Lightning Introduction

Aircraft_Helicopter_Lightning. Lightning certification of aircraft. This series of articles goes over the many steps involved with the certification of aircraft for lightning events and lightning strikes. Lightning protection of aircraft.

Lightning Certification of Transport Aircraft

A summary of all required tasks

Lightning protection of aircraft is a large, multi-year program for any aircraft certification effort. The tasks are further complicated by the use of composites as the structural material.

The approach presented has emerged as a best practice for lightning certification. By leveraging state-of-the-art simulation and analysis tools, this is the best approach to lightning protection programs that save overall total man-hours and schedule, as documented in the literature.5 Testing is optimized to validate a simulation approach.

The modern approach to lightning protection is to build a simulation model of the aircraft. This model provides a scientific foundation for the certification plan development and its execution. Benefits of this model include:

  • It is directly traceable to official CAD drawings.
  • It can be configuration controlled and become part of the certification documentation.
  • It can rapidly provide design trade-off analysis.
  • It can provide structural currents and voltages to guide direct effects testing.
  • It can provide ATL, EDTL, etc. for vendor flow down requirements
  • It can be modified to provide certification support for aircraft upgrades and derivatives, payload integration, and other future activities.

There is historical4 and recent success in certification and cost savings in all aspects of lightning certification programs for major aircraft systems. This work has been widely publicized and is visible in technical publications.1,2,3


EMA Lightning Resources- Table of Contents

1: Indirect Effects of Lightning (IEL)

1.1 Specify Initial Transient Control Levels (TCLs)

1.2 Design Guidance for IEL

1.3 Design Circuit Protection

1.4 Evaluate the Performance of Structures, Interfaces and Cables

1.5 Determine Actual Transient Levels (ATLs) for the Aircraft

1.6 Validate Analysis

1.7 Conformed Equipment and Subsystem Testing

1.8 Corrective Measures

1.9 Team Experience in IEL

1.10 Benefits of Technical Approach

2: Direct Effects of Lightning (DEL)

2.1 Zoning Assignment

2.2 Set Current Distribution in Structures

2.3 Evaluate the Performance of Structures to Lightning Arc Attachment and Conducted Current

2.4 Design Guidance for DEL

2.5 Component and Representative Coupon Testing at High Current/High Voltage

2.6 Corrective Measures

2.7 Team Experience in DEL

3: Lightning Fuel System 25.981

3.1 Introduction

3.2 Regulatory and Program Planning/Reporting

3.3 Characterization of the EM Performance of Individual Structures and Systems

3.4 Establishment of Failure Modes

3.5 Determination of the Electromagnetic Environment during All Scenarios

3.6 Coupon Testing

3.7 Experience in Fuel Tank 25.981


[1] B. Walgren, M. Backstrom, R. Perala, P. McKenna, “The use of finite difference electromagnetic analysis in the design and verification of modern aircraft”, 1989 International Conference on Lightning and Static Electricity (University of Bath, UK)

[2] D. Lalonde, J. Kitaygorsky, W. B. S. Tse, J. Kohler and C. Weber, “COMPUTATIONAL ELECTROMAGNETIC MODELING AND EXPERIMENTAL VALIDATION OF FUEL TANK LIGHTNING CURRENTS FOR A TRANSPORT CATEGORY AIRCRAFT,” 2015 International Conference on Lightning and Static Electricity (Toulouse, France), 2015.

[3] C. Weber, D. Lalonde, W. Tse, S. Brault, J. Ahmad and J. Kitaygorsky, “LIGHTNING RESPONSE OF A COMPOSITE WING TEST BOX: A VALIDATION OF SIMULATION RESULTS,” 2015 International Conference on Lightning and Static Electricity (Toulouse, France), 2015.

[4] K. SATAKE, S. YAMAMOTO, H. YAMAKOSHI, T. AOI, A. IYOMASA and K. MURAKAMI, “Development of Electromagnetic Simulation Supporting Lightning Protection Design of Mitsubishi Regional Jet,” Mitsubishi Heavy Industries Technical Review, vol. 49, no. 4, pp. 79-84, 2012.

[5] T. Rudolfph, B. D. Sherman, T. He, and B. Nozari, “MD-90 Transport Aircraft Lightning Induced Transient Level Evaluation by Time Domain Three Dimensional Finite Difference Modeling”, 1995 International Aerospace and Ground Conference on Lightning and Static Electricity, Williamsburg, VA, USA

Indirect Effects

Indirect Effects of Lightning (IEL)


Table of contents:

1. Indirect effects of Lightning
1.1 Specify Initial Transient Control Levels (TCLs)
1.2 Design Guidance for IEL
1.3 Design Circuit Protection
1.4 Evaluate the Performance of Structures, Interfaces and Cables
1.5 Determine Actual Transient Levels (ATLs) for the Aircraft
1.6 Validate Analysis
1.7 Conformed Equipment and Subsystem Testing
1.8 Corrective Measures
1.9 Team Experience in IEL
1.10 Benefits of Technical Approach
2. Direct Effects of Lightning
3. Lightning Fuel Systems


1.1: Specify Initial Transient Control Levels (TCLs)

It is important to establish the proper lightning requirements. It is not advantageous to over-design lightning protection, as the cost in mass and system capability is too great. Alternatively, under-designing lightning protection leaves systems, missions, and lives at stake. A careful, data-based TCL assessment is required in order to achieve the proper balance.

EMA can prepare a basic simulation of the aircraft using CAD drawings and verbal discussions about the type and location of various systems. This will improve greatly upon the fidelity of the levels versus using the generic levels from the standards documentation. We can incorporate all the structural seams and junctions as well as they are known. In addition, EMA can create nominal cable routes to estimate the lightning transients.

EMA can use simulation to derive the estimated levels of the electronics at the system and sub-system level.

EMA’s team can validate an acceptable margin approach. The margin is added to the simulated transient and it is categorized according to DO-160G. The TCL plus margin is the Estimated Test and Design Level (ETDL). The resulting table of ETDLs is the required first step in evaluating electronics and designing a mitigation approach. It is the basis of verification once box testing is complete.
Relationship of actual transient levels, transient control levels, and equipment transient design levels
(from AC 20-136A)

Relationship of actual transient levels, transient control levels, and equipment transient design levels (from AC 20-136A)

Relationship of actual transient levels, transient control levels, and equipment transient design levels (from AC 20-136A)

 

These levels will be used in determining whether each electronic device will survive, by comparing them to existing tests of equipment, or estimating the likely susceptibility of each interface. The susceptibility of each item will be determined based on the following criteria.

  1. If the electronic device has not been developed, the EDTL will be used as the design requirement and passed to the design team. If the requirement is deemed too high for the design team, then they will communicate their desired susceptibility test level.
  2. If the device is vendor-supplied but has not been developed, the EDTL will be passed on as a requirement. If the requirement is deemed too high for the design team, then they will communicate their desired susceptibility test level.
  3. If the device has already been developed and used a previous project and exisitng lightning test data is available, that test level will be converted to the equivalent DO-160 G test level as the device susceptibility.
  4. If the device has already been developed but has not been tested, then the circuit components will be analyzed to estimate the susceptibility. The device should be tested according to DO-160 G section 22 procedures to verify or adjust this assumed level.

Next, EMA can compare the determined susceptibility to the EDTL table, in light of the hazard assessment for the criticality of each system. For any devices in which the EDTL exceeds the determined susceptibility, and if the impact is hazardous or catastrophic, mitigations should be performed. If the hazard is severe or lower, EMA will consult with the mitigation to determine the trade-offs in performance, schedule, weight and cost.

The mitigations can include aircraft-level mitigations, cable mitigations, or box-level mitigations. The consulting team and the electromagnetic effects team should cooperate to determine the mitigation that is most appropriate in each case.

 

1.2: Design Guidance for IEL

EMA can provide specific recommendations for design changes that will improve the lightning indirect effects protection of the vehicle. These recommendations may be supported by additional simulations or trade-studies that show the resulting pin levels when some element of the design is altered. We will coordinate with other groups to balance the trade-offs and impact to other areas from any proposed design changes.

Indirect Effects of Lightning refers to the effects of an aircraft lightning attachment on electronic systems inside the aircraft that are exposed to coupled lightning currents. EMA can use simulation to predict the lightning transient levels seen at avionics interfaces. This allows for hardening to be added at the aircraft level (more overbraid, for example) or at the box level (terminal protection devices inside of electronics systems). The approach to Indirect Effects hardening is described below.

1.2.1: Aircraft-level mitigations

These mitigations include reducing the bond impedance between major structural elements, addition of new conductors to carry the lightning current, addition of or increase in the gauge of ECF or other surface metallization, and addition of bond-straps. In any case that an aircraft-level mitigation is pursued, the full-aircraft simulation should be re-performed to take the mitigation into account.

