10.10.4.2 Calculate the ratio between the induced wire bundle current and the illuminating antenna field strength and normalize this ratio to 1 V/m. This provides the transfer function in terms of induced current per unit external field strength. Then the current induced by the applicable external HIRF environment can be calculated by multiplying the transfer function by the external HIRF field strength. The calculated HIRF currents for all transmit antenna positions for each aircraft wire bundle being assessed should be overlaid to produce worst-case induced current for each wire bundle.
These worst-case induced currents can be compared with the current used during the integrated system test in step 5.
10.10.5 Step 10d — Low-Level Swept-Field Test.
Low-level swept-field testing is typically used from 100 MHz to 18 GHz. The test procedures for the low-level swept-field test are similar to those used for the low -level swept-current test; however, in the low-level swept- field test, the internal RF fields in the vicinity of the equipment are measured instead of the wire 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.
10.11 Step 11 — Generic Transfer Functions and Attenuation - Display Systems Only.
10.11.1 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.
10.11.2 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 A to this AC. Appendix A to this AC also contains guidelines f or 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 li nearly by HIRF environment I, II, or III, as appropriate, to determine the integrated system test levels.
10.11.3 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.
10.11.4 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.
10.12 Step 12 — Aircraft Similarity Assessment .
10.12.1 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. To use this assessment, 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.
10.12.2 The similarity assessment for the new aircraft should consider the aircr aft differences that could impact the internal HIRF environment affecting the level A systems and associated wiring. The comparison should consider equipment and wiring installation locations, airframe materials, construction, and apertures that could affect attenuation for the external HIRF environment.
10.12.3 If the assessment finds negligible differences, with respect to HIRF aspects, 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.
10.13 Step 13 — Assess Immunity.
10.13.1 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. You should include your comparison method 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.
10.13.2 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 step 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 step 10d, 11 , or 12, then corrective measures are needed (see step 14).
10.13.3 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 laboratory integrated system test 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 syste m tests and the aircraft tests.
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 HIRF test plan (see paragraph 9.2.1). If the HIRF susceptibilities are not acceptable, then corrective measures may be needed (see step 14).
10.13.4 HIRF susceptibilities that were not anticipated or defined in the test plan pass/fail criteria may be observed during aircraft high-level tests or laboratory integrated system tests.
10.13.5 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).
10.13.6 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.
10.13.7 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 cir cuits 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.
10.13.8 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 ma y be needed (see step 14).
10.13.9 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 an d the RF response at 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 prepar ed before implementation of HIRF requirements, the RF receiver pass/fail criteria should be coordinated with the FAA.
10.13.10 You should provide the similarity assessment and supporting rationale to the FAA for approval.
10.14 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. RTCA/DO -160E, section 20 (as specified in AC 21-16) equipment tests, integrated system tests, and aircraft tests, in whole or in part, may need to be repeated to show HIRF compliance.
10.15 Step 15 — HIRF Protection Compliance .
Submit the test results and compliance report to the cognizant AIR certification branch for approval as part of the overall aircraft type certification or supplemental type certification process.
10.16 Compliance for HCL A Systems on Level 1, 2, 3, and 4 Airplanes Certificated Under Part 23.
Airplane certification levels 1, 2, 3, and 4 are defined in 14 CFR 23.2005 (established by Amendment 23-64). Airplane certification classes I, II, III, and IV are de fined in AC
23.1309-1. The following test levels may be used for HCL A integrated systems on normal category level 1, 2, 3, and 4 airplanes (certification class I, II, III, and IV) .
Equipment testing is acceptable if you show that equipment in the HCL A systems can be operated and tested independently and effectively monitored to verify no adverse effects to the system and function. Otherwise, you should perform integrated system HIRF tests as discussed in step 5 (refer to paragraph 10.5) . The system should comprise all equipment and wiring needed to perform the function. One or more of these systems with redundant channels may perform the same function. When there are multiple systems with redundant channels performing the same function, it is only necessary for one of the multiple systems ’ redundant channels to meet the requirements in § 23.1308(a) or § 23.2520(a). Level 1 airplanes certificated under part 23 meet the same requirements for HIRF as shown in paragraph 10.16.1 below. Level 2 and 3 airplanes certificated under part 23 meet the requirements for HIRF as shown in paragraph 36
10.16.2 below. Level 4 airplanes certified under part 23 meet the requirements for HIRF as shown in paragraph 10.5 (Step 5 - Integrated System Test).
