AC 2 0-136C Table 1. Lightning Failure Conditions and Certification Levels
LIGHTNING REQUIREMENTS
EXCERPTS FROM §§ 23.1306 (amendment 23-61), 25.1316,
27.1316, and 29.1316 MOST SEVERE
FAILURE
CONDITION SYSTEM
LIGHTNING
CERTIFICATION
LEVEL (LCL)
(a) Each electrical and electronic system that performs a function, for which failure would prevent the continued safe flight and landing of the [aircraft]2… Catastrophic A
(b) 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 operation condition… Note: Paragraph (b) in this table is applicable for normal category airplanes and rotorcraft (part 23 and 27, respectively) approved for instrument flight rules (IFR) operations.
Hazardous Major B C
2 Parts 23 and 25 refer to airplanes, and parts 27 and 29 refer to rotorcraft.
AC 20-136C Table 2. Lightning Failure Conditions and Certification Levels (§ 23.2515)
LIGHTNING REQUIREMENTS
EXCERPTS FROM § 23.2515 (Amendment 23-64) MOST SEVERE
FAILURE
CONDITION SYSTEM
LIGHTNING
CERTIFICATION
LEVEL (LCL)
(a) Each electrical or electronic system that performs a function, the failure of which would prevent the continued safe flight and landing of the airplane… Catastrophic A
(b) Each electrical and electronic system that performs a function, the failure of which would significantly reduce the capability of the airplane or the ability of the flightcrew to respond to an adverse operation condition… Note: Paragraphs (a) and (b) in this table are applicable for normal category airplanes approved for instrument flight rules (IFR) operations unless an applicant shows that exposure to lightning is unlikely.
Hazardous
B
7.3.3.1 LCL A Systems.
7.3.3.1.1 You must demonstrate compliance with §§ 23.1306(a) (amendment 23-
61), 23.2515(a), 25.1316(a), 27.1316(a), or § 29.1316(a), for aircraft functions and systems that perform functions whose failure would prevent continued safe flight and landing of the aircraft. Additionally, paragraph
(a)(2) of those regulations require the electrical and electronic system to automatically recover normal operation of the functions in a timely manner after an airplane is exposed to lightning, with the exception of § 23.2515(a)(2), which allows manual recovery (flightcrew action) or automatic recovery. If all electrical and electronic equipment, components, and electrical interconnections that are required for normal operation of the Level A3 functions comply with the requirements of paragraph (a), then all electrical and electronic equipment, components, and electrical interconnections that are only operational in non-normal situations may comply with paragraph (b) of these regulations, unless the FAA determines the failure is catastrophic.
3 In the context of this AC, refers to a critical function whose failure may be catastrophic (LCL A system).
AC 20-136C
7.3.3.1.2 The lig htning safety assessment should consider effects of lightningrelated failures or malfunctions on systems with lower failure classification that may affect the function of LCL A systems. You should demonstrate that any system with wiring connections to an LCL A system will not adversely affect the functions with catastrophic failure conditions performed by the LCL A system when the aircraft is exposed to lightning.
Redundancy alone cannot protect against lightning because the lightninggenerated electromagnetic fields, conducted currents, and induced currents in the aircraft, can simultaneously induce transients in all electrical wiring on an aircraft.
7.3.3.2 LCL B or C Systems.
You must demonstrate compliance with §§ 23.1306(b) (amendment 23-
61), 23.2515(b), 25.1316(b), 27.1316(b), or § 29.1316(b), for a system that performs a function whose failure would reduce the capability of the aircraft or the ability of the flightcrew to respond to an adverse operating condition. Simultaneous and common cause failures due to lightning exposure generally do not have to be assumed for LCL 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 systems (internal to the airframe) may mitigate lightning indirect effects, if there is acceptable separation between the equipment to prevent damage to multiple systems, so that the function is maintained. Therefore, simultaneous loss of all of these redundant and spatially separated LCL B or C systems due to lightning exposure does not need to be considered. However, if multiple LCL B or C systems including interface wiring 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 LCL A systems.
7.3.3.3 Failure Conditions.
The lightning safety assessment should consider all potential adverse effects due to system failures, malfunctions, or misleading information.
The lightning safety assessment may show that a system’s functions have different failure conditions in different phases of flight. Therefore, the system LCL corresponds to phase of flight with the most severe failure condition. 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.
