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 compliance 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. This 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 accessible 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 Fire Extinguishing/Suppression Systems. ............13

12 Use of Simulants for Certification 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 Determining 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 Concentration Time History ............................................................................... 10 

Tables 

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

1 

1 PURPOSE. 

This advisory circular (AC) provides guidance concerning compliance 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. This 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 and be accessible in flight to 

crewmembers. 

2 APPLICABILITY. 

2.1 The guidance in this document is for airplane manufacturers, modifiers, foreign 

regulatory authorities, and Federal Aviation Administration (FAA) transport airplane 

type-certification engineers and their designees. 

2.2 The material in this AC is neither mandatory nor regulatory in nature and does not 

constitute a regulation. It describes acceptable 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 are derived from extensive FAA and industry 

experience in determining compliance with the relevant regulations. If, however, we 

become aware of circumstances that convince 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 additional substantiation as a basis for finding 

compliance. 

2.3 The material in this AC does not change or create any additional 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 applicability of this particular method of 

compliance when the acceptable method of compliance 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 smoke 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 publication 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_policies/advisory_circulars/. 

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

 AC 25-9A, Smoke Detection, Penetration, and Evacuation 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 Modified for Cargo Service, issued 

January 6, 1994.

 AC 25-22, Certification of Transport Airplane 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 Operations 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 “suppression system” are used interchangeably 

in this AC. Systems that are located in remote (e.g., inaccessible) areas are not required 
XX/XX/XX DRAFT AC 25.851-X 

3 

to extinguish a fire in its entirety. These systems 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 standards 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 fire extinguisher/fire suppression system to 

extinguish or control any fire likely to occur in the cargo compartment. Therefore, the 

means to extinguish a fire applies to bulk-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 compartments have fire or smoke detection 

systems to alert the crew to the presence of the fire anywhere within the cargo 

compartment (i.e., anywhere cargo is designed to be carried). 

6.2 A Class F cargo compartment that uses a built-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 quantities 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 drafts within the compartment so that the 

extinguishing agent used can control any fire that may start within the compartment; 

and 

6.2.5 Flight tests are conducted to ensure compliance with the 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 concentration of 5 percent by volume in order to 

knock down a cargo fire. Subsequent concentration 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 ventilated compartment, Halon 1301 starts to 

stratify shortly after discharge and the concentration level decays faster in the upper 
XX/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 any upward pitch, or forward in any downward pitch of the 

airplane in flight. For some airplane certification programs, the concentration levels of 

Halon 1301 have been measured at various locations throughout 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 concentration level to drop below 3 percent by 

volume at individual sampling locations near the top of the cargo compartment. 

6.3.2 Testing data from the FAA William J. Hughes 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 extinguishing agent throughout the compartment to 

effectively suppress a cargo fire. If a cargo 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 vicinity 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 convective stirring caused by the heat of the 

fire may be insufficient to raise the local concentration of Halon in the vicinity of the 

fire. Therefore, compliance testing should involve the use of point concentration data 

from each probe. Additionally, the probes closest to the cargo compartment ceiling 

should be at least at the highest level that cargo and baggage can be loaded as specified 

by the manufacturer and certified by the appropriate airworthiness authority. In 

addition, certification test data acquisition should include analysis and/or data taken 

after landing at a time increment which represents the completion of an evacuation. 

Note: Due to the ozone depleting characteristics of halon, the U.S. Environmental 

Protection Agency (EPA) has established restrictions on using halon for testing. 

Applicants should instead use a halon simulant, such as pentafluoroethane (HFC-125), 

for tests. Applicants will find useful information 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 fire protection, a system for classification of cargo or 

baggage compartments was developed and adopted for transport category airplanes. 

Classes A, B, and C were initially established; 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 that is located so close to the station of 

a crewmember that the crewmember would discover the presence of a fire immediately. 

In addition, each part of the compartment is easily accessible so that the crewmember 
XX/XX/XX DRAFT AC 25.851-X 

5 

could quickly extinguish a fire with a portable fire extinguisher. A Class A 

compartment is not required 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 pilot 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 extinguisher in any part of the compartment when standing at any 

one access point, without stepping into the compartment. There must be means to 

ensure that, while the access provisions are 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 also 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 fire 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 instead of providing smoke or fire detection 

and extinguishment, Class D compartments were designed to control a fire by severely 

restricting the supply of available oxygen. Due to several uncontrollable 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 with a certification basis pre-dating 

Amendment 25-93. 

7.5 A Class E compartment (see § 25.857(e)) is found 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 cargo 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 detection system installed in accordance with 

§ 25.857(e)(2). It is not required 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 requiring a crewmember to enter the compartment. Class F 

compartments that include a built-in fire extinguisher/suppression system or require the 

use of other means acceptable to the FAA would meet these requirements. The Class F 

compartment must have a fire or smoke detection system installed in accordance with 

§ 25.857(f)(1). Unless there are other means of containing the fire and protecting 
XX/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 part 25, or other approved equivalent methods 

(see § 25.855(b)). The Class F cargo compartment 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 exclude hazardous quantities of extinguishant 

and products of combustion from occupied areas (see § 25.857(f)(3)). 

