AC 20-144A

U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: FAA Approval of Electrical Firing Cartridge Components in an Aircraft Fire Extinguishing or Suppression System Date:10/11/2018 Initiated By: AIR-670 AC No: 20-144A

This advisory circular (AC) provides acceptable means for showing compliance with the requirements of title 14, Code of Federal Regulations (14 CFR) 23.1301, amendment 23-62 or earlier, 25.1301, 27.1301, and 29.1301 (herein, collectively, 2X.1301), Function and installation, with regard to fire extinguishing or suppression systems that contain electrical firing cartridge components.

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

Dr. Michael C. Romanowski Director, Policy and Innovation Division Aircraft Certification Service

Contents Paragraph Page ii

1 Purpose1

2 Applicability1

3 Cancellation1

4 Summary of Changes2

5 Related Documents2

5.1 Regulations2

5.2 Advisory Circulars3

5.3 Orders4

5.4 Other Documents4

6 Definitions4

6.1 All-Fire and No-Fire Reliability Conditions4

6.2 Backshell4

6.3 Burst Disc4

6.4 Cartridge5

6.5 Discharge Head5

6.6 Firex5

6.7 Pressure Indicator5

6.8 Production Approval Holder (PAH)5

7 Scope5

8 Firex System Functional Overview6

9 Firex Cartridge Performance Parameters Overview6

9.1 General6

9.2 Design Parameters7

9.2.1 Physical Parameters7

9.2.2 Electrical Parameters7

9.2.3 Functional Parameters8

9.2.4 Environmental Parameters9

9.3 Cartridge Design10

9.3.1 Backshell10

9.3.2 Bridgewire11

Contents (continued) Paragraph Page iii

9.3.3 Explosive Charges11

9.3.4 Part Labeling11

9.3.5 Shorting Springs and Shunts11

9.4 Cartridge Validation12

9.5 Basic Qualification12

9.6 Pre-Production Design and Quality Control12

9.7 Quality Verification14

9.7.1 Design Development14

9.7.2 Design Change Control15

9.7.3 Design Qualification and Validation15

9.7.4 Testing15

9.7.5 Production Lot Acceptance Testing16

9.8 Service Life Issues16

9.9 Final Approval Process17

9.9.1 Qualification Document Package17

9.9.2 Final Qualification18

Figures Number Page

Figure 1. Cross-Section View of a Typical Firex Cartridge10

1 1 PURPOSE.

1.1 This AC provides acceptable means for showing compliance with the requirements of 14 CFR 2X.1301, Function and installation, with regard to fire extinguishing or suppression systems that contain electrical firing cartridge components. Airplanes having to comply with part 23 rules using amendment 23-64, effective August 30, 2017, must show compliance with § 23.2505, Function and installation.

1.2 This AC also includes the various aspects that should be considered by applicants seeking approval from the Federal Aviation Administration (FAA) of fire extinguishing or suppression systems that contain electrical firing cartridge components manufactured under a production certificate (PC), components to be approved under the part manufacturer approval (PMA) process, or design changes to components originally approved by either method. Since each environment and installation may differ, the applicant should select those portions of this document necessary to show compliance and coordinate it with the responsible FAA engineer, inspector, or their designee. This AC does not address other firex components or the complete aircraft system installation.

There may be airframe installation requirements and boundary conditions that should be met prior to installation approval.

2 APPLICABILITY.

The guidance in this AC is for applicants requesting a PC or PMA, or a new, or amended, or supplemental type certificate (STC). This guidance is also for aircraft manufacturers, modifiers, foreign regulatory authorities, FAA aircraft type certification engineers, FAA manufacturing inspectors, and the Administrator’s designees.

2.1 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 means 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 or design changes as a basis for finding compliance.

2.2 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.

This AC cancels AC 20-144, Recommended Method for FAA Approval of Aircraft Fire Extinguishing System Components, dated September 22, 2000.

2 4 SUMMARY OF CHANGES.

This AC revision removes the sections that had been reserved for later inclusion of specifications for fire extinguishing or suppression system components other than firing cartridge components: precision burst discs, fill/change fittings, pressure indicators, and discharge head assemblies. The revised title of this AC reflects that change by specifying electrical firing cartridge components. This AC adds “or suppression” to “fire extinguishing or suppression system.” In addition, this AC adds and updates references and definitions, and deletes the “Engineering Design Test Schedule,” referring the reader instead to MIL-DTL-23659F, General Design Specifications for Electrical Initiators, for the current version. Compliance with MIL-DTL-23659F and this AC satisfies the requirements of § 2X.1301.

