Advisory U.S. Department of Transportation CircularFederal Aviation Administration Subject: Aircraft Electrical and Electronic Date: 9/7/11 AC No: 20-136B System Lightning Protection Initiated by: AIR-130
1. Purpose.
a. This advisory circular (AC) provides you with information and guidance on how you can protect aircraft electrical and electronic systems from the effects of lightning. This AC describes a means, but not the only means, for you to show compliance with Title 14 of the Code of Federal Regulations (14 CFR) 23.1306, 25.1316, 27.1316, and 29.1316, Electrical and electronic system lightning protection, as they pertain to the type or supplemental type certification of your aircraft.
b. This AC is not mandatory and does not constitute a regulation. However, if you use the means described in this AC to comply with 14 CFR 23.1306, 25.1316, 27.1316, and 29.1316, you must follow it in its entirety.
c. The term “must” is used to indicate mandatory requirements when following the guidance in this AC in its entirety. The terms “should” and “recommend” are used when following the guidance is recommended but not required to comply with this AC.
d. Appendix 1 addresses related documents. Appendix 2 addresses definitions. Appendix 3 defines acronyms.
2. Applicability. 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 certification requirements of 14 CFR 23.1306, 25.1316, 27.1316, and 29.1316.
3. Scope.
a. This AC provides guidance for complying with 14 CFR 23.1306, 25.1316, 27.1316, and
29.1316 for the effects on electrical and electronic systems due to lightning transients induced or conducted onto equipment and wiring.
b. Applicants must also comply with 14 CFR 23.1306, 25.1316, 27.1316, and 29.1316 for the effects on electrical and electronic systems when lightning directly attaches to equipment, components, or wiring. This AC addresses the functional aspects of these effects for aircraft electrical and electronic equipment, components, or wiring. However, this AC does not address
lightning effects such as burning, eroding, and blasting of aircraft equipment, components, or wiring. For showing compliance for these effects, we recommend using SAE ARP 5577, Aircraft Lightning Direct Effects Certification.
c. For information on fuel ignition hazards, see AC 20-53, Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition Caused By Lightning. This AC does not address lightning zoning methods, lightning environment definition, or lightning test methods. For information on lightning zoning methods and lightning environment definition, see AC 20-155, SAE Documents To Support Aircraft Lightning Protection Certification. For information on lightning test methods, see SAE ARP 5416, Aircraft Lightning Test Methods, or RTCA/DO-160, Section 22, Lightning Induced Transient Susceptibility, and Section 23, Lightning Direct Effects.
Note: When complying with 14 CFR 23.1306, 25.1316,
27.1316, and 29.1316, use RTCA/DO-160D, Change Notice 3, or later revisions.
4. Cancellation. AC 20-136A, Protection of Aircraft Electrical/Electronic Systems Against the Indirect Effects of Lightning, dated December 21, 2006, is cancelled.
5. Background.
a. Regulatory Applicability. The standards for aircraft electrical and electronic system lightning protection are based on the aircraft's potential for lightning exposure and the consequences of system failure. The regulations require lightning protection of aircraft electrical and electronic systems with catastrophic, hazardous, or major failure conditions for aircraft certificated under 14 CFR parts 25 and 29. The requirements also apply to 14 CFR part 23 airplanes and part 27 rotorcraft approved for operations under instrument flight rules. Those 14 CFR part 23 airplanes and part 27 rotorcraft approved solely for operations under visual flight rules require lightning protection of electrical or electronic systems having catastrophic failure conditions.
b. Regulatory Requirements. Protection against the effects of lightning for aircraft electrical and electronic systems, regardless of whether these are ‘indirect’ or ‘direct’ effects of lightning, are addressed under 14 CFR 23.1306, 25.1316, 27.1316, and 29.1316. The terms ‘indirect’ and ‘direct’ are often used to classify the effects of lightning. However, the regulations do not, and are not intended to, differentiate between the effects of lightning. The focus is to protect aircraft electrical and electronic systems from effects of lightning. The regulations listed in this paragraph introduce several terms which are further explained below, including:
(1) System. A system can include equipment, components, parts, wire bundles, software, and firmware. Electrical and electronic systems consist of pieces of equipment connected by electrical conductors, all of which are required to perform one or more functions.
(2) Function. The specific action of a system, equipment, and flight crew performance aboard the aircraft that, by itself, provides a completely recognizable operational capability. For 2
example, “display aircraft heading to the pilots” is a function. One or more systems may perform a specific function or one system may perform multiple functions.
