepted, except in well-defined cases (i.e., where a sensor probe failure occurs and the use of interpolation from adjacent sensor probes is warranted). Compliance with § 25.851(b) involves the use of point-concentration data from each sensor and that the probes closest to the cargo compartment ceilin g should be at least at the highest level that cargo and baggage can be loaded as specified by the manufacturer and certified by the FAA. Other placement of concentration sensor probes within the cargo compartment should be sufficient to substantiate that ther e are adequate concentr ation levels of fire extinguishing agent throughout the compar tment to effectively control a cargo compartment fire. The sampling rate should be sufficient to establ ish a concentration level versus time decay curve. In the event that a single sensor displays a suspect time history, the use of an inter polated time-averaged value may be acceptable. If fire extinguishing agent concentration leve ls at a probe drop below the minimum requirement, it should be a temporary anomal y of short duration a nd not observed in adjacent probes. If it could be demonstrated that the temporary anomaly is associated with normal airplane maneuvers, then the data should be acceptable. The applicant should coordinate such situa tions closely with the FAA.

8.4 Typically there are two type s of extinguishing agent dispensing systems—a flood or dump (high-rate discharge or quick discharge) system and a metered system. The flood or dump system dispenses the agent with th e activation of the system, and a selected amount of agent is injected into the compar tment to suppress the fire. Once the agent concentration level approaches the mini mum sustaining level (i.e., 3 percent for Halon 1301), a second and subsequent discharg e of agent takes pl ace to assure the 3 percent concentration level is maintained for the time necessary to divert to a safe landing. The metered systems usually discharg e agent into the compartment for fire suppression (5 percent for Halon 1301) and th en adds agent in a prescribed amount to the compartment to maintain the 3 per cent concentration le vel (for Halon 1301).

8.5 Certification flight-test demonstration is used in demonstrating compliance for built-in or “dump” systems for the duration of the inte nded diversion profile. If the built-in fire suppression system includes a metering system, then the system’s acceptability may be demonstrated through a limited flight test, in which a portion of the system is actually tested, and the full capability of the system is demonstrated via anal ysis. Issues such as size of the compartment that should be tested (smallest or largest) , the test duration in flight, and whether reliable analytical methods are availa ble to predict concentration levels for various locations and heights in a given cargo compartment will have an impact on certification tests. Th is type of testing and anal ysis of the product should be documented in the applicant certification plan. A sufficient portion of the metering system capability should be demonstrated to provide enough data to establish fire extinguishing agent concentra tion and behavior for the rema ining flight. Airplane climb flight phase and the descent flight phase represent dynamic environments, and no dataXX/XX/XX DRAFT AC 25.851-X

8 need be acquired during these transient fli ght phases where cabin altitude changes would preclude accurate data acquisition. Howe ver, certification data should include analysis and/or data taken after landing at a time incr ement representative of the completion of an evacuation of all occupants. If a fire extinguis hing agent other than Halon 1301 is proposed, the method of complia nce will need to be documented in an issue paper.
Due to EPA restrictions, a halon simulant , such as HFC-125, should be used for Note: testing and would not require specific FAA approval.

9 AIRPLANE TEST CONDITIONS.

9.1 Flight tests are required (i.e ., § 25.855(h)) to demonstrate f unction and dissipation of the fire extinguishing agent in a cargo compartment. For certification test s, the airplane and relevant systems should be in the type design configuration.

9.2 The cargo compartment should be empty for the above test. However, as shown in figure 1, a compartment with cargo may be more time-critical than an emptycompartment for minimum fire extinguishing agent concentration levels. The timecritical nature depends on several factors. Even with a pure “dump” system, having cargo does not necessarily mean a marginally performing system during an empty cargo compartment test will result in a “bad” syst em with cargo. Also, metering systems, if designed properly, are relatively in sensitive to the cargo load factor.XX/XX/XX DRAFT AC 25.851-X

9 Figure 1. Effect of Cargo Load on Halon 1301 Concentration Levels

9.3 A specific example of the effect of cargo compartment loading is shown in figure 2, using the simulation in Appendix A of this AC. If the volume of the compartment is decreased to represent increasing cargo load pe rcentages, and the leakage rate and initial Halon 1301 quantity are kept constant, then the initial Halon 1301 concentrations increase and the concentration decay rates also increase. Using this approach, the concentration in an empty compartment will decay to 3 percent faster than a loaded compartment up to a load percentage of a bout 65.6 percent. With compartments loaded to a higher percentage than 65.6 percent, th e concentration will fall below 3 percent faster than an empty compartment.