5 RELATED DOCUMENTS.

5.1 Regulations.

5.1.1 The following 14 CFR regulations are related to this AC. You can download the full text of these regulations at the U.S. Government Printing Office e-CFR website. You can order a paper copy by sending a request to the U.S. Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402-0001; by calling telephone number (202) 512-1800; or by sending a request by facsimile to (202) 512-2250.

 Section 21.1, Applicability, and definitions.

 Section 21.137, Quality System.

 Section 21.303, Application.

 Part 21, subpart G, Production Certificates.

 Part 21, subpart K, Parts Manufacturer Approvals.

 Section 25.851, Fire extinguishers.

 Section 25.855, Cargo or baggage compartments.

 Section 25.857, Cargo compartment classification.

 Section 25.901, Installation.

 Section 25.1195, Fire extinguishing systems.

 Section 25.1197, Fire extinguishing agents.

 Section 25.1199, Extinguishing agent containers.

 Section 25.1201, Fire extinguishing system materials.

 Section 25.1301, Function and installation.

 Section 25.1529, Instructions for Continued Airworthiness.

 Section 27.859, Heating systems.

3  Section 27.1301, Function and installation.

 Section 27.1529, Instructions for Continued Airworthiness.

 Section 29.851, Fire extinguishers.

 Section 29.859, Combustion heater fire protection.

 Section 29.1195, Fire extinguishing systems.

 Section 29.1197, Fire extinguishing agents.

 Section 29.1199, Extinguishing agent containers.

 Section 29.1201, Fire extinguishing system materials.

 Section 29.1301, Fire and installation.

 Section 29.1529, Instructions for Continued Airworthiness.

5.1.2 In addition to the part 21 requirements in paragraph 5.1.1 above, airplanes having to comply with part 23 rules prior to amendment 23-64, effective August 30, 2017, must meet:

 Section 23.1195, Fire extinguishing systems.

 Section 23.1197, Fire extinguishing agents.

 Section 23.1199, Fire extinguishing containers.

 Section 23.1201, Fire extinguishing systems materials.

 Section 23.1301, Function and installation.

5.1.3 In addition to the part 21 requirements in paragraph 5.1.1 above, airplanes having to comply with part 23 rules using amendment 23-64, effective August 30, 2017, must meet:

 Section 23.2250(a), Design and construction principles.

 Section 23.2400(c) and (d), Powerplant installation.

 Section 23.2440(c) and (f), Powerplant fire protection.

 Section 23.2500, Airplane level systems requirements.

 Section 23.2505, Function and installation.

5.2 Advisory Circulars.

AC 21.303-4, Application for Parts Manufacturer Approval via Tests and Computations or Identicality, dated March 21, 2014, is related to the guidance in this AC. If AC 21.303-4 is revised after publication of this AC, you should refer to the latest version for guidance, which can be downloaded from the Internet at http://www.faa.gov/regulations_policies/advisory_circulars/.

5.3 Orders.

The following FAA orders are related to the guidance in this AC. The latest version of each order at the time of publication of this AC is identified below. If any order is revised after publication of this AC, you should refer to the latest version for guidance, which can be downloaded from the Internet at http://www.faa.gov/regulations_policies/orders_notices/.

 Order 8110.42D, Parts Manufacturer Approval Procedures, dated March 21, 2014.

 Order 8120.16A, Suspected Unapproved Parts Program, dated June 3, 2016.

 Order 8120.22A, Production Approval Procedures, dated January 11, 2016.

5.4 Other Documents.

The following documents are related to this AC. If any of these documents are revised after publication of this AC, you should refer to the latest version.

 MIL-DTL-23659F NOT 1, Initiators, Electrical, General Design Specifications for, dated March 18, 2015, ASSIST (DoD Specs and Standards).

 AIAA-S-113A-2016/A1-2018, Criteria for Explosive Systems and Devices on Space and Launch Vehicles, dated 2018, https://aiaa.org.

 MIL-C-22284A(1) NOT 1 (AS), Revision A, Notice 1, Container, Aircraft Fire Extinguishing System, Bromotrifluoromethane, CF3BR, dated September 18, 1996, ASSIST (DoD Specs and Standards).

 RTCA/DO-160G, Change 1, Environmental Conditions and Test Procedures for Airborne Equipment, dated December 16, 2014, https://www.rtca.org/.

 SAE AIR5060A, Electronic Engine Control Design Guide for Electromagnetic Environmental Effects, dated August 11, 2011, https://www.sae.org/.

6 DEFINITIONS.

For the purpose of this document, the following definitions apply:

6.1 All-Fire and No-Fire Reliability Conditions.

“All-fire” and “no-fire” are minima and maxima conditions for different cartridges under specific operating conditions. Refer to MIL-DTL-23659F.

