Part-66 Module 3 questions cover the electrical theory behind every aircraft system: how charge moves, how circuits behave, how magnetism turns into motion and how alternating current works. For B1, B2 and B2L candidates it is one of the longer foundation papers, and for many mechanically minded people it is the first real hurdle. This page sets out the paper size for every category under EASA and UK CAA rules, walks through all 18 sub-modules with the traps examiners use, and links free timed practice.
Part-66 Prep is an independent study aid. The syllabus is Appendix I and the exam standard Appendix II to Part-66, in Regulation (EU) No 1321/2014 (as amended by Regulation (EU) 2023/989 for EASA exams since 12 June 2024) and in the retained UK Part-66. The Part-66 modules guide shows where Module 3 fits in your licence.
Part-66 Module 3 paper size by category
| Category | EASA questions / time | UK CAA questions / time | Marks needed |
|---|---|---|---|
| A (A1 to A4) | 20 / 25 min | 20 / 25 min | 15 |
| B1.1, B1.2, B1.3 | 52 / 65 min | 52 / 65 min | 39 |
| B2 and B2L | 52 / 65 min | 52 / 65 min | 39 |
| B3 | 24 / 30 min | 24 / 30 min | 18 |
Module 3 is one of the few modules where both systems use the same number of questions for every category. The format is the usual one: three options, one correct, about 75 seconds a question, a 75% pass mark and no negative marking.
Key fact: Category A stops at AC theory. From 3.7 resistance onwards most of the syllabus is for B categories only, apart from 3.13 AC theory, which category A sits at level 1. That is why the A paper is only 20 questions.
What Module 3 covers, sub-module by sub-module
Appendix I uses knowledge levels: 1 is familiarisation, 2 is general working knowledge, 3 is detailed knowledge. In Module 3, B1, B2 and B2L work at level 2 on almost everything (level 1 on electron theory, generation of electricity, resistors and filters). B3 works mostly at level 1, and category A at level 1 on the sub-modules it sits. The figures in brackets are EASA's suggested question numbers for the 52-question paper, from AMC1 to Appendix II.
3.1 to 3.4: electron theory, static, terminology, generation (about 9 questions)
The structure of the atom, conductors, insulators and semiconductors; static charge and Coulomb's law; the units (coulomb, volt, ampere, ohm, conductance) and conventional versus electron flow; and the six ways of producing electricity (light, heat, friction, pressure, chemical action, magnetism and motion). Our free electron theory and static note covers this block.
Examiners like definition questions here: which unit is a coulomb per second, which direction conventional current flows, what a thermocouple produces. Easy marks if you learn the terms precisely.
3.5 Sources of DC electricity (about 3 questions)
Primary and secondary cells (lead-acid, nickel-cadmium, lithium and others), cells in series and parallel, internal resistance, thermocouples and photocells. Classic trap: cells in parallel add capacity, not voltage.
3.6 to 3.8: DC circuits, resistance and power (about 10 questions)
The core of the paper. Ohm's law, Kirchhoff's voltage and current laws, series and parallel resistance, the Wheatstone bridge, resistor colour codes, potentiometers and rheostats, and power, work and energy (P = VI, P = I²R, P = V²/R). Temperature coefficient of resistance appears here too.
Typical traps:
- adding parallel resistances directly instead of adding reciprocals
- using the full supply voltage across one resistor in a series chain
- mixing milli and kilo prefixes in a power calculation
The DC circuits, resistance and power note works through the methods, and the DC circuits drill (Pro) gives new numbers each time.
3.9 Capacitance (about 4 questions)
How a capacitor stores charge, the factors that set capacitance (plate area, spacing, dielectric), capacitors in series and parallel (the opposite rules to resistors), working voltage, colour codes, and charging and discharging through a resistor.
A short worked example, because the time constant comes up often. A 100 microfarad capacitor charges through a 10 kilohm resistor. The time constant is R times C: 10,000 ohms times 0.0001 farads is 1 second. After one time constant the capacitor reaches about 63% of the supply voltage; after five it is treated as fully charged, so about 5 seconds. The distractors usually come from a missed prefix (giving 1,000 seconds or 0.001 seconds) or from using 37% instead of 63% on a charging question.