Note: The applicant may also use a means of compliance, which may include ASTM consensus standards, accepted by the Administra tor under § 23.2010. (See paragraph 4.4).
10.16.1 For level 1 airplane HCL A systems, you may perform integrated system conducted and radiated susceptibility tests using RTCA/DO -160E (refer to AC 21-16), section 20, category R.
10.16.2 For level 2 and 3 airplane HCL A systems, you may perform integrated system conducted susceptibility tests per the Generic Conductive Curve — < 25 meters extrapolated to Environment I and using the applicable levels defined in RTCA/DO- 160G (refer to AC 21-16), section 20, category M.
Note: RTCA/DO-160E, section 20, category A is also acceptable.
10.16.3 For level 2 and 3 airplane HCL A systems, you may perform integrated system radiated susceptibility tests per the Aircraft Generic Attenuation Curves applied to Environment I and using the applicable levels described below (RTCA/DO-160G, refer to AC 21-16).
• 0 dB: 100 MHz – 400 MHz (using RTCA/DO-160G, section 20, category G)
• -6 dB: 400 MHz – 1 GHz (using RTCA/DO-160G, section 20, category F)
• -12 dB: 1 GHz – 18 GHz (using RTCA/DO-160G, section 20, category D)
10.16.4 You may design and install the engine control systems using the guidance provided in AC 33.28-3. You may develop and install appropriate electrical bonding features for the engine control systems using the guidance in AC 33.28 -3.
10.16.5 Electrical bonding specifications and verifications should be developed and implemented on the production drawings and ins tructions for continued airworthiness.
11 STEPS TO LEVEL B AND C SYSTEM HIRF COMPLIANCE Steps 1 through 8 listed below for HCL B and C systems HIRF compliance may be used to show compliance with § § 23.1308(b) and (c), 23.2520(b), 25.1317(b) and (c),
27.1317(b) and (c), or § 29.1317(b) and (c).
11.1 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 7.3. 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 10.
11.2 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 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.
11.3 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.
11.4 Step 4 — Equipment Test.
Level B and C systems do not require the sa me degree of HIRF compliance testing as level A systems, and therefore do not require aircraft -level testing. You should follow RTCA/DO-160E, section 20 (as specified in AC 21-16) laboratory test procedures, using the 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-160E, section 20, Category RR, satisfies the requirements of equipment HIRF test level 1. For equipment HIRF test level 2, you may use the approach in paragraph 10.11 of this AC to 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-160E, 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 for more details on modulation.
11.4.1 Define appropriate pass/fail criteria for the system, based on the system safety assessment and the appropr iate HIRF regulation (see paragraph 7.3). 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.
11.5 Step 5 — Similarity Assessment.
11.5.1 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.
11.5.2 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 affect HIRF immunity. An assessment of a new system should consider the differences in the equipment circuit interfaces, wiring, grounding, bonding, connecto rs, and wire-shielding practices.
11.5.3 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, provided there are no unresolved in-service HIRF problems related to the previously certified system. If there is uncertainty about the effects of the differences, you should conduct additional tests and analysis 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.
11.6 Step 6 — Assess Immunity.
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 and supporting rationale for any pass/fail criteria modifications to the FAA for approval. If the HIRF susceptibilities are not acceptable, then corrective measures may be needed (see step 7).
11.6.1 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).
11.6.2 Certain RF receivers with antennas conne cted 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 re ceiver pass/fail criteria should be coordinated with the FAA. Future modifications of the minimum performance standards should reflect HIRF performance requirements.
