Category: Avionics Objective Questions

Avionics Multiple Choice Questions on “Transmission Line Basics – 1”.

1. Which of the following is not a requirement of transmission lines?
a) Minimum attenuation of signal
b) Should not radiate signal
c) High power signal
d) Less noise
Answer: c
Clarification: The primary requirements of a transmission line are that the line introduces minimum attenuation, less noise to the signal and the line did not radiate any of the signals as radio energy. All transmission lines and connectors are designed with these requirements in mind.

2. What is used between the two lead wires of a parallel transmission line?
a) Plastic
b) Ceramic
c) Metal
d) Wood
Answer: a
Clarification: A parallel transmission line is a two-wire balanced line in which insulating spacers have been used to keep the wires separated. Such lines are rarely used today. The spacing between the wires is maintained by a continuous plastic insulator.

3. What is used as a surrounding for the solid center core in a coaxial cable?
a) Plastic dielectric
b) Ceramic
c) Metal
d) Wood
Answer: a
Clarification: The coaxial cable consists of a solid center conductor surrounded by a dielectric material, usually a plastic insulator such as Teflon. Outside the insulation, there is a second conductor made of fine wires.

4. For what purpose was twisted pair cable first used?
a) AM
b) Telephone
c) Wi-Fi
d) Digital data transmission
Answer: b
Clarification: Twisted-pair cable uses two insulated solid copper wires covered with insulation and loosely twisted together. This type of cable was first used in telephone wiring and is still used for that today. But it is also used for security system wiring of sensors and other equipment.

5. What is the type of transmission line in which neither wire is connected to the ground?
a) Unbalanced line
b) Twisted pair cable
c) LAN lines
d) Balanced line
Answer: d
Clarification: A balanced line is one in which neither wire is connected to ground. Instead, the signal on each wire is referenced to ground.

6. What is the phase difference between the current flowing in the two wires of a balanced line?
a) 100°
b) 180°
c) 150°
d) 90°
Answer: b
Clarification: In a balanced line, the same current flows in each wire with respect to ground, although the direction of current in one wire is 180° out of phase with the current in the other wire.

7. In a balanced line, noise is present in the transmission line but cancels at the receiver.
a) True
b) False
Answer: a
Clarification: Balanced-line wires offer significant protection from noise pickup and crosstalk. Because of the identical polarities of the signals on balanced lines, any external signal induced into the cable (noise) appears on both wires simultaneously but cancels at the receiver.

8. What is it called when the noise is present in the transmission line but gets canceled at the receiver?
a) Cancellation mode
b) Common mode cancellation
c) Common mode rejection
d) Rectification
Answer: c
Clarification: When the noise affects the transmission, its effect is on both the wires but since they are 180° out of phase when the signal reaches the receiver the noise gets cancelled out. This is called common-mode rejection, and noise reduction can be as great as 60 to 70 dB.

9. Coaxial cables are ________ lines?
a) Balanced
b) Unbalanced
c) LAN
d) Parallel wire
Answer: b
Clarification: Coaxial cables are unbalanced lines; the current in the center conductor is referenced to the braid, which is connected to ground. Coaxial cable and shielded twisted-pair cable provide significant but not complete protection from noise pickup or cross talk from inductive or capacitive coupling due to external signals.

10. What is the device that converts the balanced line to an unbalanced line called?
a) Unbalancer
b) Balancer
c) Balun
d) Equalizer
Answer: c
Clarification: It is sometimes necessary or desirable to convert from balanced to unbalanced operation or vice versa. This is done with a device called a balun, from “balanced-unbalanced.”

Avionics Questions and Answers for Aptitude test on “Radio Propagation and Noise Characteristics”.

1. Received radio energy is a function of the area of the receiving antenna.
a) True
b) False
Answer: a
Clarification: The received radio energy is a function of the area of the receiving antenna. If the transmission is omnidirectional, the received energy is proportional to the area of the receiving antenna divided by the area of the sphere of radius equal to the distance from the transmitter.