3.10 and 3.11: magnetism and inductance (about 8 questions)
Magnetic theory, permeability, hysteresis, retentivity, coercive force, magnetomotive force and field strength, flux density, reluctance, and how to care for and store magnets. Then inductance: Faraday's and Lenz's laws, self and mutual inductance, the inductor time constant and back EMF. The rules for inductors in series and parallel match those for resistors. The magnetism and inductance note has diagrams of each effect.
Exam tip: Lenz's law questions are about direction: the induced current always opposes the change that caused it. If an option says it aids the change, it is wrong.
3.12 DC motor and generator theory (about 3 questions)
Construction, the generator equation in words (EMF depends on flux, speed and number of conductors), series, shunt and compound machines, armature reaction, commutation, and starter-generators. Know which winding arrangement gives high starting torque (series) and which gives near-constant speed (shunt).
3.13 and 3.14: AC theory and RLC circuits (about 6 questions)
Sine waves, frequency and period, peak, peak-to-peak, average and RMS values, phase, and three-phase basics. Then resistive, capacitive and inductive circuits: reactance, impedance, phase angle, power factor, true, apparent and reactive power, and series and parallel resonance. The AC theory, RLC and filters note covers 3.13, 3.14 and 3.16 together, and the AC theory drill (Pro) practises the calculations.
Things to have at your fingertips: RMS is about 0.707 of peak, average (over half a cycle) about 0.637 of peak; capacitive reactance falls as frequency rises while inductive reactance rises; at resonance the two reactances are equal. Remember "CIVIL": in a capacitor current leads voltage, in an inductor voltage leads current.
3.15 Transformers (about 3 questions)
Construction, turns ratio and its effect on voltage and current, losses (copper, iron, eddy current, hysteresis), efficiency, auto-transformers and current transformers. The trap is turning the current ratio the same way as the voltage ratio: a step-up transformer steps current down.
3.16 Filters (about 1 question)
Low-pass, high-pass, band-pass and band-stop filters, built from the reactance behaviour you learned in 3.14.
3.17 and 3.18: AC generators and AC motors (about 5 questions)
Rotating-field and rotating-armature machines, single, two and three-phase alternators, star and delta connections, permanent magnet generators, and then synchronous and induction motors, how a rotating field is produced, slip, and single-phase starting methods. Know the star relationships (line voltage is about 1.73 times phase voltage) and why an induction motor cannot run at synchronous speed.
Units and relationships to know cold
A surprising number of Module 3 marks depend on knowing a unit or a relationship without hesitation. If you can fill in this table from memory, you have the vocabulary for most of the paper.
| Quantity | Unit | Key relationship |
|---|---|---|
| Charge | coulomb (C) | one ampere is one coulomb per second |
| Potential difference | volt (V) | V = IR |
| Resistance | ohm | rises with length, falls with cross-sectional area |
| Conductance | siemens (S) | reciprocal of resistance |
| Power | watt (W) | P = VI = I²R = V²/R |
| Energy | joule (J) | power multiplied by time |
| Capacitance | farad (F) | charge stored per volt; time constant RC |
| Inductance | henry (H) | time constant L/R |
| Magnetic flux | weber (Wb) | flux density is flux per unit area, in tesla |
| Frequency | hertz (Hz) | reciprocal of the period |
| Reactance and impedance | ohm | capacitive reactance falls with frequency, inductive reactance rises |
| Apparent power | volt-ampere (VA) | true power is apparent power times power factor |
Exam tip: When two options differ only by a factor of 1,000, the question is testing a prefix. Convert everything to base units before you calculate.
Revising Module 3 in the right order
The sub-modules build on each other, so the order you study them in matters more than in most modules. A sequence that works for most people:
- Foundations (3.1 to 3.5). Atoms, charge, units and DC sources. Mostly definitions; aim to finish these quickly and bank the marks.
- DC circuits (3.6 to 3.8). Spend real time here. Work series, parallel and series-parallel networks by hand until you can do one in under a minute, then add Kirchhoff and power.
- Storage elements (3.9 to 3.11). Capacitance, magnetism and inductance. Learn the time constant curves as pictures: rising to 63% when charging, falling to 37% when discharging.
- Machines (3.12). DC motors and generators, which use the magnetism you just learned.
