AC 20-144A

9.3 Cartridge Design.

Figure 1 below shows an example of a firex cartridge.

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

9.3.1 Backshell.

9.3.1.1 The backshell is typically a stainless steel housing with pins imbedded in a high-pressure hermetic seal. The pins provide the electrical connections from the external wiring or connector to the bridgewire. The environmental and glass seals are critical in providing a high-pressure hermetic seal before and after firing. They provide:

 Pressure and shock barrier between the explosive output and the electrical connections.

 Hermetic moisture seal to protect the explosive charges.

 Mechanical retention of the pins.

 Electrical isolation of the pins.

9.3.1.2 Cracked or poor seals, due to design or quality issues, can cause failures because the header pins could be explosively dislodged from the cartridge, resulting in agent leakage through the cartridge backshell. Also, cracked or poor seals can allow moisture to infiltrate and degrade the explosive charges during the environment exposure cycles of aircraft operation. This will degrade performance as well as the effective service life of the cartridge. The applicant should define the method of verifying the integrity of the seal (typically using a helium leak test).

9.3.2 Bridgewire.

The bridgewire initiates the detonation by heating the ignition charge to its ignition temperature. The specific bridgewire size and material should be selected to meet the required all-fire and no-fire reliability. See paragraphs 9.2.2.1 and 9.2.2.3 of this AC for the definitions of minimum firing current and no-fire current.

9.3.3 Explosive Charges.

The typical cartridge contains a minimum of two types of explosive charges: an ignition charge and an output charge. These charges initiate sequentially in the cartridge to generate the required energy output to rupture the firex system burst disc. The bridgewire initiates the cartridge by heating the ignition charge to its ignition temperature.

9.3.3.1 Ignition Charge.

The ignition temperature and thermal conductivity characteristics of the ignition charge have a significant effect on the all-fire and no-fire reliability and performance of the cartridge.

9.3.3.2 Output Charge.

When initiated, the output charge provides the energy focused to open the burst disc effectively without causing other damage to the firex system or aircraft.

9.3.4 Part Labeling.

To comply with § 2X.1301, firing cartridge components should be labeled. The labeling should include the applicable “manufactured on” date or “life-limited date.”

9.3.5 Shorting Springs and Shunts.

Firing cartridge components are often shipped using a shorting spring or shunt mounted on the cartridge electrical connector. There have been instances where shunts were left installed inadvertently on the cartridge connector during installation. This effectively disables the firex system. When these cases were detected, the FAA released airworthiness directives (ADs) to correct the condition. To comply with § 2X.1301 and prevent future ADs, firing cartridge components should be designed to ensure that cartridges may not be installed with such devices in place. Alternatively, where this is impractical, the device should be distinctively marked with instructions to ensure correct installation.

9.4 Cartridge Validation.

To validate compliance to § 2X.1301, the all-fire and no-fire condition tests may consist of firing a group or groups of cartridges at various current levels and noting the performance characteristics of the cartridge. The applicant should establish specific quantities of specimens for testing to be statistically significant and provide the desired confidence levels. The current levels should be statistically selected and the results analyzed to provide no-fire and all-fire current ratings at various required reliability and confidence levels (0.99 reliability at 95 percent confidence) with reference to the engineering design test schedule in MIL-DTL-23659F.

9.5 Basic Qualification.

The FAA recommends that the applicant qualify each cartridge design to meet and address the items listed in the engineering design test schedule in MIL-DTL-23659F, plus an additional test sequence to ensure open disc performance (pursuant to paragraph

9.2.3.2 of this AC) at high and low temperature extremes. The test should use a representative bottle and burst disc charged to the minimum allowable pressure using 7 bottle/disc test units per high and low temperature condition for a total of 14 firings.

(See MIL-DTL-23659F.)

9.6 Pre-Production Design and Quality Control.

The FAA will review the applicant’s quality system to ensure the inspection system that is required by part 21 is in place. The applicant should ensure that the cartridge design, development, qualified baseline, and quality aspects are maintained throughout the production life cycle and meet the requirements in §§ 21.1, 21.137, 21.303, and Orders 8110.42D and 8120.22A.

