BASIC ELECTRICAL SYSTEMS IN MODERN AIRCRAFT

This article introduces the fundamental electrical power systems of modern aircraft, explaining basic AC/DC principles, 400 Hz usage, and main power sources such as batteries, IDGs, APUs, RATs, and ground power. By linking theory with real operational examples, it aims to make aircraft electrical systems more accessible for maintenance professionals.

In this article, we will examine the basic electrical systems in modern aircraft. We will attempt to explain the electrical systems in aircraft using general and fundamental information. At times, we will elaborate on our topics with intermediate transitions from basic training level to type level. Our colleagues working in the aircraft maintenance and repair sector, most of whom have a background in mechanics, structural engineering, or cabin operations, tend to approach electrical topics with a certain distance and hesitation, whether they are new to the job or experienced. We hope this article will break the ice and pave the way. After this lengthy introduction, let us begin.

Fundamentally, there are two types of current in electrical theory. These are known as Direct Current and Alternating Current. Direct current is referred to as DC. Alternating current is referred to as AC.

Direct Current (DC) is defined as a current whose direction and intensity do not change over time, or as an electric current that remains at a constant voltage level. Batteries, accumulators and cells always produce Direct Current.

Alternating current (AC), unlike direct current whose direction remains constant, is an electric current whose magnitude and direction change periodically. The usual waveform of an AC power circuit is a sine wave, as this allows the most efficient transmission of energy. However, different waveforms such as triangular or square waves are also used in some applications. An AC generator is the simplest producer of electrical power.

Direct Current can be converted to Alternating Current using a component called an Inverter (SI  Static Inverter). Alternating Current (AC) can be converted to DC using a component called a Transformer Rectifier Unit (TRU  Transformer Rectifier Unit).

In aircraft, the voltage and current parameters are generally known as 115V (phase-to-neutral value) / 200V (phase-to-phase value) AC 400Hz 3-phase, 28V DC. I can hear you asking why aeroplanes use 400Hz when mains power uses 50Hz. Let me try to explain this a little:

Scientific research has shown that higher frequencies mean smaller and lighter electrical components. Transformers, motors and generators can be much smaller and lighter at 400Hz compared to 50/60Hz. In aviation, weight savings translate to fuel savings, making this a significant advantage. At higher frequencies, electrical machinery (generators, transformers, motors) can handle more power in a smaller size. 400Hz systems provide the same power output as 50/60Hz systems but with lower weight and size. Many avionics and flight control systems require high-speed processing. 400Hz benefits navigation, communication, and radar systems by enabling faster response times in electrical circuits. Therefore, the reason for using 400Hz instead of 50/60Hz in aircraft is to reduce weight, increase efficiency, and improve avionics response times.

Electrical Power Sources Used in Aircraft

Batteries

Aircraft batteries are fundamental components that provide power for various functions both on the ground and in the air. They produce 28V DC (direct current). Aircraft batteries are nominally classified as 24V, meaning their designed operating voltage is around 24 volts. However, when fully charged, their voltage is actually higher – typically around 28V. There are three types of batteries that have been used from the past to the present: Lead-Acid, Nickel-Cadmium (Ni-Cd) and Lithium-Ion. They are calculated in ampere-hours (Ah) and indicate how much charge a battery can store.

The basic structural unit of batteries is the cell. Connecting cells in series results in the battery output voltage (usually 24V). A typical aircraft (lead acid) lead-acid battery consists of 12 cells to provide a nominal voltage of 24V. In addition, a Ni-Cad battery has 20 cells that provide a nominal voltage of 24V. In modern aircraft, there are at least two batteries, which are connected in parallel to increase capacity. When power is supplied to the aircraft (Engine, APU or Ground Power), the battery charging units automatically start charging the battery.



The capacity of a battery is measured in ampere-hours (Ah = number of amperes multiplied by the operating time during discharge) and is the amount of electricity the battery can supply from its fully charged state to its discharged state at normal temperature. For example, the capacity of a typical Ni-Cad battery in commercial aircraft is approximately 40-65 Ah, while in wide-body aircraft it can be as high as 65-100 Ah.