AC 20-136C
7.4 D etermine the Lightning Strike Zones for the Aircraft.
T he 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 aircraft.
7.5 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. The FAA accepts analysis to estimate Zone 3 current levels that result from the external environment. The external lightning environment is:
7.5.1 Caused by the lightning flash interacting with the exterior of the aircraft.
7.5.2 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.
7.6 Determine the Lightning Transient Environment Associated with the Systems.
7.6.1 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.
7.6.2 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 7.2. 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. Refer to SAE ARP5415 / EUROCAE ED-158 for more detailed guidance. The voltage and current transient waveforms and amplitudes are dependent upon the loop impedances of the system and its interconnecting wiring.
7.7 Establish Equipment Transient Design Levels (ETDLs) and Aircraft Actual Transient Levels (ATLs).
The regulations in §§ 23.1306 (amendment 23-61), 23.2515, 25.1316, 27.1316, 13
AC 20-136C a nd 29.1316 define req uirements 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, requirements for the electrical and electronic system lightning protection can be based on the concept of ETDLs and ATLs.
7.7.1 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.
7.7.2 The ATLs are the voltage and current amplitudes and transient waveforms 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 between 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 10.12 of this AC.
Figure 1. Relationships Among Transient Levels
7.7.3 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 14
AC 20-136C know ledgeabl e estimates. These expected aircraft lightning transient levels are termed transient control levels (TCL). Specify the TCL’s 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 ATLs should not exceed the aircraft TCLs.
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.
7.8 Verify Compliance with the Regulations.
7.8.1 You must demonstrate that the systems comply with the applicable requirements of §§ 23.1306 (amendment 23-61), 23.2515, 25.1316, 27.1316, or § 29.1316.
7.8.2 You should demonstrate that the ETDLs exceed the ATLs by the margin established in your certification plan.
7.8.3 Verification may be accomplished by tests, by analysis, or by demonstrating similarity with previously certified aircraft and systems. The certification process for LCL A systems is discussed in paragraph 10. The certification process for LCL B and C systems is discussed in paragraph 11.
7.8.4 Submit your certification plan early in the program to the cognizant certification branch 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 certification branch. 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 3.
Note: You may refer to paragraph 8.1 for more information.
7.9 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.
7.10 Margins.
Margins account for uncertainty in the verification process. For lightning certification, the margin is the difference between the ETDLs and ATLs. The verification approach should be described in the certification plan with justification for margin, which requires agreement by the respective FAA certification branch. Refer to paragraph
10.12 for additional guidance on margin.
AC 20-136C Ta ble 3. Items Recommended for a Lightning Certification Plan
ITEM DISCUSSION Description of Systems Describe systems’ installation, including unusual or unique features; the system failure condition classifications or initial LCLs; the operational aspects; lightning attachment zones;
lightning environment; preliminary estimate of ETDLs and TCLs;
and acceptable margins between ETDLs and ATLs.
Description of Compliance Method Describe how to verify compliance. Typically, your verification method includes similarity, analytical procedures, and tests. If using analytical procedures, describe how to verify them. (See paragraph 10.9 of this AC.)
Acceptance Criteria Determine the pass/fail criteria for each system by analyzing how safe the system is; this information may be finalized in the test plan. 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.
Test Plans Plan each test you include as part of your certification process. As an applicant, you can decide if your test plans are separate documents or part of the certification plan. Your test plans should state the test sequence.
8 LIGHTNING COMPLIANCE.
8.1 Lightning Certification Plan.
You should establish an overall certification plan to clearly identify and define lightning certification requirements, lightning 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 lightning compliance will be assessed. You should discuss the lightning certification plan with, and submit it to, the FAA for approval before initiating lightning compliance activities. If the aircraft, system, or installation design changes after approval, you should submit a revised lightning certification plan to the FAA for approval. The lightning certification plan should include the following:
• A lightning certification plan summary;
• Identification of the aircraft systems (see section 7.2), with classification based on the safety assessment as it relates to lightning;
• The expected lightning environment for the aircraft and installed systems; and
• The verification methods, such as test, analysis, or similarity.
