⚡

⚡ Electrics & Electronics

From one electron drifting along a wire to the four paralleled generators of a long-haul jet. The first nine chapters build the DC picture — what makes current flow, how it is switched, protected and stored, and how a light aircraft's system fits together. The last seven move to AC, which is what every large aircraft runs on, and finish with the semiconductors and logic that sit inside every box on board.

16 chapters 1,000 linked questions 🧮 Calculators 📊 Diagrams Aircraft-independent

🧰 Quick tools & cheat sheet play with the numbers

Six calculators for the sums this subject asks again and again, and one screen of the numbers worth knowing by heart. Change any box and the answer updates.

Ω Ohm's law & power

Voltage V (volts) Resistance R (ohms)

🔗 Series & parallel

Resistors (Ω), separated by commas Supply voltage (V)

🔋 Battery endurance

Rated capacity (Ah) Load current (A) Capacity test result (% of rated)

🌀 Poles, RPM & frequency

Number of poles (total N + S) Speed (RPM) Induction motor slip (%)

〰️ Reactance & impedance

Frequency (Hz) Inductance (mH) Capacitance (µF) Resistance (Ω)

🔁 Transformer & three phase

Primary voltage (V) Primary turns Secondary turns Primary current (A)

📋 Cheat sheet — numbers worth knowing

Idea	Number / rule
Ω Ohm's law	V = I × R
💡 Power	P = V × I = I²R = V²/R
🔗 Series / parallel	R add in series · 1/R add in parallel
🔋 Cell voltages	Lead acid 2 V (2.2 off load) · NiCad 1.2 V · primary 1.5 V
💧 Lead acid SG	1.27 charged · 1.17 flat (recharge at 1.8 V)
🧪 Battery capacity test	≥ 80 % to stay in service · essential loads 30 min
🧷 Spare fuses	10 % of each rating, minimum 3
⚙️ Light aircraft system	14 V (12 V battery) · overvoltage trip ≈ 16.5 V
✈️ Large aircraft AC	115 V / 200 V · 400 Hz · 3 phase
🌀 Frequency	f = (poles ÷ 2) × (RPM ÷ 60)
〰️ RMS	RMS = 0.707 × peak · peak = 1.414 × RMS
⭐ Star connection	V line = √3 × V phase · I line = I phase
🔺 Delta connection	V line = V phase · I line = √3 × I phase
📐 Power factor	PF = kW ÷ kVA = cos φ
🛞 CSD frequency band	400 Hz ± 5 % (380–420 Hz)
🧲 Fleming	Left = motor · Right = generator (First-Field, seCond-Current, thuMb-Motion)

⚡ Part one — Direct current

1 DC Basic Principles Chapter 1

Four quantities — voltage, current, resistance and power — and two laws tie the whole subject together.

🎯 Free electrons🎯 Electron vs conventional flow🎯 Six sources of EMF🎯 Ohm's law🎯 Power and I²R🎯 Series & parallel🎯 Kirchhoff

💡 Key idea

Voltage pushes, resistance resists, current is what flows: V = I × R. Every calculation in this chapter is that formula, used directly or rearranged.

Where current comes from

An atom has a positive nucleus (protons and neutrons) with negative electrons in orbit; a neutral atom has as many protons as electrons. Electrons in the outer orbits are held weakly and can wander off to a neighbouring atom — these are free electrons. An atom that loses one becomes a positive ion; one that gains an extra becomes a negative ion.

Conductors

Many free electrons: silver (best), copper, aluminium.

Insulators

Very few free electrons: wood, rubber, glass, plastics.

Connect a wire across a source and the negative terminal repels the free electrons while the positive attracts them: electrons drift from negative to positive (electron flow). The older convention, used on diagrams and in every hand rule, shows current from positive to negative (conventional flow). The source keeps the flow going by taking electrons in at its positive terminal and pushing them out of the negative — so it only works round a complete circuit.

Six ways to make electrons move

Method	Device	Useful power?
Friction	Static electricity	No
Chemical action	Cells and batteries	Yes
Magnetism	Generators and alternators	Yes
Heat	Thermocouple (two dissimilar metals)	No — sensing
Light	Photoelectric cell	No — sensing
Pressure	Piezoelectric crystal	No — sensing

Volts, amps and ohms

EMF / pd → volts (V)

The electrical "pressure". Voltmeter goes across the component. 1 kV = 1000 V · 1 mV = 0.001 V · 1 µV = 10⁻⁶ V.

Current I → amps (A)

Electrons passing a point per second. Ammeter goes in series. 1 mA = 0.001 A · 1 µA = 10⁻⁶ A.

Resistance → ohms (Ω)

1 Ω passes 1 A when 1 V is applied. 1 kΩ = 1000 Ω · 1 MΩ = 10⁶ Ω · 1 mΩ = 0.001 Ω.

Current has three effects, and every one of them is used on board: heating (lamps, heaters, fuses), magnetic (motors, generators, transformers — a field always surrounds a current) and chemical (battery charging, electroplating).

Resistance depends on material, length (double the length, double the R), cross-section (thicker = lower R) and temperature. A resistor whose R rises with temperature has a positive temperature coefficient (PTC); one whose R falls has a negative coefficient (NTC). Both are used as temperature sensors. The coefficient's symbol is α.

Ohm's law and power

V = I R · I = V/R · R = V/I

Constant V: more R, less I. Constant R: more V, more I.

P = V × I

Watts — the rate of doing work. 28 V × 5 A = 140 W.

P = I² R

Use when V is unknown. 3 A in 10 Ω = 90 W.

