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Module 5 Digital techniques / electronic instrument systems: 50 practice questions

Real EASA paper: Category A 20 Q / 25 min · B1.1 40 Q / 50 min · B1.2 40 Q / 50 min · B1.3 40 Q / 50 min · B2 72 Q / 90 min · B2L 72 Q / 90 min · B3 20 Q / 25 min. Three options, 75% to pass. Answer key on the last page. From part66exam.com · independent study aid, not an official exam or a past paper.

  1. 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

    1. A. AND
    2. B. OR
    3. C. NOR

  2. 2. An ESD-protected workstation typically includes

    1. A. a carpeted floor to absorb any static from personnel
    2. B. a fan blowing very dry air over all the components
    3. C. a dissipative work mat and wrist strap connected to a common ground

  3. 3. Cache memory is

    1. A. a large, slow archive memory for old flight data
    2. B. a small fast memory holding recently used data near the CPU
    3. C. a memory that is erased by ultraviolet light

  4. 4. At very low temperatures, an LCD may

    1. A. emit excessive light and dazzle the crew
    2. B. respond slowly, so display heaters are sometimes fitted
    3. C. require a higher anode voltage to operate

  5. 5. ED-12C / DO-178C defines software levels A to E according to:

    1. A. the severity of the failure condition the software could cause
    2. B. the size and structural complexity of the program
    3. C. the processor hardware the software runs on

  6. 6. Using repeated division by 16, decimal 171 converts to which hex number?

    1. A. AB
    2. B. BB
    3. C. BA

  7. 7. On a 4-to-1 multiplexer with select inputs S1 = 1 and S0 = 0, the output is connected to input

    1. A. D1
    2. B. D3
    3. C. D2

  8. 8. How many output lines does a decoder with 5 input lines have (full decoding)?

    1. A. 32
    2. B. 10
    3. C. 25

  9. 9. In a typical EFIS installation, air data and inertial source selection switches allow the crew to

    1. A. select an alternative sensor if one source fails
    2. B. increase the brightness of all displays together
    3. C. test the TCAS aural warnings before flight

  10. 10. A cable shield grounded at both ends is mainly effective against

    1. A. only DC voltage offsets between units
    2. B. mechanical abrasion of the conductors
    3. C. high-frequency and magnetic field interference

  11. 11. An ARINC 429 bus is physically a

    1. A. single coaxial cable
    2. B. fibre optic cable
    3. C. shielded twisted pair

  12. 12. In a fibre optic data bus, 'control terminals' and 'remote terminals' are

    1. A. the bus controller and the units that respond to it
    2. B. two different types of fibre splice
    3. C. the light source and the photodetector

  13. 13. Pipelining in a processor allows

    1. A. the processor to run without any clock signal at all
    2. B. several instructions to be at different stages at the same time
    3. C. memory locations to be accessed without any addresses

  14. 14. In an aircraft, an example of an analogue input that must be digitised before a computer can use it is

    1. A. a thermocouple EGT voltage
    2. B. an ARINC 429 word
    3. C. a discrete ground/open signal

  15. 15. A 'flight deck effect' in maintenance terms is

    1. A. a structural load acting on the cockpit floor panels
    2. B. an indication or message the crew sees as a result of a fault
    3. C. the lighting level selected on the flight deck panels

  16. 16. An OR gate with an inverter on each input is equivalent to a

    1. A. NOR gate
    2. B. NAND gate
    3. C. AND gate

  17. 17. Conductive foam is used to store ICs because it

    1. A. insulates each pin electrically from all the other pins
    2. B. cools the IC during storage and prevents overheating
    3. C. shorts all the pins together so no voltage builds up between them

  18. 18. The part of a computer that holds the program and data currently in use is the

    1. A. power supply unit
    2. B. main memory
    3. C. cooling fan

  19. 19. The phosphor 'persistence' in a CRT refers to

    1. A. how long the phosphor glows after the beam has passed
    2. B. the voltage needed to deflect the beam
    3. C. the time taken for the cathode to heat up

  20. 20. Modifying the operational software of a flight control computer is permitted only when

    1. A. done in accordance with approved design data
    2. B. the aircraft is on the ground and powered
    3. C. the operator's pilots request it

  21. 21. Each digit of an octal number corresponds to a group of how many binary digits?

