f. Verify HIRF Protection Effectiveness. You should show the RF current on system and equipment wire bundles and the RF fields on the system, created by the HIRF environment, are lower than the equipment or system HIRF qualification test levels.
g. Compliance Based on Previously Issued HIRF Special Conditions. The installation of a level A electrical or electronic system, until December 1, 2012, was allowed under section
(d) of the HIRF regulation, when it could be shown the system intended for installation previously complied with HIRF special conditions prescribed under 14 CFR 21.16 and issued before December 1, 2007. Since December 1, 2012, section (d) of the HIRF regulations and paragraph 6.g of this AC were no longer applicable.
7. Margins. Margins are not required if your HIRF compliance is based on tests of the specific aircraft model and system undergoing certification. Margins are also not required if your HIRF compliance is based on analysis or similarity if the process validation is robust and the data well substantiated. Where data have limited substantiation, a margin may be required depending on the available justifications. When a margin is required, include a justification for the selected margin in the HIRF Compliance Plan, as discussed in paragraph 8.
8. HIRF Compliance.
a. HIRF Compliance Plan. An overall HIRF compliance plan should be established to clearly identify and define HIRF certification requirements, HIRF protection development, and the design, test, and analysis activities intended to be part of the compliance effort. This plan should provide definitions of the aircraft systems, installations, and protective features against which HIRF compliance will be assessed. The HIRF compliance plan should be discussed with, and submitted to, the FAA for approval before initiating HIRF compliance activities. If the aircraft, system, or installation design changes after approval, a revised HIRF compliance plan should be submitted to the FAA for approval. The HIRF compliance plan should include the following:
(1) A HIRF compliance plan summary;
(2) Identification of the aircraft systems, with classification based on the safety assessment as it relates to HIRF (see paragraph 6.b(2));
(3) The HIRF environment for the aircraft and installed systems; and
(4) The verification methods, such as test, analysis, or similarity.
b. HIRF Verification Test, Analysis, or Similarity Plan. Test, analysis and similarity are all acceptable methods. The applicant must choose the method most appropriate for their project. See paragraphs 9 and 10 of this AC, and SAE ARP5583A for additional guidance for selecting the appropriate method. Specific HIRF test, analysis, or similarity plans should be prepared to describe specific verification activities. One or more verification plans may be necessary. For example, there may be several systems or equipment laboratory test plans, an aircraft test plan, or a similarity plan for selected systems on an aircraft.
(1) Test Plan.
(a) A HIRF compliance test plan should include the equipment, system, and aircraft test objectives for the acquisition of data to support HIRF compliance verification. The plan should provide an overview of the factors being addressed for each system test requirement.
The test plan should include:
1 The purpose of the test;
2 A description of the aircraft and/or system being tested;
3 System configuration drawings;
4 The proposed test setup and methods;
5 Intended test levels, modulations, and frequency bands;
6 Pass/fail criteria; and 7 The test schedule and test location.
(b) The test plan should cover level A, B, and C systems and equipment, as appropriate. Level A systems may require both integrated systems laboratory tests and aircraft tests. Level B and level C systems and equipment require only equipment laboratory testing.
(c) The test plan should describe the appropriate aspects of the systems to be tested and their installation. Additionally, the test plan should reflect the results of any analysis performed in the overall process of the HIRF compliance evaluation.
(2) Analysis Plan. A HIRF compliance analysis plan should include the objectives, both at the system and equipment level, for generating data to support HIRF compliance verification. Comprehensive modeling and analysis for RF field coupling to aircraft systems and structures is an emerging technology; therefore, the analysis plan should be coordinated with the FAA to determine an acceptable scope for the analysis. The analysis plan should include:
(a) The purpose and scope of the analysis;
(b) A description of the aircraft and/or system addressed by the analysis;
(c) System configuration descriptions;
(d) Proposed analysis methods;
(e) The approach for validating the analysis results; and
(f) Pass/fail criteria, including margins to account for analysis uncertainty.
