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probably weren’t aware of it, and that is the way it should be. A lightning strike is a safety-critical event and happens so frequently that it can be con- sidered almost inevitable. It occurs once about every 3,000 hours — or once a year for commercial aircraft. A bolt of lightning can deliver a current of
When Lightning Strikes — Keeping Aircraft Flying I
By Jason Evans and Joseph Kane, MK Test Systems, Ltd.
f you travel regularly by air, then it is quite likely that while flying through a thunder- storm your plane was struck by lightning. You
cally or to fail. There are other ways in which electrical sys- tems can be affected, including electrical equipment
faults leading to stray current leakage, and high- intensity radiated fields (HIRF). HIRF is generated by various radio frequency emissions, such as high- power TV, radio or radar signals, all of which can have a similar effect to a lightning strike.
Staying Grounded in the Air The traditional all-metal plane fuselage acts
200kA and cause direct physical damage to an air- plane, such as perforation of the fuselage and burnout of high-resistance joints in the grounding circuits. Damage can also result from the genera- tion of electromagnetic fields and transient volt- ages that are induced into aircraft wiring and cir- cuits, which can cause systems to operate errati-
Different elements of earth bond circuits.
as a Faraday cage, protecting against the direct and indirect effects of lightning, and enabling the transfer of stray current to ground. Today, the increasing use of composite materials, which are good electrical insulators, means that all of the metallic components and elements that make up the aircraft’s structure must be electrically bonded to provide continuity and a reliable earth ground- ing circuit. This circuit allows the stray currents to be safely routed through aircraft and dissipated to an appropriate ground. The testing of that circuit to ensure a low-resistance conductive pathway, both in manufacture and MRO, is vitally important. Cable shielding between connec-
tor backshells, bonding straps, specif- ic mounting bracket fastening meth- ods, pipework, current return circuits, and the aircraft chassis itself, are all used to create the protective ground circuit. A critical aspect of this circuit is that all of these jointing elements provide a low-resistance electrical pathway. If any joint in that pathway
Lightning striking a
commercial aircraft is a safety-critical event and
happens so frequently that it can be considered almost inevitable.
becomes high-resistance, the stray current will have to work harder to force its way through that joint, resulting in a higher temperature. The higher temperature of the joint then further increases its resistance, which adds again to its temperature, which creates a vicious circle. The results can be catastrophic. Aircraft OEMs, such as Airbus
and Boeing, have teams of engineers developing methods to ensure that such damage and effect is minimized, and that the aircraft are considered airworthy following lightning strikes and electrical faults.
Methods of Test Various products are available
for bond testing, the simplest being a bond meter, which is used to meas- ure the conductive path between two elements. Measurement of the elec- trical resistance of the bond between two elements can be carried out using the four-wire Kelvin technique. This provides results that are accu- rate to fractions of a milliohm. Accuracy of the reported resistance value depends on the accuracy of the known current, as well as the accura- cy of the voltage drop measured. Testing an electrical ground
loop can be much more difficult, how- ever. A loop is made of a series of con- ductive components and results in one or more parallel resistance paths. Standard bond test methods should not be used to test the resist- ance of a joint in a loop as they will give a false measured resistance value of that joint. Specific methods have been
Continued on next page See at NEPCON China, Booth 1K30 and SMT Hybrid Packaging, Booth 7A-221
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