AC 25.851-1

U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Built-in Fire Extinguishing/Suppression Systems in Class C and Class F Cargo Compartments Date: 02/03/16 Initiated By: ANM-112 AC No: 25.851-1

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 extinguishing/suppression systems. This guidance is applicable to transport category airplanes for which a new, amended, or supplemental type certificate is requested.

If you have any suggestions for improving this AC, you may use the Advisory Circular Feedback form at the end of this AC.

Jeffrey E. Duven Manager, Transport Airplane Directorate Aircraft Certification Service

ii Contents Paragraph Page

1 Purpose1

2 Applicability1

3 Cancellation1

4 Related Documents2

5 Background3

6 Compartment Classification Regulations5

7 Testing Volumetric Concentration Levels6

8 Airplane Test Conditions9

9 Evaluation of Alternative Gaseous Extinguishing/Suppression Systems and Alternative

Agents12

10 Evaluation of Alternative Liquid Agent and Fire Extinguishing/Suppression Systems15

11 Use of Simulants for Certification Testing15

12 Establishing Duration for the Suppression System17

13 Airplane Manual Considerations19

14 Placards and Markings in Cargo Compartments19

Appendix A. Analytical Methods for Determining Halon 1301 Concentration LevelsA-1

Figures

Figure 1. Effect of Cargo Load on Halon 1301 Concentration Levels10

Figure 2. Halon Concentration Time History11

Figure A-1. Example – Halon 1301 ModelA-3

Tables

Table A-1. Terms and SymbolsA-1

1 1 PURPOSE.

This 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 extinguishing/suppression systems. This guidance applies to Class C cargo compartments with built-in fire extinguishing/suppression systems regardless of their location (lower deck, main deck, or upper deck) and to Class F cargo compartments with built-in fire extinguishing/suppression systems, which must be located on the main deck.

Note: The terms “extinguishing system,” “suppression system,” and “extinguishing/suppression system” are used interchangeably in this AC. Systems that are located in remote (e.g., inaccessible or distant) areas are not required to extinguish a fire in its entirety, because the crew would not be able to confirm that the fire is extinguished. These systems are intended, instead, to suppress a fire until it can be completely extinguished by ground personnel following a safe landing.

2 APPLICABILITY.

2.1 The guidance in this AC is applicable to transport category airplanes for which a new, amended, or supplemental type certificate is requested. This guidance 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.

3 CANCELLATION.

The following FAA policy memoranda are cancelled by this AC:

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

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

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

4 RELATED DOCUMENTS.

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

 § 25.851, Fire extinguishers.

 § 25.855, Cargo or baggage compartments.

 § 25.857, Cargo compartment classification.

 § 25.858, Cargo or baggage compartment smoke or fire detection systems.

 § 121.314, Cargo and baggage compartments.

4.2 FAA ACs.

The following ACs were current at the time of publication of this AC. You should use the latest version for guidance. You can view and download the latest version at the FAA website.

 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 25.857-1, Class B and F Cargo Compartments, issued February 3, 2016.

 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 Orders.

 FAA Order 8150.4, Certification of Cargo Containers with Self-Contained Temperature Control Systems (Active ULDs), dated August 29, 2007.

4.4 References.

The following documents are referenced in this AC:

4.4.1 Gassmann, J., Hill, R., Fire Extinguishing Methods for New Passenger/Cargo Aircraft, Federal Aviation Administration, Final Report FAA-RD-71-68, November 1971.

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

4.4.3 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.

4.4.4 Marker, T., Reinhardt, J., Water Spray as a Fire Suppression Agent for Aircraft Cargo Compartment Fires, Federal Aviation Administration, Technical Note DOT/FAA/AR- TN01/1, June 2001.

4.4.5 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.

4.4.6 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.

5 BACKGROUND.

5.1 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 extinguishing/suppression system to extinguish or suppress any fire likely to occur in the cargo compartment. The means to extinguish or suppress a fire applies to bulk-loaded cargo, cargo loaded on pallets, cargo within unit load devices, and other means of carrying cargo. 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).