6.2 Backshell.

Metal shell connecting circuit shields or overbraid to an electrical connector.

6.3 Burst Disc.

Diaphragm that ruptures to allow extinguishing agent to escape.

6.4 Cartridge.

Device used for discharging pressurized extinguishing agent. These are sometimes referred to as firing cartridges and initiators. These components meet definition of critical component under FAA Order 8110.42D, appendix K.

6.5 Discharge Head.

Device that houses cartridges and interfaces between the pressure vessel and the agent distribution system. This is sometimes referred to as the cartridge body or header assembly.

6.6 Firex.

Any fire extinguishing or suppression system that consists of the extinguishing agent container (vessel) and all attached components.

6.7 Pressure Indicator.

Device to indicate pressure or status of pressure vessel.

6.8 Production Approval Holder (PAH).

The holder of a production certificate, parts manufacturer approval, or technical standard order authorization who controls the design and quality of a product or part.

7 SCOPE.

7.1 A fire extinguishing or suppression (firex) system is required by subpart E of parts 23, 25, 27, and 29, among other part 2X provisions (for example, §§ 2X.851, 2X.854, 2X.855, 2X.857, 2X.901, 2X.1195, 2X.1197, 2X.1199, 2X.1201, 2X.1301, and 2X.1309). A firex system is composed of many components that are critical for the proper operation of the system when installed in an aircraft. One such component may be an electrical firing cartridge component. While there are no specific regulatory requirements on the design, production, or testing of an electrical firing cartridge, § 2X.901(b)(2) requires that components of the installation be constructed, arranged, and installed to ensure their continued safe operation between normal inspections and overhauls. Additionally, § 2X.1301(a)(1) and (4) requires that each item of installed equipment must be of a kind and design appropriate to its intended function and operate properly when installed. This AC does not address other firex components or the complete aircraft system installation. There may be airframe installation requirements and boundary conditions that should be met prior to installation approval.

7.2 The applicant should evaluate the firex system and establish minimum reliability standards for the components of the system so the overall system can meet the requirements of subparts E and F of parts 23, 25, 27, and 29. In accordance with § 2X.1529, the maintenance instructions should also be provided. These instructions should include required service after discharge.

7.3 The applicant should design and qualify components to meet specific operating performance, service life, and reliability requirements of the firex system established at the time of type design approval.

7.4 The FAA recommends that PAH and PMA applicants demonstrate that their candidate components meet or exceed these criteria. This document describes the critical parameters involved with the design, production, and testing of the electrical firing cartridge component.

8 FIREX SYSTEM FUNCTIONAL OVERVIEW.

8.1 There are generally three, cartridge-activated, firex systems onboard commercial aircraft. These systems provide fire extinguishment or suppression for engines, auxiliary power units (APUs), and cargo compartments. The extinguishing agent is contained in a pressurized vessel that is sealed with a precision burst disc. A discharge head, containing a cartridge and strainer, is attached to the pressure vessel.

8.2 When a fire is detected, an electrical pulse to the firing cartridge activates the firex system (activation may be automatic or commanded by a flightcrew member). The aircraft electrical system supplies the pulse, and the agent is discharged into the fire zone through a distribution system of piping and nozzles.

8.3 Most firex systems are designed to use a firing cartridge. In some types, when the firex cartridge activates, it produces a controlled shock wave that fractures the burst disc on the pressurized container, allowing the agent to escape.

8.4 In other types, the firex cartridge, upon activation, either propels a slug into the burst disc and discharges the agent, or supplies a pressurizing force to drive a piston that, in turn, moves a cutter knife that pierces the burst disc and allows the agent to disperse.

8.5 The opening in the burst disc is dependent on the application—high-rate discharge for engine or APU, or a combination of high-rated and flow-metered discharge for cargo compartments. In either case, the opening of the burst disc diaphragm should meet critical performance characteristics. A proper size opening should ensure the correct flow rate of agent to the desired destination.

8.6 A firex system may use a system of diverter valves, components of the flow metering, and filters to control where the agent travels for some cargo compartment applications.

9 FIREX CARTRIDGE PERFORMANCE PARAMETERS OVERVIEW.

9.1 General.

A firex cartridge is an explosive component of an aircraft firex system. The applicant is responsible for ensuring that the design and qualification of firex cartridges meet the specific operating performance, service life, and reliability requirements of aircraft firex system designs. This section discusses parameters for the design, production, testing,

7 and approval process. To comply with § 2X.1301, the applicant should verify that these parameters are appropriate for the installation and perform analysis and qualification testing to validate the cartridge. These analyses and tests are discussed in paragraphs 9.4, 9.5, 9.7, and 9.9.2 of this AC.

