U.S. Department 

of Transportation 

Federal Aviation 

Administration 

Advisory 

Circular

Subject: THE CERTIFICATION Date: 7/30/07 AC No: 20-158

OF AIRCRAFT ELECTRICAL AND Initiated by: AIR-100 

ELECTRONIC SYSTEMS FOR 

OPERATION IN THE HIGH-INTENSITY 

RADIATED FIELDS (HIRF) ENVIRONMENT 

1. What is the purpose of this Advisory Circular (AC)? 

a. This advisory circular (AC) will provide you with information and guidance on how to 

show compliance with Title 14, Code of Federal Regulations (14 CFR) §§ 23.1308, 25.1317, 

27.1317, and 29.1317, High-intensity Radiated Fields (HIRF) protection. 

b. 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 on an aircraft when the aircraft is 

exposed to an external HIRF environment. If you use the means described in this AC, you must 

follow it in all important respects. 

 

2. Who does this AC apply to? This AC applies to all applicants for a new type certificate 

(TC) or a change to an existing TC when the certification basis requires you to address the HIRF 

certification requirements of 14 CFR §§ 23.1308, 25.1317, 27.1317, and 29.1317. 

3. Related Material. 

a. 14 CFR. Sections 23.1308, 25.1317, 27.1317, and 29.1317, High-intensity Radiated 

Fields (HIRF) protection; §§ 23.1309, 25.1309, 27.1309, and 29.1309, Equipment, systems, and 

installations; and §§ 23.1529, 25.1529, 27.1529, and 29.1529, Instructions for Continued 

Airworthiness. You can get copies of the above 14 CFR sections from the Superintendent of 

Documents, Government Printing Office, Washington, D.C. 20402-9325, 

telephone 202-512-1800, fax 202-512-2250. You can also get copies from the Government 

Printing Office (GPO), electronic CFR Internet web site at http://www.gpoaccess.gov/cfr/.

b. European Aviation Safety Agency (EASA). Certification Specifications (CS)-23, 

Normal, utility, aerobatic & commuter aeroplanes, CS-25, Large aeroplanes, CS-27, Small 

rotorcraft, and CS-29, Large rotorcraft. You can get copies of these documents from the 
AC 20-158 7/30/07 

European Aviation Safety Agency, Postfach 10 12 53, D-50452 Köln, Germany; telephone +49 

221 8999 000; fax: +49 221 8999 099; web site: http://www.easa.eu.int . 

c. FAA ACs. AC 23.1309-1C, Equipment, Systems, and Installations in Part 23 

Airplanes; and AC 25.1309-1A, System Design and Analysis, or later revisions. You can order 

copies of these documents from the U.S. Department of Transportation, Subsequent Distribution 

Office, DOT Warehouse M30, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, 

MD 20785; telephone (301) 322-5377. You can also get copies on the Internet from the FAA’s 

Regulatory and Guidance Library (RGL) at the web site: http://rgl.faa.gov. On the RGL website, 

click on “Advisory Circulars.” 

d. RTCA, Inc. RTCA/DO-160, Environmental Conditions and Test Procedures for 

Airborne Equipment, Revision E or later. You can order copies of this document from RTCA, 

Inc., 1828 L Street NW, Suite 805, Washington, DC 20036; telephone: (202) 833-9339; web site: 

http://www.rtca.org. This document is technically equivalent to EUROCAE ED-14. Whenever 

there is a reference to EUROCAE ED-14 in this AC, you may use RTCA/DO-160. 

e. European Organization for Civil Aviation Equipment (EUROCAE). You can order 

copies of the following documents from EUROCAE, 102 rue Etienne Dolet, 92240 Malakoff, 

France; Telephone: +33 1 40 92 79 30; Fax: +33 1 46 55 62 65; web site: 

http://www.eurocae.org. 

(1) EUROCAE ED-107, Guide to Certification of Aircraft in a High Intensity 

Radiated Field (HIRF) Environment, March 2001. ED-107 and SAE ARP 5583, referenced in 

paragraph 3f(1) below, are technically equivalent and either document may serve as the 

"User’s Guide" referred to in this AC. 

(2) EUROCAE ED-14, Environmental Conditions and Test Procedures for Airborne 

Equipment, Revision E or later. This document is technically equivalent to RTCA/DO-160. 

Whenever there is a reference to RTCA/DO-160 in this AC, you may use EUROCAE ED-14. 

f. SAE International. You can order copies of the following documents from SAE World 

Headquarters, 400 Commonwealth Drive, Warrendale, Pennsylvania 15096-0001; telephone 

(724) 776-4970; web site: http://www.sae.org. 

