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