5.2 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, an applicant for a Class F cargo compartment with a built-in fire extinguishing/suppression system must ensure that:

5.2.1 No discharge of the extinguisher can cause structural damage;

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

5.2.3 There are means to exclude hazardous quantities of smoke, flames, or extinguishing agent from any compartment occupied by the crew or passengers; and

5.2.4 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.

Note: Flight tests are conducted to ensure compliance with the provisions in §§ 25.855(h), 25.857(f), 25.858, and 25.1301(a).

5.3 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 (i.e., flames are no longer visible from the source of the fire) 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.

5.3.1 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 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.

5.3.2 Testing data from the FAA William J. Hughes Technical Center (see paragraph 4.4 of this AC) 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, compliance demonstration should include data acquired after landing at a time representative of the completion of an evacuation of all occupants; as with the transient flight phases, if real-time data cannot be acquired using approved instrumentation, applicants may submit the results of analysis.

5 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.

6 COMPARTMENT CLASSIFICATION REGULATIONS.

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 has since been eliminated from the regulations. The classification is based on the means by which a fire can be detected and the means available to control a fire. 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 pertaining to the particular class involved.

6.1 A Class A compartment (see § 25.857(a)) is located so close to the station of a crewmember that the crewmember would easily discover the presence of a fire. In addition, each part of the compartment is easily accessible so that the crewmember could quickly extinguish a fire with a portable fire extinguisher. A Class A compartment is not required to have a liner.

6.2 A Class B compartment (see § 25.857(b)) is accessible in flight but more remote from the crewmember’s station than a Class A compartment and must, therefore, incorporate a smoke or fire 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 that meets the requirements of § 25.855(b). In flight, a crewmember must have sufficient access to a Class B compartment to effectively fight a fire in any part of the compartment with a hand fire extinguisher when standing at the compartment’s one designated access point, without stepping into the compartment. Therefore, Class B cargo compartments are limited to the main deck. 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.

6.3 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 that is controllable from the flight deck. A Class C compartment must have a liner and a smoke or fire detection system that meets the requirements of §§ 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)).

6.4 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

6 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.

6.5 A Class E compartment (see § 25.857(e)) is found only 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–deck, 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 smoke or fire detection system installed in accordance with § 25.857(e)(2). It is not required to have a built-in fire suppression system.

6.6 A Class F compartment (see § 25.857(f)) must be located on the main deck of the airplane. Class F compartments must have means to control or extinguish a fire without requiring a crewmember to enter the compartment. Class F compartments that include a built-in fire extinguishing/suppression system or require the use of fire containment covers (FCCs) or fire resistant containers (FRCs) meeting the criteria identified for these components in paragraph 6.3.3 of AC 25.857-1 may meet these requirements.

6.6.1 We introduced the Class F cargo compartment as a practicable and safe alternative to the previous practice of providing large Class B cargo compartments. Class B compartments are limited to the main deck for accessibility reasons. Likewise, Class F cargo compartments must be limited to the main deck for accessibility. Lower-deck cargo compartments should not be approved as Class F cargo compartments. All lowerdeck or remote cargo compartments in airplanes carrying passengers need to comply with the Class C cargo compartment requirements of § 25.857(c).

6.6.2 The Class F compartment must have a smoke or fire detection system installed in accordance with § 25.857(f)(1). Unless there are other means of containing the fire and protecting 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)). For example, the FAA would accept a Class F cargo compartment requiring the use of FCCs, FRCs, or other means meeting the liner requirements (i.e., part 25, appendix F, part III flame penetration resistant means) as equivalent to installation of a fixed liner.

7 TESTING VOLUMETRIC CONCENTRATION LEVELS.

7.1 The applicant must 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

7 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 , if required for the duration of the flight, should be introduced via a metering or other appropriate system to ensure that point concentration levels do not drop below the minimum level appropriate for the agent (e.g., for Halon 1301, the minimum level is 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 (ETOPS) with two-engine airplanes, 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.