9.4 This simulation of cargo loading assume s that the Halon 1301 concentration is homogeneous throughout the compartment, a nd 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 in an actu al loaded compartment in flight.XX/XX/XX DRAFT AC 25.851-X

10 Figure 2. Halon Concentration Time History

9.5 Analysis should be provided to ensure that the suppression agent concentration levels will not fall below the minimum requiremen t with a cargo load factor as follows:

9.5.1 For cargo compartments using only standard cargo containers, the maximum possible volume occupied by containerized cargo should be determined for the product, and this value should be used as the cargo load factor. This maximum volume becomes an airplane limitation.

9.5.2 For all other configurations, a minimum cargo load factor of 75 percent by volume should be used for the product.

9.6 Appendix A to this AC provides guidan ce on analyzing Halon 1301 concentration levels.

9.7 The suppression system certification test s hould be conducted, as a minimum, during steady-state cruise with a maximum cabin-to-ambient pressure differential. The ventilation system should be configured acco rding to the airplane flight manual (AFM) procedures for a cargo compartment fire. The system should also be demonstrated Theoretical Halon Decay

0 3 6 9 12 15 18

0 5 10 15 20 25 30 35 40 45 Time (Minutes)Percent Halon Empty
25% Full
50% Full
65.6% Full
75% Full c=c 0 *exp(-t/ T ) T = Effective time for one air change Comp. Leakage= 60 CFM Comp. Volume= 2357 ft 3 Halon Quantity= 49.6 lbs.XX/XX/XX DRAFT AC 25.851-X

11 acceptable for unpressurised flight conditi ons unless there is a restriction on unpressurised flight for the airplane.

9.8 Cargo compartment leakage rates will vary between airplanes. This is especially significant for changes introduced by a suppl emental type certific ate (STC) modifying airplanes that have been in service. So me preliminary testing should be done to determine the maximum leakage rates seen/exp ected in service. For new type designs the issue of wear and tear on the compartm ent (e.g., cargo compartment door seals, fire resistant tape used on adjacent liner materi al, etc.) should also be addressed when establishing the decay rate in a brand new ai rplane 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.
Due to EPA restrictions, a halon simulant , such as HFC-125, should be used for Note: testing and would not require specific FAA approval.

10 EVALUATION OF ALTERNATE GASEOUS EXTINGUISHING/SUPPRESSION SY STEMS AND ALTER NATE AGENTS.

10.1 The Montreal Protocol, in existence since 1987, is an international agreement to phase out production of ozone-depleting substan ces, including halogenated hydrocarbons, also known as halon. The Montreal Protocol pr ohibits the manufacture or import of new halon in all developed countries as of Ja nuary 1, 1994, and will extend this prohibition to developing countries in the future. Th e U.S. EPA has subsequently released a regulation banning the intentiona l release of halons during re pair, testing, and disposal of equipment containing halons and during t echnician training. However, the EPA has provided the aviation industry an exemption fr om their ban on the intentional release of halons in determining compliance with airworthiness standards. A European (EU) regulation 1 also contains provisions that allow ex emptions for critical uses of halon, including fire extinguishing in aviation. Th e EPA/EU exemption is predicated on the basis that there is currently no suitable alte rnate 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.

10.2 To date, the FAA Technical Center testing of alternate gaseous extinguishing/suppression agents has not yielded any acceptable alternate Halon replacement agents for use in cargo comp artments. For example, testing at the Technical Center using HFC-125 demonstrated the need for large concentrations of this agent that would carry weight penalty and toxicity concerns. In addition, HFC-125 may exhibit significant over pressure events when sub-inerting concentr ations of the agent are introduced into a fire under certain environmental cond itions (e.g., small amount of

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.XX/XX/XX DRAFT AC 25.851-X

12 agent, intense fire, low ventilation rate, et c.). The Technical Center will continue to pursue this line of research to identif y alternate gaseous, liquid, and other fire extinguishing/suppression agent systems. Acceptable means of compliance for these immature systems are beyond the scope of this AC. Future revisions to this AC will be accomplished as soon as suitable standard s are developed for these systems.

