Aircraft Electrical Systems

Most aircraft require some form of electrical power to operate navigation-, taxi-, landing-, strobe lights, one or more COM and NAV radio's, transponder, intercom and other electronic systems. The electrical system consist of a battery and an alternator (or generator on older type aircraft) to recharge the battery, fuses and switches and lights for indication purposes. The remainder of the basic electrical system consists of a master switch, one or more bus bars, ammeter and or voltmeter and fuses or circuit breakers and switches to control the radio's, lights and other electrical devices. Master Switch Bus bar and fuses Monitoring Volt and Amps Aircraft wiring and ground Aviation hardware All of this is connected through several meters (kilometers in large aircraft) of wire and connectors, attached to the airframe with insulation material as cushion clamps, ty-wraps and what not. Airplane power basics On an airplane, the electrical system produces, controls and distributes power to all the other systems that need it — flight deck displays, flight controls, in-flight entertainment and more. It’s much like the electrical system in your house, which carries electricity throughout the rooms to power your lights, television and so forth. Unlike your house, though, the airplane generates electricity as it flies. Airplanes don’t fly on battery power. Generators on the engines make power in flight. v The traditional airplane: electrical and pneumatic systems On a traditional airplane, power is extracted from the engines in two ways to power other airplane systems: Generators driven by the engines create electricity. A pneumatic system “bleeds” air off the engines to power other systems (e.g., hydraulics). Modern jet engines are very efficient, but removing that high-energy air robs them of some energy. A pneumatic system means that the engines produce less thrust, so they must be bigger, work harder and use more fuel. The system also means more weight, fuel burn and maintenance due to the heavy ducts and equipment needed to manage that hot air. The 787: A more-electric system The 787 Dreamliner uses more electricity, instead of pneumatics, to power airplane systems such as hydraulics, engine start and wing ice protection. Benefits of the 787’s innovative, more-electric design include: More efficient power generation, distribution and use — including new remote power distribution units, which reduce wiring and save weight (approximately 20 miles, or 32 km, less wiring than on the 767). Better fuel efficiency — better for airlines and the environment. Lower maintenance costs and fewer maintenance tasks. Less drag and noise. Because the 787 uses more electricity than do other Boeing airplanes, the 787 generates more electricity, via six generators: two on each engine and two on the auxiliary power unit (APU, a small turbine engine in the tail). On the ground, the 787 can be started without any ground power: The APU battery starts the APU generators, which start the APU to power the engine generators, which then start the engines. In flight, the four engine generators are the primary sources of electrical power; the APU generators are secondary. Power runs from the generators to four alternating current (AC) buses, where it is either distributed for use as is (235 V AC) or converted to what other systems need. Other power sources for the 787 include the main battery, used primarily for brief ground operations and braking; the APU battery, which helps start the APU; and ground power, which can connect through three power receptacles. The main battery, APU battery and ram air turbine also are available as backup power in flight in the unlikely event of a power failure. As with every Boeing airplane, the 787 includes many layers of redundancy for continued safe operation, and the electrical system is no exception. For example, Boeing has demonstrated that the 787 can fly for more than 330 minutes on only one engine and one of the six generators and land safely. Boeing designs to preclude failure — that is, so that systems won’t fail. Then Boeing goes further, assuming failure will occur and designing for the proper protections. Boeing also designs so that no single failure will cause an accident; for example, by including redundant systems, separating systems in space and functions — so that the loss of one doesn’t cause the loss of another — and providing standby and protective systems. The 787 completed 5,000 hours of flight testing and an equal amount of test time on the ground. That testing demonstrated that the airplane performs as designed. The 787 electrical system was certified along with the airplane on Aug. 26, 2011. Aircraft Electrical Systems Definition An Aircraft Electrical System is a self contained network of components that generate, transmit, distribute, utilize and store electrical energy. General Description An electrical system is an integratal and essential component of all but the most simplistic of aircraft designs. The electical system capacity and complexity varies tremendously between a light, piston powered, single engine GA aircraft and a modern, multiengine ______ jet aircraft. However, the electrical system for aircraft at both ends of the complexity spectrum share many of the same basic components. All aircraft electrical systems have components with the ability to generate electricity. Depending upon the aircraft, generators or alternators are used to produce electricity. These are generally engine driven but may also be powered by an APU, a hydraulic motor or a Ram Air Turbine (RAT). Generator output is normally 115-120V/400HZ AC, 28V DC or 14V DC. Power from the generator may be used without modification or it may be routed through transformers, rectifiers or inverters to change the voltage or type of current. The generator output will normally be directed to one or more distribution Bus. Individual components are powered from the bus with circuit protection in the form of a Circuit Breaker or fuse incorporated into the wiring. The generator output is also used to charge the aircraft