7.2 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.

7.3 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, except where a sensor probe failure occurs and the use of interpolation from adjacent sensor probes is warranted. If the FAA finds the use of the adjacent sensors is not warranted, then the flight test must be repeated. Compliance with § 25.851(b) requires the use of point-concentration data from each sensor and that the sensors closest to the cargo compartment ceiling 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 sensors 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

8 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 the applicant can demonstrate 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.

7.4 Cargo airplane designs typically use two types of built-in fire suppression systems. The first type, known as a “flood,” “dump,” or “quick discharge” system, rapidly introduces extinguishing agent throughout the cargo compartment upon initial indication of fire.

These systems often discharge once more, after a set amount of time. After these initial discharges, a second type of system, known as a “metered” or “slow-discharge” system, maintains the concentration of agent needed to suppress the fire until the airplane can safely land and be evacuated. Regardless of the type of system, compliance with §§ 25.855(h) and 25.857 requires demonstration flight tests.

7.4.1 Applicants whose flood or dump systems use Halon 1301 fire extinguishing agent should demonstrate an initial concentration of 5 percent of agent by volume. After any second discharge by the dump system, as the concentration approaches the minimum sustaining level (3 percent for Halon 1301) the metered system should maintain that minimum sustaining level for the time necessary for a safe landing and evacuation.

7.4.2 Certification flight test demonstrations are required for demonstrating compliance for built-in systems for the duration of the intended diversion profile. If the built-in fire suppression system includes a metering system, then the metering 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. This type of testing and analysis should be documented in the applicant’s 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 historically have represented a challenge for accurate data acquisition. While real-time data should be acquired during all phases of airplane flight, applicants may submit the results of analysis for these transient flight phases if the intended instrumentation would preclude accurate data acquisition during these flight phases and the FAA agrees with the use of the instrumentation. In addition, compliance demonstration should include data acquired after landing at a time representative of the completion of an evacuation of all occupants; as with the transient flight phases, if real-time data cannot be acquired using approved instrumentation, applicants may submit the results of analysis. 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 1: Due to EPA restrictions, an FAA-accepted halon simulant, such as HFC-125, should be used for testing (see section 11 of this AC).

9 Note 2: Future fire extinguishing systems may be adaptive in providing superior control of a fire by zonal fire control in lieu of the flooding control provided by today’s Class C cargo compartment fire suppression systems. Such systems will require development of an issue paper and may result in special conditions or other regulatory means to obtain FAA approval.

8 AIRPLANE TEST CONDITIONS.

8.1 Flight tests are required per § 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.

8.2 The cargo compartment should be empty for the above tests. However, as shown in figure 1 of this AC, the fire extinguishing agent concentration level decreases more quickly in a compartment with cargo than it would in the same compartment without cargo. This is because discharging the same quantity of fire extinguishing agent into a smaller volume results in a larger percentage of extinguishing agent by volume. For a cargo compartment with a constant leakage rate, the result of the smaller volume and constant leakage rate will be that the fire extinguishing agent concentration level decreases more quickly. The duration that the concentration is maintained (or conversely, how quickly it decreases) depends on several factors (e.g., cargo load factor, location of leakage within the cargo compartment such as seals around cargo loading door and light fixtures, etc.). Rather than requiring numerous flight tests with varying load factors to demonstrate compliance, applicants may conduct analysis to account for these effects as shown in Figure 2 of this AC.

10 Figure 1. Effect of Cargo Load on Halon 1301 Concentration Levels 0% 50% 75% Cargo Load Factor

Increasing Cargo Load TIME

CONCENTRATION V empty = Empty Cargo Compartment Volume V cargo = Cargo Volume V cargo V empty/Cargo Load Factor =

8.3 A specific example of the effect of cargo compartment loading is shown in figure 2 of this AC, 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.