Modern transport category aircraft are equipped with loads of avionics and electronics that enhance safety and provide the crew with a wide range of information; traffic avoidance technology, terrain avoidance and warning systems, weather radar, and aircraft engine and systems monitoring equipment. These systems are excellent additions to any cockpit as well as invaluable tools for flight crews. All of them would be unusable, however, without electrical power.
As with any aircraft system, the electrical system has several levels of built-in redundancy. Unlike basic trainer aircraft that usually feature a single alternator and battery, most Transport aircraft have two or three levels of redundancy with several generators, batteries, and even back-up batteries.
Each engine is usually equipped with two generators in addition to the auxiliary power unit (APU), which is also equipped with a generator. A generator is a lot like an alternator, but it provides DC current, and the current provided is dependent upon engine rpm. With the extremely high rpm of a turbine engine, generators are usually the choice for most Transport aircraft powerplant electrical sources.
Each generator is usually capable of providing enough power for all avionics and electronics as a standalone source of power; however, most aircraft are equipped with automatic load-sharing and load-shedding logics that will isolate electrical buses in the event of a single or multiple generator failures. This ensures that emergency power is provided to the essential avionics, and it automatically sheds any unnecessary items.
In an engine failure, the electrical system will experience the loss of two generators. With a load-sharing and load-shedding system in place, the electrical system will automatically disconnect buses that contain unnecessary equipment such as galley power or aircraft lighting that isn’t critical for safety. This ensures suitable power remains for flight instrumentation, engine and systems monitors, navigation and communication radios, and stand-by instruments.
With an APU that has an additional generator, if a dual generator failure occurred in flight—either with or without an associated engine failure—it’s possible to start the APU and restore power to all electrical buses.
In addition to several electrical generators, most aircraft have main batteries and back-up batteries. In the unlikely event that you experience a total electrical failure and all generators are inoperative, you would still have back-up battery power. Although the electrical load-sharing and load-shedding system would eliminate almost all avionic conveniences, you would be left with stand-by instruments and back-up engine and systems monitors, allowing for a very tense, but safe, emergency landing.
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