Figure 1. Routes to HIRF Compliance - Level A Systems De fine Airoraft and System Safety System HIRF Assessment Level A System Protection
(2) Aircraft-Level System -Level Verification Verification Select Select Airoraft System Assessment Assessmen
(3) Airoraft Test
- Level A Display Only Similarity Equipment Test Decision * Airorat Generic Transter Functions / Test (4) Similarity Attenuation (B}
(11) Integrated Airoratt Airoraft Low-Level Coupling Tests Airora t Similarity System Test (5 High-Level Tests Assessment
(10)
(9) See Figure 2 (12) System Similarity Assessment Assess Corrective Immunity - Unacceptable- Measures
(13) (14) Acceptable
(n) = Step number as described in Paragraph 10 of this AC HIRF Protection Compliance
(15) 21
Figure 2. Aircraft Low-Level Coupling Tests - Level A Systems From Figure 1 Step 10 Aircraft Low-Level •Airframe Resonant- 10 kHz to the 1 g -100 MHzto 18 GHz Coupling Tests Frequency
(10) 500 kHzto 400 MHz Aircraft Skin Current Analysis (10a) LowLevel LoM Level Low Level Direct-Drive Test Swept-Current Test Swept-Field Test (10b) (10c) (10d) Back To Figure 1 Step 13
(n) = Step number as described in Paragraph 10 of this AC 22
Figure 3. Routes to HIRF Compliance - Level B and C Systems System Safety Assessment Level B or C System Define Aircraft and System HIRF Protection (2) Select Compliance Method Similarity Equipment-Level Verification Similarity Equipment Corredive Assessment Test Measures
(5) (4) (7) Unacceptable Assess Immurity
(6) Acceptable HIRF Protection Compliance (B)
(n) = Step number as described in Paragraph 11 of this AC 23
10 STEPS TO LEVEL A SYSTEM HIRF COMPLIANCE Steps 1 through 15 listed below for HCL A system HIRF compliance may be used to show compliance with §§ 25.1317(a), 27.1317(a), or § 29.1317(a). Airplanes certificated under part 23 may use steps 1 through 15 to show compliance with § 23.1308(a) or § 23.2520(a), but acceptable compliance for HCL A systems on level 1, 2, 3, and 4 airplanes certificated under part 23 may also be shown using the method in paragraph 10.16 below.
10.1 Step 1 — HIRF Safety Assessment .
10.1.1 Determine the system failure condition classification f or the systems being certified on your aircraft, using a system safety assessment as discussed in paragraph s 7.2 and 7.3.
For systems classified with catastrophic failure conditions (HCL A systems), follow compliance steps 2 through 15 listed below, as appropriate. Figures 1 and 2 of this AC also depict these compliance steps, which are not necessarily accomplished sequentially. Systems classified with hazardous or major failure conditions (HCL B and C systems) should follow the compliance steps outlined in paragraph 1 1.
10.1.2 When showing compliance with § § 23.1308(a), 23.2520(a), 25.1317(a), 27.1317(a), or § 29.1317(a), the level A electrical and electronic system includes all electrical and electronic equipment, components , and electrical interconnections required to perform the function whose failure would prevent continued safe flight and landing. This electrical and electronic system must also recover normal operation of the level A functions in a timely manner to comply with §§ 23.1308(a)(2), 23.2520(a)(2),
25.1317(a)(2), 27.1317(a)(2), or § 29.1317(a)(2). The level A electrical and electronic system is not required to include :
• Equipment, components, and electrical interconnections required only for nonnormal situations; or
• Equipment, components, and electrical interconnections required only for dispatch under a minimum equipment list.
10.1.3 Some systems include mechanical, hydraulic, and/or pneumatic channels as well as electrical and electronic elements or channels t hat perform functions whose failure would prevent continued safe flight and landing. The HIRF safety assessment for §§ 23.1308(a), 23.2520(a), 25.1317(a), 27.1317(a), or § 29.1317(a) only applies to functions performed by electrical and electronic systems. The HIRF safety assessment should consider electrical or electronic failures that would adversely affect the function of the mechanical, hydraulic, and/or pneumatic channels. If electrical or electronic equipment, components, and electrical interconnections are used to assist, augment, or monitor for control loop feedback, the mechanical, hydraulic, and/or pneumatic channels in performing functions with failures that would prevent continued safe flight and landing during normal operation, then the electrical and electronic channel(s) must comply with §§ 23.1308(a), 23.2520(a), 25.1317(a), 27.1317(a), or § 29.1317(a).
