What Is a Large Multi-Engine Aircraft Electrical System?

Large aircraft are typically equipped with multiple electrical power sources as they necessitate extremely reliable power distribution systems. One type of power source that is often implemented is an AC generator. Most aircraft have two or more main AC generators controlled by aircraft turbine engines, and one backup AC generator. Other power sources implemented in large aircraft include DC systems as well as an aircraft battery to provide emergency power in the case of multiple failures.

Often called an alternator, an AC generator produces three-phase 115-volt AC at 400 Hz. As most aircraft are designed with two engines, thus there are usually two main AC generators. In some instances, an APU will drive an AC generator which is on standby during flight if one of the main generators fails. Driven by an emergency ram air turbine, another generator is available in the case that the two main generators and one auxiliary generator malfunction.

Since there are four AC generators and back-up mechanisms in place to ensure that aircraft perform optimally despite internal power failures, it is unlikely that a complete power failure will occur. In this blog, we will be providing a brief overview of aircraft electrical systems, how they work, and their various types.

AC Power Systems

To provide consistent comfort for passengers, aircraft require power sources to supply energy for lighting, audio visual systems, food warmers, and beverage coolers. More than that, they require a myriad of electrical systems for the flight control systems, electronic engine controls as well as the communication and navigation systems.

The output capacity of one engine-driven AC generator is used to power all of the aforementioned systems while a second engine-driven generator is operated during flight to distribute the electrical load and provide redundancy. It is important to keep in mind that the AC electrical system should not exceed its power limits and that the generators are connected to the proper distribution busses.

There are two electronic line replaceable units that are used to control electrical power on aircraft, which include the generator control unit (GCU) and the bus power control unit (BPCU). The GCU is used to control the varying functions of AC generators while the BPCU controls the distribution of electrical power between the various distribution busses. Together, these two units are responsible for controlling electrical power, detecting faults, making adjustments, and reporting any potential failures to the central maintenance system.

Found around the main power cables of the electrical distribution system, a current transformer (CT) is tasked with receiving an induced voltage and is connected to the BPCU, providing accurate current readings for the system. The BPCU controls the electrical connections between distribution busses, each bus varying in design and function. These include split-bus, parallel-bus, and split-parallel variations.

Split-Bus Power Distribution Systems

Modern twin-engine aircraft are designed with a split-bus power distribution system, wherein the busses are kept in a split position in relation to each other. It is important to note that two generators can never power the same bus as the output current of the generators is not phase regulated, and the generators must remain isolated from one another to avoid damage.

Parallel-Bus Power Distribution Systems

Multi-engine aircraft employ a parallel-bus power distribution system in which the generators are parallely operated. All the generator output current must be phase regulated, and their output frequency must be adjusted to guarantee that the AC output reaches both the positive and negative peaks at the same time. In the case of a generator failure, busses are already connected and only the defective generator needs to be isolated from the system. During flight, a paralleling bus, or synchronizing bus, is utilized to connect the generators and requires the flight crew to monitor and manually control bus contactors.

Split-Parallel Power Distribution Systems

As its name suggests, a split-parallel bus combines the functions of the previously mentioned systems. Found on the Boeing 747-400, a split-parallel power distribution system contains four generators that are powered by the main engines and two APU-driven generators. The system is usually only operated in split-bus mode under failure conditions or when utilizing external power. Using four GCUs and two BPCUs, these units work similarly to those discussed in a split-bus system. The Boeing 747-400, like all large aircraft, contains a DC power distribution system as well. Typically employed for battery and emergency operations, the DC system is powered by TR units that are connected to the AC busses and convert AC into 26-volt DC. 


If you find yourself in need of AC power systems, DC power connectors, APU generators, air turbines, and other related components necessary for power distribution systems, rely on NSN Orbit. NSN Orbit is a premier purchasing platform consisting of thousands of authentic and reliable parts and components, all of which have been subjected to a number of rigorous quality assurance measures. When you choose to purchase from NSN Orbit, you are guaranteed cost savings and rapid lead-times; call us today!


May 17, 2023
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