Part-66 Module 5 questions are where many mechanical candidates first meet binary, logic gates and data buses, and where avionics candidates face one of their longest basic papers. This page sets out the Module 5 paper for every category under EASA and UK CAA rules, goes through all 15 sub-modules with the points examiners test, shows which ones category A and B3 skip, and finishes with a practical study plan.
Part-66 Prep is an independent study aid, not an authority or a Part-147 organisation. The syllabus is Appendix I and the examination standard is Appendix II to Part-66: see the EASA Easy Access Rules for Continuing Airworthiness for the EU text and UK Part-66 on legislation.gov.uk for the UK. For the full list of modules, see the Part-66 modules guide.
Part-66 Module 5 exam size by category
Module 5 has more variation between categories than most papers. The time follows from 75 seconds per question.
| Category | EASA questions / minutes | UK CAA questions / minutes | Correct answers needed (75%) |
|---|---|---|---|
| A (A1 to A4) | 20 / 25 | 16 / 20 | 15 (EASA) / 12 (UK) |
| B1.1 and B1.3 | 40 / 50 | 40 / 50 | 30 |
| B1.2 | 40 / 50 | 20 / 25 | 30 (EASA) / 15 (UK) |
| B2 and B2L | 72 / 90 | 72 / 90 | 54 |
| B3 | 20 / 25 | 16 / 20 | 15 (EASA) / 12 (UK) |
There are no essays in Module 5 under either system. Under EASA rules, a paper longer than 90 minutes may be split into partial exams; the B2 Module 5 paper is exactly 90 minutes, so it is normally sat in one go.
Key fact: The UK B1.2 paper is half the EASA size. If you are a piston-aeroplane candidate under UK rules, each question is worth 5% of the paper, so one bad topic hurts more.
Why B2 sits a much bigger paper
Module 5 is avionics foundation material. B2 engineers troubleshoot digital systems, so Appendix I examines them at level 2 on every sub-module, including the inside of a computer: microprocessor operation, integrated circuits, multiplexing and logic diagram interpretation. B1 engineers need to understand the systems they work around, so they sit most sub-modules at level 1 or 2 and skip the deepest four. Category A and B3 sit only the parts about displays, terminology, ESD and the systems themselves. If you are choosing between categories, the B2 licence guide explains what this depth is for.
Who sits which sub-module
Levels: 1 = familiarity, 2 = understanding and application, 3 = detailed knowledge. A dash means not examined.
| Sub-module | A | B3 | B1 | B2 / B2L |
|---|---|---|---|---|
| 5.1 Electronic instrument systems | 1 | 1 | 1 | 1 |
| 5.2 Numbering systems | — | — | 1 | 2 |
| 5.3 Data conversion | — | — | 1 | 2 |
| 5.4 Data buses | — | — | 2 | 2 |
| 5.5(a) Logic circuits: identification and applications | — | — | 2 | 2 |
| 5.5(b) Interpretation of logic diagrams | — | — | — | 2 |
| 5.6(a) Computer terminology and technology | 1 | 1 | 2 | 2 |
| 5.6(b) Computer operation | — | — | — | 2 |
| 5.7 Microprocessors | — | — | — | 2 |
| 5.8 Integrated circuits | — | — | — | 2 |
| 5.9 Multiplexing | — | — | — | 2 |
| 5.10 Fibre optics | — | — | 1 | 2 |
| 5.11 Electronic displays | 1 | 1 | 2 | 2 |
| 5.12 Electrostatic-sensitive devices | 1 | 1 | 2 | 2 |
| 5.13 Software management control | — | 1 | 2 | 2 |
| 5.14 Electromagnetic environment | — | 1 | 2 | 2 |
| 5.15 Typical electronic/digital aircraft systems | 1 | 1 | 1 | 1 |
Practise at your size: B2 Module 5 paper 1, B1.1 Module 5 paper 1, or the hubs for category A, B1.2 and B3.
Sub-module walkthrough
5.1 Electronic instrument systems
The layout of cockpit electronic instrument systems: primary flight and navigation displays, engine and system displays, and how they replace electromechanical instruments. Level 1 for everyone, so expect "what does this display show" questions rather than detail. Our free electronic instruments and displays note covers 5.1 and 5.11 together.
