Pilots receive a great deal of generalized information and edu cation about thunderstorms and their avoidance, but the dangers of one specific hazard associated with convective weather — in-flight lightning strikes — are hardly ever raised. Maybe that's because you don't often hear about lightning hitting airplanes. Or maybe it's because of a vague conviction that lightning somehow poses no threat to aircraft. After all, some of us might say, aircraft structures are usually made of aluminum, aluminum is a good conductor, and therefore any transient electricity will harmlessly pass through an aircraft as it makes its way to strike zones more favorable to real damage — such as tall towers and other ground- bound objects.
Wrong-o. The part about aluminum's being a good conductor is true enough, but aircraft can indeed be severely damaged by lightning strikes. This past May, for example, a Beech King Air carrying Illinois Governor Jim Edgar and five others was hit by lightning while flying near an area of convective activity south of Peoria. According to reports, a bolt of lightning hit the airplane's nose, then exited via the tailcone, where it blasted a hole eight to 10 inches long. A cabin fire ensued, with flames coming from a light fixture and smoke filling the cabin. The crew depressurized the airplane (it was flying at 14,000 feet at the time of the strike), an action that was credited with extinguishing the flames. Edgar and the rest of the passengers breathed through their drop-down oxygen masks until the airplane was landed just 12 minutes after the incident occurred.
Of course, there have been other such incidents. Most don't result in any publicity, which helps to perpetuate pilot complacency. The most recent, well-investigated fatal airline accidents linked to lightning strikes took place in 1960 and 1963. In the 1960 accident, lightning struck a TWA Lockheed Constellation as it was flying near Milan, Italy. The 1963 accident involved a Pan American Airways Boeing 707 that was struck over Elkton, Maryland. In both cases, it's suspected that lightning ignited the fuel system.
While catastrophic aircraft lightning strikes such as the ones described here may be infrequent, the point is that strikes do occur. In fact, strikes that damage but don't kill happen more often than most of us might like to think. Major airlines use a rule of thumb for lightning-strike frequency, and that rule is once every 3,000 hours.
Lightning damage can range from very light to potentially disastrous, as Edgar's King Air incident illustrates. Very often, lightning attaches to an airplane's extremities — wing tips, propeller spinners, nosecones, antennas, pitot probes — then passes through the structure to an exit point situated at another extremity, such as the tailcone or another wing tip. At the entry or exit point, lightning can burn away a telltale hole or two, or demolish any composite material. Needless to say, avionics can be fried in an instant. Circuit breakers or fuses are generally useless, because the lightning's voltage surge passes through an aircraft so quickly that they can't pop or blow fast enough.
Other possible effects of a lightning strike include:
- Magnetization of the airframe. This leaves the aircraft with a charge that ruins compass accuracy and destroys power sources such as alternators and batteries. To demagnetize the aircraft, a time-consuming process called degaussing is required.
- Fusing of control-surface hinges and other airframe gaps. Most airframes are made of aluminum, all right, but they're not made in continuous, solid structures. Gaps at hinges, in control linkages, and in electric motors can be jumped by lightning surges. This can melt, fuse, or jam these components.
- Destruction of composite structures. Nosecones, radomes, wing tips, tail surfaces, and other portions of the airframe are often enclosed or faired in with composite materials. Because composite structures are poor electrical conductors, they stand in the way of a lightning bolt's passage and are therefore the site of a lot of lightning damage. Their location at airplane extremities makes them especially vulnerable.
- Fuel tank ignition. Sparks can jump gaps in unprotected fuel caps (those that aren't properly grounded) and fuel quantity sensing units, with predictable results.
- In turbojet or turbofan aircraft, compressor stall and engine flameout. It's believed that the tremendous pressure waves generated by the rapidly expanding, superheated air around a stroke of lightning can disrupt the flow of air into jet engines, causing them to flame out. There have been cases of double-engine flameouts attributed to lightning strokes. In each case, in-flight relights were successful.
- Burned wiring. Huge induced voltages can vaporize the small wires used in aircraft electrical systems, causing failure of navigation, communication, and autopilot equipment. Fire can result, too.
How can all this havoc come about? Unfortunately, the issues surrounding cloud electrification are still the subject of great debate. No one really knows how lightning comes about. One theory holds that when water droplets are ruptured within a turbulent cumulonimbus cloud, the drops create a separation of electrical charges. The smaller droplets take on negative charges, this theory goes, and the bigger ones become positively charged. Nice thought, but critics point out that this doesn't explain why most storm clouds have negatively charged bases and positively charged tops. The droplet theory would have the big, positively charged droplets at the bottom of the clouds, since larger droplets have higher fall velocities.
Collision of ice crystals has also been posited. In this approach, it's colliding ice crystals that cause charge separation. Large crystals become negative, small ones positive. This explains the charge distribution in most clouds, but critics say that the scale of ice-crystal collision activity is too small to generate lightning.
Yet another theory focuses on melting ice crystals. Here, the cores of the ice crystals become negatively charged and the thin layers of water surrounding them turn positive. The heavier ice cores with their negative charge fall to the cloud bottom, while the shed, positively charged water layers turn to microdrops and rise to the cloud tops. Again, nice try, but this theory can't explain why lightning routinely comes from clouds that are well above freezing temperature.
Experts, however, do agree on many aspects of lightning propagation and activity. Building cumulus and cumulonimbus clouds do have negatively charged bases and positively charged tops. The earth carries a net negative charge. When a charged cloud moves over the earth, a change occurs in the electrical field: The earth can take on a positive charge as the negative cloud base repels electrons beneath it.