1.2.2: Cable Mitigation

A useful way to improve the Indirect Effects of Lightning ETDL transients is to add cable shielding, increase the gauge of shields, or provide parallel bond-straps to the cable shields. In any case that a cable mitigation is pursued, the cable harness simulation should be re-performed to take the mitigation into account.

1.2.3: Box-level Mitigation

This includes terminal or circuit protection. Verification of the mitigation and the increased box hardness will be via DO-160G section 22 testing.

1.2.4: Lightning Indirect Effects Control Items

The lightning EDTLs are a control item. If any aspect of the aircraft or system will significantly alter the EDTLs, they should be re-calculated. The hardness of the electronics systems is a control item. If any part of the electronics system is substituted, it should be re-tested or evaluated for its lightning susceptibility. Any aircraft level, sub-system level, system-level, or box-level item that impacts the full aircraft Indirect Effects hardness should be carefully tabled in order to trace verification for initial effectiveness and to maintain the level over the craft lifecycle.

 

1.3: Design Circuit Protection

For new development items or even for off-the-shelf items, circuit protection devices are an effective means of providing lightning mitigation. EMA can convert the TCL levels to a short circuit threat into a candidate transient voltage suppression diode. The total power dissipated by the diode compared to the de-rated diode power determines if the selected part is acceptable.

 

1.4: Evaluate the Performance of Structures, Interfaces and Cables

Accurate results from a computational electromagnetic simulation can only be expected if the relevant electromagnetic properties of the modeled object are accurately known and have been properly incorporated in the numerical model. Our experience is that critical values are often not available in the form needed for reliable EM modeling unless a separate set of focused measurements is made.

EMA can prepare test plans in advance and provide them. We can perform a set of measurements that will provide the required input values needed for simulation purposes.  EMA’s team would need a set of basic material samples fabricated by the structures group. The samples measured may include:

  • Composites with and without ECF
  • Joints and interfaces between two composite panels of a representative seam
  • Fastener configurations

The input parameter measurements will be made using a variety of current injection methods and measurement of resulting induced voltages and currents.

  • DC current injection
  • Low level swept CW current injection
  • Lightning-like pulse current injection

Some of the conductivity measurements will require samples cut to specialized geometries depending on the anisotropic properties of the CFRP.

(Left panel) Schematic describing the testing to characterize the composite panels with fasteners. (Right panel) photograph of an actual test setup at our team’s labs

 

1.5: Determine Actual Transient Levels (ATLs) for the Aircraft

EMA can prepare a full simulation of the aircraft using our proprietary simulation tools. Our team has pioneered this approach for over 25 years and is the leading provider of these services. The model can be used for multiple purposes:

  • To determine the ATLs during design phases
  • As a substitute to full aircraft testing for indirect effects
  • As a method of verification for fuel ignition
  • It can be reused in related efforts, such as HIRF and EMI/EMC

The end product of this effort is to create a simulation model (validated in the following subtasks) that will include the proper attachment and detachment locations and hundreds of voltage/current probes necessary to set all coupon test levels needed in the following tasks. Simulation multiplies the number of current probes and gives results early in the program to begin testing. The modeling sets the appropriate and not overly conservative pass/fail criteria for the sparking threshold testing.

Further, the critical cables will be modeled down to the pin level to determine the lightning indirect effects levels.

 

1.6: Validate Analysis

One example of a correlation assessment of test to simulation for currents of the CSeries aircraft.

One example of a correlation assessment of test to simulation for currents of the CSeries aircraft.

Validation is required to use analysis as the basis of certification.

Model of a full-aircraft with a return conductor system for the simulation. The cockpit and empennage have been omitted as they are unnecessary in this wing simulation.

Model of a full-aircraft with a return conductor system for the simulation. The cockpit and empennage have been omitted as they are unnecessary in this wing simulation.

An example of a successful correlation assessment for the Bombardier CSeries is shown in the figure below. On the left axis is the normalized test current. The bottom axis is the simulation by EMA3D. The red line shows perfect agreement, showing most data points are within 3 dB. These are actual blind predictions done before the test.

A picture of the model of a recent test of the CSeries aircraft is shown in the figure below.

Model of a full-aircraft with a return conductor system for the simulation. The cockpit and empennage have been omitted as they are unnecessary in this wing simulation.

 

1.7: Conformed Equipment and Subsystem Testing

The EDTLs described above should be used as the basis for equipment testing for all critical electronic systems based. The testing is based on DO-160G Section 22. EMA can support this testing as well as provide guidance on deviations and acceptable test methods.

 

1.8: Corrective Measures

Corrective measures can be at the system-level or equipment-level. System-level mitigations may include:

  • Additional harnessing
  • Reduction of cable loop areas
  • Grounding and bonding changes
  • Application of metallization to composite items
  • Changes in the bonding of structural items

Equipment-level mitigations include:

  • Additions of transient voltage suppression diodes
  • Changes in equipment front-end impedance
  • Changes in the front end circuit elements to withstand larger voltages/currents

 

1.9: Team Experience in IEL

Past Performance Success Demonstration: EMA has performed indirect effects design and certification effort directly for many programs. These have resulting in impressive correlation between simulation and testing. Past customers include:

  • Bombardier C-Series
  • Mitsubishi Regional Jet
  • Orbital Sciences Missile Defense Agency GMD Vehicle
  • Lockheed Martin NASA Orion MPCV
EMA provided material property measurements to the MRJ team

EMA provided material property measurements to the MRJ team

1.10: Benefits of Technical Approach

1.10.1: Case Study 1: McDonald Douglas MD-90
EMA helped Douglas Aerospace save $1.6 M on the MD-90 indirect effects certification.

EMA helped Douglas Aerospace save $1.6 M on the MD-90 indirect effects certification.

EMA helped Douglas Aerospace save $1.6 M on the MD-90 indirect effects certification.

The first known application of CEM for civilian certification of a complex transport aircraft occurred on the MD-90.[1] In this project, simulation was used to determine the induced lightning transients at avionics box cable interfaces (indirect effects transient control levels). The simulation approach was validated using existing experimental data obtained for certification of the MD-80 program. This validation was accepted by the FAA so that the need for full-scale testing was eliminated.

The group noted that the simulation approach, including the verification experiments and analysis costs, were much lower than the cost to build the same number of physical models of reasonable size and perform the direct effects testing. The savings were estimated to be at least $1.5 M ($2.2 M adjusted for inflation).

Since that time, the FDTD approach has become common for indirect effects qualification programs. EMA’s codes have been used on a number of these programs and have achieved a high level of acceptance in the aerospace certification community.

1.10.2: Business Justification

Below are general advantages that indicate qualitatively that integrators can reduce costs and program risk during the design phase of their lightning and HIRF certification programs by utilizing a CEM simulation-aided approach. Some of the major factors include:

  • Early specification of accurate interface control levels for downstream vendors – One of the most critical tasks for indirect effects programs it to develop the requirements for line-replaceable units (LRUs). If the requirements are overly conservative, this results in unnecessary program costs if vendors do not have an off-the-shelf LRU capable of meeting the conservative requirement. If the requirements are too low, then a late-stage redesign during the certification phase could be necessary, delaying certification and driving up program costs. CEM simulations can generate LRU interface control levels for lightning and HIRF with proven historical accuracy and EMC community acceptance.
  • Reduce the number of development tests – The use of CEM simulation tools reduces the need for testing to determine the interface transients and interference levels. Further, trade studies can be performed via simulation that would be prohibitively costly otherwise.
  • Provide data for FAA designated engineering representatives (DERs) – The DERs will need data to support their case to the FAA. By preparing a certification simulation with the proper fidelity and with material property measurement inputs, the DERs will have a stronger case. CEM simulation is routinely used by IEL DERs in recent
  • Provide evidence of IEL and HIRF considerations for FAA Acceptance – The FAA will want to know that IEL and HIRF considerations have been part of the design process from the beginning. By providing CEM simulation results from the both the design and the certification phases, the FAA can see that initial concerns have been identified and
  • Exploit synergies in the various EM environments – Once a full aircraft model has been developed based on CAD drawings and discussions with the design team, it can be reused with minor modification/cost for analyzing:
    • Lightning indirect effects
    • HIRF
    • Lightning direct effects fuel system ignition prevention
    • Antenna design (especially for antennas embedded in CFRP structures)
    • P-static wick sizing and placement
  • Exploit synergies among program phases – Once a full aircraft model has been developed during the design phase, it can be reused with some modification in the certification
  • Reduce program risk – By using simulation techniques to determine the lightning/HIRF interface transients/levels early in the design phase, the risk of having a late-certification phase issue with a proposed LRU device is
  • Shorten schedule – There is often a chicken-and-egg problem with completing a design to mitigate lightning/HIRF issues and having a testable prototype. By providing

trade-study feedback to designers early in the design phase, this issue is mitigated. Further, the design team for structures and the LRU design/specification teams can now work in parallel since estimated control levels can be generated earlier in the program.