(3) Similarity Plan. A similarity plan should describe the approach undertaken to use the certification data from previously certified systems, equipment, and aircraft in the proposed HIRF compliance program. The similarity plan should include:
(a) The purpose and scope of the similarity assessment;
(b) Specific systems addressed by the similarity assessment;
(c) Data used from the previously certified systems, equipment, and aircraft;
(d) Details on significant differences between the aircraft and system being certified and the similar aircraft and system from which the data will be used; and
(e) When data have limited substantiation, a description and justification for margins to account for similarity uncertainty. See paragraph 7 for additional information on margins.
c. Compliance Reports. One or more compliance reports may be necessary to document the results of your test, analysis, or similarity assessments. For new or significantly modified aircraft, HIRF compliance reports include many system and equipment test reports, aircraft test reports, and HIRF vulnerability analysis reports. For these types of HIRF certification programs, a compliance summary report may be useful to summarize the results of tests and analysis. For HIRF certification programs of relatively simple systems, a single compliance report is adequate.
(1) Test Reports. Comprehensive test reports should be produced at the conclusion of HIRF compliance testing. The test reports should include descriptions of the salient aspects of equipment or system performance during the test, details of any area of noncompliance with HIRF requirements, actions taken to correct the noncompliance, and any similarity declarations.
You should also provide supporting rationale for any deviations from system performance observed during testing.
(2) Analysis Reports. Analysis reports should describe the details of the analytical model, the methods used to perform the analysis, and the results of the analysis. The reports should identify any modeling uncertainty and justify the margins established in the analysis plan.
(3) Similarity Reports. Similarity reports should document the significant aircraft, system, equipment, and installation features common between the aircraft or system that is the subject of the similarity analysis and the aircraft or system that previously was certified for HIRF.
Identify all significant differences encountered, along with the assessment of the impact of these differences on HIRF compliance. These report s should also justify the margins established in the similarity plan.
d. Methods of Compliance Verification.
(1) Various methods are available to aid in demonstrating HIRF compliance. Methods acceptable to the FAA are described in paragraphs 9 and 10 of this AC. Figures 1 and 2 below outline the steps to HIRF compliance for systems requiring level A HIRF certification. Figure 3 below outlines the steps to HIRF compliance for systems requiring level B or C HIRF 15
r--------------------- r---------, L __________________ J L ______ _
certification. The steps in these figures are not necessarily accomplished sequentially. Wherever a decision point is indicated on these figures, you should complete the steps in that path as described in paragraphs 9 and 10 of this AC.
(2) Other HIRF compliance techniques may be used to demonstrate system performance in the HIRF environment; however those techniques should be approved by the FAA before using them.
Figure 1. Routes to HIRF Compliance – Level A Systems Corrective Measures
(14) System Safety Assessment
(1) Level A System Define Aircraft and System HIRF Protection
(2) Equipment Test
(4) Integrated System Test
(5) Select System Assessment
(3) Test System Similarity Assessment
(6) Similarity System-Level Verification Aircraft Similarity Assessment
(12) HIRF Protection Compliance
(15) Level A Display Only Acceptable Unacceptable Select Aircraft Assessment
(7) Aircraft Test Similarity Aircraft High-Level Tests
(9) Aircraft Low-Level Coupling Tests
(10) See Figure 2 Aircraft Test Decision
(8) Generic Transfer Functions / Attenuation
(11) Assess Immunity
(13) Aircraft-Level Verification
(n) = Step number as described in Paragraph 9 of this AC 16
Figure 2. Aircraft Low-Level Coupling Tests – Level A Systems Aircraft Skin Current Analysis (10a) Low-Level Swept-Current Test (10c) Low-Level Direct-Drive Test (10b) Low-Level Swept-Field Test (10d) From Figure 1 Step 10 Aircraft Low-Level Coupling Tests
(10) 10 kHz to the 1st Airframe Resonant Frequency 100 MHz to 18 GHz 500 kHz to 400 MHz Back To Figure 1 Step 13
(n) = Step number as described in Paragraph 9 of this AC 17
I Figure 3. Routes to HIRF Compliance – Level B and C Systems System Safety Assessment
(1) Equipment Test
(4) Similarity Assessment
(5) HIRF Protection Compliance
(8) Corrective Measures
(7) Define Aircraft and System HIRF Protection (2) Assess Immunity
(6) Select Compliance Method
(3) Level B or C System Similarity Unacceptable Acceptable Equipment-Level Verification
(n) = Step number as described in Paragraph 10 of this AC 18
9. Steps to Level A 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 catastrophic failure conditions (Level A systems), follow compliance steps 2 through 15 listed below, as appropriate. These compliance steps are also depicted in figures 1 and 2 of this AC, and are not necessarily accomplished sequentially. Systems classified with hazardous or major failure conditions (Level B and C systems) should follow the compliance steps outlined in paragraph 10.
b. 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 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.
