021 Airframe, Systems and Powerplant topic guide
Aircraft Electrical Systems
An airliner's electrical system exists to turn mechanical rotation from the engines into stable electrical power, then get that power to every system that needs it, in the right form, with enough redundancy that no single failure leaves a critical system dark. The topic rewards a structural view: generation, distribution, protection, and backup are four distinct jobs, and most exam questions are really asking which of the four a described component belongs to.
The recurring exam pattern is a chain of failures: lose a generator, then a bus, then ask what still has power and from where. That style of question cannot be answered from memorised component lists alone; it requires actually tracing the path power would take after each step, which is why this topic consistently produces the multi-part failure-logic questions students find hardest.
Generation
Primary AC power on most turbine aircraft comes from generators driven by the engines themselves. Because engine speed varies with power setting while most aircraft systems need a stable frequency, many designs interpose a constant-speed drive between the variable-speed engine and the generator, holding the generator's input speed constant so its output frequency stays constant regardless of engine RPM. Where the constant-speed drive and the generator are combined into one unit, it is commonly called an integrated drive generator, or IDG.
The output is commonly quoted as 115 V AC at 400 Hz on many transport aircraft, a higher frequency than mains power specifically because it allows smaller, lighter transformers and motors for a given power output, a genuine benefit given how much weight matters in an airframe.
Distribution and bus architecture
Generated power is not delivered to every system directly; it is fed onto buses, and those buses are arranged in a priority structure so that if generation is reduced, the least essential loads are shed first while flight-critical systems keep power for as long as possible. Bus tie arrangements allow a healthy generator to feed a bus that has lost its own source, which is exactly the mechanism exam questions probe when they describe one generator failing and ask what the remaining generator can still cover.
Not every system runs on AC. Transformer rectifier units, TRUs, convert AC to DC for the many systems, lighting, some avionics, battery charging among them, that need direct current rather than alternating current. A TRU failure is therefore a DC-side problem even though its input side is AC, a distinction that trips up students who assume a rectifier fault must be an AC generation fault.
Battery, standby power, and protection
The battery and any standby inverter exist for the scenario where normal generation is lost entirely, providing a last-resort supply to a reduced set of essential buses: basic instruments, communication, and control systems, not the whole aircraft. Essential and battery buses are kept simple and directly protected because they are the fallback of last resort.
Circuit protection devices, breakers and current limiters among them, isolate a faulted circuit before it can damage the wiring or the source feeding it. A tripped breaker removes one specific load from one specific bus; it does not by itself explain a wider loss of power unless the question builds that chain.
- Engine-driven generator plus constant-speed drive (or IDG): stable-frequency AC generation
- Bus tie: lets a healthy generator cover a bus that lost its own source
- TRU: converts AC to DC for direct-current loads
- Battery and standby inverter: last-resort supply to essential buses only
- Circuit breaker: isolates one faulted circuit, not a whole generation source
Worked example
Worked example: tracing a TRU failure
Not quite. The correct answer is B.
Correct answer: B
- A. Generators supply the AC buses directly; the TRU sits downstream converting some of that AC to DC. A TRU failure does not remove power upstream at the AC generation source.
- B. Correct. The TRU only converts AC to DC for the loads wired to its output; its failure is a DC-distribution problem downstream of two otherwise healthy AC generators.
- C. The battery is a last-resort supply for a reduced set of essential buses when generation is lost entirely, not a general substitute for one failed TRU while both generators are still healthy.
- D. This ignores that any DC load normally fed by that specific TRU genuinely loses its normal source, even though AC generation itself is untouched.
Step by step
- Place the TRU correctly in the chain: it sits downstream of AC generation, converting AC to DC for specific loads.
- Confirm the generators are unaffected, since the fault is downstream of them, not at the generation source.
- Identify what actually loses power: only the DC loads that particular TRU was feeding.
- Rule out a wider effect: healthy AC buses and a functioning second TRU, if fitted, are not touched by this single failure.
Common mistakes
Assuming a DC-side component failure implies an AC generation failure
TRUs and other DC equipment sit downstream of the generators. A fault there is a distribution problem for DC loads, not evidence that the generators themselves have failed, and treating the two as the same collapses an otherwise answerable failure-chain question.
Treating the battery as a general backup for any single failure
The battery and standby inverter are sized and wired for a reduced essential-loads scenario used when normal generation is lost, not as an automatic substitute for every individual component fault described in a question.
Losing track of the failure order in a multi-step scenario
These questions are built as a sequence, generator lost, then a bus tie lost, then a specific system asked about, and answering from the final state without tracing each step in order is how a correct-sounding but wrong option gets picked.
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Important information
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Last reviewed July 2026