    1. A. 4
    2. B. 2
    3. C. 3

  22. 22. The data rate on a time-division multiplexed line carrying eight channels, each at 1 kbit/s, must be at least

    1. A. 8 kbit/s
    2. B. 1 kbit/s
    3. C. 0.125 kbit/s

  23. 23. A decimal-to-BCD encoder is used, for example, to

    1. A. convert a keypad digit into a 4-bit code
    2. B. drive a seven-segment display from BCD
    3. C. select one memory chip from an address

  24. 24. The purpose of the standby (integrated standby) instrument on an EFIS-equipped aircraft is to

    1. A. give attitude, airspeed and altitude independent of the main displays
    2. B. repeat the engine parameters during engine start
    3. C. show maintenance and status pages on the ground

  25. 25. 'Swept stroke' lightning attachment occurs because

    1. A. the lightning channel moves faster than light
    2. B. the static wicks attract the stroke
    3. C. the aircraft moves forward through the lightning channel

  26. 26. In an ARINC 629 system, LRUs are connected to the bus cable by

    1. A. soldered T-junctions in the bus
    2. B. current-mode couplers clamped around the bus
    3. C. fibre optic star couplers only

  27. 27. A fusion splice joins two optical fibres by

    1. A. clamping them in a mechanical sleeve with gel
    2. B. soldering them with a low-melting-point alloy
    3. C. melting the fibre ends together with an electric arc

  28. 28. The accumulator is

    1. A. the register that holds one ALU operand and receives the result
    2. B. the register that holds the next instruction address
    3. C. the counter that divides the clock frequency

  29. 29. Why is an operational amplifier often used at the output of a DAC?

    1. A. To turn the ladder current into a buffered voltage
    2. B. To convert the analogue output into binary form
    3. C. To add a parity bit to the output signal

  30. 30. A flight data acquisition unit collects parameters and sends them to the

    1. A. cockpit voice recorder
    2. B. flight data recorder
    3. C. emergency locator transmitter

  31. 31. A rectangular IEC gate symbol is marked '&'. Which function does it perform?

    1. A. OR function
    2. B. AND function
    3. C. NOT function

  32. 32. Which surface is most suitable for placing an ESD-sensitive component on during maintenance?

    1. A. A polystyrene foam tray
    2. B. A grounded static-dissipative mat
    3. C. A painted wooden bench

  33. 33. Static RAM stores each bit in

    1. A. a tiny capacitor that must be refreshed
    2. B. a fuse that is blown or intact
    3. C. a flip-flop circuit

  34. 34. A precaution when handling or removing a CRT display unit is that

    1. A. the tube may hold a high-voltage charge and can implode
    2. B. it must be kept below 0 °C at all times
    3. C. it must be stored face-down in a water bath

  35. 35. DO-178C software level E applies to software whose failure would have

    1. A. a catastrophic effect
    2. B. no effect on safety
    3. C. a hazardous effect

  36. 36. Subtract the binary number 0101 from 1100.

    1. A. 1001
    2. B. 0111
    3. C. 0110

  37. 37. Which alternative name is used for a demultiplexer?

    1. A. data distributor
    2. B. data selector
    3. C. priority encoder

  38. 38. How many inputs and outputs does an 8-to-3 line encoder have?

    1. A. 3 inputs and 8 outputs
    2. B. 8 inputs and 8 outputs
    3. C. 8 inputs and 3 outputs

  39. 39. In a typical EFIS installation, which two displays are located directly in front of each pilot?