10.3 Should the FAA be approached with the intent of using an alternat e agent or alternate gaseous fire extinguishing system in lieu of a Halon 1301 system for the product, then the recommended approach would be to (1) perform testing that meets the minimum performance standards for that applicati on as developed by the International Halon Replacement Working Group, and (2) address installation issues. The International Halon Replacement Working Group was esta blished 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 ex tinguishers, and lavatory trash receptacles. The International Halon Replacement Worki ng Group has been expanded to include all system fire protection resear ch and development for aircraft and now carries the name, International Aircraft Systems Fire Pr otection 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. Concentrati on testing should be conducted to ensure the installation provides acceptable concentr ation levels anywhere within the cargo compartment where cargo can be carried. A dditional testing including full scale fire extinguishing testing within an actual cargo compartment or high-fidelity simulator may be necessary. An applicant proposing to use a non-halon fire extinguishing agent should contact the FAA as soon as possible to en able the FAA to consider their design, determine what testing is required, and to pr opose any additional testing to ensure the applicant’s design is acceptable.

10.4 To ensure compliance when using the means provided by in this AC, the following should be provided for gaseous agent systems:

10.4.1 The test data and gaseous agent distribution profiles that meet the certification criteria and in the minimum performance standards as developed by FAA T echnical Center as part of the International Ha lon Replacement program. See Appendix B of this AC for a list of references.

10.4.2 A system description document that include s a description of the distribution of the gaseous agent under the test condi tions in the cargo compartment.

10.4.3 A detailed test plan.

10.4.4 Chemical data which describes th e agent and any toxicity data.

10.5 Pre-Test Considerations:

10.5.1 An analyzer (for example, Statham-derivativ e analyzer) capable of measuring the agent distribution profile in th e form of volumetric co ncentration is required.XX/XX/XX DRAFT AC 25.851-X

13 10.5.2 An analyzer (for example, Statham-derivativ e analyzer) and associated hardware should be configured for the particular application.

10.5.3 The fire suppression system should be co mpletely conformed prior to the test.

10.5.4 The fire extinguisher bottle(s) s hould be serviced and prepared for the prescribed test(s).

10.6 Test Procedures:

10.6.1 Perform the prescribed distribution test in accordance with the test plan. See paragraph 8 for guidance on probe placement.

10.6.2 An analyzer (for example, Statham-derivativ e analyzer) should r ecord the distribution profile as volumetric concentration for the agent.

10.7 Test Result Evaluation:

10.7.1 Produce the data from the analyzer (for ex ample, Statham-derivative analyzer) in graphical format. This format should be the volumetric concentration of the agent versus time. A specific percent volumetric in itial concentration and a specific percent volumetric metered concentration for the lengt h of the test duration, as determined by previous testing conducted according to the established minimum performance standards, is required for airworthine ss approval of cargo compartment systems.

10.7.2 Using the appropriate minimum performance st andards evaluation criteria, evaluate the distribution profile of the agen t for acceptable performance. The acceptability of the test data would be dependent upon the distribu tion profile and duration exhibited by each probe according to paragraph 10.7.1 above and paragraph 8 for cargo compartment fire extinguishing systems.

11 EVALUATION OF ALTERNATE LIQUID AGENT AND FIRE EXTINGUISHING/SUPPRESSION SYSTEMS.

11.1 The FAA Technical Center released a techni cal note (see Appendix B for applicable references) that represents the latest mini mum performance standards for a water spray system. However, as mentioned within the body of the report, a dditional 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 alternate liquid and other fire extinguishing/suppression systems. Accep table means of compliance for these immature systems are beyond the scope of this AC. Future revisions to this AC will be accomplished as soon as suitable standard s are developed for these systems.

11.2 If the proposed design will use a liquid fire extinguishing agent or system, or any nonhalon agent, the FAA should be contacted. Th e FAA may initiate a ce rtification review item addressing the use of an alternate fire extinguishing agent or system.XX/XX/XX DRAFT AC 25.851-X

14 12 USE OF SIMULANTS FOR CERTIFICATION TESTING.

