Munich Airport, a day in May. In the distance, lightning flashes down to the city from an almost black sky. On the runway an A320 awaits clearance for its flight to Berlin. Gusts of wind sweep over the aircraft. Then comes the pilot's announcement: the aircraft will postpone its takeoff until the storm has passed. A sigh of relief from the passengers.
"No pilot wants to take off in a storm," explains Markus Kirschneck of the 'Cockpit' pilot association. "Even during the flight, he will always make a detour around a storm." The reason for this precautionary measure is, first and foremost, the comfort of the passengers. No one likes to fly through turbulence. However, as far as lightning protection is concerned, the passengers don't need to worry. Just like a car or railway carriage, the aircraft acts as a Faraday's Cage, diverting the energy from the lightning strike over the metal bodywork. Although a discharge generates up to 200,000 amps and several million volts, the passengers are in no direct danger from the electrical field.
However, this law of physics can no longer be optimally applied when – as is the case with modern aircraft – the metallic components are replaced by weight-saving composite parts. The metal structure is interrupted and a number of the aircraft's on-board systems are exposed to the energy generated by the lightning. In addition, when a lightning bolt strikes, a considerable rise in temperature occurs. The aircraft's nose as well as protruding parts such as engine air intakes, wingtips or vertical fins are particularly at risk. Serious damage is rare. An aircraft will be hit by lightning only once or twice a year on average. But even this minimal risk should be avoided.
"The swift increase in the use of non-metallic composite materials has made it necessary to find new solutions for the protection of aircraft and their on-board systems," explains Richard Perraud, an electromagnetic compatibility expert at EADS Innovation Works in Suresnes near Paris. In the lightning protection laboratory at EADS Innovation Works, researchers are examining the behaviour of a wide range of composite components when struck by lightning. Batteries and capacitors generate lightning strikes of the same intensity as occurs in nature. The effect on the test items is then compared and analyzed on the computer using special methods for the calculation of electromagnetic fields. The results are incorporated into the development of new aircraft. To enable lightning currents to be better deflected along the composite material skin of the aircraft, the researchers have developed a method by which very thin metal meshes – usually copper wire – are applied to the skin, thereby electrically bonding all the outer elements of the aircraft.
But it is not only the hardware that needs protection: the avionics software of modern aircraft is also at risk from lightning strikes. Every lightning bolt generates a strong electromagnetic pulse. Cables and wiring receive these disturbances like an antenna and conduct them directly to the equipment. However, it only requires an overvoltage of a few volts to put the highly sensitive aircraft electronics out of action. In the worst case, this could cause the flight control to fail, for example. "Therefore we have to develop new test procedures for lightning protection through which we can verify the suitability of both the hard- and software. This calls for using simulation at all levels of development – at the component supplier, at the system supplier and at the aircraft manufacturer responsible for specifying the avionic systems," says Michel Crockaert, who is in charge of the systems environment department at Airbus. Protection solutions are, however, not automatically applicable from one aircraft to another without modification. Depending on the individual configuration of an aircraft, the data for each model have to be re-checked every time. EADS Innovation Works has developed the ASERIS software package for this task.
Where resistance to the electromagnetic environment is concerned, even greater demands are placed on military transport aircraft. In addition to lightning, these aircraft also have to face the radiation field of ground radar and airborne radar. In a parachuting configuration with the cargo-door open, electromagnetic fields can easily couple to the wiring and disrupt the aircraft's electrical systems. The researchers in Suresnes are also working on effective shielding against such influences.
These are all plans that are to provide aircraft with optimal protection against the effects of a lightning strike. However, scientists at the Friedrich Schiller University in Jena, for example, are now working on ways to prevent lightning strikes from even occurring. Their intention is to actually "suck" an approaching storm out of the sky. Using a three billion watt laser beam, they are changing the air's properties so that it becomes as conductive as a wire. At the approach of a storm, the scientists intend to use the laser beam to establish a link to the cloud through which the lightning is to be discharged. However – as yet – this procedure appears to be both complicated and impractical. But, given time and further technology development, this may change. And then the A320 could also enjoy a trouble free takeoff from Munich Airport – despite the impending storm.
In the lightning protection laboratory at EADS Innovation Works, researchers are examining the behaviour of a wide range of composite components when struck by lightning. Batteries and capacitors generate lightning strikes of the same intensity as occurs in nature.
