bundle currents. Various techniques can be used to ensure the maximum internal field in the vicinity of the equipment is measured. Depending on the size of the aircraft and the size of the aircraft cabin, flight deck, and equipment bays, multipoint measurement or mode stirring can be used to maximize the internal field in the vicinity of the equipment. See the User's Guide for detailed low-level swept-field test procedures.
k. Step 11 — Generic Transfer Functions and Attenuation - Level A Display Systems Only.
(1) Level A displays involve functions for which system information is displayed directly to the pilot. For level A display systems, the aircraft attenuation data may be determined using generic attenuation and transfer function data. This approach should not be used for other level A systems, such as control systems, because failures and malfunctions of those systems can more directly and abruptly contribute to a catastrophic failure event than display system failures and malfunctions; therefore, other level A systems should have a more rigorous HIRF compliance verification program.
(2) The integrated system test levels specified in step 5 may be derived from the generic transfer functions and attenuation for different types of aircraft. Acceptable transfer functions for calculating the test levels are given in appendix 1 to this AC. Appendix 1 to this AC also contains guidelines for selecting the proper generic attenuation. The generic transfer functions show the envelope of the currents that might be expected to be induced in the types of aircraft in an external HIRF environment of 1 V/m. The current levels should be multiplied linearly by HIRF environment I, II, or III, as appropriate, to determine the integrated system test levels.
(3) The internal HIRF electric field levels are the external HIRF environment divided by the appropriate attenuation, in linear units. For example, 20 dB or a 10:1 attenuation means the test level is the applicable external HIRF environment electric field strength reduced by a factor of 10.
(4) The internal HIRF environments for level A display systems can also be measured using on-aircraft low-level coupling measurements of the actual system installation (see step 10).
This procedure should provide more accurate information to the user, and the test levels may be lower than the generic transfer functions or attenuation, which are worst-case estimates.
l. Step 12 — Aircraft Similarity Assessment.
(1) The aircraft attenuation and transfer functions tests performed for a previously certified aircraft may be used to support aircraft-level verification for a similar aircraft model.
The aircraft used as the basis for similarity must have been previously certified for HIRF compliance, using HIRF attenuation and transfer functions determined by tests on that aircraft.
(2) The similarity assessment for the new aircraft should consider the aircraft differences that could impact the internal HIRF environment affecting the level A systems and associated wiring. The comparison should consider equipment and wiring locations, airframe materials and construction, and apertures that could affect attenuation for the external HIRF environment.
(3) If the assessment finds only minimal differences between the previously certified aircraft and the new aircraft to be certified, similarity may be used to determine aircraft attenuation and transfer functions without the need for additional aircraft tests, providing there are no unresolved in-service HIRF problems related to the existing aircraft. If there is uncertainty about the effects of the differences, additional tests and analysis should be conducted 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 aircraft and the aircraft previously certified. If significant differences are found, similarity should not be used as the basis for aircraft-level verification.
m. Step 13 — Assess Immunity.
(1) Compare the test levels used for the integrated system test of step 5 with the internal RF current or RF fields determined by the aircraft low-level coupling tests (see step 10), the generic transfer functions and attenuation (see step 11), or the aircraft similarity assessment (see step 12). The actual aircraft internal RF currents and RF fields should be lower than the integrated system test levels. Your comparison method should be included in the HIRF compliance plan. The method should enable a direct comparison between the system test level and the aircraft internal HIRF environment at the equipment or system location, using current for frequencies from 10 kHz through 400 MHz, and using electric field strength for frequencies from 100 MHz through 18 GHz.
(2) If the conducted RF susceptibility test levels used for the integrated system test (see step 5) were too low when compared with the aircraft-induced currents determined in steps 10b, 10c, 11 or 12, then corrective measures are needed (see step 14). If the radiated RF susceptibility test levels used for integrated system tests (see step 5) were too low when compared with the aircraft internal fields determined in steps 10d, 11 or 12, then corrective measures are needed (see step 14).
(3) When comparing the current measured during low-level swept-current tests in step 10c with the current used during the integrated system tests in step 5, there may be differences.