(1) SAE Aerospace Recommended Practice (ARP) 5583, Guide to Certification of 

Aircraft in a High Intensity Radiated Field (HIRF) Environment, January 2003. SAE ARP 5583 

and ED-107, referenced in paragraph 3e(1) above, are technically equivalent and either 

document may serve as the "User’s Guide" referred to in this AC. 

(2) SAE ARP 4754, Certification Considerations for Highly-Integrated or Complex 

Aircraft Systems, November 1996. 

(3) SAE ARP 4761, Guidelines and Methods for Conducting the Safety Assessment 

Process on Civil Airborne Systems and Equipment, December 1996. 

Page 2 
7/30/07 AC 20-158 

4. Background. 

a. Aircraft Protection. Concern for the protection of aircraft electrical and electronic 

systems has increased substantially in recent years because of— 

(1) Greater dependence on electrical and electronic systems performing functions 

required for continued safe flight and landing of an aircraft; 

(2) Reduced electromagnetic shielding afforded by some composite materials used in 

aircraft designs; 

(3) Increased susceptibility of electrical and electronic systems to HIRF because of

increased data bus and processor operating speeds, higher density integrated circuits and cards, 

and greater sensitivities of electronic equipment; 

(4) Expanded frequency usage, especially above 1 gigahertz (GHz); 

(5) Increased severity of the HIRF environment because of an increase in the number 

and radiated power of radio frequency (RF) transmitters; and 

(6) Adverse effects experienced by some aircraft when exposed to HIRF. 

b. HIRF Environment. The electromagnetic HIRF environment exists because of the 

transmission of electromagnetic RF energy from radar, radio, television, and other ground-based, 

shipborne, or airborne RF transmitters. The User's Guide (SAE ARP 5583/EUROCAE ED-107) 

provides a detailed description of the derivation of these HIRF environments. 

5. Definitions. 

a. Adverse Effect: HIRF effect that results in system failure, malfunction, or misleading 

information to a degree that is unacceptable for the specific aircraft function or system addressed 

in the HIRF regulations. A determination of whether a system or function is adversely affected 

should consider the HIRF effect in relation to the overall aircraft and its operation. 

b. Attenuation: term used to denote a decrease in electromagnetic field strength in 

transmission from one point to another. Attenuation may be expressed as a scalar ratio of the 

input magnitude to the output magnitude or in decibels (dB). 

c. Bulk Current Injection (BCI): method of electromagnetic interference (EMI) testing 

that involves injecting current into wire bundles through a current injection probe. 

d. Continued Safe Flight and Landing: capability for continued controlled flight and 

landing at a suitable location, possibly using emergency procedures, but without requiring 

exceptional pilot skill or strength. Some aircraft damage may be associated with a failure 

condition during flight or upon landing. 

 Page 3 
AC 20-158 7/30/07 

e. Continuous Wave (CW): RF signal consisting of only the fundamental frequency with 

no modulation in amplitude, frequency, or phase. 

f. Coupling: process whereby electromagnetic energy is induced in a system by radiation 

produced by an RF source. 

g. Current Injection Probe. inductive device designed to inject RF signals directly into 

wire bundles when clamped around them. 

h. Direct Drive Test: EMI test that involves electrically connecting a signal source 

directly to the unit being tested. 

i. Equipment: component of an electrical or electronic system with interconnecting 

electrical conductors. 

j. Equipment Electrical Interface: location on a piece of equipment where an electrical 

connection is made to the other equipment in a system of which it is a part. The electrical 

interface may consist of individual wires or wire bundles that connect the equipment. 

k. External High-intensity Radiated Fields Environment: electromagnetic RF fields at 

the exterior of an aircraft. 

l. Field Strength: magnitude of the electromagnetic energy propagating in free space 

expressed in volts per meter (V/m). 

m. High-intensity Radiated Fields (HIRF) Environment: electromagnetic environment 

that exists from the transmission of high power RF energy into free space. 

n. HIRF Vulnerability: susceptibility characteristics of a system that cause it to suffer 

adverse effects when performing its intended function as a result of having been subjected to a 

HIRF environment. 

o. Immunity: capacity of a system or piece of equipment to continue to perform its 

intended function, in an acceptable manner, in the presence of RF fields. 

p. Interface Circuit: electrical or electronic device connecting the electrical inputs and 

outputs of equipment to other equipment or devices in an aircraft. 

q. Internal HIRF Environment: the RF environment inside an airframe, equipment 

enclosure, or cavity. The internal RF environment is described in terms of the internal RF field 

strength or wire bundle current. 

r. Margin: difference between equipment susceptibility or qualification levels and the 

aircraft internal HIRF environment. Margin requirements may be specified to account for 

uncertainties in design, analysis, or test. 

s. Modulation: process whereby certain characteristics of a wave, often called the carrier 

wave, are varied in accordance with an applied function. 