10.1.4 Sections 23.1308(a), 23.2520(a), 25.1317(a), 27.1317(a), and 29.1317(a) do not require you to assume preexisting failure conditions when classifying the functional failure conditions and the scope of the level A systems. You should consider total losses, partial losses, and malfunctions of the systems, including hazardously misleading information presented to the flightcrew during and after the aircraft is exposed to HIRF.
10.1.5 Sections 23.1308(a)(2), 25.1317(a)(2), 27.1317(a)(2), and 29.1317(a)(2) require that the level A systems automatically recover normal operation of the function in a timely manner after exposure to HIRF Environment I. Section 23.2520(a)(2) require s that the level A systems recover normal operation of the function in a timely manner after exposure to the HIRF environment. Automatic or manual recovery applies to all redundant active channels of the level A system required for normal operation unless the recovery conflicts with other operational or functional requirements of the system.
The exception for recovery conflicts should be based on aircraft operational or functional requirements independent of HIRF exposure. The exception should not be a mitigation for level A system effects observed after exposure to HIRF Environment I.
10.1.6 Appendix B provides examples of system scope for transport category airplanes based on the guidance above. The HIRF safety assessment examples for level A systems contained in appendix B may also be applicable to normal category airplanes and to rotorcraft.
10.2 Step 2 — Define Aircraft and System HIRF Protection .
Define the HIRF protection features to be incorporated into the aircraft and system designs, based on the HIRF environments that are applicable to your aircraft and its level A systems. Equipment, system, and aircraft HIRF protection design may occur before you perform aircraft-level tests, and before you determine the actual internal HIRF environment. Therefore, you should base the equipment, system, and aircraft HIRF protection design on an estimate of the expected internal HIRF environment. You should consider all aircraft configurations that may affect HIRF protection, such as open landing gear doors (see step 7).
10.3 Step 3 — System Assessment Decision .
Determine if you will perform integrated system HIRF tests on the level A system, or if you will base the system verification on previous integrated system HIRF tests performed on a similar system. Aircraft and system tests and assessments need not be performed for the HIRF environments above 18 GHz if data and design analysis show the integrated system tests results (see step 5) satisfy the pass criteria from 12 GHz to 18 GHz, and the systems have no circuits that operate in the 18 GHz to 40 GHz frequency range.
10.4 Step 4 — Equipment Test.
10.4.1 Radiated and conducted RF susceptibility laboratory tests of RTCA/DO -160E, section 20 (as specified in AC 21-16), may be used to build confidence in the equipment's HIRF immunity before conducting laboratory integrated system tests in step 5. The equipment should be tested in accordance with the test levels (wire bundle currents and RF field strengths) of RTCA/DO-160E, section 20, or to a level estimated for the aircraft and equipment installation using the applicable external HIRF environment.
10.4.2 Equipment HIRF tests may be used to augment the integrated system HIRF tests where appropriate. For equipment whose HIRF imm unity is evaluated as part of the integrated system-level HIRF tests discussed in step 5, the individual equipment’s HIRF testing described in this step is optional.
10.5 Step 5 — Integrated System Test.
10.5.1 Radiated and conducted RF susceptibility laboratory tests on an integrated system should be performed for level A systems. The HIRF field strengths and wire bundle currents selected for this test should be based on the attenuated external HIRF environment determined in the aircraft assessment (see steps 10, 11, or 12). In many cases, the integrated system test is performed before the aircraft assessment is complete.
In these cases, the integrated system test field strengths and currents should be selected based on the expected aircraft attenuation or transfer fun ction.
10.5.2 The installation details for the laboratory integrated system tests should be similar to the installation in the aircraft. For example, the bonding and grounding of the system, wire size, routing, arrangement (whether parallel or twisted wires), con nector types, wire shields, and shield terminations, and the relative position of the elements to each other and the ground plane in the laboratory should closely match the system installation on the aircraft to be certificated. In addition, the laboratory integrated system rig should have an FAA conformity inspection prior to conducting any FAA certification credit testing.
