Part-66 Module 4 Electronic Fundamentals Questions

Part-66 Module 4 is the electronics module: how diodes, transistors and integrated circuits behave, how circuit boards are built and handled, and how servomechanisms use feedback to move things accurately. If you are searching for Part-66 Module 4 questions, the first thing to know is that the paper changes a lot with your category. A B2 candidate sits twice as many questions as a B1 candidate, at a deeper knowledge level, and a UK B3 candidate sits only eight.

This page sets out the paper size for every category under both systems, walks through each sub-module with the topics examiners test and the traps that catch people, and ends with a study plan. Part-66 Prep is an independent study aid, not EASA, the UK CAA or a Part-147 organisation. For where Module 4 sits in the full licence, see the Part-66 modules guide.

Module 4 paper size by category: EASA and UK CAA

Every Module 4 paper is three-option multiple choice, 75 seconds per question, 75% to pass and no negative marking. There are no essays in Module 4 under either system.

Category	EASA questions / time	UK CAA questions / time
A	not required	not required
B1.1, B1.2, B1.3	20 / 25 min	20 / 25 min
B2	40 / 50 min	40 / 50 min
B2L	40 / 50 min	40 / 50 min
B3	20 / 25 min	8 / 10 min

The figures come from Appendix II to Part-66: the EU version as replaced by Regulation (EU) 2023/989, applicable from 12 June 2024, and the retained UK text on legislation.gov.uk. The B3 gap is the largest relative difference in any module: the UK paper is less than half the EASA length.

Key fact: On a 20-question B1 paper you can drop five questions and still pass; on the 8-question UK B3 paper you can drop two. Short papers leave little room for a bad topic.

Who sits Module 4 and at what depth

Module 4 is sat by every B1 subcategory, B2, B2L and B3. Category A does not take it, which is one of the few exceptions in Modules 1 to 10. The syllabus in Appendix I gives each topic a knowledge level from 1 (familiar with the basics) to 3 (detailed, able to apply it). In Module 4 the split is clear:

Topic	B1 and B3 (EASA)	B2 and B2L
4.1.1 Diodes, description and characteristics	2	2
4.1.1 Diode circuits and special diodes in detail	not required	2
4.1.2 Transistors, description and characteristics	1	2
4.1.2 Transistor circuits, amplifiers, oscillators	not required	2
4.1.3 Integrated circuits, basic description	1	2
4.1.3 Op-amp applications and feedback	not required	2
4.2 Printed circuit boards	1	2
4.3 Servomechanism principles	1	2
4.3 Synchro and servo construction, fault-finding	not required	2

Under UK rules the B3 column is narrower still: basic diode, transistor and integrated circuit knowledge only, with no printed circuit board or servomechanism content. That is why the UK B3 paper has just 8 questions. If you are a B1 candidate, the left column is your whole syllabus: you need to recognise a component, know what it does and read its characteristics, not design or troubleshoot a circuit.

The free Module 4 semiconductors note shows the levels for every sub-topic, and the B1 vs B2 licence guide explains why avionics engineers carry the heavier electronic load.

Sub-module 4.1: semiconductors

Semiconductors take most of the questions in every category. The EASA acceptable means of compliance (AMC1 to Appendix II) suggest about 15 of the 20 B1 and B3 questions and 32 of the 40 B2 questions come from 4.1, with the rest split between printed circuit boards and servomechanisms. Our papers follow the same spread.

Semiconductor materials and the P-N junction

Start with the material. Silicon and germanium have four valence electrons. Doping with a five-electron (pentavalent) impurity gives N-type material, where electrons are the majority carriers; a three-electron (trivalent) impurity gives P-type, where holes are the majority carriers. Minority carriers are created by heat and are the source of leakage current, which is why leakage rises with temperature.

At the junction a depletion layer forms with a barrier potential of roughly 0.6 to 0.7 V for silicon and 0.2 to 0.3 V for germanium. Forward bias narrows the depletion layer and current rises steeply once the barrier is overcome; reverse bias widens it until breakdown. Examiners like questions that swap "forward" and "reverse" or give the germanium figure as a distractor for silicon.

