1: EMA HIRF Background (IEL)
2: HIRF Certification Requirements
2.1 Relevant Requirements Documents
2.2 Application of Certification Documents to CEM HIRF Analysis
Chapter 2: HIRF Certification Requirements
2.1 Relevant Requirements Documents
There are four relevant documents for HIRF testing of commercial aircraft. The top level document – and the only document that defines a legal requirement for manufacturers — is FAA CFR parts 23.1308, 25.1317, 27.1317, and 29.1317 . The other FAA document that applies to HIRF testing is Advisory Circular 20-158 . This document defines one acceptable path to demonstrate compliance with the CFR. However, it is not a regulation and does not constitute a legal requirement for manufacturers. SAE ARP5583 Rev A  expands on AC 20-158 and provides actual test procedures that can be used to demonstrate compliance. The last document is RTCA DO-160F . This document deals only with testing at the box level and does not have any information on aircraft level testing. It is, however, referenced by ARP5583.
In summary, there is no regulation that specifically states exactly how to perform tests for HIRF certification. There are guidelines laid out in AC 20-158, ARP5583, and DO-160F, but they are not binding. A manufacturer is free to define its own test procedures as long as they are accepted by the FAA.
2.1.1 FAA 14 CFR Parts 23, 25, 27, 29 (2007)
FAA 14 CFR has four sections that are relevant to HIRF certification of aircraft.
- Part 23 applies to “normal, utility, acrobatic, and commuter category airplanes.”
- Part 25 applies to “transport category airplanes.”
- Part 27 applies to “normal category rotorcraft.”
- Part 29 applies to “transport category rotorcraft.”
Each of the sections provides tables for the HIRF environment:
- HIRF environment I
- HIRF environment II
- HIRF environment III
HIRF environment III only applies to rotorcraft. The values in these tables are root mean square (RMS) values not amplitudes, and any calculated data should be reported as RMS values. Each table also has an entry for peak field and average field, but there is no discussion of what “peak” and “average” mean or how they are used in testing. This is clarified in SAE ARP 5583. Each section also provides 3 test levels for systems of different criticality. In appendix J to each part, there are some directions for performing “equipment level” tests. There are no requirements for aircraft level tests.
2.1.2 Advisory Circular 20-158 (2007)
AC 20-158 provides a route, but not the only route, to HIRF certification. On page 1 in section 1.b, AC 20-158 states, “This AC is not mandatory and does not constitute a regulation. It describes an acceptable means, but not the only means, for you to show compliance with the requirements for protection of the operation of electrical and electronic systems when the aircraft is exposed to an external HIRF environment.” It defines three classes of systems: A, B, and C – catastrophic, hazardous, and major, respectively. Each classification is tied to the HIRF environments and test levels defined in the CFR. Flow charts for each system classification show how to achieve HIRF compliance. Figure 1 on page 17 is for category A systems, Figure 2 on page 18 is for category B systems, and Figure 3 on page 19 is for category C systems. In these flow charts (and the text to explain them) is the first mention of specific aircraft level test procedures: Aircraft high level tests and Aircraft low level coupling tests. There are some specifics for the tests given on pages 23-25, but there is not enough detail to actually perform a test. AC 20-158 also provides some general transfer functions for conducted susceptibility derived from measurements of multiple aircraft. These transfer functions are based on a 95% confidence level and are presented in Appendix 1 of the ARP.
2.1.3 ARP5583 Rev A (2010)
The ARP is the first document that gives specific test procedures for aircraft level HIRF tests for measuring conducted susceptibilities and field magnitudes interior to the aircraft. The descriptions of the aircraft level tests for category A (see AC 20-158) systems are in section 6 of the ARP. The aircraft level tests fall into two categories: high level tests and low level tests. The ARP also describes system level tests similar to those in DO-160F.
220.127.116.11 High level tests
The high level tests are described in section 6.3 of the ARP. There are two types of tests: direct drive and radiated.
18.104.22.168.1 Direct drive
The direct drive test consists of RF current applied directly to the skin of the aircraft while measuring the conducted current on cables and cable bundles. The relationship of the direct drive test to actual irradiation is unclear. The field values and modulation schemes are given in Table 9 on page 57 of the ARP. This table contains the first mention of how to apply the peak or average HIRF levels given in FAA 14 CFR. Note that the pulse duration and modulation schemes are not consistent with FAA 14 CFR.
