📖 The lesson
The shape of a transport electrical system
Most transport aircraft generate 115/200 V, 400 Hz, three-phase AC as their main power and convert part of it to 28 V DC. 400 Hz (instead of the 50/60 Hz of ground mains) allows much lighter transformers and motors. Light aircraft generally use 14 V or 28 V DC throughout; the B787 and A380 use variable-frequency AC, and the B787 also ±270 V DC for big motor loads.
- Sources: one generator per engine (an IDG or a variable-frequency generator), the APU generator, external power, the batteries, and an emergency source — usually a ram air turbine (RAT).
- Priority: each bus takes the best source available. On many types an engine generator has priority over external power, which has priority over the APU generator, so connecting a source never "fights" another.
- Split-bus systems (most modern twins) never parallel two generators; each powers its own side, and bus-tie contactors transfer a bus to another source if its generator fails. Older three- and four-engine types often paralleled their generators, which needs frequency and phase synchronisation and real and reactive load sharing.
- A bus power control unit (BPCU) and the generator control units (GCUs) manage the contactors, priorities, external power and load shedding.
Batteries
| Lead-acid | Nickel-cadmium | Lithium-ion | |
|---|---|---|---|
| Cell voltage | 2.0 V (≈ 2.1 V open circuit) | 1.2 V nominal | ≈ 3.6–3.7 V |
| Cells for 24 V | 12 | 19–20 | typically 8 (in series) |
| Electrolyte | Sulphuric acid | Potassium hydroxide (alkaline) | Organic electrolyte |
| State of charge | Specific gravity of the electrolyte (≈ 1.275–1.300 charged) | Cannot be found from specific gravity, which hardly changes; use voltage / capacity test | Battery management system |
| Main hazard | Acid, hydrogen gas on charge | Thermal runaway; electrolyte level drops on discharge | Thermal runaway and fire — contained, vented overboard |
| Spill neutraliser | Sodium bicarbonate solution | Boric acid solution or vinegar | — |
- Capacity is quoted in ampere-hours. The main battery supplies APU starting, ground power when nothing else is available and emergency power for a limited time; a separate APU battery is common.
- Nickel-cadmium batteries can suffer thermal runaway: at high temperature their internal resistance falls, so a constant-voltage charger drives more current, which heats them further. Temperature sensors, charger control and overheat warnings guard against it. Their electrolyte level is checked only when fully charged, and they are deep-cycled in the workshop to equalise cells.
- Never use the same tools, hydrometers or workshop area for lead-acid and nickel-cadmium batteries: acid contaminates and destroys a Ni-Cd battery.
- Installation: vented battery boxes with vent tubes overboard (lead-acid boxes may have a sump jar with neutralising pad), quick-disconnect connectors, and terminals torqued correctly. With ordinary terminals, disconnect the negative lead first and reconnect it last. Wear eye protection and remove rings and watches — a battery short circuit can deliver thousands of amperes.
- Most transport aircraft charge their batteries from dedicated chargers or the DC bus through charge relays, with charging currents and temperatures monitored.
DC generation
- DC generators (older and light aircraft) have a rotating armature, commutator and brushes; output is controlled by varying the field current with a voltage regulator. A reverse-current relay (cut-out) disconnects the generator from the bus when its voltage falls below the battery's, so the battery cannot motor the generator. Paralleled generators use equalising circuits to share load.
- Starter-generators (turboprops, business jets, APUs) are DC machines that act as the engine starter motor and then as a 28 V generator.
- Alternators on light aircraft produce AC that built-in diodes rectify to DC; the regulator controls the rotor field current. They charge at low engine speed and need battery voltage to excite the field.
AC generation
Frequency depends on speed and the number of poles: f = (P × N) / 120, where P is the number of poles and N the speed in rpm. A 4-pole generator therefore needs 12 000 rpm for 400 Hz. Because engine speed varies, a constant frequency needs a constant-speed drive (CSD) — a hydro-mechanical differential transmission that adds or subtracts speed so the generator always turns at the same rate.
- An integrated drive generator (IDG) combines the CSD and the generator in one oil-cooled housing (typically 90 kVA or more on a narrow-body). The crew can disconnect it from the engine gearbox if it overheats or loses oil pressure; a disconnected IDG can be reconnected only on the ground with the engine stopped. IDG oil level, filters and quick-disconnect mounts are line maintenance items.
