Lightning Strikes

April 16, 2007
A look at how they affect aircraft and avionics.

When it comes to natural phenomenon, lightning has probably been the most observed throughout the ages. It has been worshiped, studied, and feared. Lightning is a result of static electricity and has been seen in volcanic eruptions, intense forest fires, surface nuclear detonations, heavy snowstorms, in large hurricanes, and even in the exhaust of large turbine engines.

However, it is most often seen in thunderstorms.

Ben Franklin usually gets the credit for concluding lightning is based on the theory of static electricity.

The Franklin experiment is as follows: He got the idea of using a flying object, such as a kite. During the next thunderstorm, which was in June 1752, accompanied by his son as an assistant, he raised a kite. On his end of the string he attached a key and tied it to a post with a silk thread. As time passed, Franklin noticed the loose fibers on the string stretching out; he then brought his hand close to the key and a spark jumped the gap. The rain had soaked the line and made it conductive.

Please do not attempt this at home!

At any given moment, there can be as many as 2,000 thunderstorms occurring across the globe. This translates to more than 14,500,000 storms each year. The National Aeronautic and Space Administration (NASA) satellite network indicates they produce lightning flashes about 40 times a second worldwide.

Positive and negative charges

A thunderstorm gathers a pool of positively charged particles as it moves along the ground and they travel with the storm. As the differences in charges continue to increase, positively charged particles rise up taller objects such as trees, houses, and telephone poles. A channel of negative charge, called a "stepped leader" will descend from the bottom of the storm toward the ground. It is invisible to the human eye, and shoots to the ground in a series of rapid steps, each occurring in less time than it takes to blink your eye. As the negative leader approaches the ground, a positive charge collects in the ground and in objects on the ground.

This positive charge "reaches" out to the approaching negative charge with its own channel, called a "streamer." When these channels connect, the resulting electrical transfer is what we see as lightning. After the initial lightning strike, if enough charge is leftover, additional strikes will use the same channel and will give the bolt its flickering appearance.

Thunder is a product of lightning and is a direct result of the shockwave generated by the rapid air movement caused by the streamer. As light travels faster than sound, we will observe the flash of lightning prior to the rumble of the resulting thunder. A general precaution is to consider lightning to be a local hazard if the thunder occurs within 30 seconds of the flash.

Some lightning originates in the cirrus anvil or upper parts near the top of the thunderstorm, where a high positive charge resides. Lightning that forms in this region follows the same situation as previously described, but the descending stepped leader will carry a positive charge while its subsequent ground streamers will have a negative potential. These bolts are known as "positive lightning" because there is a net transfer of positive charge from the cloud to the ground.

Positive has negative effect

Positive lightning makes up less than 5 percent of all strikes. However, despite a significantly lower rate of occurrence, positive lightning is particularly dangerous. Since it originates in the upper levels of a storm, the amount of air it must burn through to reach the ground is usually much greater. Therefore, its electric field typically is much stronger than a negative strike. Its flash duration is longer, and its peak charge and potential can be 10 times greater than a negative strike; as much as 300,000 amperes and 1 billion volts!

Some positive strikes can occur within the parent thunderstorm and strike the ground beneath the cloud. However, many positive strikes occur near the edge of the cloud or strike more than 10 miles away, where you may not perceive any risk nor hear any thunder. Positive flashes are believed to be responsible for a large percentage of forest fires and power line damage. Thus, positive lightning is much more lethal and causes greater damage than negative lightning.

There are two principal sources of static electrification on aircraft. The Autogenous Field is caused by frictional charges resulting from contact between the aircraft and various particles such as dust as it moves through the atmosphere. Exogenous Fields are caused by the presence of the aircraft in a thunderstorm which can cause both positive and negative streamers.

Aircraft lightning strikes

In the United States, the Federal Aviation Administration (FAA) has a system in place to track lightning strikes on commercial aircraft. The reported statistical results indicate that lightning strike frequency is such that every commercial aircraft gets one and a half strikes per year and commercial pilots experience this phenomenon once every 3,000 flight hours. Although general aviation aircraft are not required to report these incidents, the U.S. Department of Transportation has conducted a research project which was completed in 2004 and is titled "General Aviation Lightning Strike Report and Protection Level Study."

This report analyzed 95 lightning strike reports from general aviation business jet aircraft that occurred over a five-year period. The analyses were conducted to determine which variables most affect the severity of indirect lightning effects damage of in-service aircraft and their systems and to assess the effect of the level of lightning and High-Intensity Radiated Fields (HIRF) protection design and implementation.

After validating the data, three variables were studied with respect to lightning damage: aircraft age, aircraft flight hours, and the level of lightning and HIRF protection. The level of protection for each aircraft model in the database was categorized as no protection, avionics protection, or full protection.