2. The ratio of terminating impedance to the power density of the incident wave is called as?
a) Maximum effective aperture
b) Maximum allowable power
c) Directivity
d) Gain
Answer: a
Clarification: Maximum effective aperture is defined as the ratio of terminating impedance to the power density of the incident wave. It is also called the effective area of an antenna and is given by Dλ/4π.

3. The factor by which a receiver fails to reach the theoretical internal noise limit is known as ________
a) Noise factor
b) Noise
c) Coefficient of noise
d) Noise figure
Answer: d
Clarification: The factor by which a receiver fails to reach the theoretical internal noise limit is often expressed as a ratio, in decibels, and is known as the noise figure. It can also be described as the ratio between the noise power output of a practical receiver and the noise power output of an ideal receiver.

4. What is the maximum limit on the frequency that can be used for ground wave communication?
a) 6MHz
b) 2MHz
c) 5MHz
d) 3MHz
Answer: d
Clarification: Up to 3MHz, an appreciable amount of energy follows the curvature of the earth and are called as the ground wave. Above 3MHz, the radio waves do not follow the curvature of the earth and are used for a line of sight communication.

5. What type of polarization is used for ground waves?
a) Vertical polarization
b) Right circular polarization
c) Horizontal polarization
d) Left circular polarization
Answer: a
Clarification: At lower frequencies, along the surface of the earth, vertical polarization is universally used. This way minimum signal is radiated into the ground.

6. Why can we not use sky waves for navigation?
a) Low power
b) More noise
c) Unpredictable path
d) Not economical
Answer: c
Clarification: The sky waves makes some types of long range communication possible, but is of less value to navigation system because it’s transmission path is unpredictable. A rough location where the sky waves might get reflected can be determined by Snell’s law.

7. Ground waves are not adversely affected by which of the following factors?
a) Conductivity of earth
b) Dielectric of earth
c) Lightning strikes
d) Day/night changes
Answer: d
Clarification: Propagation of ground waves depends on several additional factors. Some of which are conductivity and dielectric constant of earth. Weather changes like thunderstorms and lightning strikes also create extra atmospheric noise and degrade signal.

8. The propagation velocity is not constant in ground waves.
a) True
b) False
Answer: a
Clarification: A characteristic of ground waves is that their propagation velocity is not entirely constant. While the variation is quite small, it is sufficient to limit the ability to obtain fixes at extreme ranges as good as the instrumentation might otherwise permit.

9. The effect of signal simultaneously taking different paths to reach the receiver is called as _______
a) Multipath effects
b) Differential path effects
c) Propagation noise
d) Atmospheric distortion
Answer: a
Clarification: Serious distortion of signal modulation may occur due to different paths simultaneously travelled by the signal between transmitter and receiver. These are called multipath effects.

10. ______ is the error in determining position relative to an earth referenced coordinate system?
a) Absolute error
b) Repeatable error
c) Relative error
d) Differential error
Answer: a
Clarification: The absolute or predictable error is the error in determining position relative to an earth referenced coordinate system. The earth frame is one such coordinate system that has its origin at the Earth’s center of mass and its axes fixed on Earth.

11. The method in which the user’s receiver and the reference station are assumed not to be synchronized in time is called as _______
a) Redundancy method
b) Pseudoranging
c) Integrity method
d) Differential method
Answer: b
Clarification: In this method, the user’s receiver and the reference station are assumed not to be synchronized in time. By measuring several such pseudoranges, the user’s three dimensional position and its time offset can be determined.

Avionics for Aptitude test,

Avionics Questions and Answers for Campus interviews on “Helicopter, Optical and Hyper Sonic Air Data”.

1. Airspeed is not an essential quantity for rotorcrafts.
a) True
b) False
Answer: a
Clarification: Unlike fixed wing aircraft, where knowledge of airspeed is essential for safe flight, a helicopter’s airspeed is not an essential pilotage quality, except for certain engine failure conditions where hover capability is lost.