P = V² / R

Use when I is unknown. 24 V on 12 Ω = 48 W.

⚠️ Why fuses exist Heat goes with the square of the current: double the current and the heat is four times as great. Every component has a power rating (a 60 W lamp, say); a fuse or circuit breaker opens the circuit before that rating is exceeded.

Series and parallel

Series: one failure stops everything. Parallel: each load is independent — which is why aircraft loads are wired in parallel.

Series

R_T = R1 + R2 + R3. Same current everywhere; less current and power; loads cannot be switched on their own; one failure kills the circuit.

Parallel

1/R_T = 1/R1 + 1/R2 + 1/R3. Same voltage on every branch; each load works alone. R_T is always less than the smallest resistor. For two: R1·R2 / (R1 + R2).

Worked example — series–parallel

A 12 Ω and an 8 Ω resistor in parallel, in series with 5.2 Ω, across 28 V.

Parallel pair: (12 × 8) ÷ (12 + 8) = 96 ÷ 20 = 4.8 Ω.
Add the series resistor: 4.8 + 5.2 = 10 Ω.
Current: 28 ÷ 10 = 2.8 A.

The usual slip is to add all three (25.2 Ω, 1.1 A) — always collapse the parallel group first.

Kirchhoff's two laws

Σ I in = Σ I out

First law, at any junction. 5 A + 3 A in, 2 A out on one wire → 6 A out on the other.

Σ voltage drops = V applied

Second law, round a closed loop. 1, 2, 3 Ω on 24 V → 4 A → drops of 4, 8 and 12 V.

2 DC Switches Chapter 2

Every circuit is started and stopped by something — a pilot's finger, a door, a temperature, a speed or a clock.

🎯 Toggle & switch light🎯 Momentary vs alternate🎯 Guards🎯 Microswitch🎯 Proximity detectors🎯 Thermal, time, centrifugal

💡 Key idea

Match the switch to its trigger: a person (toggle, switch light), a position (micro or proximity), a temperature (bimetal), a time (time switch) or a speed (centrifugal).

Switches the crew operate

Toggle switch

General-purpose lever, common in older aircraft. Two-position (on/off), or multi-position sprung to the centre — press and hold in the direction wanted.

Switch light

Push switch + indicator in one. Momentary: active only while held. Alternate: press on, press again off. The light shows the selected state or warns that a selection is needed.

Guarded switch

A cover must be lifted first, to stop inadvertent operation of critical functions — generator disconnect, fuel dump.

Switches the system operates

Switch	Triggered by	Typical use
Microswitch	Very small plunger movement ("make and break")	Door open/closed; now mostly replaced by proximity detectors
Bimetallic (thermal)	A set temperature	Fire detection, battery overheat, oil temperature warning
Time switch	A set time after starting	APU air intake door closing a set time after shutdown
Centrifugal	RPM rising or falling past a value	Starter motor cut-out

Proximity detectors — sensing without touching

Inductive

A coil whose inductance changes when a ferromagnetic target comes near. Used on landing gear: sensor on the structure, target beside it, printed circuit card in the electronic unit.

Capacitive

Capacitance changes near any material — metal or not, solid or liquid. Can detect high/low levels in hydraulic or fuel systems.

Magnetic

Coil on a magnet: a moving ferrous object changes the flux and induces an EMF (size ∝ rate of change). A gear wheel gives a pulse train at the tooth-passing frequency — a speed sensor.

✈️ Why gear uses them The sensor is hermetically sealed and never touched by its target, so dirt, ice and water don't affect it — far more reliable than a microswitch.

3 Circuit Protection & Capacitors Chapter 3

Faults bring overcurrent and overvoltage; fuses and breakers stop them. Then the capacitor — the component that stores charge and blocks DC.

🎯 Fuse vs breaker timing🎯 Three fuse types🎯 Fuse rating rules🎯 Spare fuses🎯 Trip-free🎯 Capacitance🎯 C in series/parallel

💡 Key idea

A fuse opens before the fault current peaks; a circuit breaker opens after it. So with a breaker, both it and the component must survive full fault current briefly — but a breaker can be reset and used as a switch.

Fuses

Cartridge

Glass or ceramic tube, brass end caps, element of tinned copper, silver or zinc strip (zinc for heavy duty).

High rupture capacity (HRC)

Robust ceramic body filled with powdered marble or quartz sand. More accurate and faster, no flame, no ageing, unaffected by ambient temperature. High-current circuits.

Current limiter

High melting point, so it carries a large overload before rupturing. Waisted tinned-copper strip in ceramic with an inspection window. Heavy distribution — e.g. a TRU output.

⏱️ Blow time is inversely related to current: the bigger the overload, the faster it goes.
🎯 Rating: not below the normal circuit current, but blowing below the cable's safe limit. Only the specified fuse may be fitted. Range about 0.5–500 A (high ratings only HRC and limiters).
🔁 A blown fuse may be replaced once. If it blows again, leave it until the circuit has been investigated.
🧷 Spares: 10 % of each rating, minimum 3 (50 installed → 5; 12 installed → 3).
🟥 Dummy fuse: red plastic with a red streamer, fitted where a circuit is unused or isolated for work. A CB-protected service gets a warning flag on the tripped breaker instead.

Circuit breakers

A CB is fuse + switch: an automatic thermal trip (some add electromagnetic and reverse current trips) and a manual or electric switch. Small push-pull types run 5–45 A and show a white band when tripped; large reverse current CBs go up to about 600 A.