If the cloud base's electrical charge generation mechanism (whatever it may be) becomes strong enough, surges of electrons head for the positively charged earth. These surges are called step leaders.
As these negatively charged leaders travel, they ionize the air along the way, increasing their negative charge and increasing the positive charge ahead of them.
By the time the leaders approach the ground, the earth's now- positive charge is building at a high rate. From tall objects and other conductive points, this positive charge emits ground streamers, which radiate up toward the oncoming leaders. When leader and streamer meet, a return stroke is produced. It's the return stroke that we observe as the lightning flash. After the return stroke, one or more small "dart leaders" may zoom down the channel of air that formed the pathway for the first meeting of leader and streamer. This all takes place in a half-second or so.
But that's the ground, I hear you say. What's any of this got to do with flying? Plenty, if you may be flying in and around electrically charged clouds. Fact is, you fly in electrically charged fields more than you think. Precipitation static, which interferes with communications frequencies and loran signal reception, is created when an airplane flies through moisture-laden clouds, and it is evidence of a very mild form of electrical charge.
Fly in or around convection, though, and the stakes — and current — go up. Way up. Negative cloud charges seeking their way to positive ones (either on the ground or in an another cloud), or return strokes going the other way, may "seek out" a nearby airplane simply because it's such a great conductor. With the way leaders and streamers jump around randomly, your airplane may become a part of a lightning bolt. And with the risk of electrical values anywhere from 50,000 to 200,000 amps, and 500,000 volts per meter — not to mention temperatures up to 30,000 degrees Kelvin — we thought you might like to know.
A phenomenon known as St. Elmo's fire can also occur when an aircraft is flying in or near strong electrical fields. This shows up as an eerie greenish corona discharge that can surround antennas, windshields, propellers, and even the entire airplane. See Moby Dick for a pretty good special-effects representation of St. Elmo's fire. In the movie, the masts of the Pequod radiate the discharge; and when Captain Ahab picks up a harpoon it, too, comes alive with electricity. Ahab proves his courage by grasping the harpoon and quenching the fire with his bare hands. Ahab, who was pushing his luck on that trip already, may have really been dancing with the devil when he grabbed that harpoon. There have been several cases (real life) where St. Elmo's fire was a precursor to a stroke of lightning.
Back to aviation.
Certification rules have been changed in recent years in order to minimize the chance of a disastrous lightning hit. Radomes are now fitted out with metal strips designed to divert lightning from the radome's composites and ground it to the airframe. New fuel caps prevent arcing of electricity in or near fuel tanks. Grounding straps connect control surfaces to the rest of the airframe.
In several cases, some of these rule changes were responses to modern innovations. Composite airframes like the Beech Starship needed to be properly grounded with safe electrical pathways, so we inherited the first set of lightning protection rules for composite- construction airplanes. Electronic ignition systems, such as the one pioneered in the Porsche-powered Mooney PFM, had to be protected against the effects of a wide range of electrical interference — including that created by air defense radars — so we learned more about how to protect vital electrical components from high transient voltages. The fly-by-wire control systems used in some modern airliners also called for new standards to help minimize the chance of a disabling lightning strike.
Though in-flight lightning research is scanty, existing data suggest the following strategies for avoiding lightning strikes:
- Circumnavigate all thunderstorms by at least 25 nm. This is always a good rule, but just remember that lightning can and does occur well away from cumulonimbus clouds. "Bolts from the blue" have happened.
- Avoid flying in temperatures between five and minus five degrees Celsius when near cumulonimbus. For some reason, more aircraft strikes occur in this temperature range than any other.
- Avoid flying in clouds when cumulonimbus are nearby. Again, another good, all-purpose rule. This recommendation is influenced by the fact that many strikes have taken place in turbulent clouds.
- Stormscopes and Strikefinders do fair jobs identifying areas with the strongest and/or most frequent lightning strokes, but they do have shortcomings. Use them to steer well clear of the biggest clusters of lightning returns. But this equipment often cannot accurately depict your range from the most active lightning strokes. Instead, it shows them closer than they really are. Weaker returns (i.e., those that update comparatively slowly when the display screen is cleared) are more likely to be shown at the proper distance and bearing. Always remember that lightning detection equipment range indications are more products of marketing than science.
- Conventional radar gives few clues about lightning location. If you have the luxury of Doppler weather radar, however, look for display signatures indicating strong inflows toward the centers of any storm cells. Research seems to bolster the view that inflows and updrafts breed lightning.
A pilot's discussion of lightning just wouldn't be complete without a dip into the phenomenon's rich anecdotal stew. Have you heard about the time a pilot's head, shoulders, and hands sprouted streamers just before lightning struck? How about the old yarn that you can "hear lightning coming" when the static level in your headset rises to a deafening roar? Silence comes only after the stroke's discharge, which another folk dictum says can be avoided by keying the microphone — to "wick away" the airplane's building charge. (Fat chance. The only wicking that's likely to happen is through the airplane's static wicks, which are designed to reduce the effects of precipitation static.) Or what about ball lightning, which allegedly materialized at the base of an airline pilot's center pedestal? It grew to six inches in diameter, had a greenish glow, and went through the cockpit door, down the center aisle, and past the cheap seats. Remembering that ball lightning can explode on contact and electrocute the curious, the pilot allegedly chased the green ball down the aisle, shouting "Don't touch it!" at the top of his lungs. No one did, and the ball disappeared through the rear bulkhead with a flash and a bang. No damage, except to everyone's nerves.
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