CASE
2
Coping With Lightning: A Lethal Threat to Flight
Barrett Tillman and John L. Tillman
The beautiful spectacle and terrible power of lightning have always inspired fear and wonder. In flight, it has posed a significant challenge. While the number of airships, aircraft, and occupants lost to lightning have been few, they offer sobering evidence that lightning is a hazard warranting intensive study and preventative measures. This is an area of NASA research that crosses between the classic fields of aeronautics and astronautics, and that has profound implications for both.
Case-2 Cover Image: A lightning strike reveals the breadth, power, and majesty of this still mysterious electromagnetic phenomenon. NOAA.
“I learned more about lightning from flying at night over Bosnia while wearing night vision goggles than I ever learned from a meteorologist. You’d occasionally see a green flash as a bolt discharged to the ground, but that was nothing compared to what was happening inside the clouds themselves. Even a moderate-sized cloud looked like a bubbling witches’ cauldron, with almost constant green discharges left and right, up and down. You’d think, “Bloody hell! I wouldn’t want to fly through that!” But of course you do, all the time. You just don’t notice if you don’t have the goggles.”[1]
So stated one veteran airman of his impressions with lightning. Lightning is an electrical discharge in the atmosphere usually generated by thunderstorms but also by dust storms and volcanic eruptions. Because only about a fourth of discharges reach the ground, lightning represents a disproportionate hazard to aviation and rocketry. In any case, lightning is essentially an immense spark that can be many miles long.[2]
Lightning generates radio waves. Scientists at the National Aeronautics and Space Administration (NASA) discovered that very low frequency (VLF) waves cause a gap between the inner and outer Van Allen radiation belts surrounding Earth. The gap offers satellites a potential safe zone from solar outburst particle streams. But, as will be noted, protection of spacecraft from lightning and electromagnetic pulses (EMPs) represents a lasting concern.
There are numerous types of lightning. By far the most common is the streak variety, which actually is the return stroke in open air. Most lightning occurs inside clouds and is seldom witnessed inside thunderstorms. Other types include: ball (spherical, semipersistent), bead (cloud to ground), cloud-to-cloud (aka, sheet or fork lightning), dry (witnessed in absence of moisture), ground-to-cloud, heat (too distant for thunder to be heard), positive (also known as high-voltage lightning), ribbon (in high crosswinds), rocket (horizontal lightning at cloud base), sprites (above thunderstorms, including blue jets), staccato (short cloud to ground), and triggered (caused by aircraft, volcanoes, or lasers).
Every year, some 16 million thunderstorms form in the atmosphere. Thus, over any particular hour, Earth experiences over 1,800. Estimates of the average global lightning flash frequency vary from 30 to 100 per second. Satellite observations produce lower figures than did prior scientific studies yet still record more than 3 million worldwide each day.[3] Between 1959 and 1994, lightning strikes in the United States killed 3,239 people and injured a further 9,818, a measure of the lethality of this common phenomenon.[4]
Two American regions are notably prone to ground strikes: Florida and the High Plains, including foothills of the Rocky Mountains. Globally, lightning is most common in the tropics. Therefore, Florida records the most summer lightning strikes per day in the U.S. Heat differentials between land and water on the three sides of peninsular Florida, over its lakes and swamps and along its panhandle coast, drive air circulations that spin off thunderstorms year-round, although most intensely in summer.
Lightning: What It Is, What It Does
Despite recent increases in understanding, scientists are still somewhat mystified by lightning. Modern researchers might concur with stone age shaman and bronze age priests that it partakes of the celestial.
Lightning is a form of plasma, the fourth state of matter, after solids, liquids, and gases. Plasma is an ionized gas in which negatively charged electrons have been stripped by high energy from atoms and molecules, creating a cloud of electrons, neutrons, and positively charged ions.
As star stuff, plasma is by far the most common state of matter in the universe. Interstellar plasmas, such as solar wind particles, occur at low density. Plasmas found on Earth include flames, the polar auroras, and lightning.