8 TESTING VOLUMETRIC CONCENTRATION LEVELS. 

8.1 The applicant should demonstrate that the cargo fire extinguishing system provides 

adequate concentration levels of extinguishing agent to combat a fire anywhere baggage 

and cargo is placed within the cargo compartment for the time duration required to land 

and evacuate the airplane. A combination of flight testing and analysis may be used. If 

Halon 1301 is used, an initial minimum concentration of 5 percent by volume is 

required to knock down a cargo fire. Subsequent gaseous extinguishing agent should, if 

required for the duration of the flight, be introduced 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 airplanes (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 tests should be conducted to show, considering 

approved maximum diversion in still air (including an allowance for 15-minute holding 

and/or approach and landing), that the ability of the system to suppress or extinguish 

fires is adequate to ensure safe flight and landing at a suitable airport (e.g., a flight test 

simulated landing may be used in lieu of an actual landing). The minimum 

extinguishing agent concentration levels should 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, flight-test measurements do not have to be 

made in compartment areas that are designated 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 concentration levels throughout the 

cargo compartment in these planes. Averaging 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, where interpolation of probe concentration data from other 

nearby probes to yield an estimate of missing agent concentration data may be allowed 

by the FAA. In the event such interpolation is necessary, then a linear interpolation of 

the data will provide an acceptable means of approximating the missing data. The 

applicant should coordinate such situations 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, vents, etc.), which can disrupt the local 

concentration levels. The concentration levels should not be less than the minimum 

established for that fire extinguishing agent at any point within the compartment. 

Arithmetic averaging of individual sampling locations to determine the concentration 

levels is not acceptable. The use of averaged 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 ceiling 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 there are adequate concentration levels of fire 

extinguishing agent throughout the compartment to effectively control a cargo 

compartment fire. The sampling rate should be sufficient to establish a concentration 

level versus time decay curve. In the event that a single sensor displays a suspect time 

history, the use of an interpolated time-averaged value may be acceptable. If fire 

extinguishing agent concentration levels at a probe drop below the minimum 

requirement, it should be a temporary anomaly of short duration and 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 situations closely with the FAA. 

8.4 Typically there are two types 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 the activation of the system, and a selected 

amount of agent is injected into the compartment to suppress the fire. Once the agent 

concentration level approaches the minimum sustaining level (i.e., 3 percent for 

Halon 1301), a second and subsequent discharge of agent takes place to assure the 

3 percent concentration level is maintained for the time necessary to divert to a safe 

landing. The metered systems usually discharge agent into the compartment for fire 

suppression (5 percent for Halon 1301) and then adds agent in a prescribed amount to 

the compartment to maintain the 3 percent concentration level (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 intended 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 analysis. 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 available to predict concentration 

levels for various locations and heights in a given cargo compartment will have an 

impact on certification tests. This type of testing and analysis 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 concentration and behavior for the remaining flight. Airplane climb 

flight phase and the descent flight phase represent dynamic environments, and no data 
XX/XX/XX DRAFT AC 25.851-X 

8 

need be acquired during these transient flight phases where cabin altitude changes 

would preclude accurate data acquisition. However, certification data should include 

analysis and/or data taken after landing at a time increment representative of the 

completion of an evacuation of all occupants. If a fire extinguishing agent other than 

Halon 1301 is proposed, the method of compliance will need to be documented in an 

issue paper. 

Note: Due to EPA restrictions, a halon simulant, such as HFC-125, should be used for 

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 function and dissipation of the 

fire extinguishing agent in a cargo compartment. For certification tests, 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 empty 

compartment 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” system with cargo. Also, metering systems, if 

designed properly, are relatively insensitive 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 percentages, 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 about 65.6 percent. With compartments loaded 

to a higher percentage than 65.6 percent, the concentration will fall below 3 percent 

faster than an empty compartment. 

9.4 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 in an actual 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 requirement 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 guidance on analyzing Halon 1301 concentration 

levels. 

9.7 The suppression system certification test should be conducted, as 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 

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=c0*exp(-t/T)

T= Effective time for one air change

Comp. Leakage= 60 CFM

Comp. Volume= 2357 ft3

Halon Quantity= 49.6 lbs.
XX/XX/XX DRAFT AC 25.851-X 

11 

acceptable for unpressurised flight conditions 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 supplemental type certificate (STC) modifying 

airplanes that have been in service. Some preliminary testing should be done to 

determine the maximum leakage rates seen/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. 

Note: Due to EPA restrictions, a halon simulant, such as HFC-125, should be used for 

testing and would not require specific FAA approval. 

