? Lightning is a bright flash of electricity which produced by a thunderstorm. All of the thunderstorms produce lightning and are very dangerous. When we hear the sound of thunder, then we are in danger from the lightning. When we consider United States, there are an estimated 25 million lightning flashes in each year. During the past 30 years, lightning killed an average of 66 people per year. The documented lightning injuries in the United States average about 300 per year, undocumented injuries likely much higher.
? Also Lightning is a discharge of electricity between highly charged cloud formations, or in between a charged cloud and the ground. When we consider the case that an aircraft is flying, or on the ground in the vicinity of such a cloud formation, the discharge may strike to the aircraft and it is resulting in very high voltages and high current passing through the structure. All of the separate parts of an aircraft are electrically bonded together to conduct a lightning strike away from areas where damage may hazard the aircraft. As an example ,fuel tanks or flying controls, and during manufactures special precautions are often taken with non-metallic components such as wing tips, external fuel tanks and nose cones.
? What causes Lightning?
? Basically, lightning is an electric current, Within a thundercloud many small bits of ice or frozen raindrops bump into each other or collide as they move around in the air. Those kinds of collisions create an electric charge.
? After some time, the whole cloud fills up with many electrical charges. The positive charges, or protons, are formed at the top of the cloud and the negative charges, or electrons, are formed at the bottom of the cloud. Because of their opposites attract, that causes a positive charge to build up on the ground beneath the cloud.
? The ground’s electrical charge concentrates around anything that sticks up, such as mountains, people, or single trees. The charge coming up from these points eventually connects with a charge reaching down from the thundercloud and lightning strikes!
? When storms are developing ,those clouds become charged with the electricity. Scientists are still not sure exactly what causes this, but they do know that when the voltage becomes high enough for the electricity to leap across the air from one place to another, lightning flashes.
? As shown in FIGURE 1, Lightning can strike within a cloud, from one cloud to another or from a cloud to the ground, or from the ground to a cloud.
FIGURE 1- Lightning Examples
? How powerful is Lightning?
? Each spark of lightning can reach over five miles in length, soar to temperatures of approximately 54,000 degrees Fahrenheit (30,000 Celsius), and contain 100 million electrical volts.
? Lightning is a Random, Chaotic and a Dangerous Fact of Nature
? At any given moment, there are about 1,700 thunderstorms in progress somewhere on the earth. This amounts to 16 million storms each year. Scientists who studies lightning, have a better understanding today about the process that produces lightning than the past ,but there is still more to learn. We know that the conditions of clouds needed to produce lightning, but can’t forecast the locations or the time of next strike of lightning. As an example, There are many lightning detection Systems in the United States and they monitor an average of 25-26 million flashes of lightning from the cloud to ground each & every year.
? Lightning has been seen in volcanic eruptions, extremely intense forest fires, surface nuclear detonations, heavy snowstorms, also in large hurricanes. However, lightning is most often seen in thunderstorms. A thunderstorm forms in air that has three components. They are;
3) Something such as a cold front to cause the air to rise.
Continued rising motions within the storm can build the cloud to a height of 30,000 to 55,000 feet (6 to 10 miles) above the sea level. Temperatures are higher in the atmosphere are colder; ice forms in the higher parts of the cloud.
? How Lightning Develops Between the Cloud and the Ground
? When we consider a moving thunderstorm that gathers another pool of positively charged particles along the ground that travel with the storm. When the differences in charges continue to increase, positively charged particles rise up taller objects such as houses, trees, towers and telephone poles .
? The negatively charged area in the storm will send a charge towards the ground which is called ‘a stepped leader’. That is invisible to our human eye, and moves in steps in less than a second toward the ground. When it gets close to the ground, it is attracted by all of positive charged objects, and a channel develops. We see that the electrical transfer in this channel as the lightning. There could be several return strokes of electricity within the established channel that we will see as ‘flickering lightning’.
FIGURE 2- How lightning happen
FIGURE 3-Starting upward leader defining the point of strike
? Lightning Strikes effect to Aircraft and its protection
When we consider lightning , its effect of the aircrafts is a major issue. Mostly Commercial aircraft are struck by lightning once every 1,000-10,000 hours of flight,( or about once a year). It is estimated that each of commercial aircraft gets hit by lightning once every year as an average. The probability value of lightning strike for short-distance aircraft is higher than for long-distance aircrafts. In most of these cases, 90% approximately, happens when the aircraft itself triggers lightning when flying through a heavily charged region of a cloud and the other 10% happens when the aircraft intercepts a natural lightning channel. When we consider up to the year of 2014, lightning strikes have damaged more than 2500 aircrafts. Within this context the most severe accident registered so far happened in 1963 when a lightning strike ignited a fuel tank crashing a Boeing 707 and causing 81 people to perish.
Electrical currents as high as 200,000 Amperes (A) travel through the least resistant parts of the aircraft, and if the aircraft is not well protected, bad consequences such as vaporization of critical aircraft parts can be happened. Aircraft were usually made of aluminum before composite materials were introduced around 40 years ago. It provided the necessary electrical conductivity to keep the aircraft and its systems, such as onboard electronics or metallic control cables, safe. The aircraft antenna is protected by a radome, which diverts the strike to the grounded section of the aircraft while allowing electromagnetic waves used for communications
The effects of lightning can be categorized into,
1) Direct effects
2) Indirect effects.
The direct case is related to physical damage occurring at the attachment point and indirect effects are associated with the interference due to electromagnetic coupling with systems or cabling. Until the late 60’s, aircraft were built essentially with a structure of metal materials, which is considered ideal for mitigating the lightning strike effects once it helps the electrical current flow, reduce the Joule effect and works as a Faraday’s cage.
However, development of new materials and the desire for aircraft weight reduction led to the use of composite materials on its structure. These materials have anisotropic characteristics and low values of electrical and thermal conductivities therefore they are more susceptible to the effects of lightning strikes. Carbon fiber reinforced plastics (CFRP) have electrical resistance two thousands larger than of an aluminum material.