10.5.3 For integrated system test of an engine and engine control system, y ou should also refer to the guidance provided in AC 33.28-3. When the engine certification occurs prior to a known specific aircraft installation, the engine manufacturer should make reasonable installation assumptions for engine-to-aircraft electrical interfaces. The engine installation manual or operating instructions should specify wire characteristics, shielding, connector types, shield terminations, and electrical bonding features for HIRF protection that are required when the engine is installed. Systems that are part of the engine certification must be installed in accordance with the engine manufacturer’s requirements. The applicant should perform the required HIRF tests with the same wiring, shielding, and electrical bonding configuration as specified in the engine installation manual or operating instructions.
10.5.4 The integrated system should be tested with the system operating, to include connected displays, sensors, actuators, and other equipment. Place the system in various operating modes to ensure the integ rated system is tested when operating at its maximum sensitivity. If the connected equipment is not related to the functions with catastrophic failures, these items may be simulated by test sets, if the test sets accurately represent the terminating circuit impedance of the sensor. However, the connected equipment should meet the appropriate HIRF requirements for their failure condition classification.
10.5.5 The test levels should be selected based on the expected aircraft internal HIRF environment determined thr ough aircraft tests (see step 10), generic transfer functions “for level A display systems only” and attenuation (see step 11), or aircraft similarity assessment (see step 12), using the applicable external HIRF environment. Integrated system test procedures are described in detail in the user’s guide.
10.5.6 Wire bundle current injection should be used for frequencies from 10 kHz to 400 megahertz (MHz). RF currents are injected into the integrated system wiring via a current transformer. Each wire bundle in the system should be injected and the induced wire bundle current measured. If a system wire bundle branches, then each wire bundle branch should also be tested. Simultaneous multi -bundle current injection may be necessary on systems with redundant or mult i-channel architectures.
10.5.7 High-level radiated susceptibility tests should be used at frequencies greater than 100 MHz. The radiating antenna should be far enough away to ensure the total volume of the equipment and at least half a wavelength of the wiring i s simultaneously and uniformly illuminated during the test.
10.5.8 Define appropriate pass/fail criteria for the system, based on the system safety assessment and the appropriate HIRF regulation. Any system susceptibility, including system malfunctions such as di splaying hazardously misleading information, upset, or damage should be recorded and evaluated based on these previously defined pass/fail criteria.
10.5.9 Using only the modulation to which the system under evaluation is most sensitive may minimize the test time. The user’s guide provides guidance on modulation selection and suggested default modulations and dwell times.
10.5.10 The equipment tests in step 4, using the techniques in RTCA/DO -160E, section 20 (refer to AC 21-16), are not normally sufficient to show HIRF compliance for step 5.
However, these standard RTCA/DO -160E, section 20 tests may be sufficient if paragraphs 10.5.2 through 10.5.4 of this step are met.
10.5.11 If the level A system consists of multiple similar channels, you may propose using one or more channels in the laboratory test setup for the integrated system, instead of all similar channels. You should demonstrate that the laboratory test setup adequately performs the functions to demonstrate compliance with §§ 23.1308(a), 23.2520(a),
25.1317(a), 27.1317(a), or § 29.1317(a). Ensure that the laboratory test setup represents and monitors any cross-channel interactions, such as cross -channel data links, redundancy management, and system health monitoring.
Note: If pin programming or software is used to identify or configure equipment of similar elements or channels, you should assess whether these differences impact the functions performed.
10.6 Step 6 — System Similarity Assessment.
10.6.1 The integrated system HIRF tests performed for a system previously certified on one aircraft model may be used to demonstrate system verification for a similar system.
Each system considered under the similarity approach needs to be assessed independently even if it may use equipment and installation tec hniques from previous certification projects.
10.6.2 The system used as the basis for similarity must have successfully completed integrated system HIRF tests. Similarity assessment requires comparison of both equipment and installation differences that could adv ersely affect HIRF immunity. The assessment should evaluate the differences between the previously HIRF -certified system and the equipment circuit interfaces, wiring, grounding, bonding, connectors, and wire - shielding practices of the equipment that comprise the new system.
10.6.3 If the assessment finds negligible differences, with respect to HIRF aspects, between the previously certified system and the new system to be certified, similarity may be used as the basis for system-level verification without the need for additional integrated system tests, providing there are no unresolved in -service HIRF problems related to the previously certified system. If there is uncertainty about the effects of the differences, additional tests and analysis should be c onducted as necessary and appropriate to resolve the uncertainty. The amount of additional testing should be commensurate with the degree of difference identified between the new system and the system previously certified. If significant differences are fo und, similarity should not be used as the basis for system-level verification.