[1] T. Rudolfph, B. D. Sherman, T. He, and B. Nozari, “MD-90 Transport Aircraft Lightning Induced Transient Level Evaluation by Time Domain Three Dimensional Finite Difference Modeling”, 1995 International Aerospace and Ground Conference on Lightning and Static Electricity, Williamsburg, VA, USA

Direct Effects

Direct Effects of Lightning(DEL)


Table of contents:

1. Indirect effects of Lightning
2. Direct Effects of Lightning

2.1 Zoning Assignment
2.2 Set Current Distribution in Structures2.3 Evaluate the Performance of Structures to Lightning Arc Attachment and Conducted Current
2.4 Design Guidance for DEL
2.5 Component and Representative Coupon Testing at High Current/High Voltage
2.6 Corrective Measures
2.7 Team Experience in DEL

3. Lightning Fuel Systems


2.1: Zoning Assignment

EMA can determine the likely lightning attachment locations for the aircraft during all applicable mission configurations and control surface positions. The primary attachment locations are mainly based on the outer mold line (OML) and the material properties of exterior items.

The initial attachment locations will be determined by electric field modeling approaches, as prescribed in SAE ARP 5414A. EMA can initially use engineering judgment to determine the attachments. This determination will be verified and modified by E-field modeling using low frequency methods in a quasi-static condition considering all OML surfaces. The initial attachment work should consider the proper mission configurations, including the most extreme control surface extensions.

Next, the initial attachments are used as the basis for the zone assignment for all OML areas of the aircraft. WMA will use SAE ARP 5414A in order to accomplish this task. Various mission parameters, such as the velocity profile at various altitudes are needed for this task.

An example zoning is shown in the figure below. We can use an acceptable method to determine the initial attachment locations in the present effort.

An example of Zoning of a wingtip. A full 3D CAD zoning can be delivered. Direct effects of Lightning on aircraft. Attachment and current distribution simulation

An example of Zoning of a wingtip. A full 3D CAD zoning can be delivered.

 

2.2: Set Current Distribution in Structures

EMA’s simulation can be used to estimate the lightning current distribution in all critical aircraft structures. In certain assemblies and components, further direct effects effort can be eliminated by showing the current level is below the threshold to risk damage. For other items, the current may be used as a basis for the testing at high current labs.

 

2.3: Evaluate the Performance of Structures to Lightning Arc Attachment and Conducted Current

It will be important to understand what will be required to protect structures to the direct effects of lightning. EMA has significant experience in determining methods to protect composite materials from the damaging effects of lightning. EMA’s team has experience setting the mesh thickness as well as the appropriate application of the composite interface protection schemes. EMA’s team knows the conditions under which novel material solutions, such as micron-sized silver dispersed in co-polymer can be used to protect composites, and these in which a more robust bonding surface must be provided.

Further, our team’s multi-physics simulation capabilities allow for prediction of the damage level from lightning to composites to further optimize the design to meet the performance and mass requirements.

 

2.4: Design Guidance for DEL

EMA can apply the Zoning and Analysis levels to each structure, sub-system, and system and consider their susceptibility to the prescribed levels. In areas in which there is a concern, we can consider the criticality based on the hazard assessment. In cases in which the criticality is catastrophic or hazardous, mitigation should be applied. In cases in which the criticality is severe or lower, mitigations will be considered in light of mass and performance requirements, using analysis methods to assess the benefit and cost of each mitigation.

EMA can consider structures in the vehicle that may carry lightning current, assess each structure’s possible failure modes and determine if that failure might result in a catastrophic, hazardous, or severe consequence to continued safe flight, per the hazard assignment. The main areas of Direct Effects concern that will be analyzed in this manner are the integrity of carbon fiber reinforced polymer composite (CFRP) surfaces, sparking across structural junctions and joints, control surfaces and actuators, fuel ignition, and ordnance ignition.

EMA can design mitigations to the Direct Effects of Lightning based on the Lightning Zoning requirements. The Lightning Zone requirements along with the Hazard Analysis of the aircraft are the inputs to this process, following the steps in SAE ARP 5f77. The effort has an associated test and analysis program. Corrective measures will be specified in the event of failures from test and analysis.

2.4.1: Integrity of Carbon Fiber Reinforced Polymer Composites

Attachments to carbon fiber areas will be given special attention. Carbon Fiber Reinforced Polymer Composites are characterized by a much smaller conductivity (104 S/m) than that of Aluminum (3.5 x 107 S/m).

The Zoning and analysis maps the required current levels to every composite structure. Several factors can affect the susceptibility of the CFRP material:

  • Thickness and type of resin used for each layer
  • Number of and type of reinforcing carbonaceous fibers (such as weave, unidirectional)
  • Surface roughness
  • Nature of and connection to fasteners
  • Addition of metallic foils and expanded foils
  • Connectivity to neighboring and underlying structures

 

EMA’s team can use engineering judgment, analysis, and write sample coupon test plans to determine the anticipated effect on the CFRP material in each required location and current level. Next, the estimated damage will be assessed for the hazard imposed on the system. For example, if CFRP layers delaminate to expose the underlying items to the outside aerodynamic environment, what affect will this have on the continued safe flight?

Once this analysis is complete, EMA can develop mitigations for each component that presents an unacceptable hazard in the lightning environment. The possible mitigations include:

  • Addition of Expanded Copper Foil (ECF) co-cured as the outermost layer of the composite material
  • Adjustment of the thickness of the composite part
  • Adjustment or metallization of dielectric coatings or paints
2.4.2: Structural Junctions and Joints

The simulation analysis will determine if any critical seams or junctions are susceptible to sparking, with for example standard limits of 500 V on exterior surfaces. The CFRP-fastener interface in many cases will be a critical interface to assess.

Analysis that considers an evaluation of the joint seams will reveal the voltage in all cases. These will be used to set the junction impedance levels for all major interfaces. Follow-on analysis and coupon testing of representative joints will be used to verify the mitigation of these concerns.

2.4.3: Control Surfaces and Actuators

Actuators and control surfaces will be carefully considered so that excessive current does not damage moving components and joints. The analysis and Zoning levels will be used with the material properties to determine the potential for heating. If it exceeds the estimated susceptibility of the parts, mitigation will be designed. The mitigation will include additional bond paths or changes in the gauge or thickness of parts. In some cases, testing may be used to determine the susceptibility of parts.

1.4.4: Determine Physical Damage

First, EMA can estimate the CFRP and other material effects from the lightning levels. For example, we can estimate the number of CFRP layers that will be delaminated in the worst case configuration and attachment scenario. Additionally, it will determine how much coating material may be stripped away by the attachment. These estimates will be based on experienced engineering judgment for similar levels in similar material. EMA can write example test plans for any coupon testing may be required for exotic or critical material/level combinations.

 

2.5: Component and Representative Coupon Testing at High Current/High Voltage

All critical structures that are deemed to have appreciable lightning current from the analysis as well as potential for arc attachment will need testing per the provisions of SAE ARP 5416.

The high voltage testing determines risk of attachment and punctures of transparencies. The high current testing determines the risk of physical damage to the aircraft that would affect continued safe flight.

EMA has significant experience in performing such testing, writing test plans, and witnessing testing.

Method to determine DEL test article. EMA has experience in testing for the direct effects of lightning on aircraft

Method to determine DEL test article

 

2.6: Corrective Measures

EMA has significant experience in providing design changes to meet IEL requirements in the event of test failure. Our multi-physics tools that co-simulate the electrical and thermal equations allow for us to reproduce the failure and design the appropriate action to prevent the problem.

Our team has developed protection schemes for bonding and panel protection that allow us to prevent damage and harmful effects from lightning.