10 EVALUATION OF ALTERNATE GASEOUS 

EXTINGUISHING/SUPPRESSION SYSTEMS AND ALTERNATE AGENTS. 

10.1 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 

halons in determining compliance with airworthiness standards. A European (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 alternate 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 compartments. 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 concentrations of the agent 

are introduced into a fire under certain environmental conditions (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, etc.). The Technical Center will continue to 

pursue this line of research to identify 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 standards are developed for these systems. 

10.3 Should the FAA be approached with the intent of using an alternate 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 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 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. 

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 Technical Center as 

part of the International Halon Replacement program. See Appendix B of this AC for a 

list of references. 

10.4.2 A system description document that includes a description of the distribution of the 

gaseous agent under the test conditions in the cargo compartment. 

10.4.3 A detailed test plan. 

10.4.4 Chemical data which describes the agent and any toxicity data. 

10.5 Pre-Test Considerations: 

10.5.1 An analyzer (for example, Statham-derivative analyzer) capable of measuring the agent 

distribution profile in the form of volumetric concentration is required. 
XX/XX/XX DRAFT AC 25.851-X 

13 

10.5.2 An analyzer (for example, Statham-derivative analyzer) and associated hardware should 

be configured for the particular application. 

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

10.5.4 The fire extinguisher bottle(s) should 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-derivative analyzer) should record the distribution 

profile as volumetric concentration for the agent. 

10.7 Test Result Evaluation: 

10.7.1 Produce the data from the analyzer (for example, 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. 

10.7.2 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 upon the distribution 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 technical note (see Appendix B for applicable 

references) 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 alternate liquid and other fire 

extinguishing/suppression 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 standards 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. The FAA may initiate a certification 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 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 alternate extinguishing agent 

or alternate 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 affective 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. 

12.2 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 B.1.5 and B.1.6 ). 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 above in paragraph 8 

should be provided. 

12.3 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, etc., it may be 

more appropriate to use a simulant in the application. If an applicant proposes a 

simulant, they should complete adequate analysis and testing to establish the validity of 

the simulant. 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 means in this AC, the following should 

be provided to the FAA to corroborate the applicant’s findings: 

12.3.1 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 Appendix B of this AC for a list of references. 

12.3.2 A system description document that includes 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 development 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 simulant 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 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. 

12.4.2 The fire extinguisher bottle containing the simulant is pressurized with nitrogen in an 

identical manner required by the Halon 1301 charge weight. 

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

12.5 Pre-Test Considerations: 

12.5.1 An analyzer (for example, Statham-derivative analyzer) capable of measuring the 

simulant distribution profile in the form of volumetric concentration is required. 

12.5.2 An analyzer (for example, Statham-derivative 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) should 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-derivative analyzer) should record the distribution 

profile as volumetric concentration for the simulant. 

12.7 Test Result Evaluation: 

12.7.1 Produce the data from the analyzer (for example, 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. 
XX/XX/XX DRAFT AC 25.851-X 

16 

12.7.2 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 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-in system” is understood to mean, that there 

is sufficient quantity of agent to combat the 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 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. 

13.2 The applicant should work with the airplane owner/operator and the civil aviation 

authority providing operational approval to ensure that the cargo fire extinguishing 

system provides the required protection time (i.e., proper sizing of the cargo fire 

extinguishing system) 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-engineout considerations. 

13.3 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 alternate 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 alternate landing site and account for the completion of an 

evacuation of all occupants. 

13.4 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. 

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 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. 

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 

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

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 

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

14 AIRPLANE MANUAL CONSIDERATIONS. 

To ensure fire protection/fire suppression system effectiveness and safe continuation of 

flight and landing, the applicable airplane manuals should contain appropriate 

directives, for example: 

14.1 Any procedures related to fighting a cargo compartment 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 according 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 circumstances and despite clear instructions in 

the applicable airplane documentation, cargo 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 visually indicate the applicable loading 

restrictions to each person responsible for cargo loading activities in a compartment, 

placards and markings for certified 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, 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. 
mm/dd/yy D R A F T AC 25.851-X 

 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). 

= 1 - ( 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) 
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 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. 

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 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. 

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: 

C(t) = C(0) e -E t /V 

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. 

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

A.3.7.1 The metered discharge of Halon 1301 starts 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 Halon 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 cargo 
mm/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 Administration, 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, Final 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 Report DOT/FAA/AR-00/28, September 2000. 

B.1.4 Marker, T., Reinhardt, J., “Water Spray as a Fire Suppression Agent for Aircraft Cargo 

Compartment Fires,” Federal Aviation Administration, Technical Note DOT/FAA/ARTN01/1, June 2001. 

B.1.5 Ingerson, Douglas A., “Simulating the Distribution of Halon 1301 in an Aircraft Engine 

Nacelle With HFC-125,” Federal Aviation 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 Aircraft,” Federal Aviation Administration, 

Technical Note DOT/FAA/AR-TN98/32, July 1998.