Power generation and distribution on a transport aircraft

A flying power station

Modern transport aircraft are essentially flying electrical power stations. Flight controls, avionics, lighting, environmental systems, fuel pumps, galleys and passenger systems all depend on a reliable electrical supply. To achieve the reliability required for commercial aviation, aircraft use multiple power sources, redundant distribution paths, automatic switching logic and several layers of backup power.

The design philosophy is simple: no single electrical failure should leave the aircraft without power for essential systems.

The big picture

Together, these systems ensure electrical power remains available throughout all phases of flight, even following multiple equipment failures. A transport aircraft's electrical system consists of:

• Power generation: engine-driven generators and APU generators.
• Power conversion: TRUs, inverters and power converters.
• Power distribution: AC and DC buses.
• System control and protection: GCUs and bus control units.
• Backup and emergency power sources: RATs, batteries and standby generators.

Engine-driven generators

On a typical twin-engine airliner, each engine drives an electrical generator capable of supplying the majority of the aircraft's electrical demand. Historically, many transport aircraft used integrated drive generators (IDGs). An IDG combines a constant-speed drive (CSD) and an AC generator. The constant-speed drive allows the generator to maintain a steady output frequency even though engine RPM changes throughout the flight.

Why constant speed matters

Aircraft AC systems traditionally operate at 115/200 V AC, three-phase, 400 Hz. Without a constant-speed drive, generator frequency would rise and fall with engine speed. The IDG compensates for engine speed changes so the generator continuously supplies stable 400 Hz power.

Generator capacity

Large transport aircraft commonly use generators rated around 90 to 120 kVA or more, depending on aircraft type. Each generator can often power most or all essential aircraft systems by itself if necessary.

Variable frequency systems

Many newer aircraft no longer use traditional IDGs. Instead, they use variable-speed generators, power electronics and frequency conversion systems. Examples include more-electric aircraft such as the Boeing 787. Removing the mechanical constant-speed drive reduces:

• Weight
• Complexity
• Maintenance requirements

Auxiliary power unit (APU) generator

The auxiliary power unit (APU) is a small gas turbine engine usually located in the tail of the aircraft. The APU can drive its own generator and provide electrical power on the ground, during engine start and in flight when needed. An APU generator often provides power comparable to a main engine generator, and the APU is one of the most important backup systems on a transport aircraft. Typical APU uses:

• Aircraft servicing at the gate
• Engine starting
• Backup electrical supply
• Supplemental power during abnormal situations

External ground power

When the aircraft is parked, electrical power is often provided by airport ground power units or fixed electrical ground power systems. Power enters the aircraft through external power receptacles. Ground power is normally used whenever available. Benefits include:

• Reduced APU operating time
• Lower fuel consumption
• Less engine wear
• Reduced noise

Generator control unit (GCU)

Every generator is managed by a generator control unit (GCU). It performs voltage regulation, frequency regulation, load control, fault detection and generator protection. If a problem is detected, the generator may be automatically disconnected from the system. The GCU monitors for conditions such as:

• Overvoltage
• Undervoltage
• Overfrequency
• Underfrequency
• Generator faults

Bus power control unit (BPCU)

The bus power control unit manages electrical distribution throughout the aircraft. Modern transport aircraft perform much of this switching automatically, and the crew may not even notice some generator transfers occurring. Its job is to decide:

• Which source powers which bus
• When transfers occur
• How buses are isolated
• How backup sources are used

No-break transfers

During generator transfers, electrical power interruptions must be minimized. To accomplish this, generator frequencies are synchronized, voltage is matched and transfer timing is precisely controlled. The result is often a no-break transfer, where aircraft systems continue operating without interruption. This is particularly important for avionics and flight-critical equipment.

Integrated drive generator (IDG) care

An IDG is a sophisticated piece of equipment with its own dedicated oil system. The oil performs two critical functions: lubrication and cooling.

A low charge pressure or drive indication often signals a problem within the IDG oil system. If oil pressure is lost, internal damage can occur rapidly and the generator may require disconnection.

IDG disconnect

Aircraft are equipped with a disconnect mechanism that isolates the IDG from the engine gearbox. This prevents further damage when abnormal conditions occur. Reconnection is generally a maintenance action performed on the ground. Once disconnected:

• The IDG remains disconnected for the remainder of the flight.
• Electrical power must come from other sources.