 

2.7: Team Experience in DEL

Past Performance Success Demonstration: EMA has provided design guidance directly for many programs. Past customers include:

  • Bell Helicopter Relentless 525
  • Orbital Sciences Missile Defense Agency GMD Vehicle
  • Lockheed Martin AC-130
  • Stratolaunch Roc
  • XAC MA 700
  • Sierra Nevada Dream Chaser
  • Mitsubishi Regional Jet
  • Bombardier CSeries
  • Lockheed Martin AC-130
  • NASA, Lockheed Martin Orion MPCV
  • Sierra Nevada Dream Chaser
2.7-lightning-simulation-full-aircraft-model: EMA provided a lightning effects support for the Bombardier CSeries. Simulating the direct effects of lightning to aircraft

EMA provided a lightning effects support for the Bombardier CSeries

Fuel Systems 25.981

Lightning Fuel Systems 25.981


Table of contents:

1. Indirect effects of Lightning
2. Direct Effects of Lightning
3. Lightning Fuel Systems

3.1 Introduction
3.2 Regulatory and Program Planning/Reporting
3.3 Characterization of the EM Performance of Individual Structures and Systems
3.4 Establishment of Failure Modes
3.5 Determination of the Electromagnetic Environment during All Scenarios
3.6 Coupon Testing
3.7 Experience in Fuel Tank 25.981


Full 25.981 compliance for electromagnetic effects compliance is a large, multi-year program for any aircraft certification effort. The tasks are further complicated by the use of composites as the fuel tank structural material. In the recent past, there was a great deal of change and regulatory clarification into what was actually required of airworthiness applications. Fortunately, the regulatory environment has solidified into a set of standard processes.

Our team is a pioneer in composite fuel tank lightning verification. Further, our team has participated with remarkable success in the many visible and recent 25.981 fuel tank lightning certification programs.

The large effort of a 25.981 compliance program for lightning and electrostatic effects may be further divided into five major tasks:

  • Regulatory and Program Planning/Reporting
  • Characterization of the EM Performance of Individual Structures and Systems
  • Establishment of Failure Modes
  • Determination of the Electromagnetic Environment during All Scenarios
  • Coupon Testing

The main electromagnetic environments of 25.981 are lightning, electrostatic discharge and power fault. EMA has historical and recent success in supporting all aspects of these programs for major transport aircraft systems. This work has been widely publicized and is visible in technical publications.

1-25.981-lightning-protection-fuel-sustems-simulation-program-flow-chart. Flow chart breaking down the steps for a propper 25.981 compliance program for fuel systems lightning certification.

 

 

1-25.981-lightning-protection-o-aircraft-fuel-sustems-phases-of-work. Table describing the phases of work included in a fuel tank lightning certification program for 25.981 compliance

3.1: Introduction

The effort required for 25.981 compliance is best introduced by a quotes from electromagnetic effects engineers leading a project in a recent major 25.981 certification efforts.

Aircraft compliance with 14 CFR 25.981(a)(3) requires consideration of failure modes for both systems and structure design such that lightning direct effects testing is conducted on representative sub-assemblies that contain the failures. Definition of the threat levels can be supported by simulation and analysis, through a well-defined process of verification and validation that follows an increasing level of complexity. . . Generally, simulation tools provide more extensive means to rationalize threat level predictions than by using development testing alone, but also more extensive means to evaluate specific design features.

-David Lalonde, Bombardier Aerospace (CS100)[1]

 

3.2: Regulatory and Program Planning/Reporting

Because of the size and complexity of the 25.981, careful planning is a necessity. Further, ensuring that the regulations, which are not always straightforward, are fully understood across the organization will require education and collaboration.

Our team’s effort on this task is divided into several steps described in the following subtasks.

3.2.1:Program Regulatory Planning

Enunciation and description of compliance program steps for permanent structures

The 25.981 program regulatory approach is complex with many high level decisions that must be made early in the program. Previously, many manufacturers had to decide whether to pursue an exemption or special condition. Recently ANM-112-08-002[2] formalized this process. The requirements and program approach implied by the current regulatory environment will be specified and described.

Next, a choice must be made concerning organizing the program based on the failure modes or identification of two independent layers of protection against each potential ignition threat. This best choice in this area depends on the presence of novel design features and the service and manufacturing history that allows for collection of aging and manufacturing escape statistics and descriptions. EMA can assist in deciding the approach and will describe the program effort implied by the choice.

3.2.2: Bonding Paradigm Definition

This determination leads directly to a selection of the protection methodology for tank systems. Typically, one selects between bonding, in which low impedance connections are established for all systems, and isolation, in which specially-designed isolation is engineered into the system to prevent harmful currents from flowing on systems. EMA can help select the protection methodology for systems and provide guidance on how to implement its prescriptions on the design.

3.2.3: Planning for Special Items

Overview of a path to compliance for portions of aircraft wing design that are typically deemed impractical to protect via two layers of protection

For items that are typically impractical to protect with the same approach for structures and systems such as those in the preceding sections, the regulations offer an opportunity to demonstrate that ignition probability is extremely remote, as compared to 1E-9 failures per flight hour. The steps to demonstrate this regulatory approach are extensive and are typically only applied to a small number of items. EMA can give the methodology and steps in order to achieve regulatory acceptance as well as assist in identifying components that are potentially impractical to protect otherwise.

3.2.4:Program Presentation and MOC

Provide statement of method of compliance and steps to perform for each effort needed to achieve 25.981 certification for each component classification and outline a format for management and presentation of the program effort

The preceding sections described individual approaches to compliance that are applied to separate components of the aircraft. In this task, EMA can assist in classifying all system components according to which of the above approaches to compliance is applicable. Next, we can write certification plans and method of compliance matrices for each grouping that describes at a high level the specific tasks that will need to be undertaken. Finally, EMA can generate a reporting matrix that may be used for certification planning, a dashboard of progress and presentation to the airworthiness authorities.

3.2.5: Project Management

Describe a project timeline with the effort required in each step and the prerequisites and outputs required at each step

EMA can create a Gantt chart with links to the effort needed in each program step in a mutually-agreed upon project management software. Each step will be linked with prerequisites and deliverables that will allow for changes in the timeline or other changes to be modeled.

3.2.6: Airworthiness Authority Coordination

Support for meetings with airworthiness authorities

EMA experience has shown that early and transparent communication with the airworthiness authorities is essential to prevent misunderstandings and delays later in the program. EMA’s team can support preparation and planning for such meetings as well as attend the discussions.

 

3.3: Characterization of the EM Performance of Individual Structures and Systems

3.3.1: System survey

The first step in this task is to identify the fuel system design features and associated lightning protection features. EMA can:

  • Detail each structural category or fastener type exposed to lightning direct attachment
  • Detail each category of structural joint or fastener type exposed to conducted lightning currents
  • Detail each system installation exposed to lightning direct attachment or conducted current
  • Detail each structural category, joint/fastener and system installation exposed to conducted fault currents
  • Detail each structural category, joint and system installation exposed to static discharge
  • Classify each according to lightning, power fault or static discharge performance

 The design features and lightning protection features will be grouped, bounded and categorized as much as possible to reduce the matrix size, which will limit cost during later program phases.

3.3.2: Lightning Ignition source identification

The next step is critical and time consuming. EMA can guide and support a research program to identify all ignition sources present in a fuel tank. The benefit is that it allows one to tailor the protection mechanism to the specific ignition cause and installation location. This program is typically a combination of a simulation and a testing program.

The test or simulation cases consist of a representation of the design features described above. The goal of the tests and simulations is to determine the ignition mechanisms present in each feature. Next, the program attempts to determine the current and voltage threshold that will initiate one of the possible ignition mechanisms. This understanding is aided by the impedance parameter determination in next sub-task. The failure modes should also be considered in this step as well.

The testing should be conducted in a calibrated, ignitable atmosphere with the appropriate photography equipment to capture any ignition events. The typical ignition events are listed below:

  • Voltage Spark
  • Thermal Arc
  • Edge Glow
  • Outgassing

EMA can assist in classifying the conditions that lead to ignition, the type of ignition event, and the current/voltage threshold for its onset for the identified features and failure modes defined in the previous task.

 

 Illustration of the typical fuel system ignition event types [5]. Important step in the fuel tank lightning certification 25.981

Illustration of the typical ignition event types [5]

3.3.2-2-image-of-lightning-fuel-tank-ignition-Illustration of photographic evidence of an ignition even in a calibrated, ignitable atmosphere. Lightning fuel tank ignition example

Illustration of photographic evidence of an ignition even in a calibrated, ignitable atmosphere

 

3.3.3: Electrostatic Ignition Source Identification

The physical mechanism for fuel ignition is very different for electrostatic effects and lightning. For lightning, the possible ignition sources are arcing from high current on structures and sparking from excessive voltage/edge glow from composites. For electrostatic discharge, the problem occurs during refuel or from fuel sloshing inside the tank. Triboelectrification results in an unbalance of charge or charge accumulation on different parts of the tank. A spark could occur that would ignite fuel vapors.

EMA can assist in identifying materials and components inside the tank/fuel system that could lead to charge accumulation. EMA can measure the properties of surfaces and estimate the threat of charging. We can assess the conductivity of materials to determine the ability for charge to bleed off.