    1. A. Engine display and system display
    2. B. Standby attitude indicator and compass
    3. C. Primary flight display and navigation display

  40. 40. HIRF stands for

    1. A. high-impedance radio frequency
    2. B. high-intensity radiated fields
    3. C. harmonic interference rejection filter

  41. 41. AFDX redundancy is normally provided by

    1. A. a spare end system fitted in each LRU
    2. B. retransmitting any lost frames on request
    3. C. two independent networks carrying the same frames

  42. 42. Attenuation in an optical fibre is normally expressed in

    1. A. ohms per metre (Ω/m)
    2. B. decibels per kilometre (dB/km)
    3. C. volts per metre (V/m)

  43. 43. The ALU receives its operands from

    1. A. registers such as the accumulator, via the internal bus
    2. B. the power supply rails of the processor
    3. C. the output of the clock generator circuit

  44. 44. A synchro-to-digital converter is used to

    1. A. drive a synchro receiver from a digital data bus
    2. B. convert an angular position signal into a digital word
    3. C. convert AC power into DC for the computer

  45. 45. Fly-by-wire systems achieve high reliability by using

    1. A. a single powerful computer
    2. B. mechanical back-up only for the cabin doors
    3. C. redundant, dissimilar computers and power supplies

  46. 46. Q is taken from a NOR gate. One NOR input comes from an X-OR of A with B; the other NOR input is C. Which input combination gives Q = 1?

    1. A. A = 0, B = 1, C = 1
    2. B. A = 1, B = 1, C = 1
    3. C. A = 0, B = 0, C = 0

  47. 47. Low relative humidity in a workshop

    1. A. prevents static charge completely
    2. B. increases the build-up of static charge
    3. C. has no effect on static charge

  48. 48. A microcomputer's control bus carries signals such as

    1. A. read/write, clock and interrupt lines
    2. B. the binary address of each location
    3. C. the operands of each instruction

  49. 49. Why are CRT-based EFIS displays being replaced by LCDs in retrofit programmes?

    1. A. CRTs cannot show attitude information
    2. B. LCDs are lighter, cooler and more reliable
    3. C. LCDs need no symbol generation

  50. 50. Under DO-178C, the software level (DAL) assigned to a program depends on

    1. A. the number of lines of code it contains
    2. B. the programming language chosen
    3. C. the severity of the failure condition it could cause