Non-trip-free

Can be held in against a fault and the circuit is made — dangerous.

Trip-free

Trips internally even if held in — the circuit cannot be made while the fault persists.

Reverse current CB

Protects supply systems against large reverse fault currents (bigger than a cut-out handles) and stays locked out until the fault is cleared.

Capacitors

Two plates and an insulating dielectric. A capacitor stores charge in an electric field, blocks DC once charged, and appears to pass AC by charging and discharging each way — though no current ever crosses the dielectric. A polarised capacitor must have its positive lead to the positive supply.

Unit: farad (F)

1 µF = 10⁻⁶ F · 1 nF = 10⁻⁹ F · 1 pF = 10⁻¹² F.

C ↑ with area, ↓ with gap

And ∝ dielectric constant k: air 1, waxed paper ≈ 3 (three times the capacitance).

Working voltage

Largest DC or peak AC allowed. Exceed it and the dielectric breaks down for good.

Parallel — add

C_T = C1 + C2 (bigger plate area). 4 µF ∥ 6 µF = 10 µF.

Series — reciprocals

1/C_T = 1/C1 + 1/C2 (wider gap). 4 µF and 6 µF in series = 2.4 µF.

🧠 Memory hook Capacitors combine the opposite way to resistors.

4 Batteries Chapter 4

The battery starts the engine and keeps the essentials alive when the generators stop. Know its chemistry, its numbers and its one dangerous habit.

🎯 Primary vs secondary🎯 Capacity (Ah)🎯 Cells in series/parallel🎯 Lead acid & SG🎯 NiCad🎯 Thermal runaway🎯 Checks & charging

💡 Key idea

Lead acid tells you its charge through its acid (SG); NiCad tells you nothing — its SG and voltage barely change — and it can run away thermally on charge.

Cells and capacity

A cell turns chemical energy into electrical. A primary cell (≈ 1.5 V) eats its negative electrode and is thrown away; a secondary cell is recharged by driving current backwards through it, turning electrical energy back into chemical.

Capacity = A × h

Set by plate area. Quoted at the 1-hour rate: 40 Ah = 40 A for 1 h (the rated load), or 4 A for 10 h. Faster discharge gives less.

Series: V add, Ah same

Six 2 V 20 Ah cells → 12 V 20 Ah.

Parallel: V same, Ah add

Three 2 V 15 Ah cells → 2 V 45 Ah.

Lead acid and NiCad side by side

	Lead acid	Nickel cadmium (alkaline)
Positive plate	Lead peroxide	Nickel oxide
Negative plate	Spongy lead	Cadmium
Discharged plates	Lead sulphate on both	Nickel and cadmium hydroxide
Electrolyte	Sulphuric acid + water	Potassium hydroxide
SG	1.27 charged → 1.17 flat (falls as it discharges)	1.24–1.30, does not change
Cell voltage	2 V on load · 2.2 V off load · recharge at 1.8 V	≈ 1.2 V, nearly constant until near the end
Cells for 24 V	12	20
State of charge	Hydrometer (SG)	Only a measured discharge (capacity) test
Spill neutraliser	Sodium bicarbonate solution	Boric acid
Storage	Charged (prevents sulphation)	Can be stored discharged — longer shelf life

The NiCad's flat discharge is why large aircraft prefer it: a steady voltage after total generator failure.

💨 Lead acid cells vent hydrogen; charging too fast causes gassing that can boil the cell dry. Keep electrolyte just above the plates with distilled water.
🥶 Cold raises internal resistance and cuts performance; a flat lead acid battery can freeze — keep it charged in winter. Absorbed-liquid types are less prone to spill.

⚠️ Thermal runaway (NiCad) Low thermal capacity → heat builds faster than it escapes → internal resistance falls → charging current rises → more heat. Unchecked, the battery is destroyed and may explode. A built-in thermal switch disconnects it from the charger (often with a flight deck warning) until it cools.

Checks and charging

Worked example — capacity test

A 50 Ah battery discharged at its 1-hour rate lasts 45 minutes.

45 min = 0.75 h, so it gave 0.75 of its rating.
Actual capacity = 75 %.
The minimum to stay in service is 80 % → it is removed.

The 80 % rule exists so the battery can carry essential loads (attitude, essential radio, lighting, pitot heat) for 30 minutes after a generator failure.

🔎 On-load check: apply the rated load briefly (as little as 10–20 s on modern aircraft); voltage must stay steady above a stated value. A preflight drop below the value = low state of charge.
🔌 Constant voltage charging (most lead acid): the generator sits 2 V above every 12 V of battery — 14 V for 12 V, 28 V for 24 V.
⚡ NiCads may use a dedicated constant current charger that watches temperature and voltage; pulse charging starts at about 85 % capacity.
📈 After a battery start the charge reading is high and should fall quickly. Staying high, or rising, suggests a faulty battery.

5 Magnetism Chapter 5

Magnetism is the link between electricity and movement: every generator, motor, relay and transformer depends on it.

🎯 Lines of flux🎯 Like poles repel🎯 Soft iron vs hard steel🎯 Permeability & residual🎯 Corkscrew & grasp rules🎯 Relays🎯 Motor force

💡 Key idea

A current always makes a magnetic field, and a field acting on a current makes a force. Those two facts give you the relay, the motor and — in the next chapter — the generator.

Fields and poles

🧲 A magnet attracts iron and steel and, freely suspended, points north–south. The end that points north is its north-seeking (N) pole.
➰ Lines of flux run N → S outside the magnet, are always continuous and never cross. Same-direction lines reinforce (higher flux density); opposing lines cancel.
↔️ Like poles repel, unlike poles attract. Effects are strongest at the poles.