These differences may be due to variations between the actual aircraft installation and the integrated system laboratory installation, such as wire bundle lengths, shielding and bonding, and wire bundle composition. The worst-case current signature for a particular wire bundle should be compared to the current induced at the particular test level or equipment malfunction over discrete frequency ranges such as 50 kHz to 500 kHz, 500 kHz to 30 MHz, and 30 MHz to 100 MHz. This comparison should be broken into discrete frequency ranges because the resonant frequencies may differ between the integrated system tests and the aircraft tests.
(4) If you used aircraft high-level tests (see step 9) for aircraft HIRF verification, you should determine if there were any level A system susceptibilities. Any level A system susceptibilities should be evaluated based on the pass/fail criteria as established in the test plan (see paragraphs 8b(1)). If the HIRF susceptibilities are not acceptable, then corrective measures may be needed (see step 14).
(5) HIRF susceptibilities that were not anticipated or defined in the test plan pass/fail criteria may be observed during aircraft high-level tests or integrated system laboratory tests.
The pass/fail criteria may be modified if the effects neither cause nor contribute to conditions that adversely affect the aircraft functions or systems in the HIRF regulations. You should provide an assessment and supporting rationale for any modifications to the pass/fail criteria to the FAA for approval. If the HIRF susceptibilities are not acceptable, then corrective measures may be needed (see Step 14).
(6) If the level A systems show no adverse effects when tested to levels derived from the applicable HIRF environment I or III, this also demonstrates compliance of the system with HIRF environment II.
(7) If the integrated system tests results (see step 5) satisfy the pass criteria from 12 GHz to 18 GHz, and design analysis shows the system has no circuits operating in the 18 GHz to 40 GHz frequency range, this demonstrates by analysis the system is not adversely affected when exposed to HIRF environments above 18 GHz. If these conditions are satisfied, further aircraft and system tests and assessments above 18 GHz are not necessary.
(8) Review the actual system installation in the aircraft and the system configuration used for the integrated system test (see step 5). If significant configuration differences are identified, corrective measures may be needed (see step 14).
(9) Certain RF receivers with antennas connected should not be expected to perform without effects during exposure to the HIRF environments, particularly in the RF receiver operating band. Because the definition of adverse effects and the RF response at particular portions of the spectrum depends on the RF receiver system function, refer to the individual RF receiver minimum performance standards for additional guidance. However, because many RF receiver minimum performance standards were prepared before implementation of HIRF requirements, the RF receiver pass/fail criteria should be coordinated with the FAA.
(10) You should provide the similarity assessment and supporting rationale to the FAA for approval.
n. Step 14 — Corrective Measures. Take corrective measures if the system fails to satisfy the HIRF immunity assessment of step 13. If changes or modifications to the aircraft, equipment, system or system installation are required, then additional tests may be necessary to verify the effectiveness of the changes. The RTCA/DO-160E or latest version, Section 20 equipment tests, integrated system tests, and aircraft tests, in whole or in part, may need to be repeated to show HIRF compliance.
o. Step 15 — HIRF Protection Compliance. Submit the test results and compliance report to the cognizant FAA aircraft certification office for approval as part of the overall aircraft type certification or supplemental type certification process.
10. Steps to Level B and C System HIRF Compliance.
a. Step 1 — System Safety Assessment. Determine the system failure condition classification for the systems being certified on your aircraft, using a system safety assessment as discussed in paragraph 6b(2). For systems classified with hazardous or major failure conditions (Level B and C systems), follow compliance steps 2 through 8 listed below, as appropriate.
These compliance steps are also depicted in figure 3 of this AC, and are not necessarily accomplished sequentially. Systems classified with catastrophic failure conditions (Level A systems) should follow the compliance steps outlined in paragraph 9.
b. Step 2 — Define Aircraft and System HIRF Protection. Define the HIRF protection features incorporated into the aircraft and system designs, based on the HIRF test levels applicable to your aircraft and its level B and C systems. Equipment, system, and aircraft HIRF protection design may occur before aircraft-level tests are performed, and before the actual internal HIRF environment is determined. Therefore the equipment, system and aircraft HIRF protection design should be based on an estimate of the expected internal HIRF environment.
c. Step 3 — Select Compliance Method. Determine if you will perform equipment HIRF tests on the level B and C systems, or if you will base the compliance on previous equipment tests performed for a similar system.
d. Step 4 — Equipment Test.