Diodes and rectifiers

You need diode parameters (peak inverse voltage, maximum forward current, power dissipation, leakage, switching speed) and the reason diodes are placed in series (to share reverse voltage) or parallel (to share current), each with balancing resistors. Rectifier questions are common: half-wave, centre-tap full-wave, bridge and three-phase bridge, with the number of diodes and the ripple frequency.

A worked example of the kind of method you should be comfortable with: a single-phase bridge fed from a 400 Hz supply gives an 800 Hz ripple, because both half-cycles are used. A half-wave rectifier on the same supply gives 400 Hz. A common trap is to double the frequency for a half-wave circuit.

Special diodes appear in every category: the Zener in reverse breakdown as a voltage reference, the LED, the photodiode, the varactor whose capacitance changes with reverse voltage, the varistor for surge suppression and the silicon controlled rectifier (thyristor). The classic SCR question is about turn-off: once triggered it stays on until the anode current falls below the holding current, regardless of the gate. B2 candidates add clippers, clampers, voltage doublers and triplers, and Schottky diodes.

Transistors

Every category needs symbols, the three terminals and basic behaviour: base-emitter forward biased, collector-base reverse biased, and a small base current controlling a larger collector current. The emitter arrow shows conventional current, so an arrow pointing out is NPN.

B2 and B2L go much further: common emitter, common base and common collector configurations, which one inverts the signal (common emitter), amplifier classes A, B, AB and C with their conduction angles, biasing and decoupling, multistage coupling, oscillators and the three multivibrator types. Field-effect transistors are in the B2 detail too, along with the warning that MOSFETs are easily damaged by static, which links to the ESD sub-module in Module 5.

Integrated circuits and operational amplifiers

B1 and B3 need a basic description of logic and linear integrated circuits. B2 and B2L need op-amp applications: the inverting amplifier (gain equals minus the feedback resistor divided by the input resistor), the non-inverting amplifier, the integrator, differentiator, voltage follower and comparator, and the effect of positive and negative feedback. Negative feedback reduces and stabilises gain and widens bandwidth; positive feedback is used in oscillators. Questions often describe a circuit in words and ask which application it is.

Exam tip: Most transistor and op-amp questions turn on one fact: which configuration inverts, which feedback stabilises, which class conducts for 180°. Make a one-page table and test yourself on it until it is automatic.

Sub-module 4.2: printed circuit boards

This is a short sub-module with a handful of questions. Know single-sided, double-sided and multilayer boards, plated-through holes and vias, through-hole versus surface-mount components, solder mask and conformal coating. Questions also touch on handling: boards often carry static-sensitive devices, so they are handled by the edges with wrist straps and conductive packaging, and repairs beyond replacing the unit are workshop tasks done to the component maintenance manual.

For B1 this is level 1, so a description is enough. B2 and B2L sit it at level 2 and should be ready for questions on typical defects such as cracked tracks, lifted pads, dry joints and delamination.

Sub-module 4.3: servomechanisms

Servomechanisms link Module 4 to autopilots, flight control actuators and engine controls, and they are where many B2 candidates lose marks.

Control principles and terms

An open-loop system has no feedback, so errors are not corrected. A closed-loop system measures the output, compares it with the command and drives the error to zero. The vocabulary is examined directly:

Term	What it means
Feedback	signal representing the actual output
Null	zero error, the system stops
Overshoot	output passes the commanded position
Damping	reduces overshoot and oscillation
Deadband	small error band the system ignores
Hunting	continuous oscillation around null

Know the damping cases: an under-damped system overshoots and may hunt, an over-damped system is sluggish, and a critically damped one reaches the command fastest without overshoot. Questions often give a symptom, such as continuous oscillation, and ask for the cause (too little damping or too small a deadband).

