Generation makes the power; distribution decides who gets it, and, more importantly, who keeps getting it when a source dies. This lesson covers the busbar and the family of named buses, the structure as the common earth, external power and the pecking order of sources, load sharing and load shedding, and the skill the syllabus asks for by name: reading an FCOM electrical schematic well enough to tell a supply failure from a bus failure, on anything from a single-engine trainer to a transport.
The busbar
A busbar is the electrical system's distribution rail: a solid conductor to which a source delivers its power and from which many consumers draw theirs, each through its own circuit breaker, side by side in parallel. It solves distribution's bookkeeping in one stroke: instead of a wire from every source to every consumer, each consumer connects once, to the bar, and each source feeds the bar. Everything the fundamentals lesson said about parallel circuits applies: every consumer sees the full bus voltage and can be switched or protected without disturbing its neighbours. 'Bus' is the working shorthand for the same thing.
The family of buses
Transport aircraft carry not one bus but a named family, organised by how important their consumers are and how directly they touch the battery:
AC bus — the main alternating-current rails, fed by the generators, carrying the heavy consumers.
DC bus — the direct-current rails, fed through the TRUs, carrying the avionics and control loads.
Essential AC and DC buses — the rails carrying what the flight genuinely needs (primary instruments, radios, computers), wired so they can be re-fed from more than one source and are the last to be abandoned.
Emergency AC and DC buses — the minimum core kept alive in the worst case, fed from the battery (through the static inverter for the AC side) or the RAT when everything else is gone.
Battery bus — the rail fed directly from the battery through the battery switch: alive whenever the battery is selected on.
Hot bus — permanently wired to the battery, live even with every switch off and the aircraft dark: for the handful of items that must never lose power, such as the fire-extinguisher squibs (the cartridges that fire the bottles) and the clock.
Ground servicing or maintenance bus — a rail that external power can energise on the ground for cleaning, galley loading and maintenance, without waking the rest of the aircraft.
The structure as earth
The aircraft's metal structure can serve as part of the circuit: the common earth. Wire each consumer's supply side from its bus, connect its return side to the nearest structure, and the airframe itself carries every return current back to the battery's earthed negative terminal: half the aircraft's wiring, saved. The implication is that bonding becomes electrically load-bearing: the fundamentals lesson's bonding straps are no longer only about static and lightning, they are the return conductor of every circuit aboard, and a corroded or missing bond means resistance in series with every consumer beyond it: dim lights, lazy motors, and gauges that read wrong for no visible reason.
External power, and who feeds the bus
External power is the airport feeding the aircraft: a ground power unit plugged into the aircraft's receptacle energises the buses with engines and APU shut down, saving the battery for the start, sparing the APU's fuel and noise, and letting maintenance power everything at leisure. The flight deck sees it as one more source, with its own availability light and selection.
Because several sources can offer power at once, a priority sequence exists, on the ground and in flight: the system (or the crew, on simpler aircraft) connects each bus to the best source available and drops to the next when it fails. A typical order gives each engine's own generator its bus first, with the APU generator, external power (on the ground) and the opposite side's generator ranked behind it in an order that varies by type; in flight the battery is the last name on every list. The sequence is why a generator failure produces a brief flicker and a new feeder rather than a negotiation.
Load sharing and load shedding
Matching demand to supply is an arithmetic with two halves:
Load sharing — the running sources dividing the aircraft's total demand between them: two generators feeding the split system carry roughly half each, and where sources are run connected together, their controls actively equalise the burden so no machine carries more than its share.
Load shedding — the other half of the arithmetic: when supply shrinks (a generator lost, or battery-only flight), demand must shrink to match, so the system disconnects the least important consumers, automatically on modern aircraft, by checklist on older ones.
The classic shed loads are the cabin's comfort: galley power and its ovens, the passenger entertainment system, and similar non-essentials, which is why a generator failure is smelt in the cabin as cooling coffee before it is seen from seat 14A.
Reading the schematic
Interpreting the electrical schematic is its own learning objective, and the FCOM's drawing is built from exactly the pieces this module has taught: sources at the top, contactor squares connecting them to buses (a contactor is a heavy, remotely operated switch: the electromagnetism lesson's relay, grown up to busbar currents), tie breakers joining left to right, TRUs stepping down to the DC side, and the battery anchoring the bottom. Reading it is the logic-gate drill again: establish which sources live, follow what each bus is connected to, and every downstream consequence falls out.