Page 4 
7/30/07 AC 20-158 

t. Radio Frequency (RF): frequency useful for radio transmission. The present practical 

limits of RF transmissions are roughly 10 kilohertz (kHz) to 100 gigahertz (GHz). Within this 

frequency range, electromagnetic energy may be detected and amplified as an electric current at 

the wave frequency. 

u. Reflection Plane: conducting plate that reflects RF signals. 

v. Similarity: process of using existing HIRF compliance documentation and data from a 

system or aircraft to demonstrate HIRF compliance for a nearly identical system or aircraft of 

equivalent design, construction, and installation. 

w. Susceptibility: property of a piece of equipment that describes its inability to function 

acceptably when subjected to unwanted electromagnetic energy. 

x. Susceptibility Level: level where the effects of interference from electromagnetic 

energy become apparent. 

y. System: piece of equipment connected via electrical conductors to another piece of 

equipment, both of which are required to make a system function. A system may contain pieces 

of equipment, components, parts, and wire bundles. 

z. Transfer Function: ratio of the electrical output of a system to the electrical input of a 

system, expressed in the frequency domain. For HIRF, a typical transfer function is the ratio of 

the current on a wire bundle to the external HIRF field strength, as a function of frequency. 

aa. Upset: impairment of system operation, either permanent or momentary. For 

example, a change of digital or analog state that may or may not require a manual reset. 

bb. User’s Guide: refers to SAE document ARP 5583 or EUROCAE document ED-107.

6. Approaches to Compliance. 

a. General. The following activities should be elements of a proper HIRF certification 

program. The iterative application of these activities is left to you. Adherence to the sequence 

shown is not necessary. You should— 

(1) Identify the systems to be assessed; 

(2) Establish the applicable aircraft external HIRF environment; 

(3) Establish the test environment for installed systems; 

(4) Apply the appropriate method of HIRF compliance verification; and 

(5) Verify HIRF protection effectiveness. 

 Page 5 
AC 20-158 7/30/07 

b. Identify the Systems to be Assessed. 

(1) General. The aircraft systems that require HIRF assessment must be identified. 

The process used for identifying these systems should be similar to the process for showing 

compliance with 14 CFR §§ 23.1309, 25.1309, 27.1309, and 29.1309, as applicable. These 

sections address any system failure that may cause or contribute to an effect on the safety of 

flight of an aircraft. The effects of an encounter with HIRF, therefore, should be assessed in a 

manner that allows for the determination of the degree to which the aircraft and its systems 

safety may be influenced. The operation of the aircraft systems should be assessed separately 

and in combination with, or in relation to, other systems. This assessment should cover— 

(a) All normal aircraft operating modes, stages of flight, and operating 

conditions; 

(b) All failure conditions and their subsequent effect on aircraft operations 

and the flightcrew; and 

(c) Any corrective actions required. 

(2) Safety Assessment. A safety assessment related to HIRF must be performed to 

establish and classify the equipment or system failure condition. Table 1 provides the 

corresponding failure condition classification and system HIRF certification level for the 

appropriate HIRF regulations. The failure condition classifications and terms used in this AC 

are similar to those used in AC 23.1309-1C and AC 25.1309-1A, as applicable. Only those 

systems identified as performing or contributing to functions whose failure would result in 

catastrophic, hazardous, or major failure conditions are subject to HIRF regulations. Based on 

the failure condition classification established by the safety assessment, the systems should be 

assigned appropriate HIRF certification levels, as shown in table 1. Further guidance on 

performing the safety assessment can be found in AC 23.1309-1C, AC 25.1309-1A, SAE ARP 

4754, and SAE ARP 4761. 