5.2 Numbering systems and 5.3 Data conversion
Binary, octal, hexadecimal and binary-coded decimal, and converting between them and decimal; then analogue and digital data, and how analogue-to-digital and digital-to-analogue converters work, with their limitations.
What examiners like: converting a decimal number to binary and hex, recognising a valid BCD group, and the effect of resolution (number of bits) on a converter. Traps: BCD codes each decimal digit separately in four bits, so 1010 is not a valid BCD digit; and octal groups bits in threes while hex groups them in fours. Drill this with the numbering systems topic and read the numbering and data conversion note.
Exam tip: Do conversions by grouping bits, not by converting through decimal. Binary 1101 0110 is D6 in hex at a glance; going through decimal (214) doubles the chances of a slip.
5.4 Data buses
How aircraft systems share data over serial buses, the ARINC specifications and aircraft networks including Ethernet-based ones. This is the sub-module people search for most, and the one with the most detail per mark.
What examiners like about ARINC 429: one transmitter and up to 20 receivers, a twisted shielded pair, 32-bit words with the label in bits 1 to 8, the sign/status matrix, odd parity in bit 32, and the two speeds (high speed 100 kbit/s, low speed around 12 to 14.5 kbit/s). Then the contrasts: ARINC 629 allows several transmitters on one bus, MIL-STD-1553B uses a bus controller and remote terminals, and AFDX (ARINC 664) is switched full-duplex Ethernet with virtual links and redundant networks.
Trap: questions that describe a bidirectional or multi-transmitter bus and offer ARINC 429 as an option. It is always one-way. Use the data buses drill and the data buses, fibre optics and multiplexing note.
5.5 Logic circuits
Gate symbols, truth tables and equivalent circuits for AND, OR, NOT, NAND, NOR and exclusive OR, their uses in aircraft systems, and (B2 only) reading full logic diagrams.
What examiners like: the output of a gate combination for given inputs, recognising that NAND and NOR are universal gates, and a described aircraft logic (for example a warning that needs two conditions). Trap: exclusive OR gives 1 only when inputs differ; candidates mix it up with OR. The logic circuits note has every truth table.
5.6 Basic computer structure and 5.7 Microprocessors
Everyone sits terminology: bit, byte, word, hardware, software, CPU, RAM, ROM, PROM, and how computers are used in aircraft. B2 and B2L go further into how a computer runs a program, buses and memory types, and then into the microprocessor: control unit, clock, registers and the arithmetic logic unit.
What examiners like: volatile versus non-volatile memory, which memory can be reprogrammed in service, and (B2) the order of the fetch-execute cycle. Read the computers and microprocessors note.
5.8 Integrated circuits and 5.9 Multiplexing (B2 and B2L only)
Integrated circuits covers encoders, decoders and the main logic families; multiplexing covers multiplexers and demultiplexers, the principle of sending several signals over one line and selecting them at the other end. Expect short, definition-based questions plus a few that ask how many select lines a multiplexer needs for a given number of inputs.
5.10 Fibre optics
Advantages and disadvantages of fibre over copper, data-bus applications, terminations, couplers, control terminals and remote terminals. Level 1 for B1, level 2 for B2. What examiners like: immunity to electromagnetic interference, weight saving, and the fact that fibre needs careful handling and specialist terminations. This links back to total internal reflection in Module 2 optics.
5.11 Electronic displays
Operating principles of the display technologies used in cockpits: cathode ray tubes, LEDs and liquid crystal displays. Level 2 for B categories. What examiners like: why an LCD needs a backlight and what limits viewing angle. Trap: LEDs emit light; LCDs do not.
5.12 Electrostatic-sensitive devices
Handling ESD-sensitive components: warning labels, wrist straps, grounded mats, antistatic packaging, and why damage may be latent rather than immediate. Everyone sits it, which makes it reliable marks. Expect at least one question asking what to do before removing a sensitive card from an LRU.
