Electrical design in aircraft is influenced by two basic requirements: The equipment must be light weight and highly reliable. These needs may be met in a variety of ways.
At inception, electrical parameters are chosen to minimize weight so the aircraft can get off the ground with its payload and gain altitude without expending unnecessary power and fuel.
One technique is to reduce the size and weight of magnetics (generators, transformers, motors) by employing a higher frequency power system. Ground-based, utility power is generally 50 or 60 Hz. Aircraft electrical systems that are ac typically operate at 400 Hz.
Even small private single-engine airplanes have complex electrical systems. Two magnetos, for redundancy, are commonplace. High-voltage outputs go to the sparkplugs, independent of other onboard electrical circuits. Small aircraft typically have 14/28-V dc systems, providing the charging voltage for 12- and 24-V wet cell or sealed lead-acid batteries. The voltage of the generator is held to specifications by means of a voltage regulator connected to the generator’s field circuit.
An example of an aircraft electrical system is that for the F-104 Starfighter. It includes two batteries, two engine-driven variable frequency ac generators, a constant frequency 400 Hz ac generator, driven by the No. 2 hydraulic system, and a ram-air-driven constant-frequency generator for emergency use. DC power is from two 28-V transformer rectifiers. The plane can land safely if only the emergency generator is operating.
NiCd batteries have distinct advantages. The volts-per-cell may be 1.4 or 1.5, so it is important to refer to the manufacturer’s recommendations. When aircraft batteries are operated at levels above ambient temperature or charging limits, the electrolyte can boil, leading to premature battery failure. An engine-driven generator applies a constant voltage to the batteries, regardless of rpm. A battery switch is installed to guard against discharge during idle time.
Because certain electrical systems require ac power, small aircraft are equipped with an inverter, originally rotary but now solid-state. Aircraft that use a large amount of electrical power carry ac alternators. Transport aircraft including the Airbus A-380 and the Boeing 757 have an ac alternator driven by each engine. There is also an auxiliary ac alternator driven by the auxiliary power unit. Additionally, most aircraft in this category have one more backup power source, such as an ac inverter or small ac alternator driven by a ram-air turbine, for a high level of redundancy. This is necessary because the flight controls are electric over hydraulic. If all electrical power were to fail, the crew could not control the machine at all.
Aircraft wiring resembles ground-based wiring in many respects. Basic designs are similar, albeit with greater emphasis on weight limitation and a high level of reliability. Separate avionics, lighting and communication systems are needed, connected or isolated from one another by means of switches as the need arises. The basics of overcurrent protection and ampacity resemble conventional wiring, but there are significant differences. For example, at a higher altitude in unpressurized areas, there is less effective heat dissipation. So conductor derating is required.
Copper and aluminum (in sizes greater that 10 AWG) are each used where appropriate. Aluminum has a weight advantage in large diameters and long runs, but specialized techniques are needed at terminations.
Aircraft electrical systems use impeccably crimped ring-type terminations, in conjunction with stainless steel junction boxes and terminal strips. Conduit is not used except in a few critical areas, owing to weight considerations. Flight control wiring is generally shielded, employing a light foil that is bonded to structural metal, which serves as the ground plain.
Some manufacturers prohibit splicing, so documentation should be consulted. Crimped splices may be made in an in-flight emergency, but they should be replaced when back on the ground. Damaged aluminum wire should not be spliced. In an emergency, it should be possible to shunt around the damaged area.
In new construction, where possible complete wiring harnesses are made up on the factory floor, carefully following specifications, color coding and labeling. Bundling, with conductors derated as needed, is precisely measured and laced, carefully observing minimum bending radii and similar mandates, so that the installation will be safe and efficient.
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