Page 6 
7/30/07 AC 20-158 

Table 1 — HIRF Failure Conditions and System HIRF Certification Levels 

HIRF REQUIREMENTS EXCERPTS FROM 

§§ 23.1308, 25.1317, 27.1317, AND 29.1317 

FAILURE 

CONDITION 

SYSTEM HIRF 

CERTIFICATION 

LEVEL 

Each electrical and electronic system that performs a 

function whose failure would prevent the continued 

safe flight and landing of the rotorcraft/airplane 

Catastrophic A 

Each electrical and electronic system that performs a 

function whose failure would significantly reduce the 

capability of the rotorcraft/airplane or the ability of 

the flightcrew to respond to an adverse operating 

condition 

Hazardous B 

Each electrical and electronic system that performs a 

function whose failure would reduce the capability of 

the rotorcraft/airplane or the ability of the flightcrew 

to respond to an adverse operating condition 

Major C 

(3) Failure Conditions. Your safety assessment should consider all potential 

adverse effects due to system failures, malfunctions, or misleading information. The safety 

assessment may show that some systems have different failure conditions in different phases of 

flight; therefore, different HIRF requirements may have to be applied to the system for different 

phases of flight. For example, an automatic flight control system may have a catastrophic failure 

condition for autoland, while automatic flight control system operations in cruise may have a 

hazardous failure condition. 

c. Establish the Applicable Aircraft External HIRF Environment. The external HIRF 

environments I, II, and III, as published in 14 CFR §§ 23.1308, 25.1317, 27.1317, and 29.1317, 

are shown in tables 2, 3, and 4, respectively. The field strength values for the HIRF 

environments and test levels are expressed in root-mean-square (rms) units measured during the 

peak of the modulation cycle, which is how many laboratory instruments indicate amplitude. 

 Page 7 
AC 20-158 7/30/07 

Table 2 — HIRF Environment I

FIELD STRENGTH (V/m) 

FREQUENCY 

PEAK AVERAGE

 10 kHz - 2 MHz 50 50 

 2 MHz - 30 MHz 100 100 

 30 MHz - 100 MHz 50 50 

100 MHz – 400 MHz 100 100 

400 MHz – 700 MHz 700 50 

700 MHz - 1 GHz 700 100 

 1 GHz - 2 GHz 2,000 200 

 2 GHz - 6 GHz 3,000 200 

 6 GHz - 8 GHz 1,000 200 

 8 GHz - 12 GHz 3,000 300 

 12 GHz - 18 GHz 2,000 200 

 18 GHz - 40 GHz 600 200 

In this table, the higher field strength applies at the 

frequency band edges. 

Page 8 
7/30/07 AC 20-158 

Table 3 — HIRF Environment II 

FIELD STRENGTH (V/m) 

FREQUENCY 

PEAK AVERAGE

 10 kHz – 500 kHz 20 20 

500 kHz - 2 MHz 30 30 

 2 MHz - 30 MHz 100 100 

 30 MHz – 100 MHz 10 10 

100 MHz – 200 MHz 30 10 

200 MHz – 400 MHz 10 10 

400 MHz - 1 GHz 700 40 

 1 GHz - 2 GHz 1,300 160 

 2 GHz - 4 GHz 3,000 120 

 4 GHz - 6 GHz 3,000 160 

 6 GHz - 8 GHz 400 170 

 8 GHz - 12 GHz 1,230 230 

 12 GHz - 18 GHz 730 190 

 18 GHz - 40 GHz 600 150 

In this table, the higher field strength applies at the 

frequency band edges. 

 Page 9 
AC 20-158 7/30/07 

Table 4 — HIRF Environment III 

FIELD STRENGTH (V/m) 

FREQUENCY 

PEAK AVERAGE

 10 kHz – 100 kHz 150 150 

100 kHz – 400 MHz 200 200 

400 MHz – 700 MHz 730 200 

700 MHz - 1 GHz 1,400 240 

 1 GHz - 2 GHz 5,000 250 

 2 GHz - 4 GHz 6,000 490 

 4 GHz - 6 GHz 7,200 400 

 6 GHz - 8 GHz 1,100 170 

 8 GHz - 12 GHz 5,000 330 

 12 GHz - 18 GHz 2,000 330 

 18 GHz - 40 GHz 1,000 420 

In this table, the higher field strength applies at the 

frequency band edges. 

Page 10 
7/30/07 AC 20-158 

d. Establish the Test Environment for Installed Systems. 

(1) General. The external HIRF environment will penetrate the aircraft and 

establish an internal RF environment to which installed electrical and electronic systems will be 

exposed. The resultant internal RF environment is caused by a combination of factors, such as 

aircraft seams and apertures, reradiation from the internal aircraft structure and wiring, and 

characteristic aircraft electrical resonance. 

(2) Level A Systems. The resulting internal HIRF environments for level A systems 

are determined by aircraft attenuation to the external HIRF environments I, II, or III, as defined 

in 14 CFR part 23 appendix J, 14 CFR part 25 appendix L, 14 CFR part 27 appendix D, and 14 

CFR part 29 appendix E, as applicable. The attenuation is aircraft- and zone-specific and should 

be established by aircraft test, analysis, or similarity. The steps for showing level A HIRF 

compliance are presented in paragraph 9 of this AC. 