Synchros, transducers and controllers

Synchros transmit an angle electrically. Know the torque transmitter and receiver pair for light indicator loads, the control transmitter and control transformer pair that produces an error voltage for an amplifier and servo motor, the differential that adds or subtracts a second angle, and the resolver that splits an angle into sine and cosine components. Two fault questions come up repeatedly: swapping two stator leads reverses the receiver's direction, and swapping the rotor leads puts it 180° out.

Transducers include potentiometers, LVDTs and RVDTs, E and I transformers, inductance and capacitance transmitters, proximity switches, and digital tachometers and encoders. B2 candidates also meet PID controllers: proportional action for speed, integral to remove steady-state error and derivative to damp. The printed circuit boards and servomechanisms note covers all of this with worked examples.

How Module 4 connects to Modules 3 and 5

Module 4 sits between two bigger papers. It assumes you are comfortable with Module 3 electrical fundamentals: Ohm's law, AC theory, capacitance and inductance, transformers and RMS values. Rectifier and op-amp gain questions are Module 3 arithmetic in a new setting. In turn, Module 5 digital techniques builds on it with logic circuits, data conversion, microprocessors and ESD. Most candidates sit Module 3 first, then 4 and 5 close together, because the topics overlap and the revision carries over.

There are calculation questions in Module 4, mostly short ones: peak and average voltage from RMS, diode drops, transistor current gain and op-amp gain. On Part-66 Prep the calculation templates get new numbers on each paper, so you practise the method rather than memorising an answer.

Module 4 for B2 and B2L candidates

For the B2 licence, Module 4 is a 40-question paper that sits alongside the 72-question Module 5 and the long Module 13. It is worth taking seriously: the depth in transistor circuits, op-amps and synchros is the foundation for troubleshooting avionic systems later. B2L candidates sit the same 40-question Module 4 regardless of which system ratings they choose.

Practise on the paper sized for your category:

• B2: Module 4 paper 1, free quick practice, and the semiconductors drill (Pro)
• B2L: Module 4 hub for B2L
• B1.1: Module 4 paper 1 for B1.1
• B1.2 and B1.3: B1.2 Module 4 and B1.3 Module 4
• B3: B3 Module 4; choose UK CAA rules on the start screen for the 8-question paper

Common mistakes in Module 4

1. Learning at the wrong depth. B1 candidates sometimes spend weeks on op-amp circuits that are not in their syllabus. B2 candidates sometimes stop at symbols and fail on circuit behaviour. Check your column in the level table above.
2. Mixing up silicon and germanium figures. Barrier potential and leakage questions use one as the distractor for the other.
3. Losing the SCR and Zener details. Holding current, gate behaviour and reverse breakdown are reliable questions.
4. Guessing synchro faults. Learn the two lead-swap results by heart.
5. Ignoring the short sub-modules. Printed circuit boards carry only a few questions, but on a 20-question paper two lost marks is 10%.

Module 4 has a bank of 437 original questions on Part-66 Prep, spread over the three sub-modules. Neither EASA nor the CAA publishes past papers, and we do not use recalled exam questions; our papers are exam-style papers built to the official sizes. If you want a paper copy, the printable Module 4 sheet has 50 questions with an answer key.

For background reading, the FAA's Aviation Maintenance Technician Handbook – General has clear chapters on semiconductors and transistors. It uses US regulatory framing, so treat it as a textbook, not a guide to Part-66 rules. The syllabus wording itself is in the EASA Easy Access Rules for Continuing Airworthiness.

Your study plan

1. Check your column in the knowledge-level table and decide how deep you need to go.
2. Read the free semiconductors note, then the servomechanisms note.
3. Take the free quick practice for your category, for example B2 quick practice, and note which sub-module costs you marks.
4. Sit the full timed Module 4 paper 1 (or the B1.1 version) under the rules you will be examined under.
5. Drill your weakest sub-module, then sit further papers. Papers 2 to 10 and the topic drills are part of Pro.
6. Book the exam when you score 85% or more on three different papers. If you have not chosen a category yet, start with the free B2 diagnostic.