11.7 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 needed, then additional tests may be necessary to verify the effectiveness of the changes. The RTCA/DO-160E, section 20 (refer to AC 21-16) equipment tests, in whole or in part, may need to be repeated to show HIRF compliance.
11.8 Step 8 — HIRF Protection Compliance .
Submit the test results and compliance report to the cognizant AIR certification branch for approval as part of the overall aircraft type certification or supplemental type certification process.
12 MAINTENANCE, PROTECTION ASSURANCE, AND MODIFICATIONS
12.1 The minimum maintenance necessary to support HIRF certification must be identified in instructions for continued airworthiness as required by §§ 23.1529, 25.1529, 25.1729,
26.11, 27.1529, or § 29.1529.
12.2 Dedicated devices or specific features may be needed to provide HIRF protection for an equipment or system installation. You should define a ppropriate maintenance procedures 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. This assurance program may propose a su rveillance program based on a sampling of the fleet for monitoring the effectiveness of the protection features and/or maintenance procedures. The user’s guide provides further information on these topics.
12.3 The maintenance procedures should address the effects of corrosion, fretting, flexing cycles, or other causes that could degrade these HIRF protection devices. Whenever applicable, you should identify specific replacement times of these devices and features.
12.4 Aircraft or system modifications shou ld be assessed for the impact any changes will have on the HIRF protection. You should base this assessment on analysis and/or measurement.
Appendix A Appendix A. Generic Transfer Functions and Attenuatio n A.1 GENERIC TRANSFER FUNCTIONS.
A.1.1 Suitable transfer functions for calculating t he bulk current injection test levels for level A display systems (see paragraph 10.11) are given in figures A -1 through A-5. These are derived generic transfer functions acquired from test results obtained from a significant number of aircraft. The test r esults were processed to establish a 95 percent population probability.
A.1.2 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 A -1, multiply 0.7 mA/V/m by 100 V/m to establish a test level of 70 milliamperes (mA).
A.1.3 Consult the user’s guide for details on the use of generic transfer functions.
Figure A-1. Generic Transfer Function —Airplane
Note: Generic transfer function normalized to 1 V/m for an airplane with a fuselage length of < 25m.
A-1
Appendix A Figure A-2. Generic Transfer Function-Airplane
1.5, 0.7 - 6, 0.7 mA/V/m 0.1 400. 0.15 -
0.01
0.01 0.1 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.
Figure A-3 — Generic Transfer Function - Airplane 10
0.5, 2.5 5, 2.5 1 mA/V/m 400, 0.15 50, 0.3
0.1
0.01.
0.01 O.1 10 100 1000 Frequency - MHz Note: Generic transfer function normalized to 1 V/m for an airplane with a fuselage length of > 50m.
A-2
Appendix A Figure A-4 Generic Transfer Function - Rotorcraft 10 5, 1.5 30, 1.5 1
• 400, 0.5
0.1
0.01 1
0. 01 0.1 10 100 1000 Frequency - MHz Note: Generic transfer function normalized to 1 V/m for a rotorcraft.
Figure A-5 Generic Transfer Function - All Aircraft 10
0.5, 2.5 5, 2.5 30, 1.5 10, 1.5 400, 0.5
0.1 -
0.01.
0.01 0.1 10 100 1000 Frequency - MHz Note: Generic transfer function normalized to 1 V/m for all aircraft.
A-3
Appendix A A.2 GENERIC ATTENUATION.
A.2.1 Figure A-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 10.11) are installed. This internal HIRF environment provides the radiated susceptibility test level for the laboratory integrated system 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.
A.2.2 Guidance on the use of the generic attenuation is given below:
A.2.2.1 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 r ange of aircraft areas.
A.2.2.2 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 minim al additional shielding, or areas on the wing leading or trailing edges, or in wheel wells.
A.2.2.3 12 dB Attenuation.
This attenuation is appropriate when the level A display equipment and associated wiring are located entirely within aircraft areas with some HIR F 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.
A.2.2.4 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 add ition, 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.
A-4