- AC (3.13 to 3.16). Waveforms first, then reactance, impedance, resonance, transformers and filters.
- AC machines (3.17 and 3.18). Last, because they need both magnetism and three-phase AC.
After each block, do a short set of questions on that block only, then a mixed set that includes earlier blocks so you do not forget them. Leave at least two weeks at the end for full timed papers.
Which categories sit Module 3, and how it links to other modules
All categories sit Module 3. It feeds directly into:
- Module 4 electronic fundamentals: semiconductors, PCBs and servomechanisms, a 40-question paper for B2.
- Module 5 digital techniques: the longest foundation paper for B2 at 72 questions.
- Module 11 and Module 13 electrical power (ATA 24), where generators, transformers and power distribution return in aircraft form.
For B2 and B2L candidates, Module 3 is the base of everything else in the licence. If you are heading that way, the Part-66 B2 licence guide sets out the full module list.
Module 3 also relies on Module 1 mathematics: powers of ten, rearranging formulas and reciprocals. If those feel shaky, fix them first.
Practising Module 3 with exam-style papers
Our Module 3 bank holds 620 original three-option questions, 101 of them free, spread across the 18 sub-modules in line with the AMC weighting. Calculation questions (Ohm's law, power, time constants, reactance, transformer ratios) come from templates, so the numbers change from paper to paper and you have to work each one.
Paper 1 is free for every category that sits the module:
- B1.1 Module 3 paper 1, 52 questions
- B2 Module 3 paper 1, 52 questions
- B3 Module 3 paper 1, 24 questions
You can also take a free 20-question quick practice with instant explanations, or print the Module 3 question sheet (50 questions with an answer key, save as PDF). EASA and the CAA do not publish past papers, so these are exam-style papers written for the app, not recalled exam questions. Papers 2 to 10, the sub-module drills and the other eight Module 3 notes come with Pro.
Common reasons people fail Module 3
- Treating it as a formula test. Many questions are about behaviour (what happens to current when frequency rises in a capacitive circuit), not numbers.
- Prefix errors. Micro, milli, kilo and mega slips produce exactly the distractors Appendix II requires.
- Leaving AC until last. AC theory, RLC, transformers and AC machines together are a big share of the B paper.
- Mixing up series and parallel rules for capacitors versus resistors and inductors.
- Weak magnetism vocabulary. Retentivity, coercivity, reluctance and permeability are easy to confuse under time pressure.
Your study plan
- Take the free B1.1 diagnostic or B2 diagnostic to see how Module 3 compares with your other modules.
- Read the free electron theory note, then work through DC circuits before you touch AC.
- Do the free quick practice by hand, without a calculator.
- Sit paper 1 under timed conditions and list every wrong answer by sub-module.
- Revise the weakest sub-modules, then book when you score 85% or more on three different papers. Move on to Module 4 while the theory is fresh.
20 free Part-66 Module 3 Electrical fundamentals practice questions with answers
Try each one, then open the answer. For the timed experience use the free exam-style paper or the printable version.
1. A single-phase supply is taken from
- A. all three phases together
- B. a DC generator with slip rings
- C. one phase winding and a neutral or return
Show answer
C. one phase winding and a neutral or return
Single-phase loads on aircraft are connected between one phase line and neutral (usually airframe earth), e.g. 115 V from a 115/200 V system.
2. The Earth's magnetic north pole behaves magnetically as
- A. a north pole, since it repels the north pole of a compass
- B. a south pole, since it attracts the north pole of a compass
- C. a neutral point with no polarity
Show answer
B. a south pole, since it attracts the north pole of a compass
Unlike poles attract. Because the north (seeking) end of a compass is attracted toward the Earth's magnetic north, that region must have south magnetic polarity.
3. In a purely resistive AC circuit, the voltage and current are
- A. in phase
- B. 90° out of phase with current leading
- C. 90° out of phase with current lagging
Show answer
A. in phase
A resistor has no energy storage, so current follows the voltage instantly and the phase angle is 0°. Power factor is 1.
4. A supply has significant internal resistance. When the load resistance is reduced, the terminal voltage
- A. falls, because more current causes a larger internal drop
- B. rises, because the load then takes less of the voltage
- C. stays equal to the EMF of the supply
Show answer
A. falls, because more current causes a larger internal drop
V_terminal = E − I·r. Reducing the load resistance increases I and hence the internal drop I·r, so less voltage reaches the load.