5.13 Software management control
Awareness of the restrictions, airworthiness requirements and possible catastrophic effects of unapproved software changes. Not examined for category A. What examiners like: who may load software, why the loaded part number must be checked against the approved configuration, and the role of field-loadable software.
5.14 Electromagnetic environment
Electromagnetic compatibility, electromagnetic interference, high-intensity radiated fields, lightning and lightning protection. What examiners like: the difference between EMC (systems working together without interference) and EMI (the interference itself), and the role of bonding and screening. The ESD, software and EMC note covers 5.12 to 5.14.
5.15 Typical electronic/digital aircraft systems
The general arrangement of systems such as ACARS, fly-by-wire, FMS, IRS, EFIS, ECAM and EICAS, GNSS, TCAS, integrated modular avionics, cabin systems and information systems, and built-in test equipment. Level 1 for everyone: know what each system does, not how to fault-find it. The detail returns in Module 11, 12 or 13; for avionics candidates that means the very long Module 13 paper. Read the typical systems note.
How Module 5 fits with Modules 3, 4 and 13
Module 5 sits on top of the electrical and electronic modules. Module 3 electrical fundamentals gives you voltage, current and circuits; Module 4 electronic fundamentals gives you diodes, transistors and amplifiers, which explain what is inside a logic gate. If you sit them in that order, Module 5 is far easier. For B2 candidates, Module 5 then feeds directly into Module 13, where the data buses and displays reappear inside real aircraft systems.
How to prepare for Module 5
A four-step method
- Get the numbers right first. Number systems and data conversion are pure method. A week of daily conversions makes them free marks for B1 and B2.
- Learn ARINC 429 cold. Draw the 32-bit word from memory until you can label every field. Then learn one distinguishing fact for each other bus.
- Truth tables by hand. Write out every gate's truth table, then practise two- and three-gate combinations.
- Finish with the descriptive sub-modules. ESD, software control, EMC and typical systems reward reading and short quizzes rather than calculation.
Free and Pro practice
- Free Module 5 quick practice for B2: 20 questions with explanations, no account needed.
- B2 Module 5 paper 1: the full 72-question timed paper, free. Choose EASA or UK CAA rules on the start screen.
- Printable Module 5 sheet: 50 questions with an answer key for paper revision or "save as PDF". These are exam-style questions written for the app; neither authority publishes past papers.
- Pro, from £11.99 a month on the pricing page, adds papers 2 to 10, drills by sub-module and every study note, for every category. There is no pass guarantee.
Know the rules before you book
You sit Module 5 at a Part-147 organisation or, in the UK, with the CAA. CAA venues do not allow calculators or other electronic devices, according to the CAA's AML examinations page, so practise conversions on paper. After a fail you wait 90 days (30 days after tailored Part-147 retraining). Under EASA rules the limit is three attempts in any 12 months; under UK rules, three consecutive attempts and then a one-year wait. Passes count towards a licence application for 10 years.
Exam tip: In a 72-question paper, it is easy to lose time on one conversion. Mark it, move on, and come back with the minutes you saved on the definition questions.
Your study plan
- Take the free B2 diagnostic or B1.1 diagnostic to see how Module 5 compares with your other modules.
- Do the free Module 5 quick practice and list the sub-modules you got wrong.
- Read the free electronic instruments and displays note, then work on number systems and data buses.
- Sit Module 5 paper 1 (or the B1.1 paper) timed, on the EASA or UK setting that matches your exam.
- Drill your weakest sub-module, then sit further papers.
- Book when you score 85% on three different papers.
20 free Part-66 Module 5 Digital techniques / electronic instrument systems 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. Inputs A, B each pass through a NAND gate wired as an inverter, and the two inverted signals feed a third NAND gate. The overall function is
- A. AND
- B. OR
- C. NOR
Show answer
B. OR
NOT(NOT A · NOT B) = A + B by De Morgan's theorem, so three NAND gates form an OR gate.
2. An ESD-protected workstation typically includes
- A. a carpeted floor to absorb any static from personnel
- B. a fan blowing very dry air over all the components
- C. a dissipative work mat and wrist strap connected to a common ground
Show answer
C. a dissipative work mat and wrist strap connected to a common ground
The mat, strap and equipment share a common ground point so all items are at the same potential.