Consider all aircraft configurations that may affect HIRF protection, such as open landing gear doors (see step 7).
c. 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.
d. Step 4 — Equipment Test.
(1) Radiated and conducted RF susceptibility laboratory tests of RTCA/DO-160E (or latest version), Section 20, may be used to build confidence in the equipment's HIRF immunity before conducting integrated system laboratory 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-160, Section 20 or to a level estimated for the aircraft and equipment installation using the applicable external HIRF environment.
(2) Equipment HIRF tests may be used to augment the integrated system HIRF tests where appropriate. For equipment whose HIRF immunity 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.
e. Step 5 — Integrated System Test.
(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 19
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 function.
(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), connector 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.
For this reason, the laboratory integrated system rig should have an FAA conformity inspection prior to conducting any FAA certification credit testing.
(3) 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 integrated 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 required for their failure condition classification.
(4) The test levels should be selected based on the expected aircraft internal HIRF environment determined through 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 (SAE ARP 5583A/EUROCAE ED-107A).
(5) 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 multi-channel architectures.
(6) 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 is simultaneously and uniformly illuminated during the test.
(7) 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 displaying hazardously misleading information, upset, or damage should be recorded and evaluated based on these previously defined pass/fail criteria.
(8) 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.
(9) The equipment tests in step 4, using the techniques in RTCA/DO-160E (or latest version), Section 20, normally are not sufficient to show HIRF compliance for step 5. However, these standard RTCA/DO-160, Section 20 tests may be sufficient if paragraphs 9e(2) and (3) of this step are met.
f. Step 6 — System Similarity Assessment.
(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 s imilar system. Each system considered under the similarity approach needs to be assessed independently even if it may use equipment and installation techniques from previous certification projects.
(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 adversely effect 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.
(3) If the assessment finds only minimal differences 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 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 system-level verification.
g. Step 7 — Aircraft Assessment Decision.
(1) Level A systems require an aircraft assessment. 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 aircraft tests, previous coupling/attenuation data from similar aircraft types (similarity). F or level A display systems only, use the generic transfer functions and attenuation in appendix 1 to this AC. Alternately, 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.
(2) Integrated display systems include the display equipment, control panels, and the sensors that provide information to the displays. In some systems, the sensors also provide information to level A systems that are not displays. For example, if the sensors also provide 21
information to Level A flight controls, you must use actual transfer functions and attenuation when demonstrating compliance for these sensors and the flight controls.
(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.
(4) If analysis is used to determine aircraft attenuation and transfer function characteristics, test data should be provided to support this analysis. Any analysis results should take into account the quality and accuracy of the analysis. Significant testing, including aircraft level testing, is required to support the analysis.
(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.
h. Step 8 — Aircraft Test Decision.
(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 in the HIRF regulations. 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.
(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 the high-level field-illumination test in step 9, which requires illumination of the entire aircraft with the external HIRF environment.
i. Step 9 — Aircraft High-Level Tests.
(1) The aircraft high-level field-illumination test requires generating RF fields external to an aircraft at a level equal to the applicable external HIRF environment.
(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 22
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.
(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.
(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.
(5) The test time can be minimized by using only the modulation 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 8b(1). The User's Guide provides guidance on modulation selection and suggested default modulations and dwell times.
(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 highlevel 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 possible to test all systems simultaneously.
j. Step 10 — Aircraft Low-Level Coupling Tests.
(1) General.
(a) 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.
(b) 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 swept-field test (see Step 10d) is used for frequencies 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 equipment case becomes a significant factor.
(2) 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 the skin current and individual equipment wire bundle 23
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 dimensional 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 analysis 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 sweptcurrent 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.
(3) Step 10c — Low-Level Swept-Current Test.
(a) 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 currents to the external field, the induced bulk current injection test levels can be related to an external HIRF environment.
(b) 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 for horizontal and vertical polarization. Measure the currents induced on the aircraft wire bundles.
(c) 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.
(4) 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 sweptfield test, the internal RF fields in the vicinity of the equipment are measured instead of the wire 24