3.3.4: Power Fault Source Identification

Another required concern for fuel ignition is a power system fault current. This can lead to a thermal arc in the same manner as lightning current. EMA can help identify the return path for currents in or near the fuel system and establish the components and system installations affected by power fault effects.

3.3.5: Safety Assessment

Perform safety assessment to determine fault tolerance

The next task is to determine if the design is capable of providing fault tolerant fuel systems electromagnetic effects protection. This requires going systematically through all design feature groupings from a previous task.

“Fault tolerant means the design can withstand a lightning encounter free of ignition sources for each failure condition”[3]

Designs can be fault tolerant without incorporating multiple protection features, such as:

  • Fuel tank structural skins whose material and thickness are sufficient to ensure ignition sources cannot be present on the tank side opposite a lightning attachment.
  • A field fastener or joint where the current density from a likely lightning attachment location is so low that an ignition source will not result even when a failure condition is present.

For locations that do not meet these criteria, a case must be developed that relies on evaluation of each feature grouping with initially assumed faults in place.

This task relies heavily on the ignition source identification task and will also require additional testing and simulation to validate the design. Some of this design analysis effort can also be used for the final validation simulation.

3.3.6: Input Parameters

Input Parameter Measurement and Validation

This task involves measurement and validation of the material properties and impedance of structural materials, joints and interfaces. There are two motivations for this effort:

  1. Ignition source identification – The effort in the preceding two tasks is made more facile if the impedances and current flow distribution is known across the range of current densities and attachment locations that are required.
  2. Support determination of the internal electromagnetic environment – Following tasks require a validated simulation of the full system. This cannot be accomplished accurately without a program to measure the material properties of the items and characterize the impedance at the interface of materials. This is often accompanied by a validation task using simulation and testing of representative aircraft interface assemblies.

The material properties of carbon fiber are not straightforward to obtain. The conductivity tensor of composite materials is anisotropic. EMA has developed techniques to accurately measure these values. We require unidirectional panels in which the carbon fiber layers are all aligned in the same direction. By injecting current uniformly into all layers in the same direction, we can determine the conductivity tensor of each layer. For quasi-isotropic or more complex arrangements, EMA can combine the individual layer properties in the correct way to obtain the overall conductivity tensor for all possible panel layup configurations. The testing validates the process by measuring quasi-isotropic panels to confirm that the combination of the layers results in the expected value.

Next, EMA measures the interface between fasteners and carbon fiber panels, between skin panels and ribs, between ribs and spars and between spars and skin panels. The inclusion of expanded copper foil (ECF) should be included where appropriate. This testing should occur at high and low current injection amplitudes.

Finally, EMA performs more detailed verification testing of combinations of fasteners and panels. The testing is followed by simulation. The composite conductivity and fastener impedance parameters from the previous steps are inputs to this model. If the current distributed in the panels (via B-dot sensors) and the fasteners (via current probes) has a high correlation between simulation and testing, then parameter measurements are accepted and a validated, self-consistent model exists for presentation to airworthiness authorities.

An example of one such test is described in the figure below on the left panel. Current is injected into fasteners on one side of the panel. Current is returned from fasteners in the center and other side of the panel. The current is monitored at the injection and the return fastener probes. B-dot field probes in the surface are indicative of the surface current density. The experimental apparatus is shown on the right panel of the figure.

The final result is the impedances of all interfaces as necessary for validated simulation in the full aircraft and test box models. An example of a coupon test simulation to extract impedance parameters is shown in the figure below.

3.3.6-3-impedance-simulation-of-compoisite-coupon- Example simulation results validating the measured impedance and material parameters of a rib and composite coupon.

Example simulation results validating the measured impedance and material parameters of a rib and composite coupon

 

3.4: Establishment of Failure Modes

Failure mode identification is a large task that requires coordination from many disciplines. EMA can be involved in establishing the program, but other groups within the organization must be involved.

3.4.1: Failure mode identification

Failure mode identification requires a rigorous process to find all nonconformities that can arise in manufacturing or service. This program will be organized to include experts in:

  • Electromagnetic Environment Effects and Hazards
  • Structural Design
  • Systems Design
  • Materials and Processes
  • Manufacturing
  • Quality
  • Customer Support

These groups should coordinate and meet regularly to systematically assess and list all potential failure modes. There are several data sources that are typically accessed:

  • Manufacturing process data assessment
  • Service history records
  • Developmental test data

All faults of the design should be analyzed. The faults can include:

  • Manufacturing Quality Escapes
  • Operational Deterioration
  • Accidental Damage

An example of common faults analyzed are shown in the figure below.

 Common manufacturing faults considered in 25.981 programs 25.981 compliance. Lightning fuel tank example

Common manufacturing faults considered in 25.981 programs.

EMA can list the common failure modes considered as well as coordinate with disparate groups to consolidate their effort. However, EMA may require significant and coordinated input from the groups listed above.

3.4.2: Identify Special Items

Single Failure modes

In some areas of the design, it is impractical to have to independent protection mechanisms. The standards allow for the airworthiness requirements to be met by showing that the combined probability is extremely remote. There are only a few components to which this approach is typically applied. EMA can assist with part selection for this process as well as the effort in showing compliance by this method. Part of the process is shown in the tree in the figure below.

3.4.2-ema-single-failure-mode-articles-airworthiness-requirements-flow-chart-tree- Part of flow chart tree for meeting airworthiness requirements for single-failure mode articles. Lightning certification and 25.981 airworthiness

Part of flow chart tree for meeting airworthiness requirements for single-failure mode articles [6]

 

3.5: Determination of the Electromagnetic Environment during All Scenarios

This process is a rigorous testing and simulation task that allows for the determination of the electromagnetic environment at all aircraft locations affected by the 25.981 effort. Testing allows the simulation to be validated, establish margin, and illuminate any refinements in the modeling effort required. Simulation reveals the engineering test levels required in the following tasks for a wide range of attachments and probe locations that would be prohibitive or impossible to obtain by testing alone.

3-25.981-lightning-testing-lightning-simulation-organization-pyramid. Pyramid describing the complexity in testing for fuel tank lightning certification and 25.981 lightning compliance

lightning-simulation-25.981-4
3.5.1: In-flight Simulation

The end product of this effort is to create a simulation model (validated in the following subtasks) that will include the proper attachment and detachment locations and hundreds of voltage/current probes necessary to set all coupon test levels needed in the following tasks. Simulation multiplies the number of current probes and gives results early in the program to begin testing. The modeling sets the appropriate and not overly conservative pass/fail criteria for the sparking threshold testing.

3.5.2: Test Box Validation

This task includes high-level lightning testing and simulation of a representative wing test box for validation. The high level will help show how well the simulation can handle any nonlinearities in impedance.

The box is constructed out of three to five ribs, but is square and does not include the curvature of the real wing, as shown in the figure below. Otherwise, the construction, materials and interfaces are representative of the actual wing. Again, the simulation and test are compared to validate the modeling approach and provide the internal environment. In this case, the test level should be at the full certification threat level. This validates that any nonlinearities in impedance characterization have been properly determined and that the modeling approach captures them correctly.

3.5.2-representative-wing-test-box-for-high-level-airctraft-lightning-simulation-verification- Example of a representative wing test box for the high level verification testing. [2]

Example of a representative wing test box for the high level verification testing. [2]

3.5.3: Full Wing Validation

The full EM environment at all locations on and inside the wing tank is needed for 25.981 certification to set the coupon test levels. The currents, structural voltages and wire voltages are measured and simulated. This testing can occur on a full aircraft or a full wing. The wing should be conformed and include all systems and structures. A small number of panels may be removed to facilitate the testing. The results are compared in order to determine the margin and evaluate the successful understanding of the internal EM environment. The currents injected tend to be low or moderate and are not at the full certification level. An example of a successful correlation assessment for the Bombardier CSeries is shown in the figure below.

One example of a correlation assessment of test to simulation for currents of the CSeries aircraft.

One example of a correlation assessment of test to simulation for currents of the CSeries aircraft.

This task requires a CAD-derived model of the wing and the material properties described in the previous task. A picture of the model of a recent test of the CSeries aircraft is shown in the figure below. Note that Bombardier elected to test on a full-aircraft rather than a full wing.

Model of a full-aircraft with a return conductor system for the simulation. The cockpit and empennage have been omitted as they are unnecessary in this wing simulation.

Model of a full-aircraft with a return conductor system for the simulation. The cockpit and empennage have been omitted as they are unnecessary in this wing simulation.