3. Cache memory is
- A. a large, slow archive memory for old flight data
- B. a small fast memory holding recently used data near the CPU
- C. a memory that is erased by ultraviolet light
Show answer
B. a small fast memory holding recently used data near the CPU
Cache stores copies of frequently used instructions and data so the processor avoids slower main memory accesses.
4. At very low temperatures, an LCD may
- A. emit excessive light and dazzle the crew
- B. respond slowly, so display heaters are sometimes fitted
- C. require a higher anode voltage to operate
Show answer
B. respond slowly, so display heaters are sometimes fitted
Liquid crystal viscosity rises in the cold, slowing response; heaters or warm-up periods are used.
5. ED-12C / DO-178C defines software levels A to E according to:
- A. the severity of the failure condition the software could cause
- B. the size and structural complexity of the program
- C. the processor hardware the software runs on
Show answer
A. the severity of the failure condition the software could cause
Level A software could contribute to a catastrophic failure and needs the most rigorous assurance; level E has no safety effect.
6. Using repeated division by 16, decimal 171 converts to which hex number?
- A. AB
- B. BB
- C. BA
Show answer
A. AB
Divide repeatedly by 16 and read the remainders from last to first, writing 10-15 as A-F. 171 = AB₁₆.
7. On a 4-to-1 multiplexer with select inputs S1 = 1 and S0 = 0, the output is connected to input
- A. D1
- B. D3
- C. D2
Show answer
C. D2
The select code S1S0 = 10 (binary) = 2, so input D2 is routed to the output.
8. How many output lines does a decoder with 5 input lines have (full decoding)?
- A. 32
- B. 10
- C. 25
Show answer
A. 32
Outputs = 2^n = 2^5 = 32.
9. In a typical EFIS installation, air data and inertial source selection switches allow the crew to
- A. select an alternative sensor if one source fails
- B. increase the brightness of all displays together
- C. test the TCAS aural warnings before flight
Show answer
A. select an alternative sensor if one source fails
Source selection (switching) lets a display be fed from another ADC or IRS so that a single sensor failure does not remove information.
10. A cable shield grounded at both ends is mainly effective against
- A. only DC voltage offsets between units
- B. mechanical abrasion of the conductors
- C. high-frequency and magnetic field interference
Show answer
C. high-frequency and magnetic field interference
Two-end grounding allows induced shield currents that cancel magnetic coupling at higher frequencies; single-end grounding is used for some low-frequency circuits to avoid ground loops.
11. An ARINC 429 bus is physically a
- A. single coaxial cable
- B. fibre optic cable
- C. shielded twisted pair
Show answer
C. shielded twisted pair
ARINC 429 uses a twisted shielded pair with the shield grounded at both ends and at breaks, for good noise rejection.
12. In a fibre optic data bus, 'control terminals' and 'remote terminals' are
- A. the bus controller and the units that respond to it
- B. two different types of fibre splice
- C. the light source and the photodetector
Show answer
A. the bus controller and the units that respond to it
As on wire buses such as MIL-STD-1553 (or the optical MIL-STD-1773), a controlling terminal manages traffic and remote terminals respond.
13. Pipelining in a processor allows
- A. the processor to run without any clock signal at all
- B. several instructions to be at different stages at the same time
- C. memory locations to be accessed without any addresses
Show answer
B. several instructions to be at different stages at the same time
While one instruction executes, the next is decoded and a third fetched, increasing throughput.
14. In an aircraft, an example of an analogue input that must be digitised before a computer can use it is
- A. a thermocouple EGT voltage
- B. an ARINC 429 word
- C. a discrete ground/open signal
Show answer
A. a thermocouple EGT voltage
Thermocouples produce small continuously varying voltages that must be conditioned and converted by an ADC; ARINC 429 is already digital.
15. A 'flight deck effect' in maintenance terms is
- A. a structural load acting on the cockpit floor panels
- B. an indication or message the crew sees as a result of a fault
- C. the lighting level selected on the flight deck panels
Show answer
B. an indication or message the crew sees as a result of a fault
Flight deck effects (e.g. ECAM/EICAS messages) are the crew-visible symptoms used as the starting point for troubleshooting.