(3) Adverse Effect. A lightning effect resulting in system failure, malfunction, or misleading information to a degree that is unacceptable for the specific aircraft function or system addressed in the system lightning protection regulations.
(4) Timely Manner. The meaning of “in a timely manner” depends upon the function performed by the system being evaluated, the specific system design, interaction between that system and other systems, and interaction between the system and the flight crew. The definition of “in a timely manner” must be determined for each specific system and for specific functions performed by the system. The applicable definition should be included in the certification plan for review and approval by the certification authorities.
6. Steps for Showing Compliance.
a. The following seven steps describe how you may comply with 14 CFR 23.1306,
25.1316, 27.1316, and 29.1316 requirements for your aircraft’s electrical and electronic systems:
(1) Identify the systems to be assessed.
(2) Determine the lightning strike zones for the aircraft.
(3) Establish the aircraft lightning environment for each zone.
(4) Determine the lightning transient environment associated with the systems.
(5) Establish equipment transient design levels (ETDLs) and aircraft actual transient levels (ATLs).
(6) Verify compliance to the requirements.
(7) Take corrective measures, if needed.
b. The steps above should be performed to address lightning transients induced in electrical and electronic system wiring and equipment, and lightning damage to aircraft external equipment and sensors that are connected to electrical and electronic systems, such as radio antennas and air data probes. Additional guidance on lightning protection against lightning damage for external equipment and sensor installations can be found in SAE ARP 5577.
c. Identify the Systems to be Assessed.
(1) General. The aircraft systems requiring lightning assessment should be identified.
Address any lightning-related electrical or electronic system failure that may cause or contribute to an adverse effect on the aircraft. The eff ects of a lightning strike, therefore, should be 3
assessed in a manner that allows for the determination of the degree to which the aircraft and/or its systems safety may be influenced. This assessment should cover:
(a) All normal aircraft operating modes, stages of flight, and operating conditions;
and
(b) All lightning-related failure conditions and their subsequent effect on aircraft operations and the flightcrew.
(2) Safety Assessment. A safety assessment related to lightning effects should be performed to establish and classify the system failure condition. Based on the failure condition classification established by the safety assessment, the systems should be assigned appropriate lightning certification levels, as shown in Table 1. The failure condition classifications and terms used in this AC are consistent with those used in AC 23.1309-1, System Safety Analysis and Assessment for Part 23 Airplanes, and AC 25.1309-1, System Design and Analysis.
Further guidance on processes for performing safety assessments can be found in those ACs, and AC 27-1, Certification of Normal Category Rotorcraft, AC 29-2, Certification of Transport Category Rotorcraft, SAE ARP 4754, Guidelines for Development of Civil Aircraft and Systems, and ARP 4761, Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment. The specific aircraft safety assessment related to lightning effects required by 14 CFR 23.1306, 25.1316, 27.1316 and 29.1316 takes precedence over the more general safety assessment process described in AC 23.1309-1, AC 25.1309-1, AC 27-1, and AC 29-2 . Lightning effects on electrical and electronic systems are generally assessed independently from other system failures that are unrelated to lightning, and do not need be considered in combination with latent or active failures unrelated to lightning.
Table 1. Lightning Failure Conditi ons and Certification Levels Lightning Requirement Provisions From:
14 CFR 23.1306, 25.1316, 27.1316, 29.1316
(a) Each electrical and electronic system that performs a function, for which failure would prevent the continued safe flight and landing of the aircraft ….
(b) Each [… each] electrical and electronic system that performs a function, for which failure would reduce the capability of the aircraft or the ability of the flightcrew to respond to an adverse operating condition,….
Failure Condition Catastrophic Hazardous Major
System Lightning Certification Level A B C 4
(a) Level A Systems. The system safety assessment should consider effects of lightning-related failures or malfunctions on systems with lower failure classification that may affect the function of Level A systems. You should show that any system with wiring connections to a Level A system will not adversely affect the functions with catastrophic failure conditions performed by the Level A system when the aircraft is exposed to lightning.
Redundancy alone cannot protect against lightning because the lightning-generated electromagnetic fields, conducted currents and induced currents in the aircraft can simultaneously induce transients in all electrical wiring on an aircraft.