5. The cut-off frequency of a filter is usually defined as the frequency at which the output
- A. has fallen to zero volts
- B. exactly equals the input
- C. is 70.7 % of maximum (−3 dB)
Show answer
C. is 70.7 % of maximum (−3 dB)
At the −3 dB point the output voltage is 0.707 of the passband value and the output power is half. It defines the edge of the passband.
6. The electrons in the outermost shell of an atom are called:
- A. valence electrons
- B. free electrons
- C. bound inner-shell electrons
Show answer
A. valence electrons
Valence electrons determine how an element bonds and how easily it conducts; conductors have few, loosely held valence electrons.
7. In an ideal inductor on AC, the average power consumed is
- A. I² × X_L watts
- B. zero watts
- C. V × I watts
Show answer
B. zero watts
With current 90° out of phase, energy taken during one quarter cycle is returned in the next, so the true power is zero. Real inductors dissipate a little power in their resistance.
8. A current of 400 mA flows through a sensor circuit. Expressed in amperes, this current is
- A. 0.4 A
- B. 4 A
- C. 400000 A
Show answer
A. 0.4 A
milli = 10⁻³, so divide by 1 000: 400 mA = 400/1000 = 0.4 A.
9. A voltage-dependent resistor (VDR or varistor) is used to
- A. protect circuits against voltage spikes
- B. provide a fixed reference voltage
- C. measure temperature
Show answer
A. protect circuits against voltage spikes
A varistor has very high resistance at normal voltage but its resistance drops sharply above a threshold, clamping transient spikes and diverting surge current.
10. A conductor 0.5 m long moves at 15 m/s through a 0.7 T field, its motion making an angle of 60° with the lines of flux. What EMF is induced?
- A. 2.625 V
- B. 5.25 V
- C. 4.547 V
Show answer
C. 4.547 V
Only the flux-cutting component counts: e = B·L·v·sin θ = 0.7 × 0.5 × 15 × sin 60° = 4.547 V.
11. One watt is equal to
- A. one joule per second
- B. one coulomb per second
- C. one volt per ampere
Show answer
A. one joule per second
1 W = 1 J/s = 1 V × 1 A. A 100 W lamp converts 100 joules of electrical energy every second.
12. Capacitive reactance
- A. is independent of frequency
- B. increases as frequency increases
- C. decreases as frequency increases
Show answer
C. decreases as frequency increases
X_C = 1/(2πfC). At higher frequency the capacitor charges and discharges more often per second, so more current flows for the same voltage.
13. What two units are combined in an integrated drive generator (IDG)?
- A. a constant speed drive and a generator
- B. a starter motor and a DC generator
- C. a transformer and a bridge rectifier
Show answer
A. a constant speed drive and a generator
The CSD converts variable engine speed into constant generator speed so the output stays at 400 Hz.
14. Interpoles in a DC generator are used to
- A. increase the strength of the main field
- B. reduce brush sparking caused by armature reaction
- C. act as the main field poles of the machine
Show answer
B. reduce brush sparking caused by armature reaction
Interpoles, connected in series with the armature, produce a field that cancels armature reaction in the commutating zone, improving commutation and reducing arcing.
15. A transformer rating plate states the rating in kVA rather than kW because
- A. heating depends on current, whatever the pf
- B. kVA ratings are always greater than 100 kW
- C. transformers only ever supply reactive power
Show answer
A. heating depends on current, whatever the pf
Losses depend on current and voltage, not on how much of the power is true power, so the limit is set in volt-amperes.
16. Electrostatic discharge (ESD) is a particular hazard to aircraft electronic equipment because
- A. static voltages can destroy sensitive semiconductor devices
- B. the discharge current melts the wiring insulation
- C. the discharge permanently magnetises the equipment chassis
Show answer
A. static voltages can destroy sensitive semiconductor devices
A person can carry several thousand volts of static. Discharging through a sensitive device (MOS gates, thin junctions) can puncture it, so wrist straps and ESD packaging are used.
