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Overview

Electrical power generation is the foundation of every modern commercial aircraft's systems architecture. Every avionics display, flight control actuator, cabin light, galley oven, and environmental control fan depends on a reliable, regulated supply of electricity. On conventional jet transports, this power originates at engine-driven generators coupled directly to each turbofan through an accessory gearbox. The resulting AC output is distributed through an elaborate network of buses, contactors, and conversion units to reach every corner of the airframe.

Two dominant generator technologies share the commercial fleet. The Integrated Drive Generator (IDG) dominated aircraft design from the 1960s through the early 2000s; it couples a constant-speed drive (CSD) to the generator so that the output frequency remains a stable 400 Hz regardless of engine speed. The Variable Frequency Generator (VFG), introduced on the Boeing 777 and adopted widely on newer types, removes the CSD and allows frequency to vary with engine RPM, trading fixed-frequency simplicity for higher reliability and reduced maintenance burden. The Boeing 787 took this philosophy furthest with its no-bleed More Electric Architecture, replacing bleed-air functions with larger, more powerful VFGs.

How It Works

Each engine drives a generator through a gearbox mounted on the engine core. On IDG-equipped aircraft the generator shaft turns at a constant speed despite engine power changes, because the CSD acts as a hydraulic transmission that compensates for turbine speed variations. The generator converts mechanical energy to three-phase 115V AC at 400 Hz. This frequency was chosen in the 1940s as a compromise between transformer weight (lower frequency requires heavier cores) and conductor skin-effect losses (higher frequency wastes conductor cross-section).

On VFG-equipped aircraft the accessory gearbox drives the generator directly at a ratio fixed to engine speed. As engine RPM varies from idle to maximum continuous, generator output frequency swings across a defined range, typically 360 to 800 Hz on the 777 family. Aircraft systems designed for variable frequency must tolerate this range, which modern power electronics handle readily. The APU drives its own dedicated generator at ground-stable speed, producing fixed-frequency 115V 400 Hz AC whether on the ramp or in flight up to its certified altitude ceiling.

Key Components

• Integrated Drive Generator (IDG): Combined constant-speed drive and generator in a single housing, typically 90 to 115 kVA per engine on narrowbody aircraft and up to 150 kVA on widebody types.
• Generator Control Unit (GCU): Electronic regulator that senses output voltage and frequency, adjusts field excitation, and disconnects the generator for faults such as overvoltage, differential protection trips, or overtemperature.
• APU Generator: Shaft-driven generator integrated with the APU gearbox, rated 90 to 120 kVA on most types, capable of supplying the entire aircraft on the ground and supplementing engine generators in some emergency scenarios.
• Bus Tie Contactor (BTC): Electrically operated switch that connects or isolates AC buses, enabling cross-connection of generators to maintain power to all buses when one generator is lost.
• External Power Receptacle (EPR): Ground power connection allowing airport ground power units (GPUs) to supply 115V 400 Hz AC during maintenance or boarding without running the APU.

Aircraft Applications

The Boeing 737-800 uses two CFM56-7B-driven IDGs each rated 90 kVA, plus an APU generator of the same rating. The Airbus A320-200 family employs a similar architecture with two IDGs and an APU generator, with a static inverter as backup for essential AC. The Boeing 777-300ER replaced IDGs with two 120 kVA VFGs per engine (four total) plus two APU generators, a configuration that eliminated the CSD as a maintenance-intensive component. The Boeing 787 escalated electrical generation dramatically, with two 250 kVA VFGs per engine (four total) and two 225 kVA APU generators to power its all-electric subsystems.

Advantages and Limitations

IDG technology offers fixed-frequency output compatible with all legacy equipment but introduces the CSD as a wear component requiring oil changes and periodic replacement. VFGs eliminate the CSD entirely, improving mean time between removals significantly, but require variable-frequency-tolerant loads throughout the aircraft. Higher electrical generation capacity enables the More Electric Aircraft concept, eliminating bleed-air and hydraulic systems with their weight and maintenance penalties. The primary limitation of centralised engine-driven generation is vulnerability: an engine failure reduces available generation, requiring the automatic shedding of non-essential loads and the engagement of backup sources to protect flight-critical buses.