battery(s). Batteries are usually either of the lead-acid or NICAD types but lithium batteries are becoming more and more common. They are used for both aircraft startup and as an emergency source of power in the event of a generation or distribution system failure. Basic Aircraft Electrical Systems Some very simple single engine aircraft do not have an electrical system installed. The piston engine is equiped with a Magneto ignition system, which is self powering, and the fuel tank is situated so it will gravity feed the engine. The aircraft is started by means of a flywheel and crank arrangement or by "hand-proping" the engine. If an electric starter, lights, electric flight instruments, navigation aids or radios are desired, an electrical system becomes a necessity. In most cases, the system will be DC powered using a single distribution bus, a single battery and a single engine driven generator or alternator. Provisions, in the form of an on/off switch, will be incorporated to allow the battery to be isolated from the bus and for the generator/alternator to be isolated from the bus. An ammeter, loadmeter or warning light will also be incorporated to provide an indication of charging system failure. Electrical components will be wired to the bus-bar incorporating either circuit breakers or fuses for circuit protection. Provisions may be provided to allow an external power source such as an extra battery or a Ground Power Unit to be connected to assist with the engine start or to provide power whilst the engine is not running. Advanced Aircraft Electrical Systems More sophisticated electrical systems are usually multiple voltage systems using a combination of AC and DC buses to power various aircraft components. Primary power generation is normally AC with one or more Transformer Rectifier Unit (TRU) providing conversion to DC voltage to power the DC busses. Secondary AC generation from an APU is usually provided for use on the ground when engines are not running and for airborne use in the event of component failure. Tertiary generation in the form of a hydraulic motor or a RAT may also be incorporated into the system to provide redundancy in the event of multiple failures. Essential AC and DC components are wired to specific busses and special provisions are made to provide power to these busses under almost all failure situations. In the event that all AC power generation is lost, a static Inverter is included in the system so the Essential AC bus can be powered from the aircraft batteries. Robust system monitoring and failure warning provisions are incorporated into the electrical system and these are presented to the pilots when appropriate. Warnings may include, but are not limited to, generator malfuntion/failure, TRU failure, battery failure, bus fault/failure and circuit breaker monitoring. The manufacturer will also provide detailed electrical system isolation procedures to be utilized in the event of an electrical fire. In compliance with applicable regulations, components such as Standby Flight Instruments and Emergency Floor Lighting have their own backup power supplies and will function even in the event of a complete electrical system failure. Provisions are virtually always provided for connecting the aircraft electrical system to a fixed or mobile Ground Power Unit. Threats Generator Failure Bus Failure Component Failure Electrial System Fire Effects Loss of some or all of primary power generation capability Loss of all components and systems powered by the failed bus Loss of an individual component Potential loss of aircraft should the fire become uncontrolable, loss of busses; systems or components due to the fire or as a result of electrical isolation procedures; smoke and/or fumes Defences Multiple primary generators and, where applicable, secondary (APU) or tertiary (RAT) generator installation. Multiple layers of redundancy greatly reduce the potential for loss of all electrical generation capability. Components connected to the bus have individual circuit protection which, in the event of a component failure protect the bus from overload and thus protect the remaining components. A bus failure is more typically the result of a failure of the power source supplying the bus and not the failure of the bus itself. As an example, the failure of a TRU could result in the loss of the DC bus that it powers. Depending upon the system design, provisions for an alternate power source may allow the bus to be restored. Circuit breakers (CB) exist to protect the system from overload in the event of a component failure and to prevent a potential fire from developing in the component itself by interupting the electrical supply. In the event a circuit breaker "pops" in flight, the crew should comply with manufacturer and company policy when deciding whether or not the CB should be reset. Should a reset CB pop a second time, further reset should NOT be attempted. Note that some CB's such as those associated with fuel pumps should never be reset in flight. In the event of smoke, fumes or fire from a suspected electrical source, QRH procedures should be applied immediately while concurrently initiating an immediate diversion. If the faulty component cannot be readily indentified, the electrical isolation procedure should be followed. Smoke and fume elimination procedures may become a necessity. Land ASAP. Typical Scenarios During a daylight VFR cross country flight in a light aircraft, the sole engine driven generator fails. The pilot reduces the electrical load by shutting off non-essential equipment, diverts to an enroute aerodrome and lands prior to depletion of the aircraft battery. Mid ocean on a trans Atlantic flight, the circuit breaker for the inflight entertainment system pops. The video tape player is noticably warm to the touch. The cabin supervisor consults with the Captain who directs that the CB is not to be reset. An inflight shut down results in all electrical busses being powered by the one remaining engine generator. The APU is started to provide a second source of power and the aircraft is diverted to a nearby airfield.