The Weather Never Sleeps

The greatest danger's on the ground

The possibility of lightning strikes causes concern among pilots. While it might seem surprising, lightning poses a much greater danger to a pilot doing a walk-around inspection than to an airplane in the sky.

A lightning bolt is a huge electrical spark that's triggered by strong positive and negative electrical charges. Positive and negative electrical charges are attracted toward each other. Voltage is a measure of the force that tries to bring different charges together.

The figure below is a simplified look at the charges in a typical thunderstorm. Some of the key areas are numbered for identification:

1. The turbulent mixture of water drops and ice crystals stirred by the storm's main updraft leaves some particles with negative charge, others with positive charge. Negative charge builds up in the lower part of the cloud.
2. Since like charges repel each other, the cloud's area of negative charge creates an area of positive charge under the cloud. Eventually, the attraction of the opposite charges causes lightning to flash.
3. At times, so-called "bolts from the blue" lightning can hit the ground as far as 25 miles from the cloud. Lightning also flashes within clouds and from clouds to the air around them.

Scientists are still trying to answer many important questions about exactly how opposite electrical charges form in clouds. In general, the smaller particles (water drops and ice crystals) become positively charged while the larger particles become negatively charged. The updrafts tend to carry the lighter, positively charged particles up, while the larger negative charges sink toward the bottom of the cloud.

Lightning begins with a dim stepped leader--an intermittently advancing column of high ionization that's the initial leader of a lightning discharge--making its way from the cloud to the ground through the air, which has high resistance to the flow of electricity. As it zigzags toward the ground, the stepped leader creates a negative channel. When the leader nears the ground streamers of positive charge, attracted by the negative charge, move up, often from the highest object in the immediate area. When a streamer meets the stepped leader the powerful electrical current that we see as lightning flows up the channel into the cloud, causing what's called a return stroke. It's the return stroke that we observe as lightning. Usually several pulses of electricity follow the stepped leader's path, which is why we see lightning flicker.

Now, let's first imagine the sequence of events when a typical lighting bolt carrying a current of maybe 100,000 amperes hits the top of an airplane's vertical stabilizer--a prime lightning target since it's probably the highest object in the immediate area on the ramp. The current is seeking a path to ground as it moves through the airplane's aluminum skin. If you happen to be touching the airplane, or even standing close to it, you're the best path to the ground. Even if you're not touching the airplane, the lightning can jump from the airplane to you and then flow to the ground. Such "side flashes" from an object that lightning hits--to a nearby person, and then to the ground--are a major cause of lightning injuries and deaths.

A lightning bolt is approximately 50,000 degrees Fahrenheit, but the heat comes and goes so quickly that victims don't suffer widespread or deep burns as they might if they come in contact with a high-voltage electrical line, says Dr. Mary Ann Cooper, a professor in the Department of Emergency Medicine at the University of Chicago and a leading lightning injury expert. Lightning's rapid heating of the air, followed by rapid cooling, creates the sound waves that we hear as thunder.

What happens if lightning hits an aircraft in the sky? The lightning's electrical current travels through the aircraft's aluminum skin and back into the air. It may cause minor damage, such as leaving marks on the skin or breaking lights, but will not shock anyone on board.

Worse has happened, however. On December 8, 1962, lighting hit a Pan American Boeing 707 flying a holding pattern over Elkton, Maryland, causing a spark that ignited fuel vapor in a tank. The resulting explosion brought the airplane down, killing all 81 aboard. The Pan Am crash was the deadliest, and the last, airline crash caused by lightning in the United States. Airplanes and rockets can trigger lightning when they fly into areas of strong electrical charge; this happened shortly after the Apollo 12 spacecraft lifted off on November 19, 1969, but the craft went on to make the second moon landing.

Lightning's danger to an aircraft doesn't end with the direct effects of electricity flowing though the skin. Any electrical current creates a magnetic field. When such a magnetic field moves across an electrical conductor, such as a wire for an airplane's avionics, it induces an electrical current in the conductor. When electricity from lightning flashes through an aircraft's skin, it creates a magnetic field that rapidly expands as the current begins flowing and collapses as the flow ends. The expansion and collapse of the magnetic field can induce currents in other conductors with potentially disastrous results. Currents induced by lightning that knock out an airplane's navigation and communication radios would be bad enough. Induced currents that caused an airplane's "fly-by-wire" computers to give an unwanted control command could be catastrophic.

Such a catastrophe, fortunately involving an unmanned spacecraft, occurred on March 26, 1989 when an Atlas-Centaur rocket carrying a naval communication satellite was launched from Cape Canaveral in Florida into a heavy overcast with falling rain and occasional lightning. Investigators said a triggered lightning flash hit the rocket, apparently causing the onboard computer to order a violent turn that caused the spacecraft to break up.

The Pan Am crash and others somewhat like it elsewhere in the world led to improved certification requirements designed to prevent such explosions and other serious damage. Among other things, these requirements aim to ensure that lightning's electrical currents won't cause sparks in aircraft fuel systems, including the tanks and lines. Certification requirements also required shielding and other protective measures to guard avionics systems from lightning, but scientists and engineers needed to know more about exactly what happens when lightning hits an airplane.

This need helped to prompt NASA to answer such questions by flying an F-106B packed with instruments to measure the results of lightning strikes. From 1979 into 1986 the jet, based at NASA's Langley Research Center in Hampton, Virginia, flew into thunderstorms 1,496 times and was hit by lighting 714 times.

In December 2007 Elizabeth Vargas of ABC's 20/20 interviewed Bruce Fisher, lead researcher for the Storms Hazards Program, for a program on travel myths. She questioned him about whether lightning could cause an airliner to crash. Fisher flew in the back seat of the F-106 monitoring the instruments as lightning hit the jet. He estimated that he was in the airplane for more than 200 lightning strikes. The flights and the data gathered were "better than sitting at a desk and doing computations," Fisher said. "The lightning was the fun part of the mission.... You know, we would go up and go down, plus or minus 3,000 or 4,000 feet with the updrafts and the downdrafts and allow the aircraft to do that."

He assured viewers that when lightning hits an airliner "you're not going to run into a risk at all. The lightning current does not get inside to the individuals or the equipment inside. The aircraft are designed to keep the lightning on the outside of the plane."

While the traditional aircraft skin made of aluminum is a good starting point for defending aircraft from lightning, composites--which aren't inherently conductive--complicate lightning protection. Engineers are coming up with methods to incorporate conductive screens or fibers in composites to enable them to safety conduct lightning's electricity.

We've established that lightning is not likely to knock your airplane out of the sky, but that is not a reason to fly near thunderstorms. The quickly shifting winds, violent turbulence, hail, and rain heavy enough to drown a jet engine offer more than enough reasons to avoid thunderstorms.

For a pilot, lightning's big threat is on the ground. In fact, lightning near an airport causes officials to suspend ground services such as refueling and loading luggage, which can be a major cause of airline delays, especially in Florida and other areas where lightning is common during warm weather. Pilots should follow the same lightning safety rules on the ramp that anyone who goes outdoors for work or recreation should follow.

The basic rule that lightning authorities have been pushing in recent years is: "If you hear it (thunder) fear it. If you see it (lighting) flee it." In other words, the first rumble of distant thunder or first glimpse of far-away lightning should be the signal to take shelter in a vehicle (your airplane is fine) or a sturdy building; not a picnic shelter with open sides. Take shelter at the first sign of lightning because a bolt from the blue could hit you or your airplane before the storm arrives with rain.

Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.