10.7 Step 7 — Aircraft Assessment Decision .
10.7.1 Level A systems require an aircraft assessment to support compliance with §§ 23.1308(a), 23.2520(a), 25.1317(a), 27.1317(a), and 29.1317(a ). The aircraft assessment should determine the actual internal HIRF environment where the level A systems are installed in the aircraft. You should choose whether you will use either aircraft tests or previous coupling/attenuation data from similar aircra ft types (similarity). For level A display systems only, you may use the generic transfer functions and attenuation in appendix A to this AC. Alternatively, the aircraft assessment may be a test that exposes the entire aircraft with operating level A systems to external HIRF environments I, II, or III (tables 2, 3, and 4, respectively), as appropriate, to demonstrate acceptable level A system performance.
10.7.2 Level A display systems include the display equipment, control panels, and the sensors that provide information to the displays. These sensors could also provide information to level A non-display systems, and in that case, you should determine the real transfer function and attenuation curves of these sensors when demonstrating compliance for this level A non-display system. For example, for air data system and inertial reference system, which sends information to the electronic flight instrument system and flight controls, the transfer function and attenuation should be determined by the aircraft lowlevel coupling test or the aircraft similarity assessment as defined in steps 10 and 12.
10.7.3 Other methods for aircraft HIRF assessment, such as analysis, may be acceptable.
However, comprehensive modeling and analysis for RF field coupling to the aircraft structure is an emerging technology. Therefore, analysis alone is currently not adequate to show HIRF compliance for level A systems and should be augmented by testing.
10.7.4 If analysis is used to determine aircraft attenuation and transfer function characteristics, you should provide test data to support this analysis. Any analysis results should take into account the quality and accuracy of the analysis. Comprehensive testing, including aircraft level testing, is required to support the analysis.
10.7.5 Aircraft and system tests and assessments need not be performed for the HIRF environments above 18 GHz if data and design analysis show the integrated system tests results (see step 5) satisfy the pass criteria from 12 GHz to 18 GHz, and the systems have no circuits operating in the 18 GHz to 40 GHz frequency range.
10.8 Step 8 — Aircraft Test Decision.
10.8.1 Various aircraft test procedures are available and accepted for collecting data for aircraft HIRF verification. The two main approaches to aircraft testing are the aircraft high-level test (see step 9) and the aircraft low-level coupling test (see step 10). The aircraft high-level field-illumination test involves radiating the aircraft at test levels equal to the applicable external HIRF environment as described in paragraph 7.4.
Aircraft low-level coupling tests involve measuring the airframe attenuation and transfer functions, so that the internal HIRF electric fields and currents can be compared to the integrated system test levels.
10.8.2 Some test procedures may be more appropriate than others because of the size of the aircraft and the practicality of illuminating the entire aircraft with the appropriate external HIRF environment. The aircraft low -level coupling tests (see step 10) may be more suitable for testing large aircraft than th e high-level field-illumination test in step 9, which requires illumination of the entire aircraft with the external HIRF environment.
10.9 Step 9 — Aircraft High-Level Tests.
10.9.1 The aircraft high-level field-illumination test requires generating RF fields extern al to an aircraft at a level equal to the applicable external HIRF environment.
10.9.2 At frequencies below 400 MHz, the distance between the aircraft and the transmitting antenna should be sufficient to ensure the aircraft is illuminated uniformly by the external HIRF environment. The transmit antenna should be placed in at least four positions around the aircraft. For airplanes, the antenna is typically placed to illuminate the nose, tail, and each wingtip. For rotorcraft, the antenna is typically placed to illuminate the nose, tail, and each side. The aircraft should be illuminated by the antenna at each position while sweeping the frequency range. Perform separate frequency sweeps with the transmit antenna oriented for horizontal and vertical polarization. The RF field should be calibrated by measuring the RF field strength in the center of the test volume before the aircraft is placed there.
10.9.3 At frequencies above 400 MHz, the RF illumination should be localized to the system under test, provided all parts of the system and at least one wavelength of any associated wiring (or the total length if less than one wavelength) are illuminated uniformly by the RF field. You may need reflection planes to illuminate relevant apertures on the bottom and top of the aircraft.