(3) Level B Systems. The internal RF environments for level B systems are defined 

in 14 CFR part 23 appendix J, 14 CFR part 25 appendix L, 14 CFR part 27 appendix D, and 14 

CFR part 29 appendix E, as applicable, as equipment HIRF test levels 1 or 2. The steps for 

showing level B HIRF compliance are presented in paragraph 10 of this AC. 

(4) Level C Systems. The internal RF environment for level C systems is defined in 

14 CFR part 23 appendix J, 14 CFR part 25 appendix L, 14 CFR part 27 appendix D, and 14 

CFR part 29 appendix E, as applicable, as equipment HIRF test level 3. The steps for showing 

level C HIRF compliance are also presented in paragraph 10 of this AC. 

e. Apply the Appropriate Method of HIRF Compliance Verification. 

(1) General. Table 5 summarizes the relationship between the aircraft performance 

requirements in the HIRF regulations (sections (a), (b) and (c)), and the HIRF environments and 

test levels. The process for showing compliance with section (d) of the HIRF regulations is 

discussed in paragraph 6g below. 

(2) Pass/Fail Criteria. Establish specific HIRF compliance pass/fail criteria for 

each system as it relates to the applicable HIRF regulation performance criteria. These pass/fail 

criteria should be presented to the FAA for approval. The means for monitoring system

performance relative to these criteria also should be established by the applicant and approved by 

the FAA. All effects that define the pass/fail criteria should be the result of identifiable 

and traceable analysis that includes both the separate and interdependent operational 

characteristics of the systems. The analysis should evaluate the failures, either singularly or in 

combination, which could adversely affect system performance. This should include failures 

that could negate any system redundancy, or failures that could influence more than one system

performing the same function. 

 Page 11 
AC 20-158 7/30/07 

Table 5 - HIRF Certification Requirements Summary 

HIRF 

ENVIRONMENT 

OR TEST LEVEL

is exposed to HIRF environment I.is exposed to HIRF environment I, unless this conflicts with other operational or functional requirements of that syste

m..

is exposed to HIRF environment II.is exposed to HIRF environment III (Parts 27 and 29 only).is exposed to equipment HIRF test level 1 or 2.is exposed to equipment HIRF test level 3.

ITEM THE 

ENVIRONMENT OR TEST 

LEVEL APPLIES TO

the airplane/rotorcraft...the airplane/rotorcraft...the airplane/rotorcraft...

the rotorcraft...

the equipment providing these 

functions...

the equipment providing these 

functions...

PERFORMANCE CRITERIA

Each function is not adversely affected during and after the time...Each electrical and electronic system automatically recovers normal operation of that function, in a timely manner after... Each electrical and electronic system isnot adversely affected during and after...Each function required during operation under visual flight rules is not adversely affected during and after...The system is not adversely affected when...The system is not adversely affected when...

HIRF FAILURE CONDITION FROM 

§§ 23.1308, 25.1317, 27.1317, AND 29.1317

Each electrical and electronic system that performs a function whose failure would prevent the continued safe flight and landing of the airplane/rotorcraft must be designed and installed so that—

 

Each electrical and electronic system that performs a function whose failure would significantly reduce the capability of the airplane/rotorcraft or the ability of the flightcrew to respond to an adverse operating condition must be designed and installed so that—Each electrical and electronic system that performssuch a function whose failure would reduce the capability of the airplane/rotorcraft or the ability of the flightcrew to respond to an adverse operating condition must be designed and installed so that—

Page 12 
7/30/07 AC 20-158 

f. Verify HIRF Protection Effectiveness. You should show that 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. Section (d) of 

the HIRF regulations allows for the installation of a level A electrical or electronic system, until 

December 1, 2012, when it can be shown that the system intended for installation has previously 

complied with HIRF special conditions prescribed under 14 CFR § 21.16 and issued before 

December 1, 2007. Appendix 2 of this AC describes a means that you can use to show 

compliance with section (d) of 14 CFR §§ 23.1308, 25.1317, 27.1317, or 29.1317. 

7. Margins. A margin is normally not necessary for HIRF compliance based on tests on the 

specific aircraft model and system undergoing certification. However, when determining 

compliance based on analysis or similarity, a margin may be required depending on the 

validation of the analysis or similarity process. Where data have limited substantiation, a margin 

may be required depending on the available justifications. The justification for a selected margin 

should be part of the HIRF Compliance Plan as discussed in paragraph 8, below. 