Soft iron — temporary magnet

Easy to magnetise, loses it readily: high permeability, low retentivity. Ideal for electromagnet cores. The little magnetism left is residual magnetism.

Hard alloy steel — permanent magnet

Hard to magnetise, keeps it well.

Permeability is a material's ability to concentrate flux and add its own. In unmagnetised iron the molecular magnets form closed chains; magnetising lines them up, and when all are aligned the iron is saturated. Magnetism is destroyed by heating, hammering or an AC solenoid (a DC solenoid magnetises).

The field of a current

Left: the corkscrew rule. Right: a solenoid behaves like a bar magnet.

Corkscrew rule

Drive a right-hand corkscrew along the conventional current; its rotation is the flux direction.

Right hand grasp rule

Fingers curled with the conventional current round the coil; the thumb points to N.

Stronger solenoid

More turns · more current · a soft iron core. Switch off and the field collapses, leaving a little residual magnetism.

Solenoids and relays

Both are remote-controlled switches: a small flight deck current energises a coil that closes contacts carrying a large current — a starter circuit, for example. A solenoid has a moving core; a relay has a fixed core and an attracted armature. The coil has no electrical contact with the circuit it switches.

Force on a conductor

Put a current-carrying wire between two poles: its field adds to the main field on one side and cancels on the other, and the wire is pushed towards the weak side. That push is the Lorentz force — the motor principle — and its direction comes from Fleming's left hand rule.

6 Generators & Alternators Chapter 6

Move a conductor through a field and you get a voltage. The rest of the chapter is about making that voltage useful, steady and shared.

🎯 Induction🎯 Right hand rule🎯 Faraday & Lenz🎯 Commutator🎯 Series, shunt, compound🎯 Flashing the field🎯 Regulators🎯 Load sharing

💡 Key idea

The only practical way to control a generator's voltage is its field current. Speed is set by the engine and the windings by design — so the voltage regulator varies a resistance in the field circuit.

Electromagnetic induction

⚡ An EMF is induced only while there is relative movement cutting flux — conductor or field may move.
📈 Size depends on speed of cutting, flux density and number of turns.
📐 Faraday: induced voltage ∝ rate of change of flux. Lenz: the induced current opposes the change that made it (→ back EMF in motors).
✋ Fleming's right hand rule (generators): First finger = Field (N→S), thuMb = Motion, seCond finger = Current. Reverse motion or field and the current reverses; reverse both and it does not.

From AC in the loop to DC at the terminals

Maximum voltage when the loop cuts the flux at right angles; zero in the neutral plane. The commutator swaps connections every 180°.

The rising and falling DC output is commutator ripple, worst at low RPM; more armature coils (and field coils) smooth it.

Three ways to wind the field

Series wound

Field in series with armature and load: few turns of heavy wire. Voltage rises with load until the field saturates — useless where constant voltage is needed, so not used on aircraft.

Shunt wound

Field across the armature: many turns of fine wire. Builds up quickly (field current flows even with no load); voltage drops only slightly with load. Controlled by a resistance in series with the field.

Compound wound

Series + shunt fields on common poles. The series field strengthens with load, cancelling the shunt machine's voltage drop: almost constant output.

✈️ Flashing the field A self-excited generator relies on residual magnetism to start building voltage. Heat, shock, an AC field or reversed polarity can destroy it; a brief current through the field in the normal direction restores it. An externally excited machine needs the battery for its field.

Alternator or generator?

DC generator	Alternator
Rotating armature, stationary field	Stationary armature, rotating field
Commutator gives DC	Rectifier gives DC
Load current through brushes → arcing	Only small field current through slip rings
Full output at about half engine speed	Full output even at low RPM; better power-to-weight

Voltage regulation

Light aircraft run at 14 V. Output depends on speed, field strength and the armature's turns — only field strength can be controlled. A regulator has a variable resistance in series with the field and a control coil across the output sensing voltage.

Carbon pile

Stack of carbon discs: more compression, less resistance. Voltage high → control coil pulls harder → pile loosens → resistance up → field current down → voltage back down.

Vibrating contact

Contacts switch a fixed resistor in and out 50–200 times a second. A current regulator in the same box limits maximum output current.

Paralleled generators and load sharing

Generator, battery and loads are all in parallel on the bus, with the generator slightly above battery voltage to keep it charged. Two generators on one bus share the load only if their voltages are exactly equal; otherwise a recirculating current flows from the higher to the lower, which carries nothing and tries to motor. Equalising coils in the regulators trim each field until the voltages match; reverse current relays protect if the sharing circuit fails. Each generator reaches the bus through a line contactor.

7 DC Motors & Actuators Chapter 7

A motor is a generator run backwards — and the voltage it generates while turning is what controls it.

🎯 Left hand rule🎯 Back EMF🎯 Slow start resistor🎯 Series vs shunt motor🎯 Starter-generator🎯 Actuators🎯 Brakes & clutches

💡 Key idea

Back EMF is proportional to speed and always less than the supply. At standstill there is none, so starting current is large; as the motor speeds up the back EMF limits the current.

Direction and back EMF

✋ Fleming's left hand rule (motors): First = Field, seCond = Current, thuMb = Motion. Reverse armature or field to reverse; both together changes nothing.
🔁 The turning armature generates a back EMF opposing the supply (Lenz). Its gap below the supply is what drives the armature current.
🐢 A slow start resistor in series with the armature limits starting current; a centrifugal or time switch bypasses it once running.

