Weather

Lightning damage to the paint of an F–106.

The thunderstorms moving across eastern North Carolina the afternoon of July 25, 1984, were on the weak side of ordinary near the ground, with no damaging wind, flooding rain, or hail. Near the tops of these thunderstorms, however—some 37,000 feet up—NASA pilot Phil Brown and flight-test engineer Victoria Chapman Rondeau, in a NASA F–106B Delta Dart, were setting a record for the most in-flight lighting strikes on an airplane during a flight. Rondeau, who was then a U.S. Air Force lieutenant, said she volunteered for the lightning flights because she likes roller coasters, and the flights would allow her to use her electrical engineering degree.

“Strike,” Brown or Rondeau said whenever one saw lighting hit the airplane. Their taped words helped Air Force and NASA engineers and scientists later correlate images from the airplane’s video and still cameras with data captured by the package of instruments in the jet’s weapons bay. When they returned to their base at NASA’s Langley Research Center in Hampton, Virginia, Rondeau and Brown knew lightning had hit the airplane 54 times, but subsequent examination of the data showed it actually had hit 72 times during the flight. The record-setting flight was part of a lightning research program from 1980 to 1986 that collected data to better protect aircraft, including spacecraft, from lightning.

Lightning doesn’t knock airplanes from the sky. When lightning hits an airplane, its electrical current travels through the aluminum skin and flashes back into the air, usually causing only minor damage—such as burn marks where the lightning enters and leaves. Aircraft made with composites, such as the Boeing 787, have a metal mesh embedded in the composite material so that it can conduct electric currents as an aluminum skin does.

The FAA says that on the average, lightning hits each air-carrier aircraft in the United States once a year. The last lightning-caused U.S. airliner crash occurred on December 8, 1963, when lightning struck a Pan American Boeing 707 over Elkton, Maryland, igniting vapor in a fuel tank. The explosion ripped off a wing and the crash killed all 73 people aboard.

This crash led to stronger lightning safety rules for aircraft. New lightning protection regulations focused on ensuring that lightning current traveling though an aircraft wouldn’t cause sparks in fuel tanks or lines. For example, the new rules required that the skin around fuel tanks be thick enough to keep electricity from a direct strike on the wing from burning a hole into a tank.

Avionics at greatest risk. NASA’s lightning flights weren’t after data that would help to better protect fuel systems from lightning explosions. They collected data on lightning’s more subtle danger to aircraft electrical and electronic systems, which were growing more sophisticated—and vital to safe operation.

By the 1970s aeronautical engineers knew that large airplanes of the future—likely the near future—would be using what are known as “fly-by-wire” systems. These systems use electric wires and computers to replace the hydraulic systems that connect a pilot’s inputs to an aircraft’s control surfaces, such as elevators and trim tabs. Such systems had been tested as far back as the 1930s. The Apollo Lunar Landing Research Vehicle, which was first flown in 1964, was the first aircraft to use a fly-by-wire system without hydraulic or mechanical backup.

When Neil Armstrong and Buzz Aldrin landed on the moon on July 20, 1969, they faced many dangers, but lightning wasn’t among them. Nevertheless, NASA and others saw the need to learn more about what happens when lightning hits an aircraft, so that they could ensure a strike wouldn’t disable a fly-by-wire system or possibly cause induced currents to give unwanted commands to control surfaces.

A lightning flash lasting less than a second consists of one to as many as 20 or 30 individual strokes, which are surges of current. Each of these surges “turns” the current on and off several times, creating waves of expanding and collapsing magnetic fields. These induce random currents in nearby unprotected conductors. In a worst-case scenario such currents could mimic a digital fly-by-wire signal that commands an unwanted maneuver, such as a violent roll.

Electromagnetic induction occurs because a current flowing through an electrical conductor—such as a wire or an airplane’s fuselage—forms a magnetic field around the conductor. When current begins to flow, its magnetic field expands. This moving magnetic field induces a current in any conductors it crosses. When the current stops, its magnetic field collapses, moving across the second conductor—where it induces another current in the opposite direction to the first one. Engineers are still using the data collected by NASA’s Delta Dart from those flights to help design shielding and other lightning protection systems for aircraft.

Biggest danger is on the ground. The biggest dangers of thunderstorms in the air are violent turbulence that can shake aircraft more than 10 miles from a thunderstorm, and balls of ice—hail—sometimes bigger than baseballs that can shatter windshields

For anyone who flies, lightning is actually a bigger danger on the ground than in the air. This is why airlines stop outdoor activities such as refueling or loading baggage when lightning is seen near an airport. In October 1989 lightning killed an airline mechanic who was wearing a headset attached to a McDonnell Douglas DC–9 that was being pushed back from the gate. Lightning hit the airplane’s tail, sending electricity though the aircraft’s skin and the headset wire to the mechanic, and then to ground.

If you see lightning or hear thunder while doing a preflight inspection, your best bet is to run into a building with electrical wiring and plumbing, and wait for a while after you hear the last thunder or see the last lightning. The plumbing or wiring in such a building will safely carry lightning to ground. Lightning can strike into picnic shelters and similar open structures without plumbing or wires.

The last thing you should do is take shelter from a thunderstorm under a wing. To a lightning flash you are nothing but a sack of salt water, which makes a great path to ground for any lightning that might hit the airplane.