The main uses of batteries in aircraft systems are as follows:

To provide emergency power for the Inertial Navigation System (INS) or Inertial Reference System (IRS).

To provide a limited amount of power to operate essential flight instruments and radio communication equipment in emergencies; to provide power to start the engine or APU.

To provide power for emergency lighting; to power avionics and other systems while on the ground and when ground power is unavailable.

IDG (Integrated Drive Generator)

It is known as the main electrical power source during flight. Previously, the constant speed drive and generator were separate components, but in modern aircraft, both have been combined into a single component called the Integrated Drive Generator. Therefore, the IDG is known as a generator that produces electricity from the torque it receives from the aircraft engine, with the gear group and generator section in a single component. The IDG is located on the gearbox (AGB  Accessory Gear Box) in the engine.

It is essential that the generator output has a nominal frequency of 400Hz with a tolerance of typically 380 to 420Hz (±5%). This is achieved throughout the engine operating speed range by using a hydraulically operated constant speed drive unit between the engine and generator.

A CSD consists of a variable hydraulic pump and motor connected so that the CSD (Constant Speed Drive) output remains constant, independent of the engine speed. A constant speed is required for the generator to have a 400 Hz output. Many modern aircraft use an integrated drive generator (IDG), which incorporates both the generator and the CSD in a single compact unit. In the event of a generator or CSD failure, the pilot can disconnect the input shaft from the drive to prevent any damage. Modern commercial aircraft have one generator per engine.

The main tasks of generators in aircraft can be defined as supplying the aircraft electrical systems, providing power to the main systems in emergencies, and charging the batteries. Its primary function is to convert the rotational motion, i.e. mechanical energy (engine rotation), into electrical energy. It thus supplies power to avionics systems, lighting, flight instruments and displays, fuel pumps, flight control systems, and cabin systems. It provides the system with a constant 115V/200V AC, 400Hz 3-phase power. It keeps the aircraft batteries (24V Ni-Cd) fully charged. The batteries serve as a backup power source in the event of generator failure.

If one generator fails, the remaining generator or generators continue to supply power to the systems. If both fail, for example in the A320, the RAT (Ram Air Turbine) drops down and continues to supply power to the system in an emergency.



To summarize briefly, the IDG (Integrated Drive Generator) is an electric generator that supplies electricity to the aircraft. The IDG supplies electrical power to all aircraft systems. It consists of a generator and a constant speed drive (CSD). The generator uses the kinetic energy of the aircraft engines and converts it into electrical energy. The CSD converts the variable speed of the engine into a constant rotational speed for the generator. This allows the generator to provide a constant frequency to the aircraft electrical system.

Oil, a hydraulic fluid, is one of the vital components of the IDG. Oil is used for cooling and lubrication and is also used by the CSD to mechanically regulate the generator’s rotational speed. The IDG is driven by the AGB in the engine. The engine is accessible through the fan cowl. The reliability and efficiency of the IDG depend on the cleanliness and correct quantity of oil used. The oil level is checked using the sight glass that indicates the oil level. As with all maintenance and repair activities, our Aircraft Maintenance Manuals (AMMs) are our sole reference source for IDG servicing, disassembly kits and testing procedures.

Contamination of the oil with water or dust can cause premature clogging of the filter and overheating. Oil containers and tools should be kept in a clean, dry environment. It is preferable to use a new container of oil when servicing the IDG.

In the event of oil overheating (means high oil temperature) or a drop in oil pressure (means low oil pressure), the amber IDG FAULT light illuminates and the ECAM/EICAS (Electronic Centralized Aircraft Monitor / Engine Indicating and Crew Alerting System) system is triggered. The IDG button must be pressed to disconnect the IDG from the gearbox (AGB  Accessory Gear Box). The switch on the cockpit electrical panel used to disconnect the IDG from the system is connected to a red protected switch and safety wire. This is because the IDG connection cannot be restored during flight once it has been disconnected. To prevent damage to the drive shaft coupling mechanism, the system can only be reconnected on the ground with the engines shut down. A mechanical reset lever (ring) attached to the IDG allows the system to be reconnected while the engine is stationary on the ground.