Model of a full-aircraft with a return conductor system for the simulation. The cockpit and empennage have been omitted as they are unnecessary in this wing simulation. [1]

3.5.4: Electrostatic Analysis

Establish Electrostatic Electromagnetic Environment

The electromagnetic environment for static charging is determined by analyzing in 3D across all affected structures in the tank the possible charging sources which are reduced over time by the static dissipation of charge.  The source is determined by measuring the rate of charging from triboelectrification from all sources identified in previous task, and is performed in the coupon testing tasks in the next section of this document. The dissipation is determined the conductivity of materials on all structures while considering the 3D geometry. Analysis can be used to combine the material properties, sources, and a geometric model of the tank to confirm that voltages do not exceed the threshold, based on the regulatory energy limit and the capacitance at each location.

3.5.5: Power Fault Analysis

The same 3D computational electromagnetic (EMA3D) model of the in-flight aircraft is also used to verify the current and current density in all structures as affected by power fault. The maximum fault current of system installations is the main additional input to this task. The results will set the fault current levels for coupon testing.

 

3.6: Coupon Testing

3.6.1: Lightning Coupon Testing

This task includes a great number (many hundreds) of fastener strikes and conducted current testing in ignitable atmosphere of representative aircraft structures and interfaces.

The internal EM environment determined from the full wing and text box validations are used as a source for coupon testing of representative specimens in the aircraft. The specimens should contain items with and without the identified faults as well as any ignition protection features present in the region the coupon represents. The testing occurs in a dark chamber with an ignitable atmosphere to determine if the 200 microjoule threshold for arcing has been exceeded. An example of a couple is shown in the figure below. This testing should be conformed and witnessed by the airworthiness authority or its designee.

Failure modes are incorporated in the test objects, to see if the failure modes can be tolerated free of ignition sources. The presence of two independent and effective features of protection must be demonstrated to meet the fault tolerant requirement. Testing occurs with one feature of protection artificially disabled (to reflect the failure mode), and then with the other protection feature disabled. Ignition source free performance must be demonstrated in both cases to substantiate fault tolerance. If it is not possible to design a test with one feature of protection artificially disabled, then detailed analysis will be needed. Test across the range of bounding or worst case aging effects and manufacturing faults.

EMA can provide the following:

  • Support in building a suitable generator on-site
  • Support in contracting with an existing test lab with an appropriate generator
  • Test design, test plan authorship, test witnessing and test reporting
Coupon and Panel Tests
Engineering Development Tests TC Tests Done on representative structures and systems coupons
Direct Fastener Attachments(arc detection)Samples include:
– T-joints (skin panels to ribs or spars)
– Panels with fuel ports or access panels
Conducted Currents (arc/spark detection) Done on representative structures and systems (simple panel connection with 1 or 2 fasteners and T-joint samples. Used to verify design and protection features.) Direct Effects Panels (ARP5416)
Direct lightning attachment to panel surfaces to demonstrate acceptable damage:
-No strike puncture
-No significant loss of mechanical strength
-No hot spot formation
(These tests may be used for certification)
Direct Fastener Attachments
(arc detection)Samples include:
– T-joints (skin panels to ribs or spars)
– Panels with fuel ports or access panels
Conducted Currents (arc detection)

Done on representative structures and systems (simple panel connection with 1 or 2 fasteners and T-joint samples Used to verify design and protection features.

Parameter measurements
Values for model inputs. Primarily fastener contact resistances and composite conductivities

 

3.6.2: Static Coupon Testing

Static effects testing involves measuring charging from jet fuel flow across a tank inner panel coupon as well as charging of fuel conveyance tubing. Further characterization of coupon resistivity is determined by careful measurements of representative coupons that bound the worst case of each type inside the tank.

3.6.3: Power Fault Coupon Testing

The testing for power fault is similar to the lightning coupon testing. The waveform is instead a nearly constant current. The coupons and levels are those identified in the analysis task in the previous section.

 

3.7: Experience in Fuel Tank 25.981

  • SAAB Gripen
  • Bombardier CSeries
  • Mitsubishi Regional Jet

[1] D. Lalonde, J. Kitaygorsky, W. B. S. Tse, J. Kohler and C. Weber, “COMPUTATIONAL ELECTROMAGNETIC MODELING AND EXPERIMENTAL VALIDATION OF FUEL TANK LIGHTNING CURRENTS FOR A TRANSPORT CATEGORY AIRCRAFT,” 2015 International Conference on Lightning and Static Electricity (Toulouse, France), 2015.

[2] Federal Aviation Administration, “ANM-112-08-002, Policy on Issuance of Special Conditions and Exemptions Related to Lightning Protection of Fuel Tank Structure.,” 2009.

[3] SAE/Eurocae, “Policy Guidance for Lightning Protection of Fuel Tank Structure and Systems,” 2014.

[4] A. Pout, “LIGHTNING PROTECTION OF FUEL TANKS – A350 CERTIFICATION APPROACH,” 2015 International Conference on Lightning and Static Electricity (Toulouse, France).

[5] SAE/Eurocae, “Policy Guidance for Lightning Protection of Fuel Tank Structure and Systems,” 2014.

Protection of Aircraft

 Lightning_Protection_of_aircraft

Lightning Protection of Aircraft

Second Edition

By: Franklin A. Fisher, J. Anderson Plumer, Rodney A. Perala

Published by: Lightning Technologies Inc.

 

This is the canonical book for engineers seeking to understand the effects of lightning on aircraft as well as practical measures for protecting aircraft against lightning effects.

The first edition was originally published in 1979. It was expanded upon in 1990 with inputs from Rod Perala, President of EMA. The second edition, published in 2004, added new materials and techniques.

This book has been the basis of instructional courses in lightning protection. EMA also uses the book as a thank-you gift to our clients from time to time. If you would like to learn about how you can get a copy of the book, please contact us and we will give you instructions to order your own.

Chapter 1: An Introduction to High Voltage Phenomena, deals with the nature of high voltage electrical sparks and arcs and with related processes of electric charge formation, ionization, and spark propagation in the air. All of these are factors that affect the way that lightning leaders attach to an aircraft and the way that the hot return stroke arc affects the surface to which it attaches. The material introduces practices and terms used for many years in the electric power industry, but which are not commonly studied by those dealing with aircraft. These terms and practices have, however, affected the tests and practices used to evaluate the direct effects of lightning on aircraft

Chapter 2: The lightning Environment provides an elementary description of cloud electrification and lightning strike formation, and follows with statistics of cloud-to-earth lightning parameters from which the aircraft lightning design and test standards have been derived. The user of this book is urged to study these two introductory chapters before proceeding with later sections of the book. The treatment of these topics is on an elementary level and is aided by simple illustrations, which should enable those with only a limited background in electricity to proceed to an adequate understanding of important principles.

Chapter 3: Aircraft Lightning Attachment Phenomena and Chapter 4: Lightning effects on Aircraft introduce the reader to the basic mechanisms of naturally occurring and aircraft initiated lightning strikes. The flight conditions when strikes have most frequently occurred, and the types where of affects these strikes may have on the aircraft may be reviewed. Only a very brief mention is made of the potential lightning effects here, since these are the topics of most of the later chapters

Chapter 5: The Certification Process reviews the history of aircraft lightning protection regulations and standards, and introduces the latest versions of these; most notably, the standards that have been published by SAE and EUROCAE since 1999. Since further updates of these criteria are expected (SAE, for example, requires that all of its documents must be reviewed every 5 years) the user of this book should always obtain the most recently published versions of each of the requirements and standards documents referenced in this book.

Chapter 6: Direct Effects Protection and Chapter 7 – Fuel System Protection contain the basic elements of protection designed for the airframe, fuel tanks, and fuel system components.  The methods presented here are basic approaches, and many variations on these, too numerous to describe in this book, have been successfully used. The reader is cautioned that all candidate designs should be tested, especially those that do not have a successful history of prior use. Fuel vapor ignition remains one of the most serious lightning hazards, and should be given careful attention in any design and certification program. It is not possible to verify adequacy of fuel system protection without lightning testing of fuel tanks and systems.

Lightning Protection of Aircraft: Direct Effects - Lightning strike damage to an aircraft spinner spinner

Direct Effects – Lightning strike damage to an aircraft spinner spinner

Lightning Protection of Aircraft: Lightning damage to radome, probably caused by an exploding pitot tube ground wire

Lightning damage to radome, probably caused by an exploding pitot tube ground wire

Chapters 8 through 17 focus on protection of electrical and avionic systems against indirect effects and form the basis for our course, Lightning Protection of Avionics. As with all aspects of electromagnetic interference and control, the prevention of damage and interference from lightning becomes more and more critical as aircraft evolve. Most of the navigation and control functions aboard modern aircraft place a computer between the pilot and the control surfaces or engines, often without mechanical backup. This makes it essential that the computer and control equipment be designed to prevent damage or upsets by lightning. Control of these indirect effects requires coordination between those who design the air-frame and its interconnecting wiring, those who design avionic systems and those who oversee the certification process, Part of the overall control process requires the selection of transient design levels and application of suit-able test standards and practices.