Lightning is like outer space conditions coming fleetingly to Earth. The leader of a bolt might zip at 134,000 miles per hour (mph). The energy released heats air instantaneously around the discharge from 36,000 to 54,000 degrees Fahrenheit (ºF), or more than three to five times the Sun’s surface temperature. The sudden, astronomical increase in local pressure and temperature causes the atmosphere within and around a lightning bolt to expand rapidly, compressing the surrounding clear air into a supersonic shock wave, which decays to the acoustic wave perceived as thunder. Ranging from a sharp, loud crack to a long, low rumble, the sound of a thunderclap is determined by the hearer’s distance from the flash and by the type of lightning.
Lightning originates most often in cumulonimbus thunderclouds. The bases of such large, anvil-shaped masses may stretch for miles. Their tops can bump up against, spread out along, and sometimes blast through the tropopause: the boundary between the troposphere (the lower portion of the atmosphere, in which most weather occurs) and the higher stratosphere. The altitude of the lower stratosphere varies with season and latitude, from about 5 miles above sea level at the poles in winter to 10 miles near the equator. The tropopause is not a “hard” ceiling. Energetic thunderstorms, particularly from the tropics, may punch into the lower stratosphere and oscillate up and down for hours in a multicycle pattern.
A Lightning Primer
The conditions if not the mechanics that generate lightning are now well known. In essence, this atmospheric fire is started by rubbing particles together. But there is still no agreement on which processes ignite lightning. Current hypotheses focus on the separation of electric charge and generation of an electric field within a thunderstorm. Recent studies further suggest that lightning initiation requires ice, hail, and semifrozen water droplets, called “graupel.” Storms that do not produce large quantities of ice usually do not develop lightning.[5] Graupel forms when super-cooled water droplets condense around a snowflake nucleus into a sphere of rime, from 2 to 5 millimeters across. Scientific debate continues as experts grapple with the mysteries of graupel, but the stages of lightning creation in thunderstorms are clear, as outlined by the National Weather Service of the National Oceanic and Atmospheric Administration (NOAA).
First comes charge separation. Thunderstorms are turbulent, with strong updrafts and downdrafts regularly occurring close to one another. The updrafts lift water droplets from warmer lower layers to heights between 35,000 and 70,000 feet, miles above the freezing level. Simultaneously, downdrafts drag hail and ice from colder upper layers. When the opposing air currents meet, water droplets freeze, releasing heat, which keeps hail and ice surfaces slightly warmer than the surrounding environment, so that graupel, a “soft hail,” forms.
Electrons carry a negative charge. As newly formed graupel collides with more water droplets and ice particles, electrons are sheared off the ascending particles, charging them positively. The stripped electrons collect on descending bits, charging them negatively. The process results in a storm cloud with a negatively charged base and positively charged top.
Once that charge separation has been established, the second step is generation of an electrical field within the cloud and, somewhat like a mirror image, an electrical field below the storm cloud. Electrical opposites attract, and insulators inhibit current flow. The separation of positive and negative charges within a thundercloud generates an electric field between its top and base. This field strengthens with further separation of these charges into positive and negative pools. But the atmosphere acts as an insulator, inhibiting electric flow, so an enormous charge must build up before lightning can occur. When that high charge threshold is finally crossed, the strength of the electric field overpowers atmospheric insulation, unleashing lightning. Another electrical field develops with Earth’s surface below negatively charged storm base, where positively charged particles begin to pool on land or sea. Whither the storm goes, the positively charged field—responsible for cloud-to-ground lightning—will follow it. Because the electric field within the storm is much stronger than the shadowing positive charge pool, most lightning (about 75 to 80 percent) remains within the clouds and is thus not attracted groundward.
The third phase is the building of the initial stroke that shoots between the cloud and the ground. As a thunderstorm moves, the pool of positively charged particles traveling with it along the ground gathers strength. The difference in charge between the base of the clouds and ground grows, leading positively charged particles to climb up taller objects like houses, trees, and telephone poles. Eventually a “stepped leader,” a channel of negative charge, descends from the bottom of the storm toward the ground. Invisible to humans, it shoots to the ground in a series of rapid steps, each happening quicker than the blink of an eye. While this negative leader works its way toward Earth, a positive charge collects in the ground and in objects resting upon it. This accumulation of positive charge “reaches out” to the approaching negative charge with its own channel, called a “streamer.” When these channels connect, the resulting electrical transfer appears to the observer as lightning.
