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Module 5: Digital Techniques / Electronic Instrument Systems
- How is bi-directional communication achieved in a standard ARINC 429 system?
- A. A single twisted-pair databus is used for both transmitting and receiving.
- B. Two separate, unidirectional databuses are required.
- C. Four separate databuses are used for redundancy.
Correct Answer: B. Two separate, unidirectional databuses are required.
Explanation: ARINC 429 is a simplex (one-way) protocol. To achieve bi-directional communication between two Line Replaceable Units (LRUs), one LRU must have a transmitter and a receiver, the other LRU must also have a transmitter and a receiver, and they must be connected by two separate databuses, one for each direction of data flow.
- How does a Line Replaceable Unit (LRU) on an ARINC 629 databus determine when it is permitted to transmit?
- A. It waits to be addressed by the bus controller.
- B. It transmits only when it senses a specific ‘terminal gap’ on the bus.
- C. It transmits whenever it senses a ‘signal gap’ on the bus.
Correct Answer: B. It transmits only when it senses a specific ‘terminal gap’ on the bus.
Explanation: ARINC 629 is a time-division multiplexed bus that does not use a central bus controller. Each LRU has a defined Terminal Gap (TG) timer. The LRU can only transmit when it has data to send AND its own TG timer has expired, ensuring that only one terminal can transmit at a time.
- What is a key feature of a simplex databus system?
- A. It has one transmitter and one receiver only.
- B. It has a single bus controller and multiple remote terminals.
- C. It has one transmitter and can have one or more receivers.
Correct Answer: C. It has one transmitter and can have one or more receivers.
Explanation: A simplex system is a one-way communication channel. Data flows in only one direction, from a single transmitting source to one or more ‘sinks’ or receivers.
- How are LRUs physically connected to the ARINC 629 databus?
- A. Through direct pin connections in a multi-pin plug.
- B. Through the use of inductive couplers.
- C. Through the use of fibre optic couplers.
Correct Answer: B. Through the use of inductive couplers.
Explanation: ARINC 629 uses a serial data cable that forms a continuous loop. LRUs are not connected directly to it. Instead, an inductive coupler is used, which allows the LRU to transmit and receive data from the bus without making a direct electrical connection, improving reliability and ease of maintenance.
- In an ARINC 429 data word, what is the function of the parity bit?
- A. To indicate to the receiver that the data word is valid.
- B. To indicate if the data is in BCD or BNR format.
- C. To indicate that the data word is digital.
Correct Answer: A. To indicate to the receiver that the data word is valid.
Explanation: ARINC 429 uses odd parity for error checking. The parity bit (the last bit of the word) is set to a 1 or 0 to make the total number of ‘1’s in the 32-bit word an odd number. The receiver counts the ‘1’s; if the count is even, it knows a transmission error has occurred and rejects the data.
- Which statement best describes the ARINC 429 databus architecture?
- A. It is a Multiple Source, Multiple Sink system.
- B. It is a Single Source, Multiple Sink system.
- C. It is a Single Source, Single Sink system.
Correct Answer: B. It is a Single Source, Multiple Sink system.
Explanation: A standard ARINC 429 bus has only one transmitter (a single source) but can have multiple receivers (up to 20 sinks) listening to the data on that bus.
- What is the primary function of a multiplexer (MUX) in a data system?
- A. It converts a single data input into multiple parallel outputs.
- B. It converts an analogue signal into a digital signal.
- C. It selects one of several input signals and forwards it to a single output line.
Correct Answer: C. It selects one of several input signals and forwards it to a single output line.
Explanation: A multiplexer acts like a high-speed rotary switch. It takes multiple input data streams and, based on a control signal, selects one of them at a time to be transmitted over a single, shared output channel. This is fundamental to creating serial data streams.
- How is light propagation achieved within a fibre optic cable?
- A. The light is continuously reflected off the inner surface of the cladding.
- B. The light is absorbed by the fibre and re-emitted along its length.
- C. The light escapes through the cladding at regular intervals.
Correct Answer: A. The light is continuously reflected off the inner surface of the cladding.
Explanation: Fibre optics work on the principle of Total Internal Reflection (TIR). The core has a higher refractive index than the cladding, causing light signals to bounce off the core-cladding boundary and propagate down the fibre with very little loss.
- In what direction(s) can a single fibre optic cable transmit data?
- A. In one direction only.
- B. In two directions, but not at the same time.
- C. In both directions simultaneously.
Correct Answer: A. In one direction only.
Explanation: A single fibre optic strand is a simplex (one-way) communication link, just like a single ARINC 429 bus. To achieve bi-directional (duplex) communication, two separate fibres are required, one for each direction.
- For a fibre optic system requiring very high bandwidth and transmission speed, what type of fibre would be used?
- A. Graded-index multimode fibre.
- B. Step-index multimode fibre.
- C. Single-mode fibre.
Correct Answer: C. Single-mode fibre.
Explanation: Single-mode fibre has a very small core diameter that permits only one mode (or path) of light to travel down it. This eliminates modal dispersion (signal distortion), allowing for the highest possible bandwidth and data rates, making it ideal for long-distance and high-speed applications.
- How does the speed of light in a fibre optic cable compare to the speed of light in a vacuum?
- A. It is always less than the speed of light in a vacuum.
- B. It is always the same as the speed of light in a vacuum.