9.6.1 Manufacturing Processes.

The applicant should use a documented and controlled system to ensure that cartridge components are manufactured or procured from a qualified source that provides closed-loop controls to ensure material acceptability. One example of the importance of this documented and controlled system is to ensure the quality of the seal in the cartridge body (header assembly), which is a critical component. Poorly manufactured seals may result in failure, as the header pins could be explosively dislodged from the cartridge, causing agent to leak through the cartridge or backshell, or allowing air or moisture contamination of the pyrotechnic mixture. The manufacturing processes that the applicant should address are identified and described below:

9.6.1.1 Bonding Assemblies.

Bonding of the assemblies is critical to ensure the cartridge meets all electrical requirements. Failure to achieve complete and proper bonded assemblies may cause failure in dielectric strength, insulation resistance, or electrostatic discharge capability. This may result in inadvertent firing of the cartridge during electrical surges from sources such as lightning, or failure to fire if the cartridge is electrically shorted. Bonding processes include, but are not limited to:

13  Proper mix ratio definition.

 Cure temperature and time definition.

 Cleanliness of bonded surfaces definition.

 Coverage of bonded surfaces definition.

 Bridgewire material selection.

9.6.1.2 Bridgewire Welding.

Bridgewire welding is critical to ensure that the proper electrical energy is transferred to the explosive material, which is achieved through the bridgewire. Typically, the bridgewire is resistance-welded to the header pins and is process-sensitive. Poor bridgewire welds may result in the cartridge failing to fire as the bridge circuit may open during normal temperature cycling, shock, and vibration. Welding parameters include, but are not limited to:

 Length of the bridgewire.

 Location of the bridgewire weld on the header pins.

 Strength of the bridgewire welds.

 Resistance values.

 Bridgewire material.

9.6.1.3 Powder Preparation.

The applicant should use documented procedures to control critical processes. These procedures should ensure handling safety, ingredient consistency, and consistency from production lot to production lot. Each powder blend should be characterized and tested for performance and accepted prior to use in production cartridges. Typically, testing includes caloric content, particle size, thermal analysis, and performance analysis.

Steps include, but are not limited to:

 Use of controlled and approved procedures.

 Acceptance testing of powder prior to use in production cartridges.

 Documentation and approval of raw materials.

 Storing and handling.

 Verification of moisture content.

9.6.1.4 Powder Loading.

Powder loading is critical to the functional performance of the cartridge.

The applicant should verify moisture content, hygroscopic capability, and volatility of the powder to ensure proper ignition and output of the explosive. Proper density of the powder should be achieved to ensure all-fire and no-fire capability. Failure to ensure moisture, volatility content, and proper pack density will typically result in failure to fire or inadvertent firing from stray voltages. Processes include, but are not limited to, control of the:

 Process environment.

 Consolidation force of the explosive into the cartridge.

 Length of time the consolidation force is applied.

 Accuracy of the powder quantity to be loaded.

9.6.1.5 Hermetic Sealing.

Environmentally sealing the cartridge by a hermetic seal ensures the cartridge meets the prescribed life requirements. Failure to achieve a hermetic seal will reduce service life as the explosive material degrades through normal environment conditions. The applicant should verify the integrity by testing 100 percent for leakage (e.g., 1 x 10-6 standard cubic centimeter per second of helium at one atmosphere).

9.6.2 The FAA or its delegates will not allow deviation to the approved manufacturing procedures at any level of assembly, unless the applicant evaluates the deviation under an FAA-approved process.

9.7 Quality Verification.

To comply with § 2X.1301, the cartridge design or changes to cartridge design, manufacture, or processes of cartridge testing should encompass the entire spectrum of criteria from cartridge design, development, qualification, and production and storage prior to installation. The performance elements of the cartridge at the firex system level that are typically used to develop the test and validation methods are:

9.7.1 Design Development.

During development, the cartridge design requirements are dictated by the firex system performance requirements. The applicant can correlate these results with other non-system-level function tests (e.g., agent flow, function time, and environments) and use the results to determine how to test the cartridge based on the engineering design test schedule in MIL-DTL-23659F, and the addition of disc opening performance tests in paragraph 9.5 of this AC.

9.7.2 Design Change Control.

The applicant should have a process for design changes in a method acceptable to the FAA. The applicant should validate all design changes to the firex system performance and reliability requirements.

9.7.3 Design Qualification and Validation.

System qualification testing should ensure that the cartridge:

 Performs the required functions at the system level.

 Can effectively open the burst disc, allowing correct agent flow under all operating and expected environmental conditions.

 Does not cause any peripheral degradation of the firex system as the result of the explosive event occurring.

9.7.4 Testing.

9.7.4.1 During cartridge qualification and validation, the applicant should perform system level tests to demonstrate the capability of a cartridge design to meet the burst disc rupture requirements. A typical number of tests at the system level is seven according to MIL-C-22284A. Cartridge qualification and validation also involve non-system-level testing where key design elements are evaluated (see the engineering design test schedule in MIL-DTL-23659F). This combined test series establishes baseline reliability and performance parameters.