APU (Auxiliary Power Unit)

The APU generator (90 kVA) supplies power to the systems in flight if both IDGs fail while the APU is operational. The Auxiliary Power Unit (APU) is a small gas turbine engine located in the tail of commercial aircraft such as the Boeing 737NG and Airbus A320. It provides electrical power and pneumatic (air) supply when the main engines are shut down or unavailable. Like the IDG example, it has power and electrical parameters of 90 kVA, 115 V/200 V AC 400 Hz.

The APU’s usage functions may vary depending on the flight phase. It supplies power for avionics, lighting and systems before the engines are started. On the ground, it provides bleed air for engine start-up and air conditioning air for the cabin. On the ground (when not connected to ground power), it supplies electrical power to the aircraft. In flight (in an emergency), it acts as a backup power source if both engine generators fail. The APU is normally shut down after engine start-up to save fuel.

The Auxiliary Power Unit (APU) has an emergency shutdown system to protect the aircraft in the event of high EGT (Exhaust Gas Temperature), fire or other hazardous conditions. If a critical problem is detected, the APU either shuts down automatically or is shut down manually by the flight crew from the cockpit, or by aircraft maintenance technicians during maintenance by pulling the APU emergency handle. During flight, the APU does not shut down automatically if there is a fire; pilots must shut it down manually.



RAT (Ram Air Turbine)

The Ram Air Turbine (RAT) is an emergency power source found on commercial aircraft such as the Airbus A320. It is a small ram air turbine that provides backup electrical and hydraulic power in the event of a complete power failure (loss of both engine generators and the APU). It automatically activates in the event of failure of both the IDGs and the APU generator.

It generates electrical power in emergencies, supplying AC or DC power to essential systems. If hydraulic pressure is lost, it provides the hydraulic power required by the system, thereby keeping the essential flight control surfaces operational. It enables aircraft to land safely in emergency situations. It is typically located under the wing or fuselage of the aircraft and is deployed by a spring-loaded mechanism. It generates power between 5kVA and 10kVA. The Boeing 737 series of narrow-body aircraft (CL&NG&MAX) do not have RAT; they rely on batteries and the APU as backup power to keep essential flight systems functional in the event of failure of both engines.

External Power / Ground Power

External power (or ground power) is an electrical power source provided by the airport to operate an aircraft’s systems while on the ground. This prevents unnecessary fuel consumption by the APU (Auxiliary Power Unit) or engines. On the Boeing 737 series, the external power connection is made from the external power panel located on the right side of the forward fuselage. On the A320 series aircraft, the external power connection is made from underneath the nose fuselage.

It is a 115V AC, 400Hz 3-phase power source from an airport Ground Power Unit (GPU) or Fixed Electrical Ground Power (FEGP). It performs the function of the generators in the engine or APU and supplies power to the aircraft’s electrical system. It is used when the aircraft is parked at the gate or undergoing maintenance. It reduces fuel consumption, emissions and noise by eliminating the need for APU use.

A Ground Power Unit (GPU) connector has six pins to safely supply 115V AC, 400Hz power to an aircraft. These pins are required for electrical power transmission, grounding, and control:

Three-Phase Power (Pins A, B, C) → It efficiently provides balanced 115V AC, 400Hz power.

Neutral Cable (Pin N) → It provides a return path for the electrical circuit. Grounding (Pin E) → Prevents electric shocks and short circuits.

Interlock Control (Pin F) → It prevents accidental disconnection or arcing. This pin also checks whether the ground power cable is properly connected. If it is not connected, the system will not supply the required electrical power.