AC 20-136C
8.2 Methods of Compliance Verification.
8.2.1 Various methods are available to aid in demonstrating lightning compliance. Paragraphs 10 and 11 of this AC describe methods acceptable to the FAA. Figure 2 outlines the steps to show lightning compliance for LCL A systems. Figure 3 outlines the steps to show lightning compliance for LCL B or C systems. 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 10 and 11 of this AC.
8.2.2 You may use other lightning compliance techniques to demonstrate system performance in the lightning environment; however, the FAA should approve those techniques before you use them.
8.3 Lightning Verification Test, Analysis, or Similarity Plan.
Test, analysis, and similarity are all acceptable methods for verification. The applicant should choose the most appropriate method for their project. See paragraphs 10 and 11 of this AC, and SAE ARP5415, for additional guidance on selecting the appropriate method. Specific lightning tests, 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, and a similarity plan for selected systems on an aircraft.
8.3.1 Test Plan.
A lightning compliance test plan should include the equipment, system, and aircraft test objectives for the acquisition of data to support lightning compliance verification. The plan should provide an overview of the factors being addressed for each system test requirement. The test plan should include:
• The purpose of the test;
• A description of the aircraft and/or system being tested;
• System configuration drawings;
• The proposed test setup and methods;
• Intended test levels, modes of operation, and monitoring;
• Pass/fail criteria; and
• The test schedule and test location.
8.3.1.1 The test plan should cover LCL A, B, and C systems and equipment, as appropriate. LCL A systems may need both laboratory integrated systems tests and aircraft tests. LCL B and C systems and equipment need only equipment laboratory testing.
8.3.1.2 The test plan should describe the appropriate aspects of the systems to be tested and their installation. Additionally, the test plan should reflect the 17
AC 20-136C resul ts of any analysis performed in the overall process of the lightning complia nce evaluation.
8.3.2 Analysis Plan.
A lightning compliance analysis plan should include the objectives, both at the system and equipment level, for generating data to support lightning compliance verification.
Comprehensive modeling and analysis for voltage and current transients 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. Aircraft testing may be necessary to support the analysis. The analysis plan should include:
• The purpose and scope of the analysis;
• A description of the aircraft and/or system addressed by the analysis;
• System configuration descriptions;
• Proposed analysis methods;
• The approach for validating the analysis results;
• Pass/fail criteria; and
• When data has limited substantiation, a description and justification for margins to account for analysis uncertainty.
8.3.3 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 lightning compliance program. The similarity plan should include:
• The purpose and scope of the similarity assessment;
• Specific systems addressed by the similarity assessment;
• Data used from the previously certified systems, equipment, and aircraft;
• Details on differences between the aircraft and system being certified and the similar aircraft and system from which the data will be used; and
• A description and justification for margins to account for similarity uncertainty when data has limited substantiation.
8.4 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, lightning compliance reports include many system and equipment test reports, aircraft test reports, and lightning vulnerability analysis reports. For these types of lightning certification programs, a compliance summary report may be useful to summarize the results of tests and analysis. For lightning certification programs of relatively simple systems, a single compliance report is adequate.
AC 20-136C
8.4.1 Test Reports.
C omprehensive test reports should be produced at the conclusion of lightning 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 lightning 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.
8.4.2 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. If applicable, these reports should identify any modeling uncertainty and verify that the margins established in the analysis plan were met.
8.4.3 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 lightning compliance. You should identify all significant differences encountered, along with an assessment of the impact of these differences on lightning compliance. If applicable, these reports should identify any similarity uncertainty and verify that the margins established in the similarity plan were met.
8.5 Applicability of the Changed Product Rule.
The provisions of 14 CFR 21.101, commonly referred to as the Changed Product Rule, require a proposed change to a type certificate to comply with the latest part 23, 25, 27, or part 29 airworthiness requirements in effect on the date of the application for change.
Applicable exceptions include changes determined not to be significant, areas not affected by the design change, or when the FAA determines compliance would be impractical or not contribute substantially to the level of safety of the product. For proposed installations of electrical and electronic systems with catastrophic failure conditions, the airplane certification basis must include §§ 23.1306 (amendment 23-61),
23.2515, 25.1316, 27.1316, or § 29.1316.