Thermal protection

Most IDG systems include thermal protection devices. Maintenance personnel must verify oil quantity and system condition before returning the unit to service. Excessive oil temperature may cause:

• Automatic disconnect
• Generator shutdown
• System protection actions

AC distribution buses

Electrical power is distributed through buses. A bus is a common electrical distribution point. Individual systems receive power from these buses through circuit protection devices and switching logic. Common examples include:

• Left AC bus
• Right AC bus
• Essential AC bus
• Transfer bus
• Standby AC bus

Transformer rectifier units (TRUs)

Most transport aircraft use AC power as their primary electrical source, but many systems require DC power. A transformer rectifier unit (TRU) converts 115 V AC into 28 V DC without relying on the aircraft battery. Think of the TRU as the aircraft's primary DC power supply while generators are operating. The TRU supplies power to:

• DC buses
• Battery charging systems
• Avionics
• Flight instruments
• Control systems

DC buses

Critical DC-powered systems are distributed among these buses to maximize redundancy. Common DC buses include:

• Main DC bus
• Essential DC bus
• Battery bus
• Hot battery bus

Aircraft batteries

Batteries provide the final layer of electrical backup. Many transport aircraft traditionally use 24 V nickel-cadmium (Ni-Cd) batteries, although lithium technologies are becoming increasingly common. Batteries ensure power remains available even when all generators are unavailable. Aircraft batteries are used for:

• APU starting
• Emergency power
• Ground operations
• System backup
• Essential instrument power

Hot battery bus

A hot battery bus is connected directly to the battery. It remains energized regardless of switch position. Technicians must be aware that power may still be present even when aircraft battery switches are OFF. Systems commonly powered by the hot battery bus include:

• Fire protection circuits
• Emergency lighting controls
• Memory functions
• Security-critical systems

Static inverters

Batteries supply DC power, but some essential equipment may require AC power. A static inverter converts DC to AC during emergency operations, providing an important bridge between battery backup and AC system requirements. Emergency AC may power:

• Essential instruments
• Flight displays
• Standby systems

Backup generators

Many aircraft include dedicated backup generators capable of powering critical buses after major generator failures. These systems provide additional redundancy before battery operation becomes necessary.

Ram air turbine (RAT)

The ram air turbine (RAT) is one of aviation's most recognizable emergency systems. In a severe power emergency, the RAT deploys into the airflow beneath the aircraft. The airflow spins a small turbine that generates hydraulic power, electrical power, or both, depending on aircraft design.

The RAT is intended to supply enough energy to keep essential flight systems operating after catastrophic power loss. It is not designed to power every aircraft system, and it represents one of the last layers of aircraft electrical redundancy. Priority is given to:

• Flight instruments
• Flight controls
• Communication systems
• Navigation equipment

Load management

Aircraft generators have finite capacity. Under normal conditions, the available generation comfortably exceeds demand. However, during abnormal operations, electrical demand may exceed supply, for example when:

• A generator fails
• An engine shuts down
• The aircraft is operating on APU power
• Emergency power sources are active

Load shedding

To prevent overload, the electrical system automatically removes non-essential loads. This process is called load shedding. The exact sequence varies between aircraft types, but non-essential systems are typically removed first, while essential and flight-critical systems remain powered. Examples of shed loads include:

• Galley equipment
• Cabin utility power
• Certain cabin services
• Non-essential fans
• Convenience systems

Load restoration

When additional power becomes available, for example when the APU generator comes online, an engine generator is restored or external power is connected, the electrical system gradually restores loads. Loads normally return in the reverse order of removal.

Electrical priorities

Aircraft electrical systems are designed around priorities. Flight-critical buses are protected at all costs. They are the last to lose power and are typically protected by multiple independent backup sources.

• Non-essential: galleys, cabin convenience outlets and passenger entertainment systems.
• Essential: communication radios, navigation systems and flight displays.
• Flight-critical: flight control computers, standby instruments and fire protection systems.

Key takeaways

• Engine-driven generators provide the primary electrical power source.
• Traditional IDGs maintain constant 400 Hz frequency despite changing engine speeds.
• The APU generator provides independent backup and ground power capability.
• GCUs regulate and protect generators. Bus control systems manage power distribution.
• TRUs convert AC power into 28 V DC for aircraft DC systems.
• Static inverters convert battery DC into emergency AC power.
• Batteries provide emergency power and APU starting capability.
• The hot battery bus remains energized even when battery switches are OFF.
• RATs provide emergency hydraulic or electrical power following major failures.
• Load shedding automatically removes non-essential systems when generation capacity is limited.
• Flight-critical buses are protected through multiple layers of redundancy.

The bottom line

A transport aircraft's electrical system is designed around redundancy, automatic management and fault tolerance. Engine-driven generators provide the primary source of 115/200-volt AC power, TRUs create the aircraft's 28-volt DC supply, and batteries, static inverters, backup generators, APUs and RATs provide multiple layers of emergency support. Through sophisticated bus management and load-shedding logic, the system ensures that essential flight and safety equipment remains powered even after significant failures, allowing the aircraft to continue operating safely in challenging conditions.