12.1 The aviation industry may continue to use halon in cargo fire s uppression 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 alternate extinguishing agent or alternate fire extinguishing system cannot be used in place of the halon during system or equipment testing for techni cal reasons. However, certain U. S. states continue to ban the release of halon for testing. The FAA Technical Center a nd the International Aircraft Systems Fire Protection Working Gr oup are concentrating efforts on evaluating alternative fire extinguishing agents and the use of simulants during certification testing. An affective simulant (HFC-125) has been used in place of Halon 1301 during certification tests of engine-n acelle fire extinguishing syst ems 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.

12.2 Testing at the FAA Technical Center successfu lly demonstrated that there is a suitable simulant (HFC-125) for cargo compartment and engine nacelle gaseous fire extinguishing systems (see B.1.5 and B.1.6 ). It appears that HFC-125 can be used as a simulant for Halon 1301 provided that suit able conversion equations are used to correlate the concentrations of Hal on 1301 and HFC-125. To ensure acceptable, successful means of compliance, the same in formation as outlined above in paragraph 8 should be provided.

12.3 A simulant is defined in this AC as a ch emical agent that adequately imitates the discharge and distribution charac teristics of a given extinguis hing agent. It need not be an actual fire suppressant. For certain cas es due to, for example, cost of the extinguishing agent, problems with supply of the extinguishing agent, etc., it may be more appropriate to use a simulant in th e application. If an applicant proposes a simulant, they should complete adequate analys is and testing to esta blish the validity of the simulant. As a minimum, corroborating info rmation would need to be provided as to the detailed chemical analysis of the si mulant and evaluation testing of the fire extinguishing system operated with the si mulant that demonstrates the equivalent behavior. To ensure compliance when using the means in this AC, the following should be provided to the FAA to corr oborate the applicant’s findings:

12.3.1 Test data and distribution profiles using the simulant that meet the approved certification criteria and meet the mini mum performance standards as developed by FAA Technical Center as part of the Inte rnational Aircraft Systems Fire Protection Working Group. See Appendix B of this AC for a list of references.

12.3.2 A system description document that include s a description of the distribution of the simulant under the test conditions in the cargo compartment.

12.3.3 A detailed test plan.

12.3.4 Chemical data that describes the simulant and any toxicity data.XX/XX/XX DRAFT AC 25.851-X

15 12.3.5 Any unique observances noted during deve lopment testing or conductance of the minimum performance standards testing will need to be addressed appropriately prior to the start of airplane certification testing.

12.4 The distribution of the simula nt in the application should be compared with Halon 1301 under the following conditions:

12.4.1 Given the same filling conditions, the simulant is loaded into the fire extinguisher bottle based on an equivalent liquid fraction to th e Halon 1301 charge wei ght required. This is an equivalent statement to the mass of the simulant being a specific percentage of the Halon 1301 charge weight required.

12.4.2 The fire extinguisher bottle containing the si mulant is pressurized with nitrogen in an identical manner required by the Halon 1301 charge weight.

12.4.3 The simulant is discharged into the te st environment, i.e., cargo compartment.

12.5 Pre-Test Considerations:

12.5.1 An analyzer (for example, Statham-deri vative analyzer) capable of measuring the simulant distribution profile in the form of volumetric concentration is required.

12.5.2 An analyzer (for example, Statham-derivativ e analyzer) and associated hardware should be configured for the particular application.

12.5.3 The fire suppression system should be completely conformed for Halon 1301.

12.5.4 The fire extinguisher bottle(s) s hould be serviced and prepared for the prescribed test(s).

12.6 Test Procedures:

12.6.1 Perform the prescribed distribution test in accordance with the test plan. See paragraph 8 for guidance on probe placement.

12.6.2 An analyzer (for example, Statham-derivativ e analyzer) should r ecord the distribution profile as volumetric concentration for the simulant.