8.5.1 Significant Change per § 21.101.
8.5.1.1 If the existing certification basis includes §§ 23.1306 (amendment 23-61),
23.2515, 25.1316, 27.1316, or § 29.1316, you must comply with paragraphs (a) and (b) of the applicable part 23, 25, 27, or part 29 lightning regulations (latest amendment).
8.5.1.2 If the airplane certification basis does not include §§ 23.1306 (amendment 23-61), 23.2515, 25.1316, 27.1316, or § 29.1316, you must demonstrate compliance with paragraphs (a) and (b) of the applicable part 23, 25, 27, or part 29 lightning regulation (latest amendment), unless an exception to paragraph (b) of the specific lightning regulation is granted in § 21.101.
AC 20-136C
8.5.2 Not A Significant Change Exception per § 21.101.
8.5.2.1 If the existing certification basis includes the §§ 23.1306 (amendment 23-
61), 23.2515, 25.1316, 27.1316, or § 29.1316, you must comply with the applicable part 23, 25, 27, or part 29 lightning regulations.
8.5.2.2 For proposed changes to part 25 airplanes originally certified under amendment 25-80 or equivalent lightning special condition, you may propose lightning compliance at amendment 25-80 or, if applicable, part 25 lightning special condition.
8.5.2.3 If the airplane certification basis does not include §§ 23.1306 (amendment 23-61) 23.2515, 25.1316, 27.1316, or § 29.1316, you must demonstrate compliance with paragraph (a) of the applicable part 23, 25, 27, or part 29 lightning regulation (amendment as agreed by the FAA) for electrical and electronic systems with catastrophic failure conditions; compliance with paragraph (b) may be demonstrated, but is not required for electrical and electronic systems with hazardous or major failure conditions.
Note: Applicants should consider whether demonstrated compliance with paragraph (b) of the applicable lightning regulation is required for certification by other civil aviation authorities even if not required by the FAA.
9 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.
9.1 Component Damage.
Component damage 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—
• Active electronic devices, especially high frequency transistors, integrated circuits, microwave diodes, and power supply components;
• Passive electrical and electronic components, especially those of very low power or voltage rating;
• Electro-explosive devices, such as squibs and detonators;
• Electromechanical devices, such as indicators, actuators, relays, and motors; and
• Insulating materials (for example, insulating materials in printed circuit boards and connectors) and electrical connections that can burn or melt.
AC 20-136C
9.2 Syst em Functional Upset.
9.2.1 Functional upset is mainly a system problem caused by electrical transients. It may permanently or momentarily upset a signal, circuit, or 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.
9.2.2 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.
10 STEPS TO LCL A SYSTEM LIGHTNING COMPLIANCE.
Steps 1 through 13 listed below in this section may be used to demonstrate LCL A system compliance with §§ 23.1306(a) (amendment 23-61), 23.2515(a), 25.1316(a),
27.1316(a), or § 29.1316(a). Figure 2 also depicts these compliance steps. Airplanes certified under part 23 may use steps 1 through 13 to demonstrate compliance with §§ 23.1306(a) (amendment 23-61) or § 23.2515(a), but acceptable compliance for LCL A systems on level 1, 2, and 3 airplanes may also be shown using the method in paragraph 10.14 below.
10.1 Step 1 – Identify LCL A Systems.
10.1.1 Identify your LCL A systems as described in paragraph 7.2. Define the detailed system performance pass/fail criteria. A certification branch should concur with this criteria before you begin testing or analyzing your LCL A system. Identify specific equipment, components, sensors, power systems, and wiring associated with each LCL A system in order to perform the ETDL verification discussed in paragraphs 10.5 and 10.6.
10.1.2 When demonstrating compliance with §§ 23.1306(a) (amendment 23-61), 23.2515(a),
25.1316(a), 27.1316(a), or § 29.1316(a), the LCL A electrical and electronic system includes all electrical and electronic equipment, components, and electrical interconnections required to perform the intended functions whose failure is catastrophic, i.e. would prevent continued safe flight and landing. This electrical and electronic system must also automatically recover (or manually for § 23.2515) normal operation of the Level A functions in a timely manner to comply with paragraph (a)(2) of these regulations.
10.1.2.1 The LCL A electrical and electronic system is not required to include—
• equipment, components, and electrical interconnections required only for non-normal situations; or
• equipment, components, and electrical interconnections required only for dispatch under a minimum equipment list.