Weather

When student pilots begin learning about the dangers of thunderstorms, the focus is on extreme turbulence and the potentially deadly winds that can blast down as microbursts. They might hear about hail—balls of ice as large as baseballs—that can break windshields.

Surprisingly, discussions of thunderstorm dangers to pilots rarely include much, if anything, about lightning.

We know all thunderstorms produce lightning because lightning causes thunder. Lightning is obviously dangerous. A lightning flash is an electrical current, which might be 100,000 amperes. It’s hot—50,000 degrees Fahrenheit or higher. Lightning’s sudden heating of the air causes the air around it to expand. When the stroke ends, the air cools and contracts. This creates the sound waves that we hear as thunder. Lightning is a bigger danger to a pilot on the ground doing a preflight inspection than it is after the airplane takes off. To see why this is so, let’s start with a brief look at lightning’s basics.

The drawing below is a simplified look at how electrical charges become separated in a thunderstorm. Lightning is a huge electrical spark jumping between areas of negative and positive charges. The text below refers to the numbers on the drawing.

1. Turbulence in the storm’s updraft strips electrons from rising particles of ice and semi-frozen ice and deposits them on descending particles. Since electrons have negative charge, a strong area of negative charge builds up in the lower parts of the thunderstorm while the storm’s top becomes positively charged.

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 from the cloud to the ground.

3. Now and then a “bolt from the blue” lightning flash 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.

A lightning flash that hits the ground begins with a dim stepped leader (the classic zigzag lightning pattern) making its way from the cloud to the ground. The air has high resistance to the flow of electricity, but the attraction between the negative charge in the cloud and positive charge on the ground is strong enough to overcome air’s resistance.

As it zigzags toward the ground, the stepped leader creates a negative channel of ionized air, which conducts electricity. When the leader nears the ground, streamers of positive charge—attracted by the stepped leader’s 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. Usually several pulses of electricity follow the stepped leader’s path, which is why we see lightning flicker.

Lightning flashes, whether between parts of a cloud or from a cloud to clear air around the cloud, follow these steps.

If a stepped leader happens to descend approximately 30 feet to the vertical stabilizer of an airplane on the ramp, a streamer from the stabilizer could connect with the stepped leader and send a current of maybe 100,000 amperes through the airplane’s aluminum skin and into the ground. The lightning could jump to the ground from a wing tip or another part of the airplane. It might flash through a wheel to the ground, blowing out a tire on the way. This sometimes happens when lightning hits automobiles.

If you’re touching the airplane or even standing near it, you could provide the path to ground. To a lightning flash you are nothing but a sack of saline solution that’s a good conductor. Since lightning takes the path of least resistance to the ground, it often jumps to a person near an electrical conductor. Such “side splashes” cause many lightning injuries.

By the way, even though a lightning flash can be 50,000 degrees F, lightning doesn’t cause deep burns since the lightning comes and goes so quickly. Roughly 25 to 30 percent of those hit by lightning, including side splashes, survive. But many victims suffer random neurological damage that can cause serious problems for years to come.

The basic lightning safety rule anywhere is: If you hear it (thunder) fear it. If you see it (lightning) flee it. This means that anytime you hear thunder or see a lightning flash in the distance, you should take shelter in a building with wiring and plumbing or in a vehicle such as an auto—with a hard top—or a metal airplane. In either case, you want to make sure not to be touching metal inside the vehicle if lightning hits. The best shelter is in a building with wiring and plumbing since lightning that hits the building will follow them to the ground. Open shelters such as those in picnic areas offer little protection.

Airlines normally stop loading baggage, refueling, and other outdoor work around aircraft when lightning is seen near an airport. At least one ramp worker wearing a headset plugged into an airplane has been killed when lightning hit the airplane.

Lightning and airborne aircraft. When lightning hits an airplane in the air, its electrical current travels through the aircraft’s aluminum skin and back into the air, often doing minor damage, such as leaving marks in the skin or breaking lights where the lightning enters and leaves the aircraft’s skin. It rarely causes serious damage, but the sudden flash of light and loud clap of thunder are likely to frighten everyone aboard. The FAA estimates that on average lightning hits each air carrier aircraft flying in the United States once a year.

The last U.S. airliner crash blamed on lightning occurred on December 8, 1962. Lightning that hit a Pan American Boeing 707 flying a holding pattern over Elkton, Maryland, caused a spark that ignited fuel vapor in a tank. The resulting explosion brought the airplane down, killing all 81 aboard.

This led to stricter regulations designed to ensure that a lightning strike cannot cause a spark in fuel tanks or lines such as by making sure joints and fasteners are snug enough that sparks won’t jump between them. Regulations have become stricter over the years and include requirements designed to protect avionics systems from the random currents that a lightning strike can produce.

Aircraft built of non-conducting composites should include embedded conductors to ensure that lightning currents flow through the skin as easily as through an aluminum skin.

Even though lightning isn’t likely to cause a crash, you have many other reasons, such as extreme turbulence, for staying far away from thunderstorms in the air.