12.7 Test Result Evaluation:

12.7.1 Produce the data from the analyzer (for ex ample, Statham-derivative analyzer) in graphical format. This format should be the volumetric concentration of the simulant versus time. A specific percent volumetric in itial concentration and a specific percent volumetric metered concentration for the lengt h of the test duration as determined by previous testing conducted according to the established minimum performance standards should be included.XX/XX/XX DRAFT AC 25.851-X

16 12.7.2 Using the Halon 1301 certifica tion criteria, evalua te the distribution profile of the simulant for acceptable performance. The accep tability of the test data would be dependent upon the distribution profile and duration exhibited by each probe.

13 ESTABLISHING DURATION FOR THE SUPPRESSION SYSTEM.

13.1 The adequacy of the capacity of the “built-i n system” is understood to mean, that there is sufficient quantity of agent to combat th e fire anywhere where baggage and cargo is placed within the cargo compartment for the time duration required to land and evacuate the airplane. Current built-in car go fire extinguishing systems use Halon 1301 as the fire extinguishing agent where protec tion 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 iden tified as a cert ificate limitation.

13.2 The applicant should work with the airpla ne owner/operator and the civil aviation authority providing operational approval to en sure that the cargo fire extinguishing system provides the required protection time (i.e., proper sizing of the cargo fire extinguishing system) for the specific rout e 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 divers ion times based on one-engineout considerations.

13.3 Operational approval for the airplane s hould include the maximum allowable time, following the discovery of a fire or other emergency situation, required to divert the airplane to an alternate la nding site. The maximum allowa ble time should include a 15- minute allowance for holding and/or approach and landing in addition to the actual time required to reach the alternate landing s ite and account for the completion of an evacuation of all occupants.

13.4 AC 120-42B and AC 135-42 provide guida nce 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 stil l 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.

13.5 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 a llowance for holding and/or approach and landing as applicable, should never be gr eater than the maximum protection time provided by the cargo fire extinguishing system.

13.6 The following examples illustrate these issues:XX/XX/XX DRAFT AC 25.851-X

17 13.6.1 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

This example includes an allowance of 15 minutes for holding and/or approach Note: and landing.

13.6.2 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

In this example, the system w ould not support the planned route. Note:

14 AIRPLANE MANUAL CONSIDERATIONS. To ensure fire protection/fire suppression sy stem effectiveness and safe continuation of flight and landing, the applicable airp lane manuals should contain appropriate directives, for example:

14.1 Any procedures related to fighting a cargo co mpartment fire should be clearly defined in the AFM.

14.2 AFMs should contain instructions to land at the nearest adequate airport (or suitable airport for ETOPS) following detection of a cargo fire.

14.3 Cargo loading restrictions ( certified type of loading accord ing to compartment, limits for loading heights and width, etc.) should be clearly described in the weight & balance manual or any other appropriate airplane manual.

14.4 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.XX/XX/XX DRAFT AC 25.851-X

18 15 PLACARDS AND MARKINGS IN CARGO COMPARTMENTS.

15.1 Experience has shown that under certain circum stances and despite cl ear instructions in the applicable airplane documentation, car go loading personnel may not follow loading restrictions. Especially pallets may be 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 vi sually indicate the applicable loading restrictions to each person responsible fo r cargo loading activities in a compartment, placards and markings for cer tified type of cargo, maximum loading height and widths should be installed in that compartment.

15.2 For the design of these indications (i.e., for shape, size, color and brightness), illumination conditions in the compartment should be considered. Markings and placards should not be easily erased, disfi gured, 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.mm/dd/yy D R A F T AC 25.851-X Appendix A

A-1 Appendix A. Analytical Methods for Det ermining 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). = V Halon 1301 / V Dimensionless

V Halon 1301 Volume of Halon 1301 in cargo compartment. Cubic feet (ft 3 ) Cubic meters (m 3 )

V Cargo compartment free volume (i.e., volume not occupied by cargo). = 1 - ( V cargo / V empty ) Cubic feet (ft 3 ) Cubic meters (m 3 )

V cargo Cargo volume. Cubic feet (ft 3 ) Cubic meters (m 3 )

V empty Empty cargo compartment volume. Cubic feet (ft 3 ) Cubic meters (m 3 )

T Time. Minutes (min)

E Cargo compartment leakage rate. Cubic feet per minute (ft 3 /min) Cubic meter per minute (m 3 /min)

S Specific volume of Halon 1301. Cubic feet per pounds (mass) (ft 3 /lbm) Cubic meters per kilogram (m 3 /kg)

R Halon 1301 flow rate. Pounds (mass) per minute (lbm/min)mm/dd/yy D R A F T AC 25.851-X Appendix A

A-2 Kilogram per minute (kg/min)

A.3 HALON 1301 CONCENTRATI ON LEVEL MODEL.