16. An OR gate with an inverter on each input is equivalent to a
- A. NOR gate
- B. NAND gate
- C. AND gate
Show answer
B. NAND gate
NOT A + NOT B = NOT(A·B) by De Morgan's theorem, which is the NAND function.
17. Conductive foam is used to store ICs because it
- A. insulates each pin electrically from all the other pins
- B. cools the IC during storage and prevents overheating
- C. shorts all the pins together so no voltage builds up between them
Show answer
C. shorts all the pins together so no voltage builds up between them
Pushing pins into conductive foam holds them at the same potential, preventing a damaging voltage between pins.
18. The part of a computer that holds the program and data currently in use is the
- A. power supply unit
- B. main memory
- C. cooling fan
Show answer
B. main memory
Main memory (RAM, with programs often copied from non-volatile storage) holds instructions and data the CPU is working on.
19. The phosphor 'persistence' in a CRT refers to
- A. how long the phosphor glows after the beam has passed
- B. the voltage needed to deflect the beam
- C. the time taken for the cathode to heat up
Show answer
A. how long the phosphor glows after the beam has passed
Persistence determines flicker and smearing; it is chosen to suit the refresh rate.
20. Modifying the operational software of a flight control computer is permitted only when
- A. done in accordance with approved design data
- B. the aircraft is on the ground and powered
- C. the operator's pilots request it
Show answer
A. done in accordance with approved design data
Software changes form part of design changes that must be approved (e.g. STC, change to type design, approved SB) before embodiment.
Frequently asked questions
+How many questions are in Part-66 Module 5?
It depends on category and system. B2 and B2L: 72 questions in 90 minutes under both EASA and UK rules. B1.1 and B1.3: 40 in 50 minutes in both. B1.2: 40 under EASA, 20 under UK rules. Category A and B3: 20 under EASA, 16 under UK rules.
+What is in Module 5 digital techniques and electronic instrument systems?
Fifteen sub-modules: electronic instrument systems, numbering systems, data conversion, data buses, logic circuits, basic computer structure, microprocessors, integrated circuits, multiplexing, fibre optics, electronic displays, electrostatic-sensitive devices, software management control, the electromagnetic environment and typical digital aircraft systems.
+Is Module 5 harder for B2?
Yes, in length and depth. The B2 paper is 72 questions against 40 for B1, and B2 candidates are examined at level 2 on topics B1 does not sit at all, such as microprocessors, integrated circuits, multiplexing and the interpretation of logic diagrams.
+Which data buses do I need to know for Module 5?
ARINC 429 is the one examined most, so know its word format, speeds and one-transmitter rule. You should also recognise ARINC 629, MIL-STD-1553B, AFDX (ARINC 664) and CAN, and what makes each different. Category A and B3 candidates are not examined on data buses.
+Do I need to convert binary and hex in Module 5?
B1, B2 and B2L candidates do. Expect conversions between decimal, binary, octal, hexadecimal and BCD, done by hand because calculators are generally not allowed. B2 is at level 2 and gets harder conversions; A and B3 do not sit numbering systems.
+Is Module 5 different for B1.2 under UK rules?
Yes. Under UK rules the B1.2 paper is 20 questions in 25 minutes, half the EASA size of 40 questions in 50 minutes. The sub-module list is the same, so the UK paper samples the same content with fewer questions.
+How long should I study for Module 5?
No official figure exists. B2 candidates usually need much longer than B1, because the paper is nearly twice the size and deeper. Book the exam when you score 85% or more on three different full papers at your category's size.
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 13 Questions for B2 and B2LPart-66 Module 13 questions for B2 and B2L: 188 EASA or 180 UK questions, partial exams, B2L system rating papers and every sub-module explained.
- 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.
- Part-66 Module 3 Electrical Fundamentals QuestionsPart-66 Module 3 questions explained: paper sizes for A, B1, B2 and B3, all 18 electrical sub-modules, common traps and a free timed practice paper.