(1) Level B and level C systems do not require the same degree of HIRF compliance testing as level A systems, and therefore do not require aircraft-level testing. RTCA/DO-160E or latest version, Section 20 laboratory test procedures should be used, using equipment test levels defined in the regulations. The test levels used depend on whether the system is categorized as level B or C. Equipment HIRF test level 1 or 2, as applicable, should be used for level B systems. RTCA/DO-160 Section 20, Category RR (using the alternative modulation for radiated susceptibility), satisfies the requirements of equipment HIRF test level 1. For equipment HIRF test level 2, you may use the approach in paragraph 9k to help determine acceptable aircraft transfer function and attenuation curves for your level B system. Equipment HIRF test level 3 should only be used for level C systems. RTCA/DO-160 Section 20, Category TT, satisfies the requirements of equipment HIRF test level 3. When applying modulated signals, the test levels are given in terms of the peak of the test signal as measured by a root-mean-square (rms) indicating spectrum analyzer's peak detector. See the User's Guide (SAE ARP 5583A/ EUROCAE ED-107A) for more details on modulation.
(2) Define appropriate pass/fail criteria for the system, based on the system safety assessment and the appropriate HIRF regulation (see paragraph 6b(2)). Any susceptibility noted during the equipment tests, including equipment malfunctions, upset, or damage, should be recorded and evaluated based on the defined pass/fail criteria.
e. Step 5 — Similarity Assessment.
(1) The equipment HIRF tests performed for a system previously certified on one aircraft model may be used to show compliance for a similar system. Each system considered for similarity needs to be assessed independently even if it used equipment and installation techniques from a previous certification.
(2) The system used as the basis for certification by similarity must have successfully completed equipment HIRF tests and previously certified for HIRF compliance on another aircraft model. Similarity assessment requires comparison of both equipment and installation differences that could adversely effect HIRF immunity. An assessment of a new system should consider the differences in the equipment circuit interfaces, wiring, grounding, bonding, connectors, and wire-shielding practices.
(3) If the assessment finds only minimal differences between the previously certified system and the new system to be certified, similarity may be used for HIRF compliance without the need for additional equipment HIRF 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 conducted 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 found, similarity should not be used as the basis for HIRF compliance.
f. Step 6 — Assess Immunity.
(1) Review the results of the equipment test to determine if the pass/fail criteria are satisfied. HIRF susceptibilities not anticipated or defined in the test plan pass/fail criteria may be observed during equipment HIRF tests. The pass/fail criteria may be modified if the effects neither cause nor contribute to conditions that adversely affect the aircraft functions or systems, as applicable, in the HIRF regulations. You should provide an assessment of, and supporting rationale for, any modifications to the pass/fail criteria to the FAA for approval. If the HIRF susceptibilities are not acceptable, then corrective measures may be needed (see step 7).
(2) Review the actual system installation in the aircraft and the configuration used for the equipment tests (see step 4). If significant differences in grounding, shielding, connectors, or wiring are identified, corrective measures may be needed (see step 7).
(3) Certain RF receivers with antennas connected should not be expected to perform without effects during exposure to the HIRF environments, particularly in the RF receiver operating band. Because the definition of adverse effects and the RF response at particular portions of the spectrum depends on the RF receiver system function, refer to the individual RF receiver minimum performance standards for additional guidance. However, because many RF receiver minimum performance standards were prepared before implementation of HIRF requirements, the RF receiver pass/fail criteria should be coordinated with the FAA. Future modifications of the minimum performance standards should reflect HIRF performance requirements.
es D. Seipel cting Manager, Design, Manufacturing, & Airworth iness Division Aircraft Certification Service 05/30/14 AC 20-158A
g. Step 7 — Corrective Measures. Take corrective measures if the system fails to satisfy the HIRF immunity assessment of step 6. If changes or modifications to the equipment, system, or system installation are required, then additional tests may be necessary to verify the effectiveness of the changes. The RTCA/DO-160E or latest version, Section 20 equipment tests, in whole or in part, may need to be repeated to show HIRF compliance.
h. Step 8 — HIRF Protection Compliance. Submit the test results and compliance report to the cognizant FAA aircraft certification office for approval as part of the overall aircraft type certification or supplemental type certification process.