Finally, a return stroke of lightning flows along a charge channel about 0.39 inches wide between the ground and the cloud. After the initial lightning stroke, if enough charge is left over, additional strokes will flow along the same channel, giving the bolt its flickering appearance.
Land struck by a bolt may reach more than 3,300 ºF, hot enough to almost instantly melt the silica in conductive soil or sand, fusing the grains together. Within about a second, the fused grains cool into fulgurites, or normally hollow glass tubes that can extend some distance into the ground, showing the path of the lightning and its dispersion over the surface.
The tops of trees, skyscrapers, and mountains lie closer to the base of storm clouds than does low-lying ground, so such objects are commonly struck by lightning. The less atmospheric insulation that lightning must burn through, the easier falls its strike. The tallest object beneath a storm will not necessarily suffer a hit, however, because the opposite charges may not accumulate around the highest local point or in the clouds above it. Lightning can strike an open field rather than a nearby line of trees.
Lightning leader development depends not only upon the electrical breakdown of air, which requires about 3 million volts per meter, but on prior channel carving. Ambient electric fields required for lightning leader propagation can be one or two orders of magnitude less than the electrical breakdown strength. The potential gradient inside a developed return stroke channel is on the order of hundreds of volts per meter because of intense channel ionization, resulting in a power output on the order of a megawatt per meter for a vigorous return stroke current of 100,000 amperes (100 kiloamperes, kA).
Negative, Positive, Helpful, and Harmful
Most lightning forms in the negatively charged region under the base of a thunderstorm, whence negative charge is transferred from the cloud to the ground. This so-called “negative lightning” accounts for over 95 percent of strikes. An average bolt of negative lightning carries an electric current of 30 kA, transferring a charge of 5 coulombs, with energy of 500 megajoules (MJ). Large lightning bolts can carry up to 120 kA and 350 coulombs. The voltage is proportional to the length of the bolt.[6]
Some lightning originates near the top of the thunderstorm in its cirrus anvil, a region of high positive charge. Lightning formed in the upper area behaves similarly to discharges in the negatively charged storm base, except that the descending stepped leader carries a positive charge, while its subsequent ground streamers are negative. Bolts thus created are called “positive lightning,” because they deliver a net positive charge from the cloud to the ground. Positive lightning usually consists of a single stroke, while negative lightning typically comprises two or more strokes. Though less than 5 percent of all strikes consist of positive lightning, it is particularly dangerous. Because it originates in the upper levels of a storm, the amount of air it must burn through to reach the ground is usually much greater. Therefore, its electric field typically is much stronger than a negative strike would be and generates enormous amounts of extremely low frequency (ELF) and VLF waves. Its flash duration is longer, and its peak charge and potential are 6 to 10 times greater than a negative strike, as much as 300 kA and 1 billion volts!
Some positive lightning happens within the parent thunderstorm and hits the ground beneath the cloud. However, many positive strikes occur near the edge of the cloud or may even land more than 10 miles away, where perhaps no one would recognize risk or hear thunder. Such positive lightning strikes are called “bolts from the blue.” Positive lightning may be the main type of cloud-to-ground during winter months or develop in the late stages of a thunderstorm. It is believed to be responsible for a large percentage of forest fires and power-line damage, and poses a threat to high-flying aircraft. Scientists believe that recently discovered high-altitude discharges called “sprites” and “elves” result from positive lightning. These phenomena occur well above parent thunderstorms, at heights from 18 to 60 miles, in some cases reaching heights traversed only by transatmospheric systems such as the Space Shuttle.
Lightning is by no means a uniformly damaging force. For example, fires started by lightning are necessary in the life cycles of some plants, including economically valuable tree species. It is probable that, thanks to the evolution and spread of land plants, oxygen concentrations achieved the 13-percent level required for wildfires before 420 million years ago, in the Paleozoic Era, as evinced by fossil charcoal, itself proof of lightning-caused range fires.