10.9.4 To ensure the systems are tested when operating at their maximum sensitivity, level A systems should be fully operational, and the aircraft should be placed in various simulated operating modes.
10.9.5 The test time can be minimized by using only the m odulation to which the system under evaluation is most sensitive. If you do this, the rationale used to select the most sensitive modulation should be documented in the HIRF test plan as discussed in paragraph 9.2.1. The user’s guide provides guidance on modulation selection and suggested default modulations and dwell times.
10.9.6 As an alternative to testing at frequencies below the first airframe resonant frequency, it is possible to inject high-level currents directly into the airframe using aircraft high - level direct-drive test methods. Aircraft skin current analysis should be performed as described in the user’s guide, or low-level swept-current measurements should be made to determine the skin current distribution that will exist for different RF field polarizations and aircraft illumination angles so that these can be simulated accurately during this test. Aircraft high -level direct-drive testing, although applicable only from 10 kHz to the first airframe resonant frequency, is advantageous because it is pos sible to test all systems simultaneously.
10.10 Step 10 — Aircraft Low-Level Coupling Tests.
10.10.1 Low-Level Coupling Tests.
The aircraft low-level coupling tests include three different tests that cover the frequency range of 10 kHz to 18 GHz (see figure 2). Detailed descriptions are available in the user’s guide. Other techniques may be valid but must be discussed with and approved by the FAA before being used.
10.10.2 Low-Level Direct-Drive Test and Low-Level Swept-Current Test.
The low-level direct-drive test (see step 10b, figure 2) and the low -level swept-current test (see Step 10c) are used for frequencies at or below 400 MHz. The low -level sweptfield test (see Step 10d) is used for frequenc ies at and above 100 MHz. There is an overlap of test frequencies from 100 MHz to 400 MHz in the low -level swept-current test and the low-level swept-field test. The division at 400 MHz is not absolute but rather depends on when HIRF penetration of the equ ipment case becomes a significant factor.
10.10.3 Steps 10a and 10b - Aircraft Skin Current Analysis and Low -Level Direct-Drive Test.
Low level direct-drive tests in conjunction with skin current analysis should be used to determine the transfer function between t he skin current and individual equipment wire bundle currents. The low-level direct-drive test is typically used for frequencies from 10 kHz to the first airframe resonant frequency. For the low -level direct-drive test to be applied successfully, a three d imensional model of the aircraft should be derived using aircraft skin current analysis. The three dimensional model can then be used to derive the aircraft's skin current pattern for the applicable external HIRF environment.
Guidance on skin current analy sis is in the user’s guide. If the relationship between the external HIRF environment and the skin current is known for all illumination angles and polarization, either because of aircraft skin current analysis or the use of the low - level swept-current test, the skin current can be set up by direct injection into the airframe. The resultant currents on the system wire bundles are measured with a current probe and normalized to 1 V/m electric field strength so they can be scaled to the appropriate external HIRF environment. The low-level direct-drive test is more effective than low-level swept-current tests for frequencies from 10 kHz to the first airframe resonant frequency and may be necessary for small aircraft or aircraft with high levels of airframe shielding.
10.10.4 Step 10c — Low-Level Swept-Current Test.
The low-level swept-current test involves illuminating the aircraft with a low -level external HIRF field to measure the transfer function between the external field and the aircraft and equipment wire bundle currents. This test is typically used in the frequency range of 500 kHz to 400 MHz. The transfer function is resonant in nature and is dependent on both the aircraft structure and the system installation. Because the transfer function relates wire bundle c urrents to the external field, the induced bulk current injection test levels can be related to an external HIRF environment.
10.10.4.1 The transmit antenna should be placed in at least four positions around the aircraft, with the distance between the aircraft and the transmitting antenna sufficient to ensure the aircraft is illuminated uniformly.
• For airplanes, the antenna is typically placed to illuminate the nose, tail, and each wingtip.
• For rotorcraft, the antenna is typically placed to illuminate the nose, tail, and each side.
The aircraft should be illuminated by the antenna at each position while sweeping the frequencies in the range of 500 kHz to 400 MHz. Perform separate frequency sweeps with the transmit antenna oriented f or horizontal and vertical polarization. Measure the currents induced on the aircraft wire bundles.