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 being implemented. If the aircraft, system, or 

installation design changes after FAA 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 6b(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. 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. 

Page 13 
AC 20-158 7/30/07 

(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— 

Page 14 
7/30/07 AC 20-158 

(a) The purpose and scope of the similarity assessment; 

(b) Specific systems addressed by the similarity assessment; 

(c) Data that will be used from the previously certified systems, equipment, 

and aircraft; and 

(d) Any significant differences between the aircraft and system installation 

proposed for certification and the aircraft and system installation from which the data will be 

used. Include appropriate margins to account for similarity uncertainty. 

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 may 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 on relatively simple systems, a single compliance 

report may be 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 reports 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. 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 

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. 

Page 15 
AC 20-158 7/30/07 

(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. 

Page 16 
7/30/07 AC 20-158 

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

Page 17 
AC 20-158 7/30/07 

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

Page 18 
7/30/07 AC 20-158 

Figure 3 — Routes to HIRF Compliance – Level B and C Systems 

Equipment

Test

(4)

Similarity

Assessment

(5)

HIRF Protection

Compliance

(8)

Corrective

Measures

(7)

System Safety

Assessment

(1)

Define Aircraft and 

System HIRF 

Protection (2)

Assess 

Immunity

(6)

Select

Compliance 

Method

(3)

(n) = Step number as described in Paragraph 10 of this AC

Level B or C System 

Similarity

Unacceptable

Acceptable

Equipment-Level Verification

Page 19 
AC 20-158 7/30/07 

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 that will 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 

opened landing gear doors. Consider these configurations in the aircraft assessment (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/DO160 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 may be considered 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 

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7/30/07 AC 20-158 

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 match closely 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, and should 

include connected displays, sensors, actuators, and other equipment. To ensure the integrated 

system is tested when operating at its maximum sensitivity, the system should be placed in 

various operating modes. 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 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 5583/EUROCAE ED-107). 

(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 also should 

be tested. Simultaneous multi-bundle current injection may be necessary on systems where there 

are 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, 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. 

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AC 20-158 7/30/07 

(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, 

for simple systems, 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 similar system. Each 

system considered under the similarity approach needs to be assessed independently even if it 

may use equipment and installation techniques that have been the subject of a previous 

certification. 

(2) The system used as the basis for similarity must have been certified previously 

for HIRF compliance on another aircraft model, and 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 

consider 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), or, for level A display systems only, 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) 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. 

Page 22 
7/30/07 AC 20-158 

(3) 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, may be required to support the analysis. 

(4) 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. 

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, typically illuminating the nose, tail, and each wingtip. The aircraft should be 

illuminated by the antenna at each position while sweeping the frequency range. Perform

separate frequency sweeps with the transmit antenna oriented for horizontal and vertical 

polarization. The RF field should be calibrated by measuring the RF field strength in the center 

of the test volume before the aircraft is placed there. 

(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. 

Page 23 
AC 20-158 7/30/07 

(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 DirectDrive 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 

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 

Page 24 
7/30/07 AC 20-158 

and normalized to 1 V/m electric field strength so they can be scaled to the appropriate external 

HIRF environment. This test method has improved sensitivity over the low-level swept-current 

test 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, typically the nose, tail, and each wingtip, with the distance between the aircraft 

and the transmitting antenna sufficient to ensure the aircraft is illuminated uniformly. 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 

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 

Page 25 
AC 20-158 7/30/07 

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. 

Page 26 
7/30/07 AC 20-158 

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 may be 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 may also be 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 the 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. If 

so, the data collected during the HIRF compliance verification process should be used to 

determine the effect of the HIRF susceptibility on the aircraft systems and functions. 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 14). 

Page 27 
AC 20-158 7/30/07 

(6) If the level A systems show no adverse effects when tested to levels derived from

HIRF environment I or III, as applicable, then 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 that operate in the 18 GHz 

to 40 GHz frequency range, then this demonstrates by analysis that 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. 

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 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 that will be incorporated into the aircraft and system designs, based on the HIRF 

environments that are applicable to your aircraft and its level B and C systems. Equipment, 

Page 28 
7/30/07 AC 20-158 

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 rootmean-square (rms)-indicating spectrum analyzer's peak detector. See the User's Guide (SAE 

ARP 5583/EUROCAE ED-107) 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 may use equipment and 

installation techniques that have been the subject of a previous certification. 

