DC Electrics - Bonding and Screening — Page 145, Lesson 150

I want to walk you through a new topic now: bonding and screening in DC electrics. This is about controlling static electricity and radio interference on an aircraft. Let's start with bonding. As an aircraft flies through the atmosphere, it will pick up, or become charged with, static electricity. Bonding is the process that prevents any part of the aircraft from building up a potential — a voltage — so great that it would create a spark and generate a fire risk. That's the core safety reason. Here's how it works. Each piece of the metal structure of the aircraft, and each component on the aircraft, is joined to the other by flexible wire strips. All those strips must be clean and free from any insulating coatings such as anodizing, paint, grease, and oxides. Why? Because those coatings would prevent electrolytic corrosion — but more importantly for our purposes, they would introduce resistance. We don't want resistance in these bonds; we want a good electrical connection. This process is called bonding, and it provides an easy path for the electrons from one part of the aircraft to another. So if static charge builds up on one metal panel, the electrons can flow freely through the bonding strips to the rest of the airframe, rather than building up to a dangerous voltage that could spark. Bonding has two additional roles. It can act as part of the earth return system in a unipole circuit — we'll come back to that concept. It also helps to prevent radio interference due to static discharges. Now let's look at the static discharge system, or static wicks. These are fitted to reduce static build-up on the airframe. Originally they were made of cotton, about the thickness of a cigarette. They are fitted to the trailing edge of the aircraft control surfaces, and the tips of wings or stabilizers. Static electricity is dispersed from them into the atmosphere. The free end of the wick becomes 'teased' — that means spread out — and a brush discharge action takes place. Modern wicks are like miniature barbed antenna, small wire brushes, or alternatively straight metal wicks. So they're designed to let the static charge bleed off gradually into the air, rather than building up and then discharging suddenly as interference. Now, what about when the aircraft lands? To ensure that no static electrical charge — with its possible fire risk — remains on the aircraft after landing, the main bond must be brought into instantaneous contact with the ground as the aircraft touches down. This is achieved by fitting nose, tail, or main wheel tyres which contain a high proportion of carbon in the rubber. The tyre is in contact with the main bond via the wheel bearing, and any static charge is dissipated to earth on touchdown. Let's move to screening. Screening is designed to prevent radio interference by absorbing electrical energy. Static electrical charges, produced by the operation of certain electrical equipment, create interference on radio circuits. This interference is overcome by fitting interference suppressors in the cables connected to the source of interference, and by total enclosure of the cables in a continuous metal sheath. Screening is required for specific equipment: ignition systems, DC generators and motors — these are commutator machines — slip ring machines operating at over 200 RPM, and also for any electrical equipment operating by making and breaking a circuit at a frequency greater than 10 Hz. So any device that switches on and off more than ten times per second needs screening. Let me also clarify the purpose of bonding with a quick summary. Bonding provides a low resistance path for earth return circuits and safely dissipates local static charges and lightning strikes. It prevents any part of the aircraft from building up a dangerous potential. It does not isolate components electrically — quite the opposite, it connects them all together electrically so the static potential is equalized across the whole airframe. Those figures show the dipole and unipole systems I mentioned — the unipole system uses the metal structure as the return path, which is why bonding is so important for that system.