Electrical systems are vital to modern passenger aircraft, powering everything from cockpit instruments to cabin lighting. The reliability and complexity of these systems ensure the smooth operation of an aircraft’s essential components. Let’s take a look at how a passenger jet’s electrical system works, using the Boeing 737-800 as an example.
Which aircraft systems are electrically powered?
In short – a lot of them. Aircraft systems powered by electrical power include essential avionics like navigation and communication systems, flight control computers, and cockpit displays. Electrical power also supports cabin systems such as lighting, air conditioning, and in-flight entertainment. Additionally, electrically-driven hydraulic pumps help operate critical components like landing gear and flight control surfaces. So – an important system! But how does it work?
How an Electrical System Works in jets
In the Boeing 737-800, the electrical system begins with engine-driven generators, but the process is more complex than simply producing power. Each of the aircraft’s two engines is connected to a generator that produces three-phase 115-volt AC power, but the challenge lies in ensuring a constant frequency, which is crucial for the stable operation of the aircraft’s electrical systems. This is achieved through a Constant Speed Drive (CSD) unit, a mechanical system that regulates the speed of the generator regardless of the engine’s actual speed. By using a CSD, the generators maintain a constant output of 400 Hz, which is the frequency required for most of the aircraft’s AC-powered systems.
The 737-800’s electrical system architecture is designed around two main sources of AC power—one from each engine’s generator. These generators supply power to AC busbars, which act as distribution points. The busbars channel electricity to various parts of the aircraft, ensuring that essential systems such as avionics, lighting, and environmental controls are always powered.
The electrical system is divided into primary and secondary distribution networks to ensure efficiency and safety. The primary AC power from the generators flows to the primary AC busbars, which in turn feed critical systems such as the flight management system (FMS), navigation radios, and flight control computers. Other systems, like passenger lighting, air conditioning, and galley equipment, draw power from secondary AC busbars, which are considered less critical during emergencies.
In addition to supplying AC power, the electrical system must also provide Direct Current (DC) for certain systems. This is where Transformer Rectifier Units (TRUs) come into play. The TRUs convert the AC power from the generators into 28-volt DC power, which is used by essential backup systems, like cockpit instruments, flight data recorders, and some communication systems.
The DC system also includes batteries that serve as a backup in the event of an electrical failure. These batteries are constantly charged by the DC power generated by the TRUs, ensuring they are ready to supply power to critical systems if needed. For example, in an emergency situation where both engine generators fail, the batteries can provide enough power to keep essential instruments, displays, and controls operational for a limited period, allowing the aircraft to continue flying safely until it can land.
What about Turboprops?
Whilst the electrical systems in jets like the Boeing 737-800 rely on stable, constant-frequency AC power, turboprop aircraft like the ATR use a different type of electrical generator known as an AC Wild generator. These generators produce variable-frequency AC power, which can range in frequency depending on engine speed.
The AC Wild system offers certain advantages, especially in terms of weight and fuel efficiency. However, it also presents unique challenges. Because the frequency varies with engine speed, it can make the design of some systems more complex, requiring additional equipment to regulate the power. In contrast, the Boeing 737-800’s constant-frequency generators provide a more stable power supply, simplifying the design of onboard systems.
The Auxiliary Power Unit
The 737-800’s electrical system also integrates a third generator connected to the Auxiliary Power Unit (APU). The APU is a small turbine engine located in the tail of the aircraft, and it provides electrical power when the main engines are not running, such as during ground operations. The APU generator can supply AC power to the busbars in place of the engine-driven generators, allowing systems like cabin lighting and air conditioning to function while the aircraft is parked at the gate. The APU is a constant speed engine, and therefore doesn’t need a constant speed drive unit.
Most turboprops do not have an APU. Instead they rely on ground power, or the use of ‘Hotel Mode’, whereby one engine can be run without spinning the propeller, providing electrical power. This ability is one of the features that make turboprops so useful for smaller and less-prepared airports.
![An example full flight summary response from the Flightradar24 API: { "data": [ { "fr24_id": "0987654321", "flight": "SK1415", "callsign": "SAS1415", "operated_as": "SAS", "painted_as": "SAS", "type": "A20N", "reg": "SE-DOY", "orig_icao": "ESSA", "orig_iata": "ARN", "datetime_takeoff": "2023-01-27T05:15:22", "runway_takeoff": "12R", "dest_icao": "EKCH", "dest_iata": "CPH", "dest_icao_actual": "EPWA", "dest_iata_actual": "WAW", "datetime_landed": "2023-01-27T06:15:10", "runway_landed": "27L", "flight_time": 3600, "actual_distance": 1007.74, "circle_distance": 6245, "category": "Passenger", "hex": "4A91F9", "first_seen": "2023-01-27T05:06:22", "last_seen": "2023-01-27T06:18:10", "flight_ended": "true" } ] }](https://www.flightradar24.com/blog/wp-content/uploads/2025/04/Flight-summary-API-cover-300x148.jpg)
![An example full flight summary response from the Flightradar24 API: { "data": [ { "fr24_id": "0987654321", "flight": "SK1415", "callsign": "SAS1415", "operated_as": "SAS", "painted_as": "SAS", "type": "A20N", "reg": "SE-DOY", "orig_icao": "ESSA", "orig_iata": "ARN", "datetime_takeoff": "2023-01-27T05:15:22", "runway_takeoff": "12R", "dest_icao": "EKCH", "dest_iata": "CPH", "dest_icao_actual": "EPWA", "dest_iata_actual": "WAW", "datetime_landed": "2023-01-27T06:15:10", "runway_landed": "27L", "flight_time": 3600, "actual_distance": 1007.74, "circle_distance": 6245, "category": "Passenger", "hex": "4A91F9", "first_seen": "2023-01-27T05:06:22", "last_seen": "2023-01-27T06:18:10", "flight_ended": "true" } ] }](https://www.flightradar24.com/blog/wp-content/uploads/2025/04/Flight-summary-API-cover-768x378.jpg)