(2) The system used as the basis for certification by similarity must have been 

previously certified for HIRF compliance on another aircraft model, and must have successfully 

completed equipment HIRF tests. 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 

Page 29 
AC 20-158 7/30/07 

without the need for additional equipment HIRF tests, providing there are no unresolved inservice 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 is 

satisfied. HIRF susceptibilities that were not anticipated or defined in the test plan pass/fail 

criteria may be observed during equipment HIRF tests. If so, you should determine the effect of 

the HIRF susceptibility on the aircraft systems and functions. 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. 

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 certification office for approval as part of the overall aircraft type 

certification or supplemental type certification process.

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7/30/07 AC 20-158 

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, 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 5583/EUROCAE ED-107) 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. 

/s/ 

Susan J. M. Cabler 

Acting Manager, Aircraft Engineering Division 

Aircraft Certification Service 

Page 31 (and 32) 

7/30/07 AC 20-158 

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 then 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 5583/EUROCAE ED-107) for details on the use of 

generic transfer functions. 

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. 

 (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. CurrentA1- 1 
AC 20-158 7/30/07 

Appendix 1 

Appendix 1. Generic Transfer Functions and Attenuation 

carrying 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. 

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- 2 
7/30/07 AC 20-158 

Appendix 1 

Appendix 1. Generic Transfer Functions and Attenuation 

FIGURE A1-1 — Generic Transfer Function - Airplane

0.001

0.01

0.1

1

0.01 0.1 1 10 100 1000

Frequency - MHz

mA/V/m

3, 0.7

400, 0.3

15, 0.7

Generic transfer function normalized to 1 V/m for an airplane 

with a fuselage length of < 25m. 

A1- 3 
AC 20-158 7/30/07 

Appendix 1 

Appendix 1. Generic Transfer Functions and Attenuation 

0.01

0.1

1

0.01 0.1 1 10 100 1000

Frequency - MHz

mA/V/m

FIGURE A1-2 — Generic Transfer Function - Airplane 

1.5, 0.7

400, 0.15

6, 0.7

Generic transfer function normalized to 1 V/m for an airplane 

with a fuselage length of > 25m and < 50m. 

A1- 4 
7/30/07 AC 20-158 

Appendix 1 

Appendix 1. Generic Transfer Functions and Attenuation 

FIGURE A1-3 — Generic Transfer Function - Airplane 

0.01

0.1

1

10

0.01 0.1 1 10 100 1000

Frequency - MHz

mA/V/m 

0.5, 2.5

400, 0.15

5, 2.5

50, 0.3 

0 3 2

Generic transfer function normalized to 1 V/m for an airplane 

with a fuselage length of > 50m. 

A1–5 
AC 20-158 7/30/07 

Appendix 1 

Appendix 1. Generic Transfer Functions and Attenuation 

0.01

0.1

1

10

0.01 0.1 1 10 100 1000

Frequency - MHz

mA/V/m

FIGURE A1-4 — Generic Transfer Function - Rotorcraft

5, 1.5

400, 0.5

30, 1.5

Generic transfer function normalized to 1 V/m for a rotorcraft. 

A1–6 
7/30/07 AC 20-158 

Appendix 1 

Appendix 1. Generic Transfer Functions and Attenuation 

0.01

0.1

1

10

0.01 0.1 1 10 100 1000

Frequency - MHz

mA/V/m

FIGURE A1-5 — Generic Transfer Function – All Aircraft

5, 2.5

400, 0.5

30, 1.5

0.5, 2.5

10, 1.5

Generic transfer function normalized to 1 V/m for all aircraft. 

A1–7 
AC 20-158 7/30/07 

Appendix 1 

Appendix 1. Generic Transfer Functions and Attenuation 

FIGURE A1-6 — GENERIC ATTENUATION VALUES — All Aircraft 

100 MHz to 18 GHz

0

5

10

15

20

25

30

35

0.1 1 10 100

Frequency – GHz

Attenuation (dB)

32 dB

20 dB

12 dB

6 dB

0 dB

A1–8 
7/30/07 AC 20-158 

Appendix 2 

Appendix 2. Demonstration of HIRF Compliance based on Previously Issued HIRF 

Special Conditions 

1. Purpose. This Appendix describes a means that you can use to comply with section (d) of 

14 CFR §§ 23.1308, 25.1317, 27.1317, or 29.1317. Section (d) allows the installation of a level 

A electrical/electronic system, until December 1, 2012, if it can be shown that the system has 

previously complied with HIRF special conditions. 

2. Background. 

a. HIRF special conditions, which have been issued since 1987, applied only to systems 

whose functions have catastrophic failure conditions (often called critical functions or Level A). 