11. Maintenance, Protection Assurance, and Modifications.
a. The minimum maintenance required to support HIRF certification should be identified in instructions for continued airworthiness as specified in 14 CFR 23.1529, 25.1529, 25.1729,
26.11, 27.1529, and 29.1529, as appropriate. Dedicated devices or specific features may be required to provide HIRF protection for an equipment or system installation. Appropriate maintenance procedures should be defined for these devices and features to ensure in-service protection integrity. A HIRF protection assurance program may be necessary to verify that the maintenance procedures are adequate. The User's Guide (SAE ARP 5583A/EUROCAE ED 107A) provides further information on these topics.
b. The maintenance procedures should consider the effects of corrosion, fretting, flexing cycles, or other causes that could degrade these HIRF protection devices. Whenever applicable, specific replacement times of these devices and features should be identified.
c. Aircraft or system modifications should be assessed for the impact any changes will have on the HIRF protection. This assessment should be based on analysis and/or measurement.
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Appendix 1 Appendix 1. Generic Transfer Functions and Attenuation
1. Generic Transfer Functions.
a. Suitable transfer functions for calculating the bulk current injection test levels for level A display systems (see paragraph 9k) are given in figures A1-1 through A1-5. These are derived generic transfer functions acquired from test results obtained from a significant number of aircraft. The test results were processed to establish a 95 percent population probability.
b. The transfer functions are normalized to a 1 V/m HIRF environment and may be multiplied linearly by the external HIRF environment to establish the bulk current injection test level requirements in the frequency range from 10 kHz up to 400 MHz. For example, if the HIRF environment is 100 V/m at 3 MHz, then using figure A1-1, multiple 0.7 mA/V/m by 100 V/m to establish a test level of 70 milliamperes (mA).
c. Consult the User's Guide (SAE ARP 5583A/EUROCAE ED-107A) for details on the use of generic transfer functions.
FIGURE A1-1 — Generic Transfer Function - Airplane 3, 0.7 15, 0.71
0.1 400, 0.3
0.01 0.1 1 10 100 1000 Frequency - MHz mA/V/m
0.01
0.001 Note: Generic transfer function normalized to 1 V/m for an airplane with a fuselage length of < 25m.
A1-1
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Appendix 1 FIGURE A1-2 — Generic Transfer Function - Airplane
0.01
0.1 1 mA/V/m
1.5, 0.7 400, 0.15 6, 0.7
0.01 0.1 1 10 100 1000 Frequency - MHz Note: Generic transfer function normalized to 1 V/m for an airplane with a
fuselage length of > 25m and < 50m.
A1-2
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Appendix 1
FIGURE A1-3 — Generic Transfer Function - Airplane mA/V/m
0.5, 2.5 400, 0.15 5, 2.5 50, 0.3 03 2
0.01 0.1 1 10 100 1000
0.1
0.01 Frequency - MHz Note: Generic transfer function normalized to 1 V/m for an airplane with a fuselage length of > 50m.
A1-3
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Appendix 1 FIGURE A1-4 — Generic Transfer Function - Rotorcraft mA/V/m 10 100 1000
10 5, 1.5 30, 1.5 1 400, 0.5
0.1
0.01
0.01 0.1 1 Frequency - MHz Note: Generic transfer function normalized to 1 V/m for a rotorcraft.
