Describe the architecture of an aircraft electrical system, identify the role of generators, batteries and bus bars, calculate electrical loads and voltage drops, and outline the role of fly-by-wire avionics

What this dot point is asking

NESA wants you to describe how an aircraft electrical and avionics system is organised, identify the role of generators, batteries and bus bars, perform basic load and voltage drop calculations, and outline how fly-by-wire flight control systems work.

The answer

Architecture of an aircraft electrical system

A typical airliner electrical system has:

• Engine-driven generators. One main generator per engine (typically 90 to 120 kVA on a 737, 250 kVA on a 787). Driven by the accessory gearbox at constant speed by an integrated drive generator (IDG) or, on the 787, at variable speed with frequency conversion in the bus controller (VFG).
• Auxiliary power unit (APU). A small gas turbine in the tail with its own generator, used on the ground and as a backup in flight.
• Ram air turbine (RAT). A small wind-driven generator that deploys from the fuselage in emergency, powering essential flight instruments and controls.
• Batteries. Sealed lead-acid or lithium-ion. Provide power during engine start and as final backup. 787 main battery is a 32 V 65 Ah lithium-ion.
• Bus bars. Distribution rails for AC and DC power. Essential and non-essential loads are split so non-essential loads can be shed if a generator fails.
• Transformer-rectifier units (TRUs). Convert 115 V AC three-phase (or 235 V on the 787) to 28 V DC for avionics.

Standard voltages

Most large airliners use 115 V AC three-phase at 400 Hz for main distribution. The higher frequency (versus 50 Hz mains) allows smaller transformers and motors, saving mass. The Boeing 787 raised the standard to 235 V AC three-phase, allowing the same power at lower current and reducing wire mass.

DC distribution is 28 V for most avionics and lighting.

Fly-by-wire flight controls

Traditional aircraft used mechanical cables and pushrods from the control column to the hydraulic actuators at the control surfaces. Modern airliners use fly-by-wire (FBW):

• Sidestick or yoke position is read by transducers.
• A flight control computer translates pilot input into desired aircraft response.
• The computer sends electrical signals to the actuators at the flight controls.
• The actuators (hydraulic on most aircraft, electric on the 787) move the surfaces.

Advantages: lower mass (no cables); envelope protection (the computer prevents pilots from over-stressing the airframe); auto-trim and ride-quality enhancement.

The flagship Airbus FBW programmes are the A320 family and the A380; Boeing implemented FBW on the 777, 787 and 747-8. Triplicated or quadruplicated computers and sensors provide fault tolerance.

Load and voltage drop calculations

For a DC system, Ohm's law gives the voltage drop along a wire:

where $R = \rho l / A$ depends on wire length and cross-section. Aircraft wiring uses copper or, in the 787, aluminium for high-current runs to save mass.

For a three-phase AC system, the apparent power is:

Active power is $P = S \cos\phi$ where $\phi$ is the power-factor angle.

Bus bar redundancy

Essential systems (flight instruments, hydraulics, fly-by-wire, communication) are powered from an essential bus that can be fed from any generator, the APU, the battery or the RAT. Non-essential systems (galleys, in-flight entertainment, cabin lighting) are on separate buses and are shed first during a generator failure.

The 787 architecture is unusual for using such a large electrical generation capacity (1450 kVA, four generators). This is the More Electric Aircraft (MEA) concept: replace traditional bleed-air, hydraulic and pneumatic systems with electrical equivalents.

Australian context

The Boeing 787-9 Dreamliners operated by Qantas use MEA architecture; the Airbus A380 fleet (still in service for Qantas international routes) uses traditional bleed-air pressurisation and hydraulic primary flight controls but FBW. Royal Australian Air Force F-35A Lightning II combat aircraft use a fully FBW flight control system with quadruply redundant computers.