9.7.4.2 After design validation, the applicant should correlate the results with other non-system-level function tests to develop criteria for suitable production lot acceptance tests. Such testing can effectively indicate manufacturing process control issues in a validated design. However, the applicant should not use the results of the testing to demonstrate the suitability of a new design for a given system. Using non-system-level test methods to demonstrate similar performance of a new design to a qualified design does not validate the suitability of a new design to work properly in an installed firex system. The applicant should substantiate the use of non-system-level tests and analysis to show equivalence as installed on the actual firex system.

9.7.4.3 The applicant should verify proper storage and handling procedures are in place to ensure that firing cartridges function properly in support of §§ 2X.1301 and 2X.1309 requirements.

9.7.5 Production Lot Acceptance Testing.

Cartridge sample lot acceptance testing is a means of assessing the manufacturing process to ensure nothing has changed that might have affected cartridge performance.

After cartridge qualification, the applicant should develop test methods and criteria to ensure production cartridges conform to the performance requirements of the qualified design. This demonstrates that the production cartridges have not changed with respect to the qualified design. Production lot acceptance tests should include, for example:

 Bridgewire resistance on 100 percent of the lot.

 Dielectric strength on 100 percent of the lot.

 Insulation resistance on 100 percent of the lot.

 Leakage on 100 percent of the lot.

 Radiographic on 100 percent of the lot.

 Electrostatic discharge on 100 percent of the lot (if applicable).

 No-fire on a lot sample.

 All-fire on a lot sample.

 Functional performance on a lot sample.

9.8 Service Life Issues.

9.8.1 The firex system is a safety system that is exposed to repeated cycles of temperature, altitude, and vibration. This system remains dormant, until it is needed, and then should perform flawlessly. Compromising on the long-term quality when approving new, modified, or replacement cartridges could result in catastrophic loss of the airplane. For example, cracked seals and poor welds can be patched with potting to pass helium leak tests during production lot acceptance. However, this low-quality fix may not survive the entire rated service life. Repeated cycles of temperature and altitude can force moisture into the explosive charges. As previously discussed, this can cause cartridge failure resulting in non-actuation of the firex system, which would compromise safety of the airplane. Section 2X.1301 requires that the systems function properly when installed.

9.8.2 To comply with § 2X.1301, the applicant should identify the method to validate the proposed service life of the cartridge. Accelerated life testing should reference AIAA S-113A-2016, section 5.5.1, Age Surveillance of Explosive Components, or equivalent, except for the following:

Note: Prior to initial approval, the applicant should complete this validation and evaluate the aircraft installation environment with respect to the service life test plan to ensure compatibility with the proposed service life of the cartridge.

1. An additional performance discharge sequence for the non-system-level function testing identified in paragraph 9.7.4.2 of this AC should be conducted in accordance with AIAA S-113A-2016.

2. When performing accelerated age testing as listed in AIAA S-113A-2016, section

5.5.1, Age Surveillance of Explosive Components, establish the temperature and time using the method referenced in the National Institute of Standards and Technology, NIST/SEMATECH e-handbook of Statistical Methods:

http://www.itl.nist.gov/div898/handbook/apr/section1/apr151.htm.

9.8.3 Determining accelerated age test temperature and time using in-service temperature and time: Most aircraft installations do not have a uniform environmental temperature where the cartridges are installed. Some cargo installations are in a location that are environmentally controlled when the aircraft is in operation, but see temperature variations when the aircraft is not in use. Other cartridges are installed outside the pressure vessel of the aircraft in the same compartments as engine bleed ducting; these see large variations in temperature between ground operation and in-flight conditions (some may be as high as 160 ºF and as low as -80 ºF).

The number of collisions that can cause reactions increases exponentially with temperature. This can sometimes double with an increase of as little as 10 ºC. It may not be correct to use the average or mean installed temperature when determining the installed temperature used in the accelerated age testing calculation. Using a lower average temperature would ignore the detrimental effects that occurs at even relatively short periods of time at the highest temperatures. Using the maximum installed temperature would conservatively cover the in-service environment but would not take credit for the time spent at the lower temperatures. The applicant must propose a method found acceptable to the FAA to determine the usage temperature to be used in the accelerated temperature life testing.

9.9 Final Approval Process.

9.9.1 Qualification Document Package.

The applicant should include the following:

 An overall certification plan.

 Detailed drawings on the proposed item.

