Using Ansys EMC Plus for Faster Assessment of Aircraft Lightning Vulnerability

For a week each summer, attention turns to the sky. The National Weather Service hosts Lightning Safety Awareness Week to call attention to lightning being an underrated killer. Since 2001, lightning deaths in the U.S. have dropped from about 55 per year to less than 30.

On average, lightning strikes the Earth 8.6 million times per day. That is 100 strikes per second. Commercial aircraft fly through these storms, being struck by lightning on average one or two times per year. Aircraft can initiate these strikes by enhancing ambient electric fields which are commonly found in thunderstorms and clear the way for electrical breakdown through air. The good news for passengers is that planes are designed and built to survive these strikes.

Aviation officials require manufacturers to engineer lightning protection into every aircraft component and system. Before the aircraft can fly, manufacturers must demonstrate that they have complied with the rules and regulations. To assess vulnerability to lightning strikes, aviation engineers must understand the specific lightning environment for the aircraft. In the past this testing has depended on performing physical lightning tests, a costly and time-consuming process.

EMA has a cost-effective solution to help aircraft receive certification, Ansys EMC Plus integrated with MHARNESS. The two tools improve the testing process by enhancing the understanding of aircraft lightning response in a fraction of the time and without needing a physical prototype.

Approaching Certification

When lightning strikes an aircraft, it can produce either direct or indirect effects. Direct effects include physical damage to structures and components. Indirect effects happen when lightning induces transients into electric cables, causing a power surge that can disrupt or physically damage flight-critical electronics systems.

As a part of certifying an aircraft for these effects, manufacturers traditionally submit results from physical tests. These tests are typically done on the tarmac by generating lightning-like energy and injecting it into a test flight vehicle. This complex and expensive process requires special equipment, additional personnel, and ample time to place the lightning-generating probes in exactly the right spots. In all, testing for an aircraft can last for as long as a month. If one part of the physical testing fails, it can extend the certification process for the entire aircraft. Additional holdups can come from delays during other testing being done on the same aircraft or any physical damage to the plane.

Simulation avoids these types of bottlenecks, but for every simulation effort, the ability of the model to accurately reproduce experimental results must be determined and demonstrated.

A manufacturer of civilian and military aircraft used Ansys EMC Plus with integrated MHARNESS to improve their understanding of aircraft system lightning response and validate a numerical approach to compliance that compared full aircraft simulations to full physical lightning transient analysis (LTA) tests.

As part of the development of a prototype transport category aircraft, the airplane manufacturer used the tools to model lightning’s indirect effects on avionic electronics. The manufacturer then compared the results against traditional full aircraft LTA tests.  These techniques measure the temporary oscillation that occurs in the system because of a sudden change in voltage and currents and are used in physical testing to establish equipment transient design levels (ETDLs) and aircraft actual transient levels (ATLs), which are the amplitudes and waveforms that the systems and equipment must withstand for functionality and safety. The simulation and full vehicle test results closely matched, validating the approach of using computational electromagnetics (CEM) simulation software to determine aircraft ATLs in a manner similar to traditional full vehicle LTA analysis. A model of the aircraft used can be seen in Figure 1.

Fig 1. Full aircraft simulations were compared to full aircraft LTA tests to validate a numerical approach.

Detail and Accuracy

The first step the airline manufacturer took in finding how lightning finds its way into the cables and wires of an aircraft was to recreate a model based on the actual design. A comparison of the fuselage can be seen in Figure 2.

Fig. 2. Fuselage section of original model (left) vs. the computational electromagnetics simulation model, showing component simplification.

EMC Plus and MHARNESS provide engineers with a workflow to model cable packing in the harness as well as the actual routing and spatial positioning of each cable and wire throughout the entire aircraft. EMC Plus can replicate the test vehicle configuration at a level of detail and accuracy that no other software can.

Other steps taken during the validation process include:

• Determining structural material properties.
• Determining cable properties to understand transfer impedance, a measure of their shielding performance.
• Simulating lighting levels on cables.
• Validating the simulation model.
• Reducing the amplitude of the waveforms to meet the DO-160 lightning protection standard. Waveforms represent how voltage and current change over time. Reducing the amplitude of the waveforms can lessen equipment malfunctions.

Fig. 3. An aircraft model was initially prepared for in-flight configuration. Adjustments were made to the in-flight model to match the test configuration.

Lightning Speed Compared to Traditional Testing

The manufacturer achieved a variety of benefits from using simulation to determine lightning response in its new aircraft, including:

• Reducing the scope of expensive aircraft testing.
• Improving test setup and configuration, the model is seen in Figure 4.
• Expanding probe points beyond what is available for physical testing without reconfiguring the aircraft.
• Eliminating the need to reconfigure or modify flight test vehicle components.
• Optimizing cable routing.
• Eliminating test generator noise issues, data acquisition limits, and probe implementation effects.

Fig. 4. Return conductor system developed for full vehicle testing that excludes the right-hand wing and horizontal stabilizer because of symmetry.

Simulation also reduced the testing period from as long as a month to a fraction of the time. This method also made it quick and easy to change the design after testing was complete. Scatter plot comparisons of the results can be seen in Figure 5.

Fig. 5. Scatter plot comparisons for pin voltage measurements, VOC, ISC, and IBC.

Conclusion

The final results show that EMC Plus and MHARNESS enabled the company to achieve simulation results that were comparable to LTA measurements. Amplitudes were on the same order of magnitude, and waveforms had similar peaks and duration. These results provide confidence in the modeling technique and the parameter inputs and suggest that the simulation approach of predicting lightning transients could be accepted for aircraft certification.

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