- C. It can be greater than the speed of light in a vacuum if the refractive index is high.
CorrectAnswer: A. It is always less than the speed of light in a vacuum.
Explanation: The speed of light is fastest in a vacuum. When light passes through any medium, such as the glass or plastic of a fibre optic core, it slows down. The measure of this slowing is the material’s refractive index.
- When using a Time Domain Reflectometer (TDR) to test a fibre optic cable, how is the location of a fault determined?
- A. The TDR measures the time it takes for a reflected pulse of light to return to the instrument.
- B. The TDR measures the total amount of light lost along the cable’s length.
- C. The TDR sends a continuous light signal and measures the voltage drop.
Correct Answer: A. The TDR measures the time it takes for a reflected pulse of light to return to the instrument.
Explanation: A TDR sends a short pulse of light down the fibre and listens for reflections. By measuring the precise time delay between sending the pulse and receiving the reflection from a fault (like a break or bad connection), it can calculate the distance to that fault with high accuracy.
- What is considered the most common cause of signal attenuation (loss) in a fibre optic system installation?
- A. Bends in the cable that exceed the minimum bend radius.
- B. The refractive index of the core material.
- C. Poorly prepared or contaminated end terminals and connectors.
Correct Answer: C. Poorly prepared or contaminated end terminals and connectors.
Explanation: While excessive bending can cause loss, the vast majority of significant signal loss problems in fibre optic systems are due to improper termination. Contamination (dirt, oil), scratches, or misalignment at the connector end faces will cause significant reflection and scattering of the light signal.
- What is a key advantage of an LCD (Liquid Crystal Display) when compared to an LED (Light Emitting Diode) display?
- A. It produces a much brighter image.
- B. It has a significantly faster response time.
- C. It consumes considerably less electrical current.
Correct Answer: C. It consumes considerably less electrical current.
Explanation: LCDs do not produce their own light; they act as shutters that block or pass light from a separate backlight. This process requires very little power compared to LEDs, which generate light directly and consume more current. This low power consumption is a major advantage of LCD technology.
- When removing a printed circuit card from an Electrostatic Discharge Sensitive (ESDS) assembly, what is the first essential step?
- A. Wear insulating gloves to prevent contact with the card’s components.
- B. Disconnect the aircraft battery before starting the procedure.
- C. Ensure you are properly bonded to the aircraft structure or approved ground point.
Correct Answer: C. Ensure you are properly bonded to the aircraft structure or approved ground point.
Explanation: Before touching any ESDS component, the technician must be at the same electrical potential as the aircraft to prevent a static discharge. This is achieved by correctly attaching a tested wrist strap to an approved grounding point on the airframe.
- What is the primary method used to achieve electromagnetic compatibility (EMC) in aircraft systems?
- A. Enclosing all components in non-conductive housings.
- B. Shielding, bonding, grounding, and the use of interference filters.
- C. Ensuring all systems transmit on the same frequency.
Correct Answer: B. Shielding, bonding, grounding, and the use of interference filters.
Explanation: EMC is achieved by a combination of techniques. Shielding contains electromagnetic fields, bonding and grounding provide a common, low-impedance path for currents to flow safely, and filters are used to block unwanted noise from entering or exiting a unit.
- What does the acronym HIRF stand for?
- A. High Intensity Radiated Field.
- B. Heavily Ionised Radio Frequencies.
- C. High Intensity Radio Frequencies.
Correct Answer: A. High Intensity Radiated Field.
Explanation: HIRF is the standard acronym for High Intensity Radiated Field, which refers to strong electromagnetic fields, typically from external sources like powerful radio or radar transmitters, that can interfere with aircraft systems.
- What information must be manually entered into an Inertial Reference System (IRS) before it can begin the alignment process?
- A. The aircraft’s current latitude and longitude.
- B. The planned cruising altitude for the flight.
- C. The longitude and latitude of the first waypoint.
Correct Answer: A. The aircraft’s current latitude and longitude.
Explanation: The IRS needs to know its starting position on the Earth to begin sensing the Earth’s rotation and gravity, which it uses to align itself and establish True North. This present position must be entered by the pilot before alignment can commence.
- How is an Inertial Reference System (IRS) constructed?
- A. It has accelerometers on a stabilized platform and gyros fixed to the airframe.
- B. It has accelerometers and ring laser gyros that are rigidly fixed to the aircraft structure.
- C. It has both accelerometers and gyros mounted on a mechanically stabilized platform.
Correct Answer: B. It has accelerometers and ring laser gyros that are rigidly fixed to the aircraft structure.
Explanation: Unlike an older Inertial Navigation System (INS) with a mechanical gimballed platform, an IRS is a ‘strapdown’ system. Its sensors (accelerometers and laser gyros) are rigidly mounted (‘strapped down’) to the aircraft structure, and all stabilization calculations are performed mathematically by its computer.
- To compute wind direction and speed, what data source does the Inertial Reference System (IRS) require?
- A. The Doppler Radar system.
- B. The Global Positioning System (GPS).
- C. The Central Air Data Computer (CADC).
Correct Answer: C. The Central Air Data Computer (CADC).
Explanation: The IRS calculates true airspeed and heading. To determine wind, it must compare this information with static pressure, total pressure, and temperature data provided by the Air Data Computer. The difference between the aircraft’s motion through the air and its motion over the ground is the wind.