17. If one phase of a running three-phase motor is lost, the motor will:
- A. usually keep running but overheat with reduced torque
- B. immediately reverse its direction of rotation
- C. speed up to exactly synchronous speed
Show answer
A. usually keep running but overheat with reduced torque
Single-phasing leaves a pulsating field: the motor may continue to run but draws high current and overheats; it will not start from rest.
18. The nominal voltage of a single lead-acid cell is about:
- A. 1.2 V
- B. 2.0 V
- C. 3.6 V
Show answer
B. 2.0 V
A lead-acid cell gives about 2.0–2.1 V, so a 24 V battery has 12 cells; Ni-Cd cells are 1.2 V.
19. An oscilloscope shows a sine wave with a peak value of 160 V. What is its RMS value?
- A. 226.24 V
- B. 101.92 V
- C. 113.12 V
Show answer
C. 113.12 V
V_rms = 0.707 × V_peak = 0.707 × 160 = 113.12 V.
20. Reversing the current through an electromagnet coil will
- A. reverse the polarity of its poles
- B. increase its strength
- C. destroy the magnetic field
Show answer
A. reverse the polarity of its poles
Pole polarity depends on the direction of current around the core. Reversing the current reverses the flux direction and swaps N and S.
Frequently asked questions
+How many questions are in Part-66 Module 3?
B1, B2 and B2L sit 52 questions in 65 minutes. Category A sits 20 questions in 25 minutes and B3 sits 24 questions in 30 minutes. The sizes are the same under EASA and UK CAA rules.
+Is Module 3 hard?
It is one of the modules candidates without an electrical background find hardest, because it is long for B categories and mixes theory with calculations. With a solid grasp of Ohm's law, series and parallel circuits and basic AC terms, most of the paper becomes manageable.
+What is in Module 3 electrical fundamentals?
Eighteen sub-modules, from electron theory, static electricity and DC sources, through resistance, power, capacitance, magnetism and inductance, to DC machines, AC theory, RLC circuits, transformers, filters, AC generators and AC motors.
+Do B1 and B2 sit the same Module 3 paper?
Yes. B1, B2 and B2L sit the same 52-question paper at the same knowledge levels. B2 engineers then go much deeper into electronics in Modules 4 and 5, where their papers are longer than B1's.
+Can I use a calculator in Module 3?
Assume not. CAA venues ban electronic devices, and many Part-147 organisations run Module 3 without calculators, so questions use numbers you can work by hand. Check with your exam centre before you book.
+Which Module 3 topics are most examined?
In EASA's suggested spread for the 52-question paper, no single sub-module dominates; capacitance and inductance get about four questions each and most others two or three. Because the questions are spread so evenly, you cannot skip any topic.
+How should I revise AC theory for Module 3?
Learn the relationships between peak, peak-to-peak, average and RMS values first, then phase angle and power factor, then reactance and impedance. Draw the phasor diagram for every RLC question; it shows whether current leads or lags.
+How long to study for Module 3?
There is no official figure. People with an electrical trade background often need a few weeks; those starting from scratch commonly spend two to three months alongside work. Book when you pass several full timed papers comfortably.
Study offline on your phone too
The Part-66 Prep: EASA & UK CAA app has the same questions plus a module board, essay coach, units drill and an attempt tracker for resit dates.
Keep reading
- Part-66 Modules 1 to 17: The Complete GuideAll 17 Part-66 modules explained: what each covers, which categories sit it, EASA and UK CAA question counts and times, and free practice papers.
- Part-66 Module 4 Electronic Fundamentals QuestionsPart-66 Module 4 electronic fundamentals: paper sizes per category under EASA and UK CAA rules, every sub-module explained, and free timed practice.
- Part-66 Module 5 Digital Techniques QuestionsPart-66 Module 5 questions with a free timed paper: EASA and UK paper sizes for every category, all 15 sub-modules, data buses and number systems.
- Part-66 Module 1 Mathematics Questions and Practice PaperPart-66 Module 1 questions: paper sizes for every category under EASA and UK rules, what each maths sub-module tests, and a free timed paper.
- The Part-66 B2 Avionics Licence ExplainedThe Part-66 B2 avionics licence explained: what a B2 engineer certifies, the 12 modules and paper sizes under EASA and UK rules, experience and routes in.