Series vs shunt motors

Series wound

Few turns of heavy wire, whole armature current through the field. High starting torque, speed varies widely with load. Off load it overspeeds and can disintegrate — never start it unloaded. Uses: starters, actuators, landing gear.

Shunt wound

Many turns of fine wire across the supply; constant field. Torque ∝ armature current. Speed falls only about 10 % from no load to full load — a constant-speed motor. Uses: fans, centrifugal pumps, motor generators.

Starter-generators

Many turbine aircraft use one compound-wound machine, permanently geared to the engine: it starts the engine as a compound motor, then a changeover relay connects its shunt field to a voltage regulator and it becomes a DC generator (typically 300 A at 28 V). One unit saves weight; it cannot hold full output at low RPM, which is why it suits turbines.

Actuators

Solenoid actuators

Open hydraulic and pneumatic selector valves by magnetic attraction.

Rotary actuators

Reversible motor and gearbox turning a shaft: fuel and air/oil shut-off valves. OPEN/SHUT selector; one of the two limit switches is always closed.

Linear actuators

Motor and screw jack pushing a ram: flaps, gear, trim tabs, inching oil cooler shutters. Trim uses a sprung-centre switch; between limits both limit switches are closed.

↔️ Split field series actuator: two opposed series windings; whichever is energised sets the direction. Limit switches in series stop it at full travel — the open switch cuts the motor and lights "open", the close switch sets up the close circuit.
🛑 Brakes are spring-loaded ON; their solenoid, in series with the armature, releases them when power is applied, and they bite instantly when it is removed — no over-run or creep.
🧩 A slipping (friction) clutch protects against mechanical overload.
👁️ Indicators: press-to-test lights (pressing the lens tests the lamp), scale indicators where position matters, and electromagnetic doll's eye and prism indicators, linked by painted flow lines. Rotary valve indicators also show power loss and travel.

8 Aircraft DC Power Systems Chapter 8

Putting it together: how a light aircraft's alternator, regulator, protection, meters and bus bars work as one system — and what to do when the alternator quits.

🎯 Dipole vs earth return🎯 Overvoltage unit🎯 Cut-out🎯 Inverters🎯 Ammeters & voltmeters🎯 Vital/essential/non-essential🎯 Load shedding

💡 Key idea

Every protection device answers one question: overvoltage unit — regulator failed high? cut-out — generator below battery? differential cut-out — is the on-coming generator high enough? warning light — is the generator off line?

Wiring the return path

Two-wire (dipole)

Needed on a non-conductive (composite) airframe: a separate negative wire back to the source.

Single pole (earth return)

Most common on metal aircraft: the airframe is the negative conductor.

Protection and control

Device	What it does
Voltage regulator	Holds 14 V (12 V battery) or 28 V (24 V battery) by controlling field or exciter current
Overvoltage unit	Regulator failure protection: opens the field at about 16.5 V in a 14 V system; may open the cut-out
Cut-out (reverse current relay)	Closes when voltage builds above battery; opens on reverse current. Not needed with an alternator — its rectifiers block reverse current
Differential cut-out	Lets an on-coming generator on line only when about 2 % above the bus, preventing circulating current
Warning light	Generator voltage below battery voltage
Master switch	Breaks the field/exciter circuit — output falls to residual

How the cut-out works

At rest a spring holds the contacts open.
Rising voltage on the voltage coil (many turns, thin wire, across the output) closes them above battery voltage.
Load current through the current coil (few turns, thick wire) helps hold them closed.
If generator voltage falls below the battery, current reverses, the current coil's field reverses and opposes the voltage coil, and the spring opens the contacts.

Static inverter

Solid state: DC (typically 18–30 V) → 115 V 400 Hz AC, using oscillators, transistors, capacitors and transformers.

Rotary inverter

A constant-speed DC motor driving an alternator.

Reading the meters

Moving coil meters: a coil on a pivot in a permanent magnet's field, restrained by two hairsprings that also carry its current. Deflection ∝ current, so the scale is even; a soft iron case screens stray fields.

Ammeter

Low resistance, in series. Range extended by a low-resistance shunt in parallel.

Voltmeter

High resistance, in parallel. Range extended by a high-resistance multiplier in series.

Centre-zero (battery) ammeter

Right of zero: alternator supplying and battery charging. Left of zero: battery discharging.

Left-zero load meter

Shows current taken from the alternator. Zero in flight: the battery is the only source.

Bus bars and categories of service

Bus bars are low-impedance copper strips or rods, insulated and covered, in junction boxes and distribution panels. Services are ranked:

Vital

Needed after a wheels-up landing — emergency lighting, crash-switch fire extinguishers. Direct to the battery.

Essential

Needed for safe flight in an emergency; can always be fed by a generator or the batteries.

Non-essential

Shed in an emergency; generator-fed only.

Either generator can feed the essential bus; if both fail, the batteries carry the essential services for a set time — normally at least 30 minutes.

Alternator failure — what you see and do

Warning light on; load meter at zero (or centre-zero ammeter showing discharge).
Shed load: switch off everything not essential (the handling notes list what).
Switch the alternator/master off — the field opens and the failed system is made safe. The reverse current relay will usually already have isolated it.

9 Bonding & Screening Chapter 9

Static charge picked up in flight must have nowhere to spark and must leave the aircraft safely — and electrical noise must be kept out of the radios.

