Aircraft Lightning

Trouble with aircraft and lightning began shortly after the start of powered flight. Some catastrophic issues occurred, and many were fortunate to survive. Research was needed to determine the cause of these problems and potential solutions. But this required obtaining data to create these models. The thought of flying an aircraft intentionally into a lightning storm requires a stronger constitution than I possess.

However, such measurements were made before enough information was obtained to perform simulations in a laboratory environment. As early as 1940, research began on simulating these effects. By 1967, the danger of lightning strikes to the aircraft had been significantly reduced. In the 1970s, working for General Electric as contractors to NASA, Lawrence C. Walko, Franklin A. Fisher, and John Anderson (Andy) Plumer helped develop techniques for performing these simulations, culminating in 1977 with Lightning Protection of Aircraft.[1] Plumer founded Lightning Technologies in 1977, which is now part of Element. With Fisher, Andy Plumer wrote DOT/FAA/TC-22/11, Lightning Protection of Aircraft Handbook.[2]

Other work is being done by the SAE, other researchers, and airframe manufacturers to ensure that the increased sensitivity of electronics is not affected by lightning. How important is this? It is said that every aircraft will be hit by a direct lightning strike about once a year. The SAE states that the average probability is one strike every 10,000 flight hours, but a study in a region prone to lightning reports it may be every 1,000 flight hours. This is based on reported events, but other strikes do occur and go unreported, and some say they may not be noticed.

An ideal direct lightning strike current, as defined by the SAE in ARP5412, has a high current initial spike or stroke of about 5 µS, decaying to a relatively long duration continuing current for less than one second, and followed by another short-duration restrike of fairly high amplitude. The initial stroke is responsible for most of the induced currents and voltages.

Direct lightning attaches itself to the aircraft in various locations, or Zones. The ray dome, or nose of the aircraft, often takes the heaviest hits, along with the leading edge of the engines, winglets, and tail. These areas are designated Zone 1A. The currents associated with the ray dome have been measured at 200 kA. But the attached lightning strike typically sweeps down the aircraft’s structure as it travels through the air. Thus, the attachment does not remain on the leading edge, and by the time the initial pulse decays, the attachment may be on the body. The currents associated with the body where the attached lightning hangs on are found to be much lower, possibly in the hundreds of amps. This is designated Zone 2A. An exception is for attachments on the wings and tail, where the attachment may sweep across the wing/tail to the trailing edge, where the hang-on attachment continues until the end of the strike. These long attachment locations are designated Zone 1B.

For nose cone strikes, the large initial pulse can induce very high fields from the fast rise time and high amplitude of induced currents. Note that this is also the location of the flight deck and electronics bay. Thus, some of the most critical equipment is in the area where the maximum fields are created.

As these currents flow through the airframe, the induced currents in the cables internal to it are determined by several factors. Is the aircraft composite or solid aluminum in which the lightning current has to flow? Are there other impedances in the aircraft structure, such as joints, lack of bonds, corrosion, and so forth? Are the cables routed near doors or windows, or other apertures? Are the cables shielded? Are they routed near the nose cone or other initial attachment locations, or are they mainly within the body of the airframe? Are the cables long or short? Is the coupling method from the airframe to the cables predominantly inductive (magnetic field-based due to the induced currents) or capacitive (electric field-based, due to the voltages induced in the airframe)? Knowledge of aircraft structure, cable lengths, locations, and routing, and so forth, is required to understand the test levels and waveforms needed for aircraft certification.

Next time, we will look at the induced waveforms and test levels.

References

[1] NASA Reference Publication 1008, Lightning Protection of Aircraft, published April 1977, by Franklin A. Fisher and J. Anderson Plurner. Available from https://ntrs.nasa.gov/api/citations/19780003081/downloads/19780003081.pdf

[2] FAA Technical Report DOT/FAA/TC-22/11, Lightning Protection of Aircraft Handbook, published November 2022, by Franklin A. Fisher and J. Anderson Plumer. Available from https://rosap.ntl.bts.gov/view/dot/66184/dot_66184_DS1.pdf