A1-4
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Appendix 1 FIGURE A1-5 — Generic Transfer Function – All Aircraft
0.01
0.1 1 10 mA/V/m 5, 2.5 400, 0.5 30, 1.5
0.5, 2.5 10, 1.5
0.01 0.1 1 10 100 1000 Frequency - MHz Note: Generic transfer function normalized to 1 V/m for all aircraft.
2. Generic Attenuation.
a. Figure A1-6 shows the generic attenuation for frequencies from 100 MHz to 18 GHz that can be used for determining the internal HIRF environment where equipment and associated wiring for level A display systems (see paragraph 9k) are installed. This internal HIRF environment provides the test level for the integrated system radiated susceptibility laboratory test. The external HIRF environment should be divided by the appropriate attenuation, in linear units, to determine the internal HIRF environment. For example, 12 dB or a 4:1 attenuation means the test level is the applicable external HIRF environment electric field strength reduced by a factor of 4.
b. Guidance on the use of the generic attenuation is given below:
(1) No Attenuation. No attenuation credit can be used when the level A display equipment and associated wiring are located in aircraft areas with no HIRF shielding, such as areas with unprotected nonconductive composite structures, areas where there is no guarantee of structural bonding, or other open areas where no shielding is provided. You may choose to use no attenuation for equipment that may be installed in a broad range of aircraft areas.
A1-5
Appendix 1
(2) 6 dB Attenuation. This attenuation is appropriate when the level A display equipment and associated wiring are located in aircraft areas with minimal HIRF shielding, such as a cockpit in a nonconductive composite fuselage with minimal additional shielding, or areas on the wing leading or trailing edges, or in wheel wells.
(3) 12 dB Attenuation. This attenuation is appropriate when the level A display equipment and associated wiring are located entirely within aircraft areas with some HIRF shielding, in aircraft with a metal fuselage or a composite fuselage with shielding effectiveness equivalent to metal. Examples of such areas are avionics bays not enclosed by bulkheads, cockpits, and areas near windows, access panels, and doors without EMI gaskets. Currentcarrying conductors in this area, such as hydraulic tubing, control cables, wire bundles, and metal wire trays, are not all electrically bonded to bulkheads they pass through.
(4) 20 dB Attenuation. This attenuation is appropriate when the level A display equipment and associated wiring are located entirely within aircraft areas with moderate HIRF shielding, in aircraft with a metal fuselage or a composite fuselage with shielding effectiveness equivalent to metal. In addition, wire bundles passing through bulkheads in these areas have shields electrically bonded to the bulkheads. Wire bundles are installed close to metal structure and take advantage of other inherent shielding characteristics provided by metal structure.
Current-carrying conductors, such as hydraulic tubing, cables, and metal wire trays are electrically bonded to all bulkheads they pass through.
(5) 32 dB Attenuation. This attenuation is appropriate when the level A display equipment and all associated wiring to and from equipment are located entirely within areas with very effective HIRF shielding to form an electromagnetic enclosure.
(6) Generic Attenuation for Rotorcraft. Display units installed in rotorcraft typically have minimal attenuation unless specific shielding is provided in the bulkhead, glare shield, panel, and doors.
c. Different attenuation values may be appropriate for different frequency ranges. For example, 0 dB attenuation may be used for the frequency range of 100 MHz to 400 MHz, 6 dB attenuation for the frequency range of 400 MHz to 1 GHz, and 12 dB attenuation for the frequency range of 1 GHz to 18 GHz. If you intend to use different attenuation values for various frequency ranges, then you should also provide the supporting rationale.
d. Consult the User's Guide for details on the use of generic attenuation.
3. Measured Transfer Functions or Attenuation. You can produce your own generic transfer functions and attenuation for your level A display systems (see paragraph 9k) based on actual measurements on your aircraft models. These transfer functions and attenuation can then be used in your HIRF compliance submission in place of the generic transfer functions and attenuation specified in this appendix. The FAA encourages this approach because it provides a more accurate reflection of the true internal HIRF environment for your aircraft models. However, if you intend to produce your own generic transfer functions and attenuation, then this approach should also be addressed in the HIRF compliance plan (see paragraph 8a) that is submitted to the FAA for approval.
A1-6