Before generators, busbars and load-shedding logic, the electrical module needs its physics: what charge is and what it does when it sits still, what it does when it moves, and the two ways it can move. This lesson covers static electricity and why aircraft are bonded and grounded, the direct-current circuit with its vocabulary of volts, amperes, ohms and watts, and alternating current, the form in which large aircraft generate almost all their power.
Static electricity
Static electricity is charge that has accumulated and has nowhere to go. Rub two dissimilar materials together and electrons transfer from one to the other, leaving one positively and one negatively charged. An aircraft in flight is a rubbing machine on a grand scale: air and everything in it streams over the skin at hundreds of knots, and each impact transfers a little charge. The build-up is worst where there is the most to rub against: flight through precipitation (rain, and especially snow and ice crystals), through clouds of ice crystals at high altitude, and through dust or volcanic ash. The phenomenon is called precipitation static, and a heavily charged airframe announces itself: radio interference that grows to a roar on the ADF (automatic direction finder) and HF (high-frequency) bands, and at night sometimes a visible bluish discharge crawling over the windshield, known to sailors and pilots alike as St Elmo's fire.
The static discharger exists to let that charge leave quietly. Charge concentrates at sharp points and leaks into the surrounding air most easily there, so the designer provides sharp points deliberately: small rods or braided wicks, each with a high-resistance path to fine sharp tips. Their typical locations are the trailing edges of the wings and tail surfaces and the wing tips: the rearmost and outermost points of the airframe, where the discharge happens far from the antennas it would otherwise deafen. The high resistance matters: it spreads the discharge into a continuous quiet bleed instead of the sharp crackling pulses that cause the interference. The pilot's role is the pre-flight inspection: walk the trailing edges and confirm the dischargers are present, unbroken and firmly attached. A missing or frayed discharger cannot be fixed in flight, and its cost is paid exactly when radio matters: in cloud, in precipitation, on instruments.
Grounding before refuelling deals with the same charge on the ground. Fuel rushing through a hose generates static electricity of its own, and an aircraft that landed carrying charge may sit at a very different electrical level from the fuel truck and the earth beneath both. If the first connection between them is the fuel nozzle meeting the filler neck, the difference equalises as a spark, adjacent to fuel vapour. So the aircraft is connected to earth and to the fuelling vehicle by bonding cables before any fuel cap opens: all three are brought to the same electrical level first, and then no spark is possible, because a spark needs a difference to jump across.
Electrical bonding applies the same logic inside the aircraft, permanently. Every major metal part (engines, control surfaces, panels, pipes) is connected to the main structure by bonding strips or leads, so that the entire airframe forms one electrically continuous body at one potential. The reasons stack up: no charge difference can build between parts, so no internal sparking (which matters most around the fuel tanks); a lightning strike finds a broad, continuous, low-resistance path through the structure instead of burning through hinge bearings and control cables; radio interference from small internal discharges disappears; and the continuous structure can serve as the return path for the aircraft's own electrical circuits, a role the next section explains.
Direct current and the closed circuit
Direct current (DC) is current that flows in one direction only, the form a battery produces: one terminal is always positive, the other always negative, and charge is pushed steadily the same way round. Throughout this module the conventional direction of current flow is used: from the positive terminal, through the circuit, to the negative terminal. (Physically, the electrons drift the other way; conventional flow is the agreed bookkeeping standard, and every aircraft schematic uses it.)
Current can only flow in a closed circuit: an unbroken conducting loop from the source's positive terminal, through the consumer, back to the negative terminal. Break the loop anywhere (an open switch, a broken wire, a blown fuse) and the current stops everywhere in the loop, instantly; there is no flow "up to the break". This is the fact every switch and circuit breaker in the aircraft exploits, and it is why the bonded airframe is so useful as a return path: connect every consumer's return side to the structure, and the metal of the aircraft itself closes every loop back to the battery's negative terminal, saving half the wiring.