Chapter 8 – Introduction to Indirect Effects introduces the subject of indirect effects and briefly summarizes the subjects covered in more detail in later chapters.

Chapter 9 – Elementary Aspects of Indirect Effects covers the basic physics common to the subsequent chapters.

Chapter 10 – The External Electromagnetic Field Environment, covers the external electromagnetic field environment.

Chapter 11 – The Internal Fields Coupled by Diffusion and Redistribution and Chapter 12 – The Internal Fields Coupled through Apertures describe how electromagnetic fields appear inside the airframe, and ways to estimate the magnitudes of these internal fields. These four chapters are the most analytically oriented of the book.

Chapter 13 – Full Vehicle Testing describes the meth-ods available for measuring the transient voltages and cur-rents induced by lightning in aircraft electrical wiring. These are known as “full vehicle” tests and are usually applied at reduced amplitudes so as not to damage the test-ed airplane.

Chapter 14 – Response of Aircraft Wiring discusses some of the practical problems of calculating the response of aircraft wiring to electromagnetic fields, and provides some examples of how basic principles can be used to esti-mate the magnitudes of induced transients in simple circuits.

Chapter – 15 Shielding reviews the physics of shield-ing effectiveness and discusses this important protection approach of shielding of aircraft wiring. This chapter also emphasizes the features that must be included in shield designs that are necessary to realize maximum effective-ness from shields.

Chapter 16 – Design to Minimize Indirect Effects dis-cusses some of the policy matters relating to control of indirect effects, tasks that must be undertaken by those responsible for setting overall design practices. Principally these relate to shielding and grounding prac-tices to be followed, and to transient design level specifi-cations to be imposed on vendors.

Chapter 17 – Circuit Design discusses some aspects of circuit design, principally those relating to surge protective devices and methods of analyzing the damage effects of surge voltages and components on electronic devices.

Control of lightning indirect effects by analysis can only be carried so far; proof of tolerance of indirect effects is most likely to come about by performing tests on individ-ual items of equipment and on interconnected systems.

Lightning Protection of Aircraft: Typical Arrangement for systems test

Typical Arrangement for systems test

Chapter 18 – Test Techniques for Evaluation of Indirect Effects presents an overview of test methods used to verify the ability of equipment to tolerate lightning-induced transients and the ability of complete systems to to tolerate those transients, particularly when applied in the multiple stroke and multiple burst waveform sets. These test methods have recently been incorporated in new or updated lightning test standards. A few comments on personnel safety are also included, since lightning tests involve the generating and applying very high voltages and currents – far exceeding the levels employed in most electrical test laboratories. They also far exceed lethal levels and have proven fatal to inex-perienced operators. Lightning tests to evaluate or verify either direct or indirect effects should be performed only by personnel experienced in this technology.

Lightning Certification

Guidance Flowchart: P-Static and Lightning Certification Steps for Transport Category (Part 25) Aircraft

 

In the aircraft certification world it is easy to get stuck in the quagmire of the various FARs, ACs, and ARPs. That’s why we at EMA put together a handy flowchart of guidance documents one can use when thinking about precipitation static (P-Static) and Lightning certification steps of a transport category (or part 25 category) aircraft.

The flowchart starts by first listing the CFRs (Code of Federal Regulations) pertaining to:

  • Systems Lightning Protection – 14 CFR 25.1316, which encompasses both direct effects and indirect effects of lightning
  • Fuel Systems Lightning Protection – 14 CFR 25.954 and 25.981, which also includes direct and indirect effects
  • Lightning Protection – 14 CFR 25.581, which includes lightning direct effects
  • Precipitation Static – 14 CFR 25.899(a)(3)

From there we list the relevant guidance documents, and the flowchart shows how everything fits together. The flowchart focuses on the Systems Lightning Protection, Fuel Lightning Protection, and Precipitation Static guidance documents.

 

Precipitation Static (P-Static) and Lightning Certification Steps flowchart

 

The table shown below lists the FAR (Federal Aviation Regulations) sections, the related guidance documents and their titles. NOTE: there will be a guidance document for 25.981, Fuel Tank Ignition Prevention, sometime in the future. Currently we are not aware of its release date or its document number.

Environmental Effect FAR Section FAR Description Guidance Doc Number Guidance Doc Title
Lightning 25.1316 Electrical and Electronic Systems Lightning Protection AC 20-136B Aircraft Electrical and Electronic System Lighting Protection
SAE ARP 5414B Aircraft Lightning Zoning
SAE ARP 5412B Aircraft Lightning Environment and Related Test Waveforms
SAE ARP 5415A Certification of Aircraft Electrical/Electronic Systems for the Indirect Effects of Lightning
SAE ARP 5416A Aircraft Lightning Test Methods
SAE ARP 5577 Aircraft Lightning Direct Effects Certification
DO-160G Sec. 22 Environmental Conditions and Test Procedures for Airborne Equipment: Lightning Induced Transient Susceptibility
DO-160G Sec. 23 Environmental Conditions and Test Procedures for Airborne Equipment: Lightning Direct Effects
25.954 Fuel System Lightning Protection AC 20-53B Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition Caused by Lightning
SAE ARP 4761 Aerospace Recommended Practice, Guidelines and Method for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment
25.981 Fuel Tank Ignition Prevention TBD Policy Guidance for Lightning Protection for Fuel Tank Structure and Systems
P-Static 25.899(a)(3) Electrical Bonding and Protection Against Static Electricity SAE ARP 5672 Aircraft Precipitation Static Certification
SAE ARP 1870A Aerospace Systems Electrical Bonding and Grounding for Electromagnetic Compatibility and Safety

 

 

 

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Lightning Zoning

By Tim McDonald

Lightning zoning for aircraft involves separating each aircraft outer surface area into a specific category based on its likelihood for lightning attachment, lightning sweep and lightning hang-on. The steps required are detailed in SAE ARP 5414A: Aircraft Zoning. The task is divided into two sections:

  1. Determination of the initial attachment locations
  2. Zoning based on the ARP guidance using the attachment locations and the vehicle operating speeds at various altitudes

There are three accepted methods for lightning zoning initial attachment determination:

  1. Scale model testing – A small model version of the aircraft is taken to a high voltage lab. The model is adjusted to see where lightning arcs are likely to attach
  2. Rolling sphere analysis – A sphere whose radius is related to the possible peak lightning current is rolled over the aircraft OML to see where an attachment is likely to occur
  3. Electric field modeling – Using 3D EM simulation, a leader is brought near an aircraft model to see where the E-field is most likely to attach.

Though all three are accepted by standards authorities, E-field modeling has the greatest technical basis. Scale model testing is less than satisfactory as the radius of curvature is known to affect the E-field enhancement near an object. By scaling the dimensions of the model, the radius of curvature is adjusted artificially, skewing the results. The rolling sphere method has a long historical heritage for terrestrial installations. However, it suffers from the fact that there has yet to be a first-principles derivation for its effects from the basic physics involved in lightning attachment.

E-field modeling on the other hand is grounded solidly in lightning attachment physics and does not suffer from the drawbacks of the other two methods. Further, E-field modeling is more than a factor of four less expensive than scale model testing. It is easier to implement than the rolling sphere method.

The most common approach is to use electric field modeling of the outer mold line (OML) and an analysis of the field enhancement with simulated lightning leaders to determine the initial leader attachment. Next, use the initial leader attachments along with the prescriptions of SAE ARP 5414A to create an updated zoning drawing of the aircraft.  

Lightning Zoning for Aircraft: Leaders approach an aircraft

Lightning leaders near an aircraft geometry

EMA has applied its Finite-Difference Time-Domain (FDTD) solver of Maxwell’s equations (EMA3D) in the determination of primary attachment regions on air vehicles previously. The approach is to model a lightning leader in proximity to the vehicle with a time-domain buildup of charge for which the simulation can be run to steady-state.  At steady-state, the distribution of the surface normal electric field over the vehicle is characterized.

Regions of high electric field become of interest as the potential primary attachment points for the vehicle in the E-field modeling approach. It is possible to take advantage of the time-domain nature of the solution as it approaches steady-state to characterize the rate of increase in electric field (and charge) in the regions of interest. Those areas building to a peak charge most rapidly are considered likely candidates for early streamer formation and as primary attachment locations. An example of the E-field along the length of the aircraft are shown in the picture below:

Lightning Zoning for Aircraft: E-field versus position along aircraft length

E-field zoning results along one axis of the aircraft to determine the likely attachment points

The EMA3D Framework can be used to quickly determine the initial attachment points for an aircraft. See a complete demonstration here. Next, these tools allow for the ARP 5414A recipes to be applied to the geometry using EMA3D CADfix. The final result from the software is a 3D model of the aircraft lightning zones by color that can be distributed to the entire team.