9.2 Design Parameters.

9.2.1 Physical Parameters.

Physical parameters include all information related to how the cartridge interfaces with the aircraft firex system. These include, but are not limited to:

 Length dimensions (e.g., threads, output cup, overall, connector).

 Diameter dimensions (e.g., threads, output cup).

 Concentricity of dimensions.

 Tolerances.

 Thread size.

 Thread type.

 Electrical connector specification.

 Mounting features (e.g., relationship of cartridge output cup to pressure vessel burst disc).

9.2.2 Electrical Parameters.

Electrical parameters include the electrical signal required to initiate the cartridge in a proper and reliable manner. Of equal importance are the electrical requirements that ensure the cartridge will not inadvertently fire. The cartridge electrical parameters should be compatible with the applicable aircraft electrical system requirements, limitations, and tolerances. The following is a list of electrical parameters:

9.2.2.1 Minimum Firing Current.1 The applicant should establish the minimum firing current required to initiate the cartridge in a reliable manner. This is typically expressed with a current level and time duration such as 3.5-ampere pulse for 10 to 50 milliseconds.

9.2.2.2 Firing Circuit.1 Each type of cartridge has a different firing circuit. The type and number of circuits are related to the connector specified for the particular aircraft.

The identification and proper orientation of the pins on the cartridge are critical for correct functioning of the firex system. The circuit diagram identifies each pin on the cartridge, typically by a letter or number. In

1 Values may depend on the installation requirements.

8 addition, the applicant should specify the circuit resistance, typically

1.0 ohm for each unit.

9.2.2.3 No-Fire Current.1 The no-fire current is the maximum current at which the device will not fire or degrade, typically 1 amp or 1 watt for five minutes.

9.2.2.4 Electrostatic Discharge.1 The cartridge should not fire when exposed to an electrostatic discharge pulse. An electrostatic discharge is typically 25,000 volts from a 500 picofarad capacitor through a 5,000 ohm resistor shorted pin(s)-tocase.

9.2.2.5 Electromagnetic Interference (EMI).

Applicants should use RTCA/DO-160G as a standard and should determine limitations on the aircraft. The cartridge should not fire when exposed to various electromagnetic fields. Radar and communication systems generally produce these types of fields. The applicant should establish EMI protection criteria based on aircraft system EMI limitations.

9.2.2.6 Insulation Resistance/Dielectric Withstanding Voltage.1 The applicant should use insulation resistance of typically 100 megohms at 500 volts DC to prevent the loss of a fire pulse due to internal shorting or arcing.

9.2.2.7 Dielectric Strength.1 The cartridge should typically have a dielectric strength of 1000 volts AC root mean square (RMS) shorted pin(s)-to-case, and the leakage rate should not be greater than 0.1 milliamps for 60 seconds.

9.2.3 Functional Parameters.

Upon initiation, the cartridge should provide sufficient energy to open the burst disc effectively, but not enough energy to cause a failure of the firex system or aircraft.

9.2.3.1 Installed Air Gap.

The installed air gap is the distance between the end of the cartridge and the surface of the ruptured state of the disc. The applicant should establish the installed air gap with the approved burst disc(s), considering all system installation variables and tolerances, and accounting for repair limits for the bottle burst disc assembly, to determine the minimum and maximum gap conditions. The applicant should use the minimum and maximum gap for system level testing.

9.2.3.2 Burst Disc Opening.

The applicant should identify the opening size of the burst disc for the applicable installation, which is a critical performance characteristic, and

9 evaluate the proper size opening to ensure the adequate flow rate of agent to the desired destination. The size of the required opening may be different for an engine or APU versus cargo compartment installations.

The applicant should establish burst disc opening with the pyrotechnic charge and electrical energy specifications at their minimum tolerance and the minimum and maximum temperatures established for the system. This disc opening should be equal to or greater than the minimum flow area of the discharge head. It is essential that the applicant establish an adequate margin of safety.

9.2.4 Environmental Parameters.

According to § 2X.1301, the cartridge should function properly for its intended function, which means it should function normally after exposure to all the environmental conditions it is expected to operate in. The applicant should consider the additional environmental parameters in RTCA/DO-160G, or equivalent specifications.

Examples include:

 High temperature (200 ºF).2  Low temperature (-65 ºF).2  Temperature shock/humidity/altitude cycling.2  Vibration.2  Shock.2  Drop test (6 feet).

 Drop test (40 feet).3

2 Values can depend on the installation requirements.

3 The cartridge does not have to function properly after a 40-foot drop but should remain safe, i.e., not explode or burst.