 Manufacturing and quality procedures and processes.

 Engineering change orders for the proposed item and configuration control.

 Statement of differences between the proposed item and the baseline item.

 Approved plan to address the testing specified in the engineering design test schedule in MIL-DTL-23659F.

 Data/report showing successful completion of all proposed component level tests on the proposed item.

 Approved qualification test plan on the applicant’s and PAH’s items.

 Data/report showing successful completion of approved qualification test plan.

 Data/report showing approved system-level test.

18  Data/report showing successful completion of all system-level tests in the system-level test plan.

 Approved production functional test methods to be used on both items with an explanation of the pass/fail criteria.

 Approved service life test plan.

 Data/report showing successful completion of the service life testing according to the approved life test plan.

 Approved production quality screening plan (lot acceptance).

9.9.2 Final Qualification.

The qualification is performed by conducting adequate analyses and testing to verify that the components will function properly when installed in the installation environment with the required reliability. With respect to appropriate testing in RTCA/DO-160G, qualification provides an accepted means for testing components in various environments. Qualification can be done to support an application for a type certificate (TC), an amended TC, or an STC. The applicant should submit a qualification test plan to the FAA or its authorized designee. The results of the qualification should be provided for approval in a qualification test report to the FAA or its designee. Additionally, applicants may produce after-market components with a PMA.

9.9.2.1 There are three ways to receive a PMA. They are identicality, identicality by licensing agreement, and by test and computation. Sections 21.301 through 21.320 and Order 8110.42D describe the responsibilities of the FAA aircraft certification offices (ACOs) and manufacturing inspection district offices (MIDOs) and applicants, and the process for approving replacement and modification articles for installation on type-certificated products when issuing a PMA. The associated procedures for the FAA (MIDO) and manufacturing personnel are in Order 8120.22A. Applicant guidance is in AC 21.303-4. When applying these procedures to firex cartridges, the applicant should address the following items:

9.9.2.1.1 PMA Approval by Identicality.

Engineering drawings and specifications of the proposed cartridge should contain adequate detailed information to define the cartridge assembly and its components accurately, item by item, to show identicality to approved parts. The applicant must obtain FAA approval for any significant differences with the original approved cartridges under the test and computation method. Examples of significant differences are charge material changes, loading changes, process changes, changes in supplier or manufacturer, and physical configuration differences.

9.9.2.1.2 PMA Approval by Licensing Agreement.

According to § 21.303(a)(4), if the applicant obtains the design of the electrical firing cartridge by a licensing agreement, the applicant must provide evidence of that agreement.

9.9.2.1.3 PMA Approval by Test and Computation.

Test and computation requires a PMA applicant to conduct analyses and tests necessary to show they produce an electrical firing cartridge that meets the performance and reliability of the production version. The test method is to fire the cartridge in charged bottles under operational conditions. RTCA DO-160 (latest revision) provides guidance for component testing in the appropriate environment. Typically, the firex system-level tests are coordinated with the FAA. They are performed during the development and qualification of a new design and should establish system performance of replacement parts.

9.9.2.2 For parts determined to be critical or life-limited, the FAA may require the applicant to perform inspections and tests and submit the results to the FAA. These test results are necessary to show airworthiness of parts produced in conformity with the proposed design to obtain design approval. If the application is based on identicality, no testing is normally required. If the application is based on test and computation, or an STC, the applicant should submit both design and qualification test results.

9.9.2.3 The applicant should address all sections of this AC in a certification plan per part 21, subpart G, and obtain FAA approval for the certification plan prior to beginning qualification activities.

9.9.2.4 The applicant should address any aspect that might degrade:

 Safety.

 Performance.

 Soundness of mechanical design.

 Resistance to environment.

 Service life.

9.9.2.5 The FAA considers all cartridges in aircraft fire extinguishing and suppression systems as critical components and therefore requires the applicant to define performance testing requirements and conduct such tests.

9.9.2.6 The FAA considers all subsequent changes to the design under the definition provided in § 21.93. The applicant should not ship any parts that are of any major change configuration prior to receiving FAA approval of that change. The applicant may need to seek guidance as to whether its proposed change should be considered major or minor. Parts shipped prior to FAA approval of any major change may be subject to the suspected unapproved parts reporting requirements of Order 8120.16A.

Advisory Circular Feedback If you find an error in this AC, have recommendations for improving it, or have suggestions for new items/subjects to be added, you may let us know by (1) emailing this form to 9-AWA-AVS- AIR-DMO@faa.gov, or (2) faxing it to (202) 267-1813.

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