Lightning Strikes Here

There are some hazards of my job that I have to deal with on an almost daily basis. One of these hazards includes lightning strikes. During the spring and summer thunderstorm season my flights often bounce between many electrical storms in order to bring passengers safely to their destinations. My friends and family often ask if I worry about being struck by lightning. The short answer: Yes.

Surprisingly, however, I have the most chance of being injured by lightning while pre-flighting my aircraft before departure. The untimely deaths of some airline employees working outside during thunderstorms caused the FAA to strictly regulate when people may be working outside on an airport during a thunderstorm. Lightning striking people on the ground causes many deaths per year. However, lightning striking an aircraft in flight rarely causes any concern at all.

All airliners are equipped with lightning protection devices. These include special grounding wires to duct the lightning away from the aircraft’s electrical system as well as special discharge devices called static wicks that can help dissipate the energy of the lightning strike. There are no devices on an airplane that keep it from getting struck by lightning in the first place.

Although lightning rarely causes severe damage to airplanes it is the job of the flight crew to try to keep an airplane from being struck by lightning. All airlines create certain standards when flying near thunderstorms. Flight crews will fly a certain distance away from a thunderstorm or fly at a certain height over a thunderstorm. Pilots are not supposed to enter a thunderstorm, but, there are particular circumstances where this may occur. Even if a thunderstorm is completely avoided an airplane may still be struck by lightning, as lightning is known to struck up to 100 miles from a storm.

So why are airplanes struck by lightning? The answer may not be as simple as you think.
Lightning occurs when a storm cloud produces an electrical charge. (How this electrical charge is produced and when or where a lightning bolt will occur is a rather in-depth scientific debate and is not yet fully understood.) This electrical charge is dissipated by sending a massive amount of electrons to another part of the cloud or into the ground. This flow of electrons is visually scene as the lightning bolt. These electrons want to find the path of least resistance their “target” charge. Electrons flow easily in metals, hence the copper wire that brings electricity (electrons) to your house. Since there are usually no metallic objects in thunderstorms, there is a special path created for electrons to flow along called a step leader.

Most airplanes are made of metal, creating a prime target for a lightning strike. However, some new airplanes are designed with composite materials and plastics. Electrons do not flow easily through these materials. Unfortunately, these airplanes are not immune to lightning strikes. In fact, lightning may strike these airplanes more often.

Space Shuttles, Plasmas, and You
As most airliners move at very high speeds through the air there is a build up of plasma on or just above the surface of the airplane. Plasma is a material created by the ionization of normal air molecules. Ions are created when one or more negatively charged electrons are removed from a neutrally charged molecule. This leaves a free floating positively charged molecule (positive ion) and electrons (negative ions).

You might remember that the Space Shuttle is surrounded by plasma when it re-enters the Earth’s atmosphere. As the Space Shuttle streaks through the atmosphere at up to 25 times the speed of sound electrons are removed from the air molecules that pass by the vehicle. This creates the colorful glow of plasma that surrounds the ship as it returns the Earth. Airplanes traveling at more earthly speeds of 500 mph create plasma in the same fashion, although at a much smaller scale. The build up of plasma on a modern jetliner is rarely visible.

Plasma is an extremely conductive material, i.e. electrons easily flow through this material. Basically, the positively charged ions are constantly looking for negatively charged electrons so that they may return to a neutral state of charge. An addition of electrons (electricity) will flow easily through the electrically unstable plasma. No matter what material an aircraft is made out of an extremely conductive blanket of plasma is developed, creating a conduit for lightning to travel through or seek out.

If an aircraft can minimize the creation of the plasma that surrounds it, it may reduce the risk of being struck by lightning. The factors that play a role in the amount of plasma that is created as airplane flies is: materials, speed, and the type of weather an airplane flies through.

While metal itself is a conductive material, it actually can reduce the amount of plasma on the surface of an aircraft. The metal found in an airplane can create a pathway to dissipate the excess charge created by the ionization of air molecules. The negative ions can easily flow out of the air through the metal skin of the aircraft. However, new aircraft made up of certain plastics and composites do not create a pathway to dissipate this charge, so the charge will continue to build up on the surface of the aircraft. Just remember when you rub a rubber (non-conductive material) balloon against your hair (also, non-conductive) there is a great build up of static-electricity, a type of plasma. If you try rubbing a piece of metal against your hair, the amount of static-electricity is much less since the metal can quickly dissipate the charge.

The slower of speed of the aircraft, the less of the plasma build up on its surface. Quite simply, a slow moving aircraft runs into less air molecules to ionize. Pilots can be well advised to slow down in the face of a thunderstorm. A slow moving aircraft produces less plasma, attracting less lightning.

Surprisingly, many airplanes are struck by lightning in a snow storm. Water in the solid form of snow or even its liquid form of rain drops are much more dense then the gases found in the atmospheres itself. An airplane flying into snow or rain runs into more molecules to ionize, hence a greater build up of plasma on the aircraft itself. This fact can easily be seen. An airplane flying through “dry” air (no solid or liquid water) rarely builds up enough plasma to be visible to the naked eye. However, when flying through a rainstorm pilots often report a coronal discharge (sometimes referred to as St. Elmo’s Fire) surrounding the aircraft. This can create a colorful display of blue and green ionized atmospheric molecules emitting from the windows and wing tips of airplanes. Pilots also often report this phenomena occurring just before a lightning strike. The thunderstorm is a double-edged sword: Not only does it create lightning, it produces the rain, snow, and ice that can make an aircraft more susceptible to a lightning strike.

The science of traveling thousands of miles high above the Earth is much more in-depth and complicated than the average passenger may want to imagine. It is the job the engineers, safety experts, and pilots to continually expanded their knowledge of Earth, atmosphere, and technology so that we may continue to travel efficiently and safely.