🎯 Bonding strips🎯 Static wicks🎯 Conductive tyres🎯 Screening🎯 200 RPM · 10 Hz

💡 Key idea

Bonding keeps every part at the same potential so nothing can spark; wicks bleed charge into the air; tyres dump what is left on touchdown; screening keeps interference out of radios.

Bonding

Flexible strips join every structural part and component. Contact faces must be clean of paint, anodising, grease and oxide — otherwise electrolytic corrosion adds resistance. Bonding also forms part of the earth return and reduces radio interference.

Static wicks

On the trailing edges of control surfaces and the tips of wings and stabilisers, discharging static into the air by brush discharge. Once cotton; now small barbed antennas, wire brushes or straight metal wicks.

Touchdown

Tyres with a high carbon content connect the main bond to the ground through the wheel bearing the instant the wheels touch.

Screening

Operating equipment produces static that interferes with radio. Suppressors in the cables and a continuous metal sheath round them absorb that energy. Screening is required for:

🔥 Ignition systems
⚙️ Commutator machines — DC generators and motors
🔄 Slip ring machines above 200 RPM
⏱️ Anything that makes and breaks a circuit faster than 10 Hz

〰️ Part two — Alternating current

10 Introduction to AC Chapter 10

Why big aircraft use AC, how to describe a sine wave, and what inductance and capacitance do to it.

🎯 Why AC🎯 Frequency from poles & RPM🎯 RMS🎯 L lags, C leads🎯 X_L, X_C, Z🎯 Resonance🎯 Power factor

💡 Key idea

CIVIL: in a Capacitor I leads V; V leads I in an inductor (L). Inductive reactance rises with frequency, capacitive reactance falls.

Why AC?

⚙️ Simpler, more robust machines with a better power-to-weight ratio, and no commutation problems at altitude.
🔁 Transformers change voltage at almost 100 % efficiency; TRUs give any DC needed.
🌀 Three-phase motors are simpler and more efficient than DC motors; high-voltage AC needs less cable weight.

Describing a sine wave

The RMS (effective) value is the DC that would heat a resistor equally. Supplies and meters are quoted in RMS.

f = (poles/2) × (RPM/60)

8 poles at 6000 RPM → 400 Hz. "Poles" = total N + S.

T = 1 / f

400 Hz → 2.5 ms.

RMS = peak / √2

115 V RMS → about 163 V peak. Instantaneous e = peak × sin θ.

Inductance and capacitance

A changing current in one coil induces a voltage in another — mutual induction, the transformer principle. A steady DC induces nothing once it is steady; switching it on and off gives a kick each way. A coil also induces a voltage in itself — self-induction, a back EMF opposing any change (Lenz). Inductance (henry, H: 1 A/s induces 1 V) rises with turns and a soft iron core.

X_L = 2πfL

10 mH at 400 Hz → 25 Ω. Frequency up → X_L up → current down. Frequency down → more current.

X_C = 1 / (2πfC)

10 µF at 400 Hz → 40 Ω. Frequency up → X_C down → current up.

Z = √(R² + (X_L − X_C)²)

R and X are 90° apart; X_L and X_C are 180° apart and subtract. R 6, X_L 10, X_C 2 → Z = 10 Ω.

Series resonance

X_L = X_C: reactances cancel, current is maximum.

Parallel resonance

X_L = X_C: supply current is minimum.

Power in AC circuits

Power	Meaning	Unit
True (real, wattful)	Power actually used: V × I × PF	W, kW
Reactive	Power swapped back and forth to build fields	VAR, kVAR
Apparent	RMS V × RMS I	VA, kVA

Pure resistance: power always positive, at twice the supply frequency, PF = 1. Pure L or C: true power zero, PF = 0. Equal R and X_L: phase angle 45°.

Worked example — the power triangle

A load draws 50 kVA and uses 40 kW.

PF = kW ÷ kVA = 40 ÷ 50 = 0.8 (= cos φ, so φ ≈ 37°).
kVAR = √(50² − 40²) = 30 kVAR — not 50 − 40.
At 115 V and 10 A with PF 0.8: true power = 115 × 10 × 0.8 = 920 W.

11 AC Alternators Chapter 11

The heart of a large aircraft's power: brushless three-phase alternators, held at 400 Hz, protected, paralleled and backed up.

🎯 115/200 V 400 Hz🎯 Star & delta🎯 Brushless + PMG🎯 Frequency wild🎯 CSD / IDG🎯 VSCF🎯 Real vs reactive load🎯 GCU, BTB, GCB🎯 RAT, APU, inverter

💡 Key idea

Real load is shared by matching drive torque (frequency); reactive load is shared by matching voltage (excitation). And nothing is paralleled until frequency, voltage and phase sequence all match.

Rotating field, three phases

Practical alternators rotate the field (DC-fed, to keep pole polarity) inside a stationary armature, so only small field current passes slip rings. They are rated in kVA, because armature heating limits output. Three windings 120° apart give three phases (sequence A-B-C) — better power-to-weight and easier paralleling.

Star is used for distribution: its neutral copes with unbalanced loads (an earth fault puts a heavy load on it). Delta has no neutral, so it is kept for things like tacho generators.

115 × 1.73 ≈ 200 V

The standard supply: 115 V phase, 200 V line, 400 Hz, three phase.

Delta: 10 A phase → 17.3 A line

In star the currents are equal.

Brushed and brushless

A brushed alternator feeds its rotor through brushes and slip rings (battery first, then rectified AC). A brushless one regulates by controlling the exciter field: very reliable, no brush wear, high power-to-weight. Self-excited types have some permanent magnetism in the exciter; externally excited types have none.

