- Lightning protection of aircraft is not exclusively an EWIS issue and requires high-level evaluation of the aircraft as a whole.
- Lightning can effect aircraft systems both directly (physical damage) and indirectly (electromagnetic interference).
- Documentation exists both within industry and government standards to offer guidance on ensuring adequate protection from lightning on aircraft.
Any airborne vehicle risks exposure to lightning strikes, and thus risks exposure to interference and damage known to be caused by lightning, simply by performing its intended operation. A complication as severe and inherent to air travel as lightning must be considered in the design of any aircraft.
Incorporating such considerations into a design starts with knowing where to find information and requirements regarding lightning protection of aircraft. As an electrical phenomenon, lightning certainly raises concerns from an Electrical Wiring Interconnection System (EWIS) perspective, however, the usual go-to standards for general information (like AS50881 and AC43.13-1B) do not have much detail about lightning protection or its associated test methods and requirements. Though aircraft safety requirements regarding protection from lightning are not as clearly defined as other electrical requirements, standard guidance does exist.
US Government Documentation
CFR § 25.581
A good starting point for lightning protection considerations on aircraft is the code of federal regulations (CFR) § 25.581 – Lightning protection which reads:
- “(a) The airplane must be protected against catastrophic effects from lightning.
- (b) For metallic components, compliance with paragraph (a) of this section may be shown by—
- Bonding the components properly to the airframe; or
- Designing the components so that a strike will not endanger the airplane.
(c) For nonmetallic components, compliance with paragraph (a) of this section may be shown by—
- Designing the components to minimize the effect of a strike; or
- Incorporating acceptable means of diverting the resulting electrical current so as not to endanger the airplane.”
Note that this is a structure regulation, rather than an EWIS regulation, but proper bonding of metallic conductive components is just as much an EWIS concern as a structure concern. Adequate bonding is essential for proper flow of electrical current, especially when lightning currents are involved.
The metallic surface of a traditional aircraft acts as a Faraday cage; a lightning strike typically attaches at an edge or point (nose, tip of a wing), travels along the outer surface of the aircraft, and exits through the tail. This function fundamentally relies on adequate bonding of conductive structural components.
MIL-B-5087 and MIL-STD-464
MIL-B-5087: Bonding, Electrical, and Lightning Protection, for Aerospace Systems was initially published in 1964 and contains some useful information and guidance on electrical bonding regarding lightning. The standard was cancelled in 1997 and the source for electrical bonding and lightning information was moved to MIL-STD-464: Electromagnetic Environmental Effects Requirements for Systems. The MIL-STD-464 is a more inclusive document; in addition to the electrical bonding and lightning information included from MIL-B-5087, it details a wider range of requirements for a wider range of electrical systems (aircraft, Naval ships, ground systems, space and launch vehicles).
FAA Lightning Protection Handbook
The FAA also has a rather comprehensive document available to the public called “Lightning Protection of Aircraft Handbook” (DOT/FAA/TC-22/11) whose most recent version was released in February 2023. This 500-page document is an excellent reference for anyone concerned with lightning safety on aircraft. The handbook is broken down primarily into two halves, the first addresses lightning environment and direct effects of lightning and the second addresses indirect effects of lightning.
Effects on aircraft resulting from lightning strikes are classified as either direct or indirect effects. A direct effect is any physical damage (holes, splintering, melting, etc.) caused by the lightning strike at the point of attachment. An indirect effect, or induced effect, is any effect caused by a resultant electromagnetic field generated by the lightning strike.
The ASTM standard for aircraft EWIS, ASTM F2639, is more helpful in this context than the AS50881. The standard contains several references to areas that require protection from lightning strikes and even provides guidance on aircraft lightning protection. This includes guidance on addressing the needs of different lightning zones, bonding requirements, types of protection, and guidance on maximum shield lengths in these regions of the aircraft. Further, the standard includes sections on control surface lightning protection bonding, control cable lightning protection bonding, and lightning protection for antennas and air data probes.
The SAE has several ARP (Aerospace Recommended Practice) standards specifically covering lightning safety on aircraft:
- ARP5412 – Aircraft Lightning Environment and Related Test Waveforms
- ARP5414 – Aircraft Lightning Zone
- ARP5415 – User’s Manual for Certification of Aircraft Electrical/Electronic Systems for the Indirect Effects of Lightning
- ARP5416 – Aircraft Lightning Test Methods
- ARP5577 – Aircraft Lightning Direct Effects Certification
- ARP6205 – Transport Airplane Fuel System Lightning Protection
Together, these standards paint a fairly detailed picture of designers/manufacturers’ responsibility for incorporating lightning protection into an aircraft.
The Radio Technical Commission for Aeronautics (RTCA) has a document called DO-160 Environmental Conditions and Test Procedures for Airborne Equipment. The DO-160 is a general testing standard, applicable to any equipment intended for use on an aircraft as a base-level evaluation. Two sections within the document address aircraft lightning protection:
- Section 22 – Lightning Induced Transient Susceptibility. Which evaluates indirect effects of lightning resulting from EMI (Electromagnetic Interference)
- Section 23 – Lightning Direct Effects. Which evaluates physical damage as a result of a lightning strike.
The standard provides guidance on lightning voltages and surge currents based on the exposure area and other factors. Typically, following these tests, a visual and functional examination is performed of the tested equipment to determine the impact of the representative lightning strike.
Lightning protection requirements are not as straightforward as looking up a numerical value for insulation resistance or verifying the interchangeability of connector pins, and it is not exclusively an EWIS issue. Adequate consideration involves high level evaluation of the aircraft as a whole.
The regulatory documents identified in this article can help guide OEMs through the process of ensuring their aircraft are lightning-ready. While Lectromec does not yet have lightning testing capabilities, we can support the development of EWIS-related lightning requirements. Contact us today to learn more.