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Thunderbolts and lightning

Storms are one of the most fascinating and violent meteorological phenomena on earth, especially if you are a navigator. For as long as humans can remember, sighting a storm on the horizon has always constituted a threat that must be taken into account. Storms are a phenomenon characterised by an encounter of two or more air masses at different temperatures, which leads to a disturbance in the atmosphere. This results in strong winds, rain, bolts and flashes of lightning, thunder, and even snow. Storms are not the ideal scenario in which to travel, but sometimes there is no choice: you have to go through them.

Several planes are struck by lightning mid-flight every day. However, this phenomenon does not impact on safety, and hardly alters the normal course of travel.

Lightning is one of the most feared storm phenomena. It is a powerful natural discharge of static electricity that generates an electromagnetic pulse capable of creating an instantaneous discharge of power of one gigawatt, or 1 billion watts.

Clearly, although current means of predicting weather are very advanced, it is not possible to predict exactly where and how lightning will strike. In an aeronautical context, aeroplanes can attract lightning, especially during take-off and when they pass through stormy areas. On average, an aircraft is struck by one lightning strike every year. This happens in a fraction of a second and without passengers noticing anything other than possibly a flash of light outside the aircraft.

So how is it possible for aircraft to come away from a lightning strike unharmed?

This is due to a physical phenomenon known as a Faraday cage. This effect is responsible for the fact that, when lightning strikes an aeroplane, the net sum of the electric field and the magnetic field inside the aeroplane is zero. The way that a Faraday cage works is based on the properties of a conductor in electrostatic equilibrium. When the metal cage (the plane) is placed in the presence of an external electric field (lightning), the electrons, which are free in a metal, move in the opposite direction to the electric field and position themselves on one side of the cage. This results in a strong negative charge on one side. Conversely, the other side is left without electrons, which results in a strong positive charge. This is known as polarization of an electrical conductor (negative pole and positive pole), which as a result creates a magnetic field of equal magnitude to the electric field that generated it, but in the opposite direction. This is how the forces of both fields are counteracted and cancel each other out. It is also important to note that the electrical current is conducted around the outside whenever the cage is closed, such as in the case of an aeroplane or car.

This phenomenon is also used to protect sensitive electronic equipment from external radio frequency interference (RFI), as well as to enclose devices that produce RFI, preventing their radio waves from interfering with other nearby equipment.

December 1963 marked a turning point in aircraft lightning safety, due to the fatal accident of the Pan Am Flight 214. A Boeing 707 flight took off from San Juan, Puerto Rico, on its way to Baltimore, Maryland. It was struck by lightning mid-flight in Elkton, Maryland. The lightning ignited fuel vapours in the fuel tank, causing an explosion in one of the wings. At 08:59 p.m., the plane fell into the Elkton cornfields. There were no survivors.

Following the accident, in 1967, the United States Federal Aviation Administration (FAA) updated fuel tank safety standards to prevent ignition due to lightning strikes. In 1970, preventive requirements were also put in place for other aircraft components. That accident made the aviation world aware of lightning safety in aeroplanes. Since then, the FAA and the various international organisations concerned with aircraft safety have been constantly updating preventive safety measures in this area.

Nowadays we know that aircraft behave like conductors because they are made of metal. At first glance, this is actually a major disadvantage for aircraft that are made out of next-generation materials. This is the case for the Boeing 787 Dreamliner and the Airbus A350 XWB, which are built with large amounts of composite materials, mainly carbon fibre. In the lightning laboratory at Cardiff University (Wales), they tested the effect a lightning strike could have on a carbon fibre composite sheet. The important conclusion was that the damage caused by a lightning strike on a material such as carbon fibre is quite significant, which could prove fatal mid-flight.

The solution that has been designed to counteract this disadvantage is simple: cover the composite material with a mesh made out of a light conductor. Having this mesh wrapped around the aircraft is enough to achieve the Faraday cage effect on the set of composite material components that make up the aircraft. New aircraft made out of composite materials are thus able to protect themselves against lightning without sacrificing their superb performance.

There are many examples of this phenomenon; here, we have singled out just two videos that show how lightning striking an aircraft does not affect its flight.

Video 1

This is a Boeing 777-300 that was flying from Schiphol, Amsterdam, to Lima, Peru. The video shows how the aeroplane is hit by lightning during take-off, and then continues its flight normally with no consequences. The video went viral before the plane reached its destination. The passengers weren’t aware of this until they landed in Lima and could watch the video themselves on their own mobile phones.

Another example of the impact of a lightning strike on an aeroplane can be seen in this video:

Video 2

This video was recorded by a university student, and in it you can see how a plane is struck by lightning mid-flight near Seattle-Tacoma airport, USA, and then continues its flight without any issues.

Have a good flight… even in the middle of a storm.

 

 

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30/09/2019

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