The lightning strike commonly attaches to an aircraft nose, wing tips, vertical fin tip, horizontal stabilizer tips, propellers, antennas, pitot booms or pylon-mounted.
These already mentioned are the places where the lightning arc attaches in a direct way to the aircraft, causing physical damages effects called Direct Effects, which correspond into skin hole at the point of flash attachment, welding or roughening of moveable hinges and puncturing or splintering of non-metallic structures.[11] The degree of exposure to lightning effect that correspond to the definition of Suscep-tibility of components, strongly depends on their location on the aircraft and using the lightning strike zone definition of Chapter 3.3, it is possible to established their vulnerability. For this reason aircraft protection techniques have been developed for protecting them from either type of lightning effect and in the particular case of Direct Effects should be include the following:
• In general electrical cables should not be exposed to the direct effect of light-ning, but when this is unavoidable these cables should be covered by enclosures of conductive material with good mechanical and electrical properties.
• The interfaces between the vehicle surfaces and the elements of the vehicle,
• It should be present appropriates non-detrimental conductive paths (lightning bonds) between structures, components, joints, extremities to conduct/atten-uate, or able to direct in a proper and safely way the applicable portion of the lightning currents and voltages.
• Design in proper ways the containers, tanks, lines and vents as not to be reached by any effects of a lightning flash because could be dangerous for the flammable fluids or gases contained in these solids.
Following are presented some common examples which occur on aircraft, in order to understand this kind of effects and the attention that need the aircraft designer when project the aircraft.
5.2.1 Direct Effects on Skin Structures
The outer skins of the aircraft and the internal framework characterized by ribs, bullheads and spars, that are the components in which the lightning current flows between the entry and exit point of the aircraft and tends to spread out between at-tachment points, making the aircraft an entire conductor. Any conductive material, metal or conductive composite with which most of these structures are fabricated becomes part of the conductive path for lightning current. Despite the current den-sity is high in a single point of the surface of the aircraft, rarely this causes a physical damage, instead it is what does happen when there is not a good electrical bond between the structural elements.
From the effects which suffers the skin, both metallic and CFC-composite, it is possible to include:
• melting or burning at the lightning attachment points and it is possible to evidence the presence of pit marks along the fuselage or holes in the final part of the edges or empennage tips. Most of the time the holes are present in skins of 1mm (0.040”) thick instead in case of trailing edge, where the leader attached the skin for longer time, it could happen a melting or burning of this thicker surfaces.
• magnetic force damage could be the result of the intense magnetic field which accompany the lightning current near the attachment points. This kind of damage is not enough significant to require an abortion of flight, but if severe bending metal is involved the damage is not more repairable.
• resistive temperature rise is due by lightning currents that deposit the energy in the conductor. When the lightning occur, the most part of metal is able to tolerate it without overheating, but this is not true for all. For this rea-sons there are methods for determines temperature rises in cases of specific materials.
• shock wave and overpressure, occur when the lightning stroke current flows in the channel leader, for a time of 5 − 10µs, causing an expansions of it with an increasing in velocity, until will propagate to the surface of the aircraft.
Taking into account the distance between the channel and the surfaces (wind-shields or the navigation light), the result will be an overpressure of several 100 atmosphere at the surface that will cause an implosion.
Once presented the problematic effects and for the particular case of melting or burning, it is possible to see some examples of lightning testing on skins, in which are highlighted the types, evaluated the several thickness and other conditions like the thickness of protection painted material.
In case of a skin panel in Aluminium, which gives a very high degree of conductivity, have been encountered damages in Zone 1A, as in Figure 5.2. Instead, considering a
Figure 5.2: Aluminium panel (0.032”) thick [24]
0.040” painted aluminum panel covered by aluminium tape, it is possible to see from the Figure 5.3 the completely reduction of the damage. As already seen in Figure
Figure 5.3: Painted aluminium panel covered by aluminium tape (0.040”) thick [24]
5.3, the protection added to Aluminium skins increased the resistance to melting
methods, which are highlighted in Figure 5.4 for the completely reduction of the damages.
Figure 5.4: Methods for protecting against melting and burning [24]
1. Consist of the increase of skin thickness;
2. Insertion between the surfaces of a barrier of dielectric;
3. Addition in the upper surface of conductive particles, which lead to the light-ning flash a major diffusion in these new multiples conduction paths.
4. Insertion of a thermal barrier between skin and inner layer.
Moreover, have been done also other studies on protection’s techniques for the Non conductive composites and carbon fiber composites (CFC), which characterize other skin materials utilized for the realization of the aircraft.