22.214.171.124.2 Radiated tests
The direct drive test can be used up to the first aircraft resonance. The radiated tests are performed using antennae or microwave horns from the first aircraft resonance to 18 GHz with the possibility of testing up to 40 GHz. The radiated tests are intended to measure two quantities: conducted susceptibility on cables and field values interior to the aircraft. Conducted susceptibility is measured over the range of 10 kHz to 400 MHz, and field values are measure from 100 kHz to 18 GHz with an option to go to 40 GHz if necessary. Testing above 18 GHz is only necessary if there are systems that operate a frequencies above 18 GHz, or if susceptibilities are found in the 12-18 GHz range. The tests are to be conducted at discrete frequencies at 10 points or more per decade between 10 kHz and 100 kHz and 100 points or more per decade between 100 kHz and 40 GHz. Alternately, the tests can be conducted with a continuously swept frequency source. Section 6.3 of the ARP gives antenna locations and other information necessary for testing using radiated fields.
126.96.36.199 Low level tests
The low level tests are described in section 6.4 of the ARP. The low level tests are similar to the high level tests with the exception that lower field values are used. The measured data (conducted currents and field values) are then normalized to the incident field and multiplied by the appropriate HIRF Certification environment from FAA 14 CFR. Unlike the high level test, the low level tests do not have any information on the number of measurement frequencies or the modulation schemes. However, later sections of the ARP that deal with system and subsystem testing have the same modulations schemes as for the high level testing. We could conclude from this that the modulations schemes for high level testing apply across the board and include the low level testing.
188.8.131.52.1 Low level direct drive (LLDD)
The LLDD test (ARP 6.4.1 and 6.4.2) is essentially the same as the high level direct drive test using direct current injection onto the aircraft. However, the data must be normalized to a 1V/m incident radiated field and then multiplied by the appropriate HIRF environment. Pages 70 and 71 in section 6.4.1 of the ARP discuss the normalization of the LLDD measurements. As with the high level tests, the direct injection stops at the first aircraft resonance.
184.108.40.206.2 Low level swept current (LLSC)
The LLSC test (ARP 6.4.3) starts at the first aircraft resonance and goes up to 400 MHz. Again, the measured values are normalized to 1V/m to generate transfer functions that can be multiplied by the HIRF environment to get the actual currents. The LLSC test requires measuring the incident field in the absence of the aircraft to find the field value for normalizing the measured currents. Section 6.4.3 of the ARP has specific information on field uniformity, antenna types, illumination directions, and instrumentation. Section 220.127.116.11 of the ARP recommends using 3D modeling to obtain a correction factor for the effects of the height of the aircraft above the ground so that the data can be corrected for an in-flight scenario.
18.104.22.168.3 Low level swept field (LLSF)
The LLSF tests (ARP 6.4.4) are intended to measure field strengths internal to the aircraft between 400 MHz and 18 GHz with an option to go to 40 GHz. Testing between 18 GHz and 40 GHz is necessary only if the aircraft has electronics that operate in this range or the aircraft shows susceptibility between 12 GHz and 18 GHz. The ARP also suggests using a mechanical mode stirrer or broad band illumination (p.81) to homogenize the field throughout the volume of interest. As with the other low level tests, the data are normalized to 1V/m field at each frequency and then multiplied by the HIRF environment.
2.1.4 DO-160F (2007)
DO-160 does not mention aircraft level tests, but deals purely with conducted and radiated susceptibility of systems and sub-systems. It is however referenced by ARP5583. It also has categories for the maximum conducted susceptibility for different types of systems. These levels appear to be loosely based on the generic transfer functions in AC 20-158 Appendix 1, p. 20-38.
2.1.5 Users Guide for AC/AMJ 20-1317 (1999)
This document is dated and is superseded by FAA 14 CFR, AC20-158, and ARP 5583.
2.2 Application of Certification Documents to CEM HIRF Analysis
The certification documents briefly described in section 1 of this chapter provide the basis for defining the nature and extent of CEM HIRF analysis that might be logically performed in support of aircraft certification. The HIRF analysis parallels the requirements for certification testing with a focus on the LLSC and LLSF modeling. If comparisons are desired between predictions and measurements, it is necessary to obtain detailed information concerning the measurement instrumentation, including antennas and the way in which the measurements were made (e.g. number of points per frequency decade, bandwidth averaging, antenna aperture size, etc.).
For general HIRF computations, it is important to include in the model enough complexity, lossy material and structural accuracy to ensure adequate model fidelity. In particular, it is most important to include:
- Realistically complex cables harnesses with representative numbers of conductors and branching throughout the aircraft
- Lossy material (distributed in aircraft cavities or loaded into the walls) which adjust the cavity Q and/or decay time to the typical measured value range
- Models of the apertures of antennas which are representative of typical receive antennas used in aircraft testing
- Post processing of prediction data to incorporate appropriate features of recommended measurement in ARP5583 which include 5% bandwidth averaging and specific numbers of points per frequency decade.