If desired, EMA3D Analysts can do the full zoning for you at a very competitive price.

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Pyro Heating

lightning strike heating protection of pyrotechnic and ordnance components- rocket_srb_separation

In terms of safety of mission, air and space pyrotechnic and ordnance components are of primary concern on many aerospace vehicles.  In this context, pyrotechnic and ordnance components refer to explosives which are designed to ignite at certain predetermined stages of a mission.  For example, the ordnance may allow a rocket to drop sections of the space vehicle as they become unnecessary during the flight and thereby save weight.  The unintended ignition of ordnance systems can have disastrous consequences, since the vehicle would prematurely eject entire sections of the rocket.

To protect against unintended ordnance ignition, one of the factors to consider is the energy deposited in the form of heat when lightning strikes.  EMA has a simulation suite that can evaluate both the currents induced on ordnance and the subsequent rise in temperature when lightning strikes.

However, the question remains: How hot is too hot?  The answer to this question depends sensitively on the specific design of the ordnance of interest, but many pyrotechnic components are at risk when the temperature reaches on the order of a few hundred degrees Celsius (C).

In the example of lightning component C heating on six fasteners connecting titanium and composite panels, we found the surrounding material could heat by as much as 40 C.  However, in that example, the current was assumed to distribute equally among all the fasteners.  If the fasteners were poorly designed such that a few fasteners had much higher impedance than others, it is possible that the current might concentrate on one or two fasteners rather than distribute on all six.

If the current was concentrated on two fasteners instead of six, the heating would increase by a factor of about nine.  The factor of nine results since the heating source is proportional to the current squared, and the current will go up by a factor of three when it is concentrated on two fasteners instead of six.  The surrounding material would then heat by about 360 C instead of 40 C which is hot enough to threaten the ordnance.

This simple analysis shows the importance of careful engineering and analysis when protecting aerospace vehicles against accidental ordnance and pyrotechnic ignition due to lightning strikes.

Contact us for more information about how EMA can help your team understand and protect against the direct effects of lightning.

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Thermal Modeling

EMA has a multi-physics platform through which to simulate and quantify the direct effects of lightning. The following content illustrates our analysis applied to a sample problem.

Thermal heating is one of the primary Direct Effects (DE) of lightning.  When large current loads are forced through regions of moderate resistance, energy is deposited into structures and significant heating can occur.  Quantifying the amount of heating that is likely to occur in sensitive regions during a lightning strike is an important factor in a lightning hardening program.  Fasteners, cables, ordnance components and other vulnerable areas should be protected to ensure safety of flight or mission.

EMA uses numerical simulation as a tool to quantify the potential for damaging thermal effects during a lightning strike. We employ a multi-physics approach to accurately obtain both induced currents and subsequent heating on the vulnerable structures in aerospace vehicles. We demonstrate our thermal simulation capabilities on a sample problem in what follows.

Consider a titanium sheet connected to a carbon fiber sheet by aluminum fasteners with one inch spacing, as seen in the figure below.  The sheets are both 5 mm thick and the fasteners are about one inch long (in the body) and have a 1/8 by 1/8 inch profile.

The fasteners to use for this lightning eirect effects thermal modeling example

A 5 mm thick sheet of carbon fiber (blue) is connected to a 5 mm thick sheet of titanium (purple) by six fasteners (orange) with 1 inch spacing. The two images show the geometry at different angles.

This sample geometry is similar to what may be found at the intersection of structures in many aerospace vehicles.  To investigate the potential for heating, we apply a 200 ampere component C lightning waveform to the sheets.  The fasteners have 0.5 Ω resistance and are the primary avenue for heating, since the current is forced through the small region where the fasteners touch the sheets.

The results are seen in the animations below.  The scale shown is temperature change in degrees Kelvin (K) due to the lightning current. The highest gain in temperature is about 93 K and occurs in the region of the fastener that touches the two sheets.  This is most easily viewed in the third animation, which has reduced opacity to allow for internal viewing.  Interestingly, the carbon fiber sheet itself gains about 40 K in the region near the fasteners and the titanium sheet heats up by about 30 K. This is also visible in the animation.

For aluminum to melt it would require a gain of about 640 K, so there is no threat of melting in this case. However, if there were ordnance components in this region, the potential for unintended ignition due to heating would need to be addressed.

Facing titanium sheet with fastener bodies visible lightning direct effects thermal modeling

Thermal heating due to lightning component C. View is facing the titanium sheet. The fastener bodies are visible.

Facing composite sheet with fastener heads visible, lightning direct effects thermal modeling

Thermal heating due to lightning component C. View is facing the carbon fiber sheet. The fastener heads are visible.

Lightning Direct Effects Thermal modeling Inside View

Thermal heating due to lightning component C. View is a cross section of the geometry with limited opacity. The part of the fasteners inside of the sheets is visible.

Users interested in tutorial content on how to perform thermal analysis within the EMA framework should watch these instructional videos:

Direct Effects Thermal Modeling Part 1

Direct Effects Thermal Modeling Part 2

Direct Effects Thermal Modeling Part 3

Direct Effects Thermal Modeling Part 4


Contact us for more information about how EMA can help you protect against thermal heating and other direct effects of lightning.

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Preventing Arcing

Designing Bond Requirements to Prevent Arcing on Aircraft Wing

EMA3D simulation can be used to determine how much current will flow through a seam or joint in a bond. This metric can then be used to determine what impedance requirement should be placed upon a particular bond. This simple example shows how an aircraft designer can use simulation to specify fastener impedance and then mitigation techniques when the requirements are not feasible.

We will consider a section of an aircraft where metal is connected to carbon fiber composite. This section could be on a wing near a fuel tank so arcing is an important concern. The metal rib is connected to the composite skin via fasteners.

 preventing arcing on an aircraft wing section

For this simple example we will assume that all the lightning detachment is on the composite, and there are no other current paths except through the fasteners at the composite-metal connection. Therefore we are concerned about arcing at those fasteners. We will use the standard value of 500 V for the threshold at which arcing occurs. Using Ohm’s law we can determine that the impedance of this entire composite-metal bond must be less than 2.5 milliohms or arcing will occur. However, let’s assume that this aircraft section is near a fuel tank so we would like to have a 20 dB margin on any requirement. Therefore the entire bond must be 0.25 milliohms.

 preventing arcing on an aircraft wing section

Assume the area of the metal that is in contact with the composite is 6” wide and 2’ long. Here is a look at fastener spacing and fastener impedance requirements that would be necessary to meet the 0.25 milliohm requirement.

 preventing arcing on an aircraft wing section graph of fastener spacing vs. fastener impedence

Let’s assume the technology at this aircraft manufacturer produces a fastener impedance of around 20 milliohms for metal and composite connections. This would require the aircraft engineer to have a fastener spacing of less than 1”, which is not mechanically feasible. Mechanically the closest the fasteners can be spaced are 1”

How does the aircraft designer meet this mechanical requirement and still prevent arcing?

The simple answer is to provide an alternative path for the lightning current. EMA has designed lightning return networks (LRN) for numerous vehicles to mitigate this problem. In this simplified example – we assume that there is a large copper conductor (bond strap) running from the metal to another section of metal near the composite.

 preventing arcing on an aircraft wing sample section

This sample bond strap takes approximately 130 kA of current and 70 kA of current continues to flow through the composite-metal bond. (This is a simplified example so we are ignoring late time current redistribution.)

 preventing arcing on an aircraft wing section graph

The orange line on the plot below shows the fastener spacing vs. fastener impedance when the LRN (in this case a bond strap) is in place. Since less current is flowing through the bond, the overall bond the impedance requirement can be relaxed to 7.1 milliohms, or 0.71 milliohms with the 20 dB margin.

From this plot one can see that with a LRN in place the aircraft designer is able to have a 1″ fastener spacing which is mechanically feasible.

EMA3D is able to predict the current and voltage which would occur at any seam or joint at a bond. EMA is able to work with designer to help determine impedance requirements. In some cases the design of a specific lightning path (LRN) is necessary to meet design requirements and prevent vehicle damage. EMA has extensive experience designing and simulating these type of systems. Trade studies of various bonding methods and lightning mitigation techniques can be rapidly preformed and are often useful for a customer to determine which type of mitigation is most effective for their program.

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