The study found that fully protected aircraft had a significantly lower percentage of electrical failure and interference due to lightning strikes when compared to aircraft with no protection or only avionics protection. The number of electrical failures reported did not increase over the age of the aircraft. Another result illustrates the areas where lightning tends to strike on various manufacturers' airframes.

This table shows that Zone 1, the radome and the wing tips, was the most frequent area of lightning attachment. Zone 2 includes areas on the bottom of the fuselage and wing tips, while Zone 3 includes large areas under the wings.

Protection and avoidance

Federal Air Regulation (FAR) 25.1316 along with Advisory Circular 20-136 provides guidance to manufacturers of Transport Category aircraft for protection against lightning strike and HIRF.

Combining new research on lightning with the lessons of the past we know it is true that some aircraft are less prone to lightning strikes. Size, shape, and speed are all aircraft-specific variables which determine susceptibility to a lightning strike. It is also true that aircraft damage varies with type. Airframe design can minimize lightning damage. However, all surfaces are susceptible to lightning strikes and all unprotected systems can be affected.

Some pilots are better at avoiding lightning strikes than others. The wider the berth given to thunderstorms, the better the chance of avoiding a lightning strike; however, the pilot who tries to pick his or her way between thunderstorm cells is asking for trouble.

Several theories proven false

The theory that if you avoid thunderstorms you will avoid all lightning strikes is false. Statistics show that many triggered strikes have occurred during flights that did not penetrate thunderstorms. Aircraft have triggered strikes in cirrus clouds downwind of previous thunderstorm activity, in cumulus clouds around the periphery of thunderstorms, and even in stratiform clouds and light rain showers not associated with thunderstorms.

The statement that if you are greater than 20 miles from radar-indicated precipitation, you are not susceptible to a lightning strike is also false. Aircraft have been struck by the proverbial "bolt from the blue" on more than one occasion. In fact, aircraft have been struck at distances out to 50 nautical miles from thunderstorms, particularly when cirrus clouds existed above or at their altitude, or when there were other developing showers nearby that had not yet reached maturity. Flying through precipitation, volcanic ash, or heavily polluted air can cause an aircraft to experience electrostatic discharge or triggered lightning. Usually these discharges cause only minor aircraft damage; however, there is always the chance for catastrophic occurance if the discharge passes through the vaporized fuel-air mixture in the fuel tank.

Cirrus clouds are made up of ice particles, a large number of which will strike the aircraft and bounce away. At each impact electric charges are transferred leaving the aircraft with a significant potential. This can reach a value of 100,000 to 200,000 volts in a matter of seconds. A discharge back to atmosphere can be expected and usually occurs at the trailing edges of flying surfaces and most frequently at wing and stabilizer tips. As the discharge occurs, the surrounding air becomes ionized and a corona pulse will depart the aircraft.

How to ensure safety

It is initially up to the aircraft manufacturer to verify safety in the event of a lightning strike. In many cases special inspections have been created to look at designated areas on a particular airframe when a lightning strike has been reported. In addition to inspecting external surfaces for visual damage attention needs to be paid to the not so obvious areas such as bonding straps, hinges, and other surface connecting components. Should a lightning strike be observed on a propeller, arcing may have resulted on various engine bearings. If retractable landing gear is the target when extended then trunnion bearings or actuators may have incurred damage.

The United Kingdom Civil Aviation Authority (CAA) has issued Airworthiness Information Leaflet (AIL) 0014 which addresses enhanced lightning strike protection in light aircraft. The information given in this leaflet outlines the problems which may be encountered on aircraft fitted with metallic or nonmetallic tip tanks or other external nonmetallic components and gives guidance on methods of reducing their vulnerability to lightning strikes.

Direct and indirect effects

Aircraft damage from lightning can be caused as a direct or indirect effect. Direct effects result when the lightning current attaches to and flows through the aircraft skin. Locations on the aircraft where lightning strikes occur experience extreme heating which causes burning and melting damage. Current flowing through the aircraft structure can result in isolated arcing or sparking and heating. If this occurs in a fuel tank, explosion and fire can result.

Indirect effects are caused by transient electrical pulses produced by the changing electric and magnetic fields due to the lightning current. Unless avionics and other systems are properly shielded, they are easily damaged.

In some cases metal encased fuel tanks have had sealant boundaries reinforced to inhibit an electrical spark occurring within the tank in the event of the tank surface being the point of lightning entry.

It is interesting to note that 57 percent of the aircraft lightning mishaps occur during the months of March through July, and the state of Florida has the highest number of lightning events along with the most strike victims.

It would appear that participating in a kite-flying festival this spring in Florida may be an electrifying experience.

Jim Sparks has been in aviation for 30 years and is a licensed A&P.