(b) Level B or C Systems. Simultaneous and common failures due to lightning exposure generally do not have to be assumed for Level B or C systems incorporating redundant, spatially separated installations in the aircraft. This is because aircraft transfer function tests and in-service experience have shown these redundant and spatially separated installations are not simultaneously exposed to the maximum lightning induced transients. For example, redundant external sensors may mitigate direct lightning attachment damage, if there is acceptable separation between the sensors to prevent damage to multiple sensors so that the function is maintained. Therefore, simultaneous loss of all of these redundant and spatially separated Level B or C systems due to lightning exposure do not need to be considered. However, if multiple Level B or C systems are designed and installed within the same location in the aircraft, or share a common wiring connection, then the combined failure due to lightning exposure should be assessed to determine if the combined failures are catastrophic. If so, these systems should be designated as Level A systems.
(c) Failure Conditions. The safety assessment may show that some systems have different failure conditions in different phases of flight. Therefore, different lightning 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.
d. Determine the Lightning Strike Zones for the Aircraft. The purpose of lightning zoning is to determine those areas of the aircraft likely to experience lightning channel attachment and those structures that may conduct lightning current between lightning attachment points. You should determine the lightning attachment zones for your aircraft configuration, since the zones will be dependent upon the aircraft’s geometry, materials, and operational factors. Lightning attachment zones often vary from one aircraft type to another.
Note: AC 20-155 provides guidance to determine the lightning attachment zones for your aircraft.
e. Establish the Aircraft Lightning Environment for Each Zone. Zones 1 and 2 identify where lightning is likely to attach and, as a result, the entrance and exit points for current flow through the aircraft. The appropriate voltage waveforms and current components to apply in those zones should be identified. By definition, Zone 3 areas carry lightning current flow between initial (or swept stroke) attachment points, so they may include contributions from all of the current components. We accept analysis to estimate Zone 3 current levels that result from the external environment. The external lightning environment is:
(1) Caused by the lightning flash interacting with the exterior of the aircraft.
(2) Represented by combined waveforms of the lightning current components at the aircraft surface.
Note: AC 20-155 provides guidance for selecting the lightning waveforms and their applications.
f. Determine the Lightning Transient Environment Associated with the Systems.
(1) The lightning environment, as seen by electrical and electronic systems, consists of voltages and currents produced by lightning current flowing through the aircraft. The voltages and currents that appear at system wiring interfaces result from aperture coupling, structural voltages, or conducted currents resulting from direct attachments to equipment and sensors.
(2) Determine the lightning voltage and current transient waveforms and amplitudes that can appear at the electrical and electronic equipment interface circuits for each system identified in paragraph 6.c. You may determine the lightning transients in terms of the wire bundle current, or the open circuit voltage and the short circuit current appearing at system wiring and equipment interface circuits. The voltage and current transient waveforms and amplitudes are dependent upon the loop impedances of the system and its interconnecting wiring.
g. Establish Equipment Transient Design Levels (ETDLs) and Aircraft Actual Transient Levels (ATLs). The regulations in 14 CFR 23.1306, 25.1316, 27.1316, and 29.1316 define requirements in terms of functional effects that are performed by aircraft electrical and electronic systems. From a design point of view, lightning protection for systems is shared between protection incorporated into the aircraft structure and wiring, and protection incorporated into the equipment. Therefore, requirement allocations for the electrical and electronic system lightning protection can be based on the concept of ETDLs and ATLs.
(1) Determine and specify the ETDLs for the electrical and electronic equipment that make up the systems to be assessed. The ETDLs set qualification test levels for the systems and equipment. They define the voltage and current amplitudes and waveforms that the systems and equipment must withstand without any adverse effects. The ETDLs for a specific system depend on the anticipated system and wiring installation locations on the aircraft, the expected shielding performance of the wire bundles and structure, and the system criticality.
Transient C Level ontrol (TCL) sient Actual Tran Level (ATL)
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(2) The ATLs are the voltage and current amplitudes and waveforms actually generated on the aircraft wiring when the aircraft is exposed to lightning, as determined by aircraft test, analysis, or similarity. The difference between an ETDL and ATL is the margin. Figure 1 shows the relationship among the ATL and ETDL. You should evaluate the aircraft, interconnecting wiring, and equipment protection to determine the most effective combination of ATLs and ETDLs that will provide acceptable margin. Appropriate margins to account for uncertainties in the verification techniques may be required as discussed in paragraph 8.i of this AC.
Figure 1. Relationships Among Transient Levels
(3) Typically, you specify the ETDLs prior to aircraft certification lightning tests or analyses to determine the aircraft ATLs. Therefore the expected aircraft transients must be based upon results of lightning tests on existing aircraft, engineering analysis, or knowledgeable estimates. These expected aircraft lightning transient levels are termed transient control levels (TCL). Specify the TCLs voltage and current amplitudes and waveforms based upon the expected lightning transients that would be generated on wiring in specific areas of the aircraft.