Getting a constant 400 Hz

Frequency wild

Driven straight from the engine: frequency follows engine speed. Cannot be paralleled. Fine for resistive de-icing heaters; constant frequency can be made by rectifying and inverting.

CSD / IDG

Engine-driven hydraulic pump → hydraulic motor → alternator. A swash plate set by a governor and load controller keeps 400 Hz ± 5 %. Own oil, separate from the engine. Quill drive = weak link. Overdrive (engine slower than generator), straight through, underdrive. IDG = CSD + generator in one.

VSCF

Frequency wild generator + electronic converter (rectify, filter, invert) → 115/200 V 400 Hz. No hydromechanical drive; the converter can sit away from the engine; built-in test.

⚠️ Drive disconnect Low oil pressure or high oil temperature warns of a CSD problem. The guarded, momentary disconnect switch separates the drive from the engine at any time the engine is running — but reconnection is only on the ground, engine stopped. Some GCUs can disconnect automatically.

Paralleling and load sharing

Real load (kW)

Related to drive torque. Current transformers in a loop sense imbalance (e.g. 7 A against an average of 5 A) and the load controllers trim each CSD's torque.

Reactive load (kVAR)

The "wattless" part that builds fields. Shared by matching voltage through excitation; a mutual reactor lets the detector see only the 90° current.

Each generator feeds its own bus through a GCB; each bus joins the load-free synchronising bus through a BTB (normally closed when paralleled, normally open in a split bus). A generator joins the sharing circuits only when paralleled. Ground power or the APU can feed the synchronising bus and reach the load buses through the BTBs.

🧮 Discriminatory circuits disconnect only the faulty part; a BPCU reconfigures the system.
🛡️ Protection: over/under voltage, frequency and excitation, and differential current (bus shorts, earth faults).
🔆 Synchronising: automatic, or the old dark lamp method — close the breaker when the lamps are dark.
🧠 The GCU regulates voltage and, on a dangerous fault, opens the exciter control relay (output to residual) and the GCB.
📟 Indications: generator fail light when the GCB trips; kW/kVAR meter (normally kW); one voltmeter and frequency meter with a selector.

Emergency and ground supplies

Source	Notes
RAT	Lowered into the airflow; 115/200 V 400 Hz for limited instruments and radio
APU	Constant-speed gas turbine in the tail — no CSD needed. Not paralleled with engine generators; feeds a bus only when nothing else does
Static inverter	From the batteries or essential DC bus, for limited instruments and radio
Ground power	115/200 V 400 Hz 3-phase; will not connect if aircraft generators are on line or phase sequence is wrong; tripped on overvoltage

12 AC Distribution Systems Chapter 12

Two ways to lay out a large aircraft's power: keep the generators apart (split bus) or tie them all together (parallel).

🎯 Split bus🎯 BTB closes on failure🎯 Essential via changeover🎯 DC isolation relay🎯 SSB & autoland🎯 Earth fault isolation

💡 Key idea

Split bus: generators never paralleled; the BTB closes only after a failure. Parallel: BTBs normally closed, and a fault is isolated by opening the right breakers in the right order.

Split bus — failures

Failure	AC	DC
None	Each generator feeds its own bus and TRU; essential AC from bus 1 via a changeover relay	TRU 1 → essential DC, TRU 2 → non-essential DC, joined by the isolation relay; batteries on the battery bus and, via a relay, essential DC
One generator	BTB closes, one generator feeds both buses; start the APU to restore full power	Both TRUs still fed through the BTB — all DC kept
Both generators	Non-essential AC lost; changeover puts essential AC on the static inverter (about 30 min with charged batteries)	Isolation relay opens; non-essential DC lost, essentials on the battery bus

On the ground external power or the APU can feed everything; in flight the APU generator may only manage one bus. Feeders from each generator run to a central compartment (under the flight deck or cabin floor) that holds the GCBs, BTBs, GCUs, current transformers, main buses, battery and charger.

Four-generator parallel system

⚡ Excite the field → output within limits → GCB closes onto the load bus and stays closed.
🔗 BTBs are normally closed, so each generator joins the synchronising bus once voltage, frequency and phase sequence match.
✂️ A split system breaker (SSB) joins or separates the two synchronising buses — separated, for example, during a dual-autopilot autoland so each autopilot has its own supply.
⚠️ One generator lost with the SSB open: its partner carries a double share. Closing the SSB lets the three remaining share equally.

⚠️ Earth fault on a load bus Opening only that generator's GCB lets the others feed the fault through the BTB. Open the faulty bus's BTB first, then its GCB.

13 Transformers & Rectifiers Chapter 13

Transformers change AC voltage; rectifiers turn AC into DC; TRUs do both; inverters go the other way.

🎯 Turns ratio🎯 Power in = power out🎯 Autotransformer🎯 Diodes & Zener🎯 Half, full, 3-phase🎯 TRU🎯 Inverter

💡 Key idea

Voltage goes with the turns ratio, current goes against it, and power stays the same.

Two separate coils on a laminated iron core: the primary on the supply, the secondary giving the output by mutual induction, 180° out of phase with the input. It needs a changing flux, so it does not work on steady DC.

r = N₂/N₁ = E₂/E₁

r > 1 step-up · r < 1 step-down. 115 V × 113/500 ≈ 26 V.

V₁ I₁ = V₂ I₂

2 A at 115 V = 230 W → 230 ÷ 26 ≈ 8.8 A on the secondary.

Three-phase

Three single-phase transformers, or one with each phase on its own iron limb.