The preamble to these special conditions described two approaches to compliance: 

(1) The applicant could demonstrate that the electrical/electronic system is not 

adversely affected when the entire aircraft is exposed to the external HIRF environment; or 

(2) The applicant could demonstrate that the electrical/electronic system is not 

adversely affected when the system is exposed to a defined laboratory test level. 

b. The first approach used an external HIRF environment defined in the preamble to the 

special conditions. This HIRF environment has been the same as, or greater than, HIRF 

Environment I in the HIRF regulations, or for rotorcraft, HIRF Environment III. This approach 

required the applicant to determine the actual attenuation levels for the aircraft, or to expose the 

operating aircraft to high power transmitters that generated the external HIRF environment. 

c. The second approach required system tests in a laboratory using test levels of 100 volts 

per meter (200 volts per meter if the system was required for rotorcraft VFR operations) or the 

external HIRF environment whichever was less. 

 

3. How to Show Compliance. 

a. If the level A system was previously certified using the approach described in paragraph 

2a(1) of this appendix, you should not apply section (d) of the HIRF regulation, but instead show 

compliance with section (a) of the HIRF regulation. Paragraph 9 of this AC provides guidance 

on possible methods (for example, by similarity assessment) that you can use. 

b. If the level A system was previously certified using the approach described in paragraph 

2a(2) of this appendix, then follow the process described in table A2-1 below, as a means of 

showing compliance to section (d) of the HIRF regulation for your level A system installation. 

Note: If you intend to export this system, then you should be 

aware that showing compliance with section (d) of the HIRF 

regulations may not be an acceptable means of compliance to other 

Civil Aviation Authority HIRF regulations for a level A system.

A2–1 
AC 20-158 7/30/07 

Appendix 2 

Appendix 2. Demonstration of HIRF Compliance based on Previously Issued HIRF Special 

Conditions 

Table A2-1. Process for Showing Compliance to HIRF Section 

(d) Requirements 

Requirement Do this by... What you should submit... 

(d)(1) The system has 

previously been 

shown to comply with

special conditions for 

HIRF, prescribed 

under § 21.16, issued 

before December 1, 

2007; 

Providing objective 

evidence to the ACO 

that the system that you 

intend to install has 

previously complied 

with HIRF special 

conditions (SC). 

• Copy of the final-issued HIRF SC that was used as the basis for 

showing HIRF compliance for the system you now intend to install. 

• The TC or STC number that approved the installation of this

Level A system and that reflects this HIRF SC number. 

• Substantiation that the level A system was approved under that

SC, such as an approved certification report or system HIRF

qualification test report that includes the system part numbers. 

Note: It is the applicant’s responsibility to show that all 

assumptions/limitations in this report are still applicable in

supporting the level A function identified in the safety

assessment for the new installation. See paragraph 6b(2) of this 

AC for additional information regarding safety assessments. 

You can view and download copies of special conditions from our web based 

Regulatory and Guidance library (RGL) at http://rgl.faa.gov. On the RGL 

website, select “Special Conditions,” then select appropriate options under 

“Final”.

(d)(2) The HIRF 

immunity 

characteristics of the 

system have not 

changed since 

compliance with the 

special conditions 

was demonstrated; 

and

Establishing that there 

have been no system

design changes that

would invalidate the 

HIRF immunity 

characteristics that were 

demonstrated under the 

SC. 

• An assessment of all changes to the level A system design since 

HIRF compliance was demonstrated that could impact HIRF 

immunity. Service experience alone is not acceptable since such 

experience in normal flight operations may not include exposure to

the HIRF environment. 

Note: If a change has been made that could impact the HIRF 

immunity characteristics of the system, and the effect of the 

change cannot be substantiated through analysis, the system

must comply with the general requirements of the HIRF rule as 

specified in paragraph (a) of each section. 

• Alternately, if the system supplier (for example, a TSO 

manufacturer) has a design change control process under ACO 

oversight that ensures HIRF compliance as changes are accomplished, 

you may utilize a statement from that supplier – concurred by the 

supplier’s overseeing ACO – that the HIRF immunity characteristics 

of the level A system have not been altered since compliance with the 

SC was demonstrated. 

(d)(3) The data used 

to demonstrate 

compliance with the 

special conditions is

provided. 

Providing the ACO 

with the appropriate 

data to support the new 

installation

certification. 

• Copy of the HIRF certification/qualification test report used to

demonstrate compliance to the HIRF SC. 

• Installation instructions, as appropriate, to support HIRF

immunity of the system (for example, wire-shielding and grounding 

techniques). 

• Instructions for Continued Airworthiness (ICA) to maintain the 

integrity of the system’s HIRF immunity. See paragraph 11 of this 

AC for more information.

A2–2