Electrical Power
AC/DC generation (IDG/CSD, TRUs, batteries, RAT), distribution, load management and protection.
Electrical power, summary notes
- Large aircraft use AC as the primary supply. To hold a CONSTANT FREQUENCY (400 Hz) despite varying engine speed, the generator is driven through a constant-speed drive, the CSD/IDG (integrated drive generator); some modern aircraft use variable-frequency systems.
- DC is produced from AC by TRANSFORMER-RECTIFIER UNITS (TRUs); batteries give start and emergency power; other sources are the APU generator, external/ground power and the ram air turbine (RAT).
- DISTRIBUTION uses busbars and bus-tie breakers with load prioritisation: if generation is lost, non-essential loads are shed and the ESSENTIAL/BATTERY bus keeps critical systems and standby instruments alive.
- PROTECTION comes from generator control units (voltage regulation, over-voltage/under-frequency/differential protection) and circuit breakers protecting the wiring.
- For the B2 technician the wiring, busbars, control units and the sensing/indication of the electrical system are central.
- ⚠ Exam trap: the CSD/IDG keeps the AC generator output at a CONSTANT FREQUENCY (400 Hz) regardless of engine RPM; TRUs convert AC to DC; the battery/essential bus feeds the critical loads if generation fails.
After a double generator failure, what keeps the essential flight instruments and radios powered?
The battery, feeding the DC essential/battery bus (and, via a static inverter, an AC essential bus for any AC-only critical loads). Load-shedding sheds galleys and other non-essential loads so the limited battery capacity powers only the essential instruments, radios and standby systems, and a RAT may deploy to restore generation for extended flight.
Electrical power concept map
Electrical Power
Electrical power quiz
Electrical Power, quiz
1. An IDG/CSD keeps the AC generator output at:
A constant frequency (400 Hz)A constant currentA variable voltage2. A TRU converts:
AC to DCDC to ACHydraulic to electric3. The nominal aircraft AC frequency is:
400 Hz50 Hz60 Hz4. If all generators fail, critical loads are kept by the:
Battery/essential busGalley busLanding-light bus5. A RAT provides:
Emergency electrical/hydraulic power from the airflowExtra thrust, by recovering energy from the airflow and returning it through the accessory driveCabin air, drawing in ram air and delivering it to the packs when bleed supply is lost6. A static inverter converts:
DC to ACAC to DCDC to hydraulic7. Bus-tie breakers are used to:
Connect/isolate busbars for load sharing/fault isolationCool the generators, by switching cooling airflow to whichever machine is most heavily loadedMeter fuel to the engines in proportion to the electrical load being drawn from each generator8. Load shedding removes:
Non-essential loads when generation is limitedEssential instruments firstThe battery