A.3.1 Cargo compartment fire extinguishing system s 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 pre ssurized bottles into the cargo compartment. This type of discharge met hod is commonly known as a high -rate discharge or ‘dump’ system.

A.3.2 The second type of Halon 1301 discharge met hod 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.

Figure A-1. Example - Halon 1301 Model

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.mm/dd/yy D R A F T AC 25.851-X Appendix A

A-3 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 a nd 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.

A.3.5 Prior to Phase I: Initial high -rate discharge of Halon 1301. This portion of the extinguishing process i llustrates the high-rate discharge method of releasing all of the fire exti nguishing 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 expone ntial “decay” versus time.

A.3.6.2 The governing equation for exponentia l “decay” during Phase I is the following: C(t) = C(0) e -E t /V

Note: “C(0)” is the initial concentr ation of Halon 1301 used to knock down a cargo fire at the beginning of Ph ase I, and “t” is the time elapsed since the beginning of Phase I.

A.3.7 Phase II: Metered Discharge of Halon 1301.

A.3.7.1 The metered discharge of Halon 1301 st arts at the beginning of Phase II. The example in figure A-1 shows that the metering rate is set to release Halon 1301 into the cargo compartment at a rate that is slightly greater than the rate Halon 1301 is lost through cargo compartment leakage.

A.3.7.2 The governing equation for metering during Phase II is the following: C(t) = [ C(0) - { R S / E } ] e -E t / V + { R S / E }

Note: “C(0)” is the concentration of Ha lon 1301 at the end of Phase I, and “t” is the time elapsed since the end of Phase I.

A.3.8 Phase III: Exponential “Decay” of Halon 1301.

A.3.8.1 The beginning of Phase III marks the end of Halon 1301 metering. As in Phase I, since no more Halon 1301 is introduced into the cargomm/dd/yy D R A F T AC 25.851-X Appendix A

A-4 compartment, the concentration of Halon 1301 undergoes an exponential “decay” versus time. The governing equation for exponential “decay” during Phase III is the same as during Phase I with one exception: “C(0)” is the concentration of Halon 1301 at the end of Phase II, and “t” is the time since the end of Phase II.mm/dd/yy D R A F T AC 25.851-X Appendix B

B-1 Appendix B. References

B.1 PURPOSE. This appendix contains documents referenced in this AC.

B.1.1 Gassmann, J., Hill, R., “Fire Extinguishing Methods for New Passenger/Cargo Aircraft,” Federal Aviation Administra tion, Final Report FAA-RD-71-68, November 1971.

B.1.2 Blake, D., “Evaluation of Large Class B Cargo Compartment’s Fire Protection,” Federal Aviation Administration, Fina l Report DOT/FAA/AR-96/5, June 1996.

B.1.3 Reinhardt, J., Blake, D., Marker, T., “Development of a Minimum Performance Standard for Aircraft Cargo Compartment Gaseous Fire Suppression Systems,” Federal Aviation Administration, Final Re port DOT/FAA/AR-00/28, September 2000.

B.1.4 Marker, T., Reinhardt, J., “Water Spray as a Fire Suppression Agen t for Aircraft Cargo Compartment Fires,” Federal Aviation Ad ministration, Technical Note DOT/FAA/ARTN01/1, June 2001.

B.1.5 Ingerson, Douglas A., “Simulating the Distribu tion of Halon 1301 in an Aircraft Engine Nacelle With HFC-125,” Federal Avia tion Administration, Technical Note DOT/FAA/AR-TN99/64, August 1999.

B.1.6 Blake, D., Marker, T., Hill, R., Reinhardt, J., Sarkos, C., “Cargo Compartment Fire Protection in Large Commercial Transport Ai rcraft,” Federal Aviation Administration, Technical Note DOT/FAA/AR-TN98/32, July 1998.