Aircraft Electrical Systems Explained

Power Generation

Commercial aircraft generate primary electrical power from Integrated Drive Generators (IDGs) mechanically coupled to each engine. The IDG includes a Constant Speed Drive (CSD) — essentially an automatic transmission that converts variable engine shaft speed into a constant drive speed for the generator. This constant speed produces 115V AC at 400 Hz, the standard for aviation electrical systems worldwide (compared to 50/60 Hz for household current).

400 Hz was chosen in the 1940s because higher frequency allows smaller, lighter transformers and motors for a given power level — critical for weight-sensitive aircraft. A typical commercial jet generates 90–120 kVA per engine-driven generator. The Boeing 777, with four generators (two per engine), generates up to 240 kVA per engine; the 787, with its "more electric" architecture, generates 500 kVA per engine.

AC vs. DC Power

Aircraft use both Alternating Current (AC) and Direct Current (DC). AC power at 115V/400 Hz or 230V/400 Hz runs heavy consumers: galley equipment, cabin lighting, air conditioning compressors, and large actuators. DC power at 28V (the aviation standard) runs avionics, flight computers, communications equipment, and backup systems — loads that need stable, regulated voltage independent of generator speed variations.

AC is converted to DC using Transformer Rectifier Units (TRUs), which step down and rectify the AC. Most aircraft also carry batteries (nickel-cadmium or lithium-ion) that provide DC backup power when generators fail. The batteries are kept charged by dedicated battery charger circuits.

Bus Architecture

Electrical power is distributed through a bus architecture — a network of busbars (copper conductors) with switching and protection equipment. Critical aircraft systems are distributed across multiple independent buses so that a single generator failure does not leave any vital system without power. Typical bus types include:

• AC Main Buses (1 and 2): Primary distribution, each fed from one engine generator. Bus tie contactors can cross-connect buses in failure scenarios.
• AC Essential Bus: Feeds critical avionics and controls; can be powered from multiple sources including the APU or ground power.
• DC Buses: Fed from TRUs; power avionics and computers.
• Battery Bus / Hot Battery Bus: Always connected to the battery, even with all other power lost.

Emergency Power

In the event of complete loss of all engine generators and APU, several emergency power sources are available:

• Battery: Provides 30–60 minutes of power for essential avionics and systems on most aircraft.
• Ram Air Turbine (RAT): A small turbine/generator or turbine/hydraulic pump that deploys into the airstream from the aircraft's belly. The RAT generates enough power for essential flight controls and instruments. The Airbus A320's RAT deploys automatically when both generators are lost; it generates 5 kVA — enough to power essential systems but not galleys or in-flight entertainment.
• Ground Power: Aircraft can be connected to airport ground power units (GPUs) via a receptacle, typically on the forward lower fuselage, eliminating the need to run APU for ground operations.

The Boeing 787 "More Electric" Architecture

The Boeing 787 made the most radical change to commercial aircraft electrical architecture in decades by adopting a "more electric" design that replaces bleed air and hydraulics with electrical equivalents wherever possible. Each 787 engine drives two 250 kVA generators (variable-frequency, 360–800 Hz rather than constant 400 Hz) for a total generation capacity of 1 MVA — four times a conventional aircraft.

This electricity powers the electric compressors that pressurize the cabin (replacing bleed air ECS), electric hydraulic pumps (replacing engine-driven hydraulic pumps), and electric wing anti-icing (replacing bleed air wing thermal anti-icing). The benefits: improved engine efficiency (bleed air extraction costs fuel), reduced maintenance complexity, and better passenger experience (lower cabin altitude, higher humidity).

The APU's Role

The Auxiliary Power Unit (APU) is a small jet engine (typically a Honeywell 131-9 on A320s and 737s, or a Pratt & Whitney APS3200 on A380s) in the tail that generates both electrical power and bleed air for ground operations. The APU allows the aircraft to be self-sufficient: crews can power up systems, test flight computers, and start air conditioning without being connected to airport ground services.

During flight, the APU can serve as an emergency generator source, and in some failure scenarios, an APU start in flight (up to about 33,000 feet on most aircraft) can restore electrical power. Modern APU performance has improved dramatically — the 787's APU generates 225 kVA alone, more than the total generation capacity of older aircraft's entire primary generation system.