In 2003, NASA-funded scientists learned that lightning produces ozone, a molecule composed of three oxygen atoms. High up in the stratosphere (about 6 miles above sea level at midlatitudes), ozone shields the surface of Earth from harmful ultraviolet radiation and makes the land hospitable to life, but low in the troposphere, where most weather occurs, it’s an unwelcome byproduct of manmade pollutants. NASA’s researchers were surprised to find that more low-altitude ozone develops naturally over the tropical Atlantic because of lightning than from the burning of fossil fuels or vegetation to clear land for agriculture.
Outdoors, humans can be injured or killed by lightning directly or indirectly. No place outside is truly safe, although some locations are more exposed and dangerous than others. Lightning has harmed victims in improvised shelters or sheds. An enclosure of conductive material does, however, offer refuge. An automobile is an example of such an elementary Faraday cage.
Property damage is more common than injuries or death. Around a third of all electric power-line failures and many wildfires result from lightning. (Fires started by lightning are, however, significant in the natural life cycle of forests.) Electrical and electronic devices, such as telephones, computers, and modems, also may be harmed by lightning, when overcurrent surges fritz them out via plug-in outlets, phone jacks, or Ethernet cables.
The Lightning Hazard in Aeronautics and Astronautics: A Brief Synopsis
Since only about one-fourth of discharges reach Earth’s surface, lightning presents a disproportionate hazard to aviation and rocketry. Commercial aircraft are frequently struck by lightning, but airliners are built to reduce the hazard, thanks in large part to decades of NASA research. Nevertheless, almost every type of aircraft has been destroyed or severely damaged by lightning, ranging from gliders to jet airliners. The following is a partial listing of aircraft losses related to lightning:
Additionally, lightning posed a persistent threat to rocket-launch operations, forcing extensive use of protective systems such as lightning rods and “tripwire” devices. These devices included small rockets trailing conductive wires that can trigger premature cloud-to-ground strokes, reducing the risk of more powerful lightning strokes. The classic example was the launch of Apollo 12, on November 14, 1969. “The flight of Apollo 12,” NASA historian Roger E. Bilstein has written, “was electrifying, to say the least.”[8]
During its ascent, it built up a massive static electricity charge that abruptly discharged, causing a brief loss of power. It had been an exceptionally close call. Earlier, the launch had been delayed while technicians dealt with a liquid hydrogen leak. Had a discharge struck the fuel-air mix of the leak, the conflagration would have been disastrous. Of course, three decades earlier, a form of lightning (a brush discharge, commonly called “St. Elmo’s fire”) that ignited a hydrogen gas-air mix was blamed by investigators for the loss of the German airship Hindenburg in 1937 at Lakehurst, NJ.[9]
Flight Research on Lightning
Benjamin Franklin’s famous kite experiments in the 1750s constituted the first application of lightning’s effect upon “air vehicles.” Though it is uncertain that Franklin personally conducted such tests, they certainly were done by others who were influenced by him. But nearly 200 years passed before empirical data were assembled for airplanes.[10]
Probably the first systematic study of lightning effects on aircraft was conducted in Germany in 1933 and was immediately translated by NASA’s predecessor, the National Advisory Committee on Aeronautics (NACA). German researcher Heinrich Koppe noted diverse opinions on the subject. He cited the belief that any aircraft struck by lightning “would be immediately destroyed or at least set on fire,” and, contrarily, that because there was no direct connection between the aircraft and the ground, “there could be no force of attraction and, consequently, no danger.”[11]
Koppe began his survey detailing three incidents in which “the consequences for the airplanes were happily trivial.” However, he expanded the database to 32 occasions in 6 European nations over 8 years. (He searched for reports from America but found none at the time.) By discounting incidents of St. Elmo’s fire and a glider episode, Koppe had 29 lightning strikes to evaluate. All but 3 of the aircraft struck had extended trailing antennas at the moment of impact. His conclusion was that wood and fabric aircraft were more susceptible to damage than were metal airframes, “though all-metal types are not immune.” Propellers frequently attracted lightning, with metal-tipped wooden blades being more susceptible than all-metal props. While no fatalities occurred with the cases in Koppe’s studies, he did note disturbing effects upon aircrew, including temporary blindness, short-term stunning, and brief paralysis; in each case, fortunately, no lingering effects occurred.[12]
