Why is an airplane safe from lightning strikes?

August 27, 2014

 

Have you ever wondered why you’re safe in an airplane that flies through a thunderstorm with lightning?

At first glance you might think that this flies in the face of the basic safety rule: when thunder roars go indoors.

In other words, when you’re on the ground, you should take shelter in a building with plumbing and electrical wiring. You’re also safe in a metal vehicle with the windows rolled up, as long as you don’t touch any metal, such as the shift lever. Here, lightning flows through the vehicle’s outside metal and jumps to the ground.

A plane’s outer skin does the same job as a metal vehicle on the ground. Lightning hits one part of the airplane, follows the outer surface of the frame and jumps back into the air, possibly leaving small burn marks where it enters and leaves.

If you’re on a Boeing 787 “Dreamliner” or another airplane with a skin made mostly of composite materials, conducting material embedded in the composite does the same job as the aluminum skin of older aircraft.

 

Today’s pilots strive to avoid thunderstorms by a wide margin mainly because of the dangers of extreme turbulence and hailstones that could possibly break the windshield and otherwise damage the airplane.

Nevertheless, airplanes can trigger lightning strikes — maybe a thunderstorm’s first strike – event when they are outside cumulus clouds. Cloud-to-ground flashes have hit airplanes as they take off or land, with the lightning passing through the airplane and continuing on to hit the ground.

The Federal Aviation Administration estimates that on average, lightning hits each airliner in U.S. service once a year.

Yet, the last U.S. airliner crash that lighting caused occurred on December 8, 1963 when lightning struck a Pan American Boeing 707 over Elkton, Md. Investigators said it ignited jet fuel vapor in a wing tank, which exploded, causing the jet to crash, killing all 73 aboard.

This crash led to aircraft lightning-safety research and lighting protection rules. This research and rule making continue today as airplanes become more complex with electronic equipment.

When lightning hits an airplane it sends currents of up to 200,000 amperes through the plane’s skin and frame, possibly from the nose to the tail, or a from a wing tip to the other wing tip.

Basic lightning protection begins with ensuring that fuel tanks and fuel lines have no places where lightning currents could cause a spark that would trigger an explosion.

 

As airplanes become more complex, lightning safety rules are updated to ensure safety. A big challenge is protecting today’s complex electrical and electronic systems, including “fly by wire” systems that use electrical wires and computers to link pilots’ controls to the to the movable parts of the wings, and the horizontal and vertical stabilizers at an airplane’s rear that cause an airplane to turn, roll, and pitch the nose up or down at pilots’ commands.

Any electrical current creates a magnetic field around the conductor (such an airplane’s skin) that’s carrying it. As this field expands it induces an electrical current in nearby electrical conductors, such as an airplane’s wires. When the current stops flowing the field collapses, creating currents flowing in the opposite direction in conductors.

A one-second lightning flash might contain 20 or more individual strokes that create waves of expanding and collapsing magnetic fields as each stroke ends and another begins. It’s easy to imagine what such on-and-off induced currents could do to simple electrical systems, such as radios, much less to fly-by-wire systems without today’s shielding and surge protection systems.

The greatest potential danger to people of lightning hitting an airplane is probably on the ground. For example, in 1989, lightning killed an airline employee who was talking to the crew using a headset connected to the airplane by a wire when lightning hit the airplane. The plane was being pushed back from a gate at Orlando International Airport.

Furthermore, letting passengers leave or board an airplane on outside metal stairs, rather than on an enclosed jet way, is also dangerous when you can see lightning or hear thunder.

Such potential accidents have prompted airlines to stop many activities around planes on the ground during lightning storms, including refueling, loading luggage, and boarding passengers when there is no enclosed jet way.

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Dan Stillman · August 27, 2014