The TCLs should be equal to or greater than the maximum expected aircraft ATLs. The TCLs for a specific wire bundle depend on the configuration of the aircraft, the wire bundle, and the wire bundle installation. You should design your aircraft lightning protection to meet the specified TCLs.
h. Verify Compliance to the Requirements.
(1) You should show that the systems comply with the applicable requirements of 14 CFR 23.1306, 25.1316, 27.1316, or 29.1316.
(2) You should show that the ETDLs exceed the ATLs by the margin established in your certification plan.
(3) Verification may be accomplished by tests, by analysis, or by demonstrating similarity with previously certified aircraft and systems. The certification process for Level A systems is discussed in paragraph 8. The certification process for Level B and C systems is discussed in paragraph 9.
(4) Submit your certification plan early in the program to the cognizant aircraft certification office (ACO) for review. Experience shows, particularly with aircraft using new technology or those that have complex systems, that early agreement on the certification plan benefits both the applicant and the cognizant ACO. The plan should define acceptable ways to resolve critical issues during the certification process. Analysis and test results during the certification process may warrant modifications in the design or verification methods. When significant changes are necessary, update the certification plan accordingly. The plan may include the items listed in Table 2.
i. Take Corrective Measures. If tests and analyses show that the system did not meet the pass/fail criteria, review the aircraft, installation or system design and improve protection against lightning.
Table 2. Items Recommended for a Lightning Certification Plan Item Discussion Description of systems Description of compliance method Acceptance criteria Describe systems’ installation, including unusual or unique features; the system failure condition classifications; the operational aspects; lightning attachment zones; lightning environment; preliminary estimate of ETDLs and TCLs; and acceptable margins between ETDLs and ATLs.
Describe how to verify compliance. Ty pically, your verification method includes similarity, analytical procedures, and tests. If using analytical procedures, describe how to verify them. (See paragraph 8d of this AC.)
Determine the pass/fail criteria for each system by analyzing how safe the system is. During this safety analysis, assess the aircraft in its various operational states;
account for the failure and disruption modes caused by the effects of lightning.
Plan each test you include as part of your certification process. As an applicant, Test plans you can decide if your test plans are separate documents or part of the compliance plan. Your test plans should state the test sequence.
7. Effects of Transients. Lightning causes voltage and current transients to appear on equipment circuits. Equipment circuit impedances and configurations will determine whether lightning transients are primarily voltage or current. These transient voltages and currents can degrade system performance permanently or temporarily. The two primary types of degradation are component damage and system functional upset.
a. Component Damage. This is a permanent condition in which transients alter the electrical characteristics of a circuit. Examples of devices that may be susceptible to component damage include:
(1) Active electronic devices, especially high frequency transistors, integrated circuits, microwave diodes, and power supply components;
(2) Passive electrical and electronic components, especially those of very low power or voltage rating;
(3) Electro-explosive devices, such as squibs and detonators;
(4) Electromechanical devices, such as indicators, actuators, relays, and motors; and
(5) Insulating materials (for example, insulating materials in printed circuit boards and connectors) and electrical connections that can burn or melt.
b. System Functional Upset.
(1) Functional upset is mainly a system problem caused by electrical transients. It may permanently or momentarily upset a signal, circuit, or a system component, which can adversely affect system performance enough to compromise flight safety. A functional upset is a change in digital or analog state that may or may not require manual reset. In general, functional upset depends on circuit design and operating voltages, signal characteristics and timing, and system and software configuration.
(2) Systems or devices that may be susceptible to functional upset include computers and data/signal processing systems; electronic engine and flight controls; and power generating and distribution systems.
8. Level A System Lightning Certification. Figure 2 illustrates a process that you can use to show that your level A system complies with 14 CFR 23.1306, 25.1316, 27.1316, and 29.1316.
a. Identify Level A Systems. Identify your Level A systems as described in paragraph
6.c. Define the detailed system performance pass/fail criteria. An ACO should concur on this criterion before you begin testing or analyzing your Level A system. Identify specific equipment, components, sensors, power systems and wiring associated with each Level A system in order to perform the ETDL verification discussed in paragraphs 8.g and 8.h.