Autotransformer

One winding with a common section carrying both currents. Cheaper (less wire) but no isolation. Typically gives the 26 V AC instruments need.

Rectification

➡️ A diode has low resistance forward, high in reverse. As a blocking diode it is an electrical non-return valve; a Zener diode conducts at a set voltage.
½ One diode = half wave, output frequency equal to the input. A bridge gives full wave; a capacitor across it smooths the output.
🔺 A six-diode three-phase bridge gives an essentially steady output.
🔌 TRU: transformer + rectifier, usually 115/200 V 400 Hz 3-phase → 28 V DC. Always multi-phase for smooth DC; air-cooled with an overheat warning; output amps shown on the panel.
🔄 Inverter: DC → AC. In a constant-frequency aircraft it is the battery-fed emergency AC source; in a frequency wild aircraft, rectified wild AC feeds static inverters for normal constant-frequency needs. Monitored for voltage and frequency.

14 AC Motors Chapter 14

A rotating field drags the rotor round: locked in step (synchronous) or always a little behind (induction).

🎯 Rotating field🎯 Synchronous speed🎯 Tacho & drag cup🎯 Squirrel cage🎯 Slip ≈ 5 %🎯 Split phase start🎯 Phase failure

💡 Key idea

Speed is set by supply frequency: RPM = f × 60 ÷ (poles/2). A synchronous motor runs at exactly that; an induction motor runs a few per cent slower.

AC motors avoid commutation and brush troubles — hot brushes sticking until the circuit opens and the motor stops. They suit constant-speed jobs because frequency sets their speed.

Synchronous

AC stator, DC rotor (built like a rotating field alternator). Locks onto the field: constant speed whatever the load. 4 poles at 400 Hz → 12 000 RPM. Not self-starting — extra induction windings bring it up to speed. Used in RPM indicators: an engine tacho-generator's frequency drives the motor, which turns a magnet and drag cup to move the needle.

Induction

AC stator, unpowered squirrel cage rotor: copper bars in a laminated core, joined by end rings, of very low resistance. The rotating field induces large rotor currents whose field produces torque. Simple, robust and cheap — the most common AC motor. Swap two phases to reverse.

Slip = (Nₛ − N) / Nₛ

Never zero — at synchronous speed nothing would cut the bars. Typically about 5 %: 12 000 RPM → about 11 400 RPM. Hence "asynchronous".

🔌 Single-phase induction motors are not self-starting: a split phase winding with current shifted about 90° creates a rotating field — by resistance, inductance, both, or capacitance (capacitor start, usually under 2 HP).
⚠️ Lose one phase of a running, lightly loaded three-phase motor and it may hum on unnoticed in its remote location — then fail to start next time.

15 Semiconductors Chapter 15

Doped silicon is behind every diode, transistor and chip on the aircraft.

🎯 Silicon & germanium🎯 Doping🎯 N-type & P-type🎯 Depletion layer🎯 Forward/reverse bias🎯 NPN & PNP🎯 Switch & amplifier

💡 Key idea

A P-N junction conducts one way only; put two junctions back to back and a small base current controls a large collector current.

N-type

Doped with arsenic or antimony (5 valence electrons): one spare free electron per impurity atom. Still electrically neutral.

P-type

Doped with aluminium or indium (3 valence electrons): a missing bond — a hole — that drifts like a positive carrier. Also neutral.

Pure silicon and germanium sit between conductors and insulators, nearer insulators. Doping at about 1 impurity atom in 10⁸ raises conductivity. In N-type, electrons drift to the positive terminal; in P-type, holes drift in the conventional direction, positive to negative.

Transistors

A thin (about 25 µm) layer of one type between two of the other: NPN or PNP, with regions collector, base, emitter. The symbol's arrow shows conventional current at the emitter — out (base → emitter) for NPN, in for PNP. With no base–emitter bias, nothing flows; forward-bias the base–emitter junction and electrons flood the thin base, few recombine, and most sweep on into the collector. A PNP works the same with the EMFs reversed.

As a switch

Turn the small base current on and off to switch the large collector current.

As an amplifier

Ride a small AC signal on the base bias; the collector current copies it, much larger.

Photo-etched doping lets transistors, diodes, resistors and capacitors be built together on one piece of silicon — the integrated circuit.

16 Logic Gates Chapter 16

Six simple building blocks, each with any number of inputs and exactly one output.

🎯 Binary 1 and 0🎯 Positive/negative logic🎯 AND · OR · NOT🎯 NAND · NOR🎯 Exclusive OR

💡 Key idea

AND needs all, OR needs any, NOT flips. Put an N in front and the output is inverted. Exclusive OR says 1 only when the two inputs differ.

1 = on, true, high, closed, engaged; 0 = off, false, low, open, disengaged. In positive logic (used by most systems) 1 is the more positive voltage; in negative logic it is the lower. Symbols run inputs on the left, output on the right; a truth table lists the output for every input.

Two-input truth table for every gate (NOT has one input: it outputs the opposite of A)
A	B	AND	OR	NAND	NOR	XOR
0	0	0	0	1	1	0
0	1	0	1	1	0	1
1	0	0	1	1	0	1
1	1	1	1	0	0	0

AND — "all or nothing"

Two switches in series with a lamp.

OR — "any or all"

Two switches in parallel with a lamp.

NOT (INVERT)

One input; a switch working a normally closed relay — switch on, lamp off.

✈️ Thinking in gates "Warn if the gear is up and the throttle is closed" is an AND. "Light up if either door is open" is an OR. An AND followed by a NOT is a NAND.