Thunder and lightning, oh my: Lightning protection in aircraft design

Lightning strikes aircraft with a higher frequency than most people would believe. The reported statistical data indicates that lightning strikes each aircraft in the U.S. commercial fleet one and a half times per year and pilots experience lightning strikes once every 3,000 flight hours. It occurs most often during the climb and descent phases of flight at an altitude of 5,000 to 15,000 feet, however, the probability of lightning strike decreases significantly above 20,000 feet. 

The highest probability for a lightning strike is to an aircraft’s outer extremities, such as the wingtips, nose, and rudder. During a direct lightning strike, the lightning current attaches to the aircraft and flows through the aircraft skin before eventually exiting off some other extremity
Most aircraft skins consist primarily of aluminum which conducts electricity very well. Some modern aircraft are made of composite materials which can be significantly less conductive than aluminum, however, to combat this, the composite materials contain an embedded layer of conductive fibers, or screens, designed to safely carry lightning currents. By making sure that no gaps exist in this conductive path, engineers can assure that most of the direct lightning strike current will remain on the exterior of the aircraft

Damage to sensitive avionic electronics can be caused by a direct or indirect lightning strike. Unless avionic electronic systems are properly protected, they can be easily damaged. As lightning currents travel on the exterior aircraft skin, it has the potential to induce electrical transients into cable wires or equipment beneath the skin. These induced transients, produced by changing electric and magnetic fields, are the indirect effects of a direct lightning strike. To mitigate the electrical transients caused by indirect effects in cables and equipment, shielding, grounding (bonding), and surge suppression devices are needed in any equipment designed for airborne use.

The commercial and military aerospace industries have developed standards to address lightning strikes. The predominately used standard for commercial aircraft is RTCA/DO-160, and for the military, MIL-STD-461. Recently, MIL-STD-461G added CS 117 which addresses lightning-induced transients (indirect effects) giving avionic electronic system engineers the transient signal levels expected for both direct and indirect strikes. This standard helps engineers provide adequate design protection against the destructive energy spikes generated by indirect induced transients, but it does not include provisions for direct effects.

After performing many such analyses we tend to see several common issues with lightning protection designs with most of our customers. The most common issue lies with the implementation of Transient Voltage Suppressor (TVS) diodes, where most design engineers calculate the power of the transient then select a TVS diode based on:

  • Rated power
  • Reverse standoff voltage
  • Reverse breakdown voltage
  • Clamping voltage

Designers often overlook the fact that TVS diodes have a Zener curve in that, as the reverse current increases, so does the clamping voltage. For faster transients, the clamping voltage can go even higher. The higher than expected clamping voltage usually exceeds the maximum ratings of downstream circuitry the TVS diodes are protecting. 

Another common issue is the series power resistors implemented to share the load with the TVS diode. Often, design engineers fail to adequately consider the amount of energy the power resistor will see during a transient event. If too much energy is applied to the power resistor, the resulting high temperature can destroy the resistor material.

Design reviews following circuit simulations should be done to ensure that your lightning protection devices will handle the waveforms detailed in DO-160. Contact us to see how Omnicon can perform a full review and analysis of your lightning protection design using pin injection and cable bundle simulations to give you the confidence that your design will pass testing, before performing costly and time consuming formal airworthiness qualification tests on hardware.