Answer key

  1. 1. B — NOT(NOT A · NOT B) = A + B by De Morgan's theorem, so three NAND gates form an OR gate.
  2. 2. C — The mat, strap and equipment share a common ground point so all items are at the same potential.
  3. 3. B — Cache stores copies of frequently used instructions and data so the processor avoids slower main memory accesses.
  4. 4. B — Liquid crystal viscosity rises in the cold, slowing response; heaters or warm-up periods are used.
  5. 5. A — Level A software could contribute to a catastrophic failure and needs the most rigorous assurance; level E has no safety effect.
  6. 6. A — Divide repeatedly by 16 and read the remainders from last to first, writing 10-15 as A-F. 171 = AB₁₆.
  7. 7. C — The select code S1S0 = 10 (binary) = 2, so input D2 is routed to the output.
  8. 8. A — Outputs = 2^n = 2^5 = 32.
  9. 9. A — Source selection (switching) lets a display be fed from another ADC or IRS so that a single sensor failure does not remove information.
  10. 10. C — 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. 11. C — ARINC 429 uses a twisted shielded pair with the shield grounded at both ends and at breaks, for good noise rejection.
  12. 12. A — 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. 13. B — While one instruction executes, the next is decoded and a third fetched, increasing throughput.
  14. 14. A — Thermocouples produce small continuously varying voltages that must be conditioned and converted by an ADC; ARINC 429 is already digital.
  15. 15. B — Flight deck effects (e.g. ECAM/EICAS messages) are the crew-visible symptoms used as the starting point for troubleshooting.
  16. 16. B — NOT A + NOT B = NOT(A·B) by De Morgan's theorem, which is the NAND function.
  17. 17. C — Pushing pins into conductive foam holds them at the same potential, preventing a damaging voltage between pins.
  18. 18. B — Main memory (RAM, with programs often copied from non-volatile storage) holds instructions and data the CPU is working on.
  19. 19. A — Persistence determines flicker and smearing; it is chosen to suit the refresh rate.
  20. 20. A — Software changes form part of design changes that must be approved (e.g. STC, change to type design, approved SB) before embodiment.
  21. 21. C — Eight values (0-7) need 2^3 = 8 combinations, so each octal digit maps to 3 bits.
  22. 22. A — The combined line must carry all channels in turn: 8 × 1 kbit/s = 8 kbit/s, plus any synchronisation overhead.
  23. 23. A — Pressing key 0-9 activates one of ten inputs; the encoder outputs the corresponding 4-bit BCD code.
  24. 24. A — Standby instruments (ISIS) provide essential flight data from independent sensors and power so the crew can fly if all main displays fail.
  25. 25. C — As the aircraft flies on, the attachment point is swept aft along the surface from the initial point.
  26. 26. B — Each terminal connects via a stub to a current-mode coupler that inductively couples signals to the unbroken twisted-pair bus, so the bus is not cut.
  27. 27. C — Fusion splicing aligns and welds the glass ends, giving the lowest loss and reflection.
  28. 28. A — In accumulator-based processors most arithmetic uses the accumulator as source and destination.
  29. 29. A — The op-amp acts as a current-to-voltage converter and buffer, giving a low-impedance voltage proportional to the digital input.
  30. 30. B — The FDAU/FDIMU samples many aircraft parameters and formats them for the crash-protected FDR (and often a quick-access recorder).
  31. 31. B — The '&' symbol marks the AND function; with an output inversion mark it becomes NAND.
  32. 32. B — A grounded dissipative mat safely removes charge from items placed on it.
  33. 33. C — SRAM cells are bistable latches that hold data while power is applied without refresh.
  34. 34. A — CRTs are under high vacuum and retain high voltage on the anode; they must be discharged and handled carefully to avoid implosion or shock.
  35. 35. B — Level E software has no safety effect, so DO-178C objectives do not apply beyond identification.
  36. 36. B — 1100 (12) − 0101 (5) = 0111 (7), borrowing from the next column where a 0 − 1 occurs.
  37. 37. A — It distributes one input stream to several outputs.
  38. 38. C — 8 input lines are encoded into a 3-bit binary code (2^3 = 8).
  39. 39. C — EFIS replaces the basic-T attitude and HSI with an electronic ADI (now the PFD) and an electronic HSI (now the ND) for each pilot. Engine and system pages are shown on the central ECAM/EICAS displays.
  40. 40. B — HIRF refers to strong external RF fields from ground radars, broadcast and other transmitters that can upset aircraft electronics.
  41. 41. C — Each frame is sent on both networks; the receiver accepts the first valid copy using sequence numbers ('first valid wins').
  42. 42. B — Optical loss is expressed logarithmically in dB, related to length (dB/km) for the fibre, plus losses at connectors and splices.
  43. 43. A — Operands are fetched into registers and presented to the ALU inputs; results return to a register.
  44. 44. B — Many aircraft sensors give angles as synchro (three-wire) signals; the S/D converter produces a digital angle for the computer.
  45. 45. C — Multiple independent channels, often with dissimilar hardware and software, ensure that a single failure does not cause loss of control.
  46. 46. C — The diagram gives Q = NOT((A ⊕ B) + C). Substituting A = 0, B = 0, C = 0 gives Q = 1; the other two combinations give Q = 0.
  47. 47. B — Dry air lets charges accumulate; higher humidity allows surface leakage that reduces static levels.
  48. 48. A — Control signals coordinate the timing and direction of transfers; addresses and data travel on their own buses.
  49. 49. B — Flat panels reduce weight, heat, power and maintenance compared with CRTs, while offering larger usable areas.
  50. 50. C — Levels A to E are set by the system safety assessment according to the worst failure condition that anomalous software behaviour could contribute to.

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