- Variable-frequency generators (VFG) drop the CSD: frequency follows engine speed (for example 360–800 Hz). They are simpler and more reliable, but loads such as motors must cope with varying frequency, usually through power electronics.
- Ratings are in kVA because the generator must supply both real power (kW) and reactive power; the ratio is the power factor.
Generator control and voltage regulation
The generator control unit (GCU) regulates the voltage at the point of regulation (POR) — near the bus — by varying the exciter field current, using the average of the three phases. It also protects the system, opening the generator control relay (field) and the generator line contactor (output) for:
- overvoltage and undervoltage, overfrequency and underfrequency;
- differential current (a short circuit on the feeder between the generator and the bus, detected by comparing current transformers at each end);
- overcurrent and open phase, and on paralleled systems unbalanced load sharing.
On DC systems the regulator does the same job by controlling the generator field current — older aircraft used carbon-pile or vibrating-contact regulators, modern ones are solid-state.
Emergency power
- If all main AC is lost, the RAT deploys automatically (it can also be deployed manually) into the airstream. Depending on the type it drives an emergency generator directly or a hydraulic pump that powers a hydraulic-motor-driven generator. It feeds the essential buses needed to fly and land: flight instruments, essential radios and navigation, flight controls and some lighting.
- Until the RAT is up to speed — and below the RAT's minimum airspeed on the landing roll — the batteries carry the essential DC loads and a static inverter supplies essential AC.
- The APU generator is an alternative source in flight within its altitude limits. Emergency lighting has its own batteries (Chapter 17).
Distribution, conversion and load shedding
| Bus | Typical role |
|---|---|
| Main AC and DC buses | Normal loads, one side per generator |
| Essential (ESS) buses | Loads needed for continued safe flight; fed from either side or the emergency sources |
| Hot battery bus | Connected directly to the battery and always live — fire extinguishers, clocks, some lights and memories |
| Ground service / handling buses | Power for cabin cleaning, cargo loading and refuelling without powering the whole aircraft |
| Galley and utility buses | Large non-essential loads — shed automatically when a generator fails or is overloaded |
- Transformer-rectifier units (TRUs) convert 115 V AC three-phase to 28 V DC (typically 150–200 A each).
- Static inverters convert battery DC into 115 V AC single-phase for essential AC loads; older aircraft used rotary inverters (a DC motor driving an AC generator).
- Transformers step 115 V down to 26 V AC for instruments and synchros; autotransformers adjust voltage; current transformers feed metering and differential protection. Rectifiers appear in TRUs, chargers and generators.
- Feeders are copper or aluminium cables; the structure carries the current return on single-wire systems (Chapter 3). Modern aircraft increasingly switch and protect loads with solid-state power controllers (SSPCs) controlled through software.
Circuit protection
The golden rule
A circuit breaker or fuse protects the wiring, not the equipment. Its rating is matched to the wire size; never fit a higher-rated breaker or fuse than the wiring diagram specifies.
- Thermal circuit breakers — a bimetal element trips on sustained overload; they are trip-free: the breaker cannot be held closed against a fault, even if the button is held in. Magnetic and thermal-magnetic types trip faster on heavy faults.
- Fuses — single-use; current limiters (slow-blow fuses) protect heavy feeders and allow short overloads such as motor starting.
- Remote-control circuit breakers (RCCBs) — relay-operated breakers for heavy loads, controlled from the flight deck.
- Arc-fault circuit breakers detect the signature of arcing that a thermal breaker might miss; SSPCs combine switching and protection electronically.
- Maintenance: use circuit-breaker collars and tags (lock-out/tag-out) when working on systems; investigate the cause before resetting a tripped breaker; follow the operator's rules about resetting breakers in flight — fuel-pump breakers, for example, are not reset.
External (ground) power
- A ground power unit (GPU) or fixed ground power supplies 115/200 V 400 Hz three-phase through a six-pin receptacle (light aircraft use 28 V DC GPUs).
- Pins A, B, C carry the phases and N the neutral; the short pins E and F carry 28 V DC from the GPU that energises the external-power contactor only when the plug is fully engaged, so the main pins never make or break under load.
- The BPCU checks voltage, frequency and phase sequence before allowing connection ("EXT PWR AVAIL"); wrong phase rotation would run three-phase motors backwards.
- Safe practice: check the GPU output, connect with the GPU switched off, then switch on; switch the aircraft's external power off before disconnecting; never pull the plug under load.