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9/7/11 AC 20-136B Figure 2. Typical Compliance Process for Level A Systems Identify Level A Systems
(6.c) Determine Aircraft Lightning Zones
(6.d) Determine Aircraft- Specific Lightning Environment
(6.e) Select Aircraft Verification Method Establish System ETDLs (8.b) Verify Aircraft ATL by Test
(8.c) Verify Aircraft ATL by Similarity
(8.e) Verify Aircraft ATL by Analysis
(8.d) Select ETDL Verification Method Verify ETDLs by System Qualification Tests
(8.g) Verify ETDLs by System Similarity
(8.h) Compare Aircraft ATLs and System ETDLs
(8.i) Margin Acceptable?
Lightning Compliance Demonstrated Yes Modify Aircraft or System Lightning Protection
(8.j) No Select Level A Displays Transient Levels
(8.f) Level A Displays Only Numbers in parenthesis refer to sections in this AC Define Aircraft and System Lightning Protection Features Aircraft-Level Verification System-Level Verification
b. Establish System ETDLs. Establish the aircraft system ETDLs from an evaluation of expected lightning transient amplitudes and waveforms for the system installation, structure and wiring configuration on a specific aircraft. Y ou should establish ETDLs that exceed the ATLs by acceptable margin. In general, the ETDLs for equipment in a complex system will not be the same for all wire bundles connecting them to other equipment in the system. You may use results of lightning tests on existing similar aircraft, engineering analysis, or knowledgeable estimates to 10
establish appropriate system ETDLs. While specific aircraft configurations and system installations may lead to ETDLs that have amplitudes and waveforms different than those defined in RTCA/DO-160 Section 22, ETDLs are often specified using the information from Section 22. The ETDLs must exceed the ATLs by an acceptable margin.
c. Determine ATLs Using Aircraft Tests. See SAE ARP 5415, User’s Manual for Certification of Aircraft Electrical/Electronic Systems Against the Indirect Effects of Lightning, and SAE ARP 5416 for guidance on how to determine the ATLs.
d. Determine ATLs Using Analysis. See SAE ARP 5415 for guidance on how to analyze aircraft to determine the ATLs. Acceptance of the analysis method you choose depends on the accuracy of the method. You should confirm your analysis method accuracy using experimental data, and gain agreement with your analysis approach from the cognizant ACO.
e. Determine ATLs Using Similarity.
(1) You may use similarity to determine the ATLs when there are:
(a) Only minor differences between the previously certified aircraft and system installation, and the aircraft and system installation to be certified; and
(b) There is no unresolved in-service history of problems related to lightning strikes to the previously certified aircraft.
(2) If significant differences are found that will affect the aircraft ATLs, you should perform more tests and analyses to resolve the open issues.
(3) To use similarity, you should assess the aircraft, wiring, and system installation differences that can adversely affect the system susceptibility. When assessing a new installation, consider differences affecting the internal lightning environment of the aircraft and its effects on the system. The assessment should cover:
(a) Aircraft type, equipment locations, airframe construction, structural materials, and apertures that could affect attenuation of the external lightning environment;
(b) System wiring size, length, and routing; wire types (whether parallel or twisted wires), connectors, wire shields, and shield terminations;
(c) Lightning protection devices such as transient suppressors and lightning arrestors; and
(d) Grounding and bonding.
(4) You cannot use similarity for a new aircraft design with new systems.
f. Determine Transient Levels Using RTCA/DO-160 Section 22 Guidance for Level A Displays Only.
(1) You may select ETDLs for your Level A display system using guidance in this section, without specific aircraft test or analysis. Level A displays involve functions for which the pilot will be in the loop through pilot/system information exchange. Level A display systems typically include the displays; symbol generators; data concentrators; sensors (such as attitude, air data, and heading sensors); interconnecting wiring; and associated control panels.
(2) This approach should not be used for other Level A systems, such as control systems, because failures and malfunctions of those systems can more directly and abruptly contribute to a catastrophic failure event than display system failures and malfunctions.
Therefore, other Level A systems require a more rigorous lightning transient compliance verification program.
(3) You should use the information in Table 3 to evaluate your aircraft and system installation features to select appropriate ETDLs for your system. Table 3 defines test levels for ETDLs, based on RTCA/DO-160 Section 22, Tables 22-2 and 22-3. Provide the cognizant ACO with a description of your aircraft and display system installation features and compare these to the information in Table 3 to substantiate the ETDL selected for your aircraft and Level A display system installation. When selecting ETDLs using guidance provided in Table 3, acceptable margin between anticipated ATLs for display system installations is incorporated in the selected ETDLs.