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Lightning Strikes: What Pilots Should Know

With all the wicked thunderstorms we’ve had this summer in the Northeast and across the country, we thought this is a good time to review the facts and fiction regarding lightning strikes and aircraft, especially small GA planes, courtesy of Scientific American.

Pilot to pilot on lightning strikes

Retired Malaysia Airlines pilot, Captain Lim, wrote about lightning strikes on his Askcaptainlim.com blog, “Lightning, though fearsome, is not dangerous to the airplane or passengers. Even if there is a direct strike, it does not penetrate the cabin nor affect the engines or fuel tanks. What happens is, when an airplane is struck by a lightning, the electrical charges just traverse the length of the aircraft and exit through the static wicks at the trailing edges of the flaps or tail plane.”

Captain Lim used the analogy of the Faraday cage principle to explain what happens to a plane during a lightning strike.

Faraday was a scientist who discovered that, if you put electricity through a metal cage, no matter how strong or high the voltage is, anything inside the cage is totally protected from the electricity. The airplane cabin or a car body is similar to the Faraday cage. Thus, pilots and passengers should always be safe in an aircraft during lightning strike.

What about the plane? Lim explains that the effect of a lightning strike on an airplane’s body should not be serious — just some small burn marks on the fuselage skin at the point of impact. He advises that pilots always report the strike incident to the engineers after landing for further inspection and repair, if necessary.

What happens when a plane is hit?

On average, each airplane in the U.S. commercial fleet is struck lightly by lightning more than once each year, according to Scientific American. Smaller business and private aircraft are thought to be hit less, because of their smaller size and because they often can avoid weather that is conducive to lightning strikes.

An aircraft can trigger lightning when it is flying through a heavily charged region of a cloud. In this case, the lightning flash originates at the airplane and extends away in opposite directions.

What generally happens is the lightning will attach to an aircraft extremity, such as the nose or wing tip. The airplane then flies through the lightning flash, which reattaches itself to the fuselage at other locations while the airplane is in the electric "circuit" between the cloud regions of opposite polarity. The current will travel through the conductive exterior skin and structures of the aircraft and exit off some other extremity, such as the tail.

Although passengers and crew may see a flash and hear a loud noise, and pilots occasionally report temporary flickering of lights or short-lived interference with instruments, nothing serious should happen because of the careful lightning protection engineered into the aircraft and its sensitive components.

Designed for safety

Engineers make great efforts to design and build lightning-safe planes. Most aircraft skins consist primarily of aluminum, which conducts electricity very well. By making sure that no gaps exist in this conductive path, the engineer can assure that most of the lightning current will remain on the exterior of the aircraft. Some modern aircraft are made of advanced composite materials, which are significantly less conductive than aluminum.

Keeping computers safe

Modern planes, especially passenger jets, have dozens of computers and miles of wires controlling myriad aspects of the aircraft. To protect them from being harmed in a strike, the lightning protection engineer must make sure that no damaging surges or transients can reach the sensitive equipment inside the aircraft.

Lightning traveling on the exterior skin of an aircraft has the potential to induce transients into wires or equipment beneath the skin. These transients are called lightning indirect effects. Careful shielding, grounding and the application of surge suppression devices avert problems caused by indirect effects in cables and equipment when necessary.

Every circuit and piece of equipment that is critical or essential to the safe flight and landing of an aircraft must be verified by the manufacturers to be protected against lightning in accordance with regulations set by the Federal Aviation Administration (FAA) or a similar authority in the country of the aircraft's origin.

Protecting the fuel system

Another main area of concern is the fuel system, where even a tiny spark could cause an explosion. Engineers ensure that the aircraft skin around the fuel tanks is thick enough to withstand a burn through.

Structural joints and fasteners must be tightly designed to prevent sparks, because lightning current passes from one section to another. Access doors, fuel filler caps and any vents must also withstand lightning.

Also, the pipes and fuel lines that carry fuel to the engines, and the engines themselves, must be protected against lightning. And new fuels that produce less explosive vapors are now widely used.

Nose cone and radar

The nose cone that contains radar and other flight instruments is another area that needs special protection. Radar cannot be contained within a conductive enclosure, so lightning diverter strips applied along the outer surface of the radome are used to protect this area.

Special concern for small airplanes

While they usually have a basic level of protection for the airframe, fuel system and engines, small general aviation aircraft usually do not have the same level of lightning strike protection as larger planes, so GA pilots should avoid flying through or near thunderstorms.

Traditionally, most small, commercially made aircraft have aluminum skins and do not contain computerized engine and flight controls, so they’re inherently less susceptible to lightning; however, numerous reports of non-catastrophic damage to wing tips, propellers and navigation lights have been recorded.

GA pilots should also be aware the FAA has a separate set of regulations governing the lightning protection of private aircraft that do not transport passengers.

Source: Scientific American and Askcaptainlim.com.

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