Understanding the Aircraft Electrical System Principles is fundamental for anyone involved in aviation maintenance, design, or operation. These systems are the lifeline of an aircraft, powering everything from cockpit instruments to critical flight controls. This article breaks down key aspects of aircraft electrical systems, from wire selection to component operation and safety.
Core Principles of Aircraft Wiring in Aircraft Electrical Systems
The integrity and reliability of an aircraft's electrical wiring are paramount for safe flight. Careful consideration goes into every aspect of wiring, from material selection to installation and protection.
Wire Selection and Protection
When selecting wire size for an aircraft electrical system, two critical factors must be considered: the current carrying ability of the wire and the voltage drop caused by current flowing through it. This ensures efficient power delivery without overheating or excessive voltage loss. Technicians also consider practical aspects like the maximum number of wires permitted on a single terminal strip stud.
Wire bundles require robust protection, especially when passing through structural elements like fuselage frames or bulkheads. To prevent chafing, the edges of such holes are covered with a flexible grommet, and the bundle is securely fastened to the structure using a cushioned clamp.
Understanding Wire Failure and Splicing
Wire failures, particularly in crimped connectors, often stem from improper installation. Common causes include the wire not being inserted far enough into the connector or the connector being excessively crimped, which can damage the wire strands.
When it comes to joining wires, solderless splices are generally preferred over soldered splices in aircraft electrical systems. Soldered joints tend to be stiff, making the wire susceptible to hardening and breakage due to vibration. In contrast, solderless splices are designed to maintain joint flexibility, allowing them to withstand vibrations without causing wire fatigue and failure.
Material Considerations for Aircraft Wiring
Aluminum wire, while lighter, has a significant disadvantage compared to copper wire: it is more brittle. This makes aluminum wire more prone to breakage if nicked or subjected to vibration. When replacing copper wire with aluminum, it's a standard practice to use an aluminum wire that is two gauge numbers larger to compensate for its different conductive properties.
Aircraft Electrical System Components
Aircraft electrical systems utilize various components, each designed for specific functions in power generation, distribution, and consumption.
Generators and Alternators
Aircraft AC generators, also known as alternators, play a crucial role in supplying electrical power. Before an AC generator can be placed on the same bus with another, three things must be carefully synchronized: the voltage, the frequency, and the phase rotation. Small DC alternators, commonly found in light aircraft, typically use a rectifier made up of six silicon diodes arranged in a three-phase, full-wave rectifier configuration.
Restoring the residual magnetism to a generator's frame is known as flashing the field. This process involves passing battery current through the field coils in the direction it normally flows during operation, ensuring the generator can begin producing power.
Electric Motors and Switches
DC electric motors draw significantly more current than their rated capacity during starting. This high initial current draw is an important factor in system design. Similarly, electrical switches must be derated from their nominal current rating when used in high rush-in circuits or those powering DC motors, which experience these high starting currents.
Aircraft engine electric starters typically utilize a series-wound motor. The direction of rotation of a DC shunt-wound electric motor's armature can be reversed by reversing the connections to the shunt field coil relative to the armature.
Maintenance and Safety Considerations
Ensuring the long-term reliability and safety of aircraft electrical systems involves specialized tools and protective measures.
Measuring Resistance and Troubleshooting
To measure very high resistance in an electrical system, a specialized instrument called a megohmmeter (often referred to by the registered trade name "Megger") is used. This tool is essential for checking insulation integrity and identifying potential shorts or open circuits.
Lightning Protection for Composite Aircraft
Modern composite aircraft require specific protection against lightning strikes due to their non-conductive materials. This protection is achieved by incorporating a conductive copper or aluminum mesh within the outer layers of the composite structure. This mesh safely dissipates the lightning current, preventing severe damage to the airframe and internal systems.
FAQ: Common Questions About Aircraft Electrical Systems
What are the primary considerations for selecting aircraft wiring?
When selecting aircraft wiring, technicians must consider the wire's current carrying ability and the voltage drop that will occur across its length due to the current flow.
Why are solderless splices preferred over soldered splices in aircraft wiring?
Solderless splices are preferred because they maintain flexibility in the joint, making them more resistant to vibration-induced wire hardening and breakage compared to stiff, soldered joints.
What three conditions must be synchronized before connecting multiple AC generators?
Before connecting multiple AC generators to the same bus, their voltage, frequency, and phase rotation must all be synchronized to prevent system damage.
How are composite aircraft protected from lightning strikes?
Composite aircraft are protected from lightning strikes by embedding a conductive copper or aluminum mesh in the outer layers of the composite structure, which helps to dissipate the electrical charge safely.
When do DC electric motors draw high currents, and why is this important for switch selection?
DC electric motors draw several times their rated current during starting. This high rush-in current means that switches in these circuits must be derated from their nominal current rating to prevent overload and failure.