Apple #503: Lightning Striking Airplanes

So I was watching a really bad and therefore entertaining disaster movie this weekend.  It was one of those global-warming-is-causing-crazy-weather-and-destroying-the-planet situations.  


The movie in question, Category 7: The End of the World. A storm with multiple tornados is on the path to colliding with a hurricane. Starring such luminaries as Randy Quaid, Shannen Doherty, Robert Wagner, Swoosie Kurtz, James Brolin, Tom Skerrit, and more. Superb 3-star entertainment.
(Photo from Amazon)


At one point in this movie, Tom Skerrit had to fly his airplane into one of these really bad storm clouds to collect weather data, and his plane got struck by lightning.  It wasn't just a simple lightning strike, of course, but one that enveloped the plane in blue zappy light.  This caused me and my movie-watching companion to wonder, what happens in real life when a plane gets struck by lightning?

• The short answer: not much.

• In 2001, an engineer for a company called Lightning Technologies Inc (LTI) wrote an article for Scientific American about lightning strikes and airplanes. His company makes equipment to protect airplanes from lightning strikes, so apparently he knows a lot about the topic. Anyway, everybody has been quoting his data ever since.

• According to this LTI guy, somebody (the FAA?) estimated that, on average, each US commercial airplane gets struck by lightning at least once a year, if not more often.

• But even though lightning strikes are rather common, they don't do a whole lot of damage. 

• The National Lightning Safety Institute says that since 1959, there have been 7 airplanes in the world that have gone down due to lightning strikes.  The last US plane to have gone down due to lightning was a Pan Am Boeing in 1963.  Given that each commercial airplane gets struck by lightning once or twice a year, the fact that none have crashed due to lightning in 48 years is a pretty good safety record. 

• Most of the time, passengers are completely unaware of it if lightning strikes an airplane in flight. Sometimes they may notice a bright flash or hear a loud noise, or the lights in the cabin may flicker.  But usually everybody flies onward, blissfully ignorant.

• After a plane has landed, the ground crews and pilots walk around the outside of the plane to check for any damage. They do this for every plane, at every landing.  Damage from lightning strikes is one of the things they look for. Such damage is usually very small, burned-looking hole about the size of a quarter, and most often at the tip of the wing or the tail.

Small burned patches where lightning entered and exited the nose of this private plane. This type of lightning damage is even smaller and less common on commercial aircraft.
(Photo from Learn to Fly)

• But most of the time if a plane does get struck by lightning, there isn't even this much damage. Usually, the plane is pretty much unaffected. This is because airplanes have been engineered, since the 1930s and more recently, to withstand lightning strikes.

• First of all, the outer "skin" of an airplane is made almost entirely of aluminum.  Aluminum conducts electricity very well, which means that if lightning strikes the plane, the bolt won't zap through the frame but instead the electricity will travel along the skin and exit at one of those pointy places, usually the tail or the end of the wing.

• If the "skin" has other metals in it that are less conductive, the manufacturers will add composites which are more conductive.  They'll line the outside of the plane with these composites in such a way that they'll act as a kind of channel or pathway for the electricity to follow along the body of the plane toward the pointy places at the extremities. 

• The nose cone (radome) contains the radar and electronic equipment associated with the flight instruments, and those can't be covered with conductive material or they won't work.  So the nose cone also gets additional strips of conductive material applied along it.  Sometimes instead of strips, the conductive metal is in the form of buttons that are spaced close together.  Closely spaced dots are visible on the photo of the radome above.

• Another lightning precaution is pointy metal things called static wicks. You've probably noticed them sticking off the wing of the airplane you're flying on.  They're connected to the pointy parts of the aircraft, and they're encased in fiberglass so that whatever charge they collect won't be transferred to the frame.  Any static electricity that builds up in the air around the plane gets dissipated through the static wicks. If lightning does strike the plane, the electricity often will travel to and along the wicks, away from the plane.  They're not supposed to be considered as lightning protection, but some people describe them as doing that.  Missing static wicks is another thing the crew look for when they're doing a visual check of the plane.

One kind of static discharge wicks mounted along the wing of an airplane.
(Photo from the 737 Technical Site)

• Additionally, all the parts that are anywhere close to the fuel supply have to be built to withstand any lightning currents and so that no sparks will enter the fuel system. The tanks have to be made to a required thickness, the joints and fasteners must be kept to a specified tightness to prevent sparks, and caps, doors, vents, and fuel lines all have to be tested to make sure they're thick enough and can withstand lightning. Fuel tanks also contain their own static discharge equipment.

• The electronics within the plane are also built -- "hardened" -- to withstand lightning and to protect against static buildup.

• Finally, pilots are trained to avoid flying through or even close to thunderstorms.  They're actually trying to avoid wind shear and turbulence, which are far more problematic for an airplane than lightning, but reducing the possibility of lightning strikes is of course helpful, too.

• Because of all these precautions, lightning strikes, though common, are considered to be a pretty low threat in air travel.

Here's a Boeing 747 taking off in a rainstorm. As it ascends, a bolt of lightning hits it. The video shows this once, then loops back to show the same lightning strike in super-slow-motion again. You'll be able to see how the bolt enters near the front of the plane and exits at the tail. The plane keeps right on flying, undisturbed.



You may also be interested in my entry on Lightning or another one on Thunderstorms.


Sources
Edward J. Rupke, What happens when lightning strikes an airplane? Scientific American, 2001 (reprinted August 14, 2006)
A. Pawlowski, Can lightning bring down a plane? CNN, August 17, 2010
National Lightning Safety Institute, Aviation Losses from Lightning Strikes 
Physlink.com, How is a plane protected from Lightning strikes?
The 737 Technical Site, Wingtips, Static Dischargers
Jack Williams, USAToday Weather, Answers: Does lightning hit airplanes, June 1, 2004