Koppe called for measures to mitigate the effects of lightning strikes, including housing of electrical wires in metal tubes in wood airframes and “lightning protection plates” on the external surfaces. He said radio masts and the sets themselves should be protected. One occasionally overlooked result was “electrostriction,” which the author defined as “very heavy air pressure effect.” It involved mutual attraction of parallel tracks into the area of the current’s main path. Koppe suggested a shield on the bottom of the aircraft to attract ionized air. He concluded: “airplanes are not ‘hit’ by lightning, neither do they ‘accidentally’ get into the path of a stroke. The hits to airplanes are rather the result of a release of more or less heavy electrostatic discharges whereby the airplane itself forms a part of the current path.”[13]
American studies during World War II expanded upon prewar examinations in the United States and elsewhere. A 1943 National Bureau of Standards (NBS, now the National Institute for Standards and Technology, NIST) analysis concluded that the power of a lightning bolt was so enormous—from 100 million to 1 billion volts—that there was “no possibility of interposing any insulating barrier that can effectively resist it.” Therefore, aircraft designers needed to provide alternate paths for the discharge via “lightning conductors.”[14] Postwar evaluation reinforced Koppe’s 1933 observations, especially regarding lightning effects upon airmen: temporary blindness (from seconds to 10 minutes), momentary loss of hearing, observation of electrical effects ranging from sparks to “a blinding blue flash,” and psychological effects. The latter were often caused more by the violent sensations attending the entrance of a turbulent storm front rather than a direct result of lightning.[15]
Drawing upon British data, the NACA’s 1946 study further detailed atmospheric discharges by altitude bands from roughly 6,500 to 20,500 feet, with the maximum horizontal gradient at around 8,500 feet. Size and configuration of aircraft became recognized factors in lightning, owing to the greater surface area exposed to the atmosphere. Moisture and dust particles clinging to the airframe had greater potential for drawing a lightning bolt than on a smaller aircraft. Aircraft speed also was considered, because the ram-air effect naturally forced particles closer together.[16]
A Weather Bureau survey of more than 150 strikes from 1935 to 1944 defined a clear “danger zone”: aircraft flying at or near freezing temperatures and roughly at 1,000 to 2,000 feet above ground level (AGL). The most common factors were 28–34 ºF and between 5,000 and 8,000 feet AGL. Only 15 percent of strikes occurred above 10,000 feet.[17]
On February 19, 1971, a Beechcraft B90 King Air twin-turboprop business aircraft owned by Marathon Oil was struck by a bolt of lightning while descending through 9,000 feet preparatory to landing at Jackson, MI. The strike caused “widespread, rather severe, and unusual” damage. The plane suffered “the usual melted metal and cracked nonmetallic materials at the attachments points” but in addition suffered a local structural implosion on the inboard portions of the lower right wing between the fuselage and right engine nacelle, damage to both flaps, impact-and-crush-type damage to one wingtip at an attachment point, electrical arc pitting of flap support and control rod bearings, a hole burned in a ventral fin, missing rivets, and a brief loss of power. “Metal skins were distorted,” NASA inspectors noted, “due to the ‘magnetic pinch effect’ as the lightning current flowed through them.” Pilots J.R. Day and J.W. Maxie recovered and landed the aircraft safely. Marathon received a NASA commendation for taking numerous photographs of record and contacting NASA so that a much more detailed examination could be performed.[18]
The jet age brought greater exposure to lightning, prompting further investigation by NOAA (created in 1970 to succeed the Environmental Science Services Administration, which had replaced the Weather Bureau in 1965). The National Severe Storms Laboratory conducted Project Rough Rider, measuring the physical characteristics and effects of thunderstorms, including lightning. The project employed two-seat F-100F and T-33A jets to record the intensity of lightning strikes over Florida and Oklahoma in the mid-1960s and later. The results of the research flights were studied and disseminated to airlines, providing safety guidelines for flight in the areas of thunderstorms.[19]
In December 1978, two Convair F-106A Delta Dart interceptors were struck within a few minutes near Castle Air Force Base (AFB), CA. Both had lightning protection kits, which the Air Force had installed beginning in early 1976. One Dart was struck twice, with both jets sustaining “severe” damage to the Pitot booms and area around the radomes. The protection kits prevented damage to the electrical systems, though subsequent tests determin