2. Which of the following cannot be used to obtain an approximate velocity?
a) GPS
b) Doppler radar system
c) Inertial navigation system
d) Primary radar
Answer: d
Clarification: Velocity from Doppler, inertial and GPS can be used as an approximation for velocity. It is possible for the radio controller on the ground to obtain the velocity of the rotorcraft but it is not possible for the pilot to get velocity data from primary radar.

3. The conventional pitot tube cannot be used under______
a) 100 knots
b) 40 knots
c) 2 knots
d) 80 knots
Answer: b
Clarification: The conventional pitot tube and the pressure transducer become ineffective as airspeed drops below 40 knots. At the lower speeds, impact pressure is equal to dynamic pressure q, and the sensitivity of this pressure to a change in velocity is minimal.

4. Why is the static source errors compensated in helicopters?
a) Due to difference in rotor down wash
b) Not economical
c) Requires high computational power
d) Static source error does not matter
Answer: a
Clarification: Static-source errors in helicopters tend to be difficult to compensate because of rotor downwash that differs significantly in and out of ground effect. Fixed wing aircraft do not compensate for static source errors in ground effect, and neither do helicopters.

5. Device that causes the magnitude of pressure to change when the aircraft speed nears zero are called ______
a) Airspeed indicator
b) Low speed indicators
c) Stall indicators
d) Rotating anemometer
Answer: d
Clarification: Rotating anemometer increases the magnitude of pressure change caused by the change in airspeed when the aircraft airspeed is near zero. They are used in low omni range airspeed systems for helicopters.

6. Which of the following is false with respect to rotating anemometer?
a) Used in low speeds
b) High sensitivity
c) Airspeed is measured in forward direction
d) Can be used within the rotor blade
Answer: c
Clarification: Rotating anemometer is like a pitot tube for helicopters to measure low velocities below 40 knots. In addition to obtaining improved sensitivity at low speeds, the rotating probe measures omnidirectional airspeed, including backward velocities.

7. Airspeed measurements can be made by measuring the vortex’s _______
a) Strength
b) Frequency
c) Vorticity
d) Direction
Answer: b
Clarification: A vortex sensor measures vortices shed by fluid flow over a deliberately-inserted obstruction. The frequency of vortices is proportional to the airspeed. This method has been used in helicopters and in ground vehicle fire control systems.

8. Which of the following systems use a gimballed pitot tube?
a) Swiveling pitot tube below rotor
b) Omni directional pitot tube
c) Rotating anemometer
d) Rotating pitot tube
Answer: a
Clarification: In the swiveling pitot tube below rotor system, a gimballed pitot tube containing vane arrangement that causes the tube to align with the airflow within the downwash field emanating from the rotor blades is used. Changes in the airflow field vector are correlated with changes in true airspeed.

9. What is required to accommodate for the ground effect in swiveling pitot tube below rotor system?
a) Secondary radar
b) Doppler
c) GPS
d) Radio altimeter
Answer: d
Clarification: The principle of the probe is that the horizontal component of the induced flow velocity is a repeatable function of horizontal airspeed but varies with ground proximity. Hence, a radar altimeter is required to accommodate the ground effect.

10. What instrument measures the velocity by Doppler shift from backscatter of aerosol particles?
a) Rotating anemometer
b) Doppler radar
c) Laser velocimeter
d) Laser pitot tube
Answer: c
Clarification: In a laser velocitimeter, optical sensors are located within the aircraft and look out through the flow into the free stream. They measure the Doppler shift from backscatter of naturally occurring aerosol particles in the free stream.

11. What is the approximate accuracy of laser velocimeter?
a) 1 knot and below
b) 2.5 knots
c) 10 knots
d) 0.14 knots
Answer: a
Clarification: Test results show an accuracy of 1 knot or better at altitudes where the particle density is adequate. Aerosol densities and particle densities vary with altitude, time and volcanic eruptions.

12. The configuration in which multiple pressure ports are located in the aircraft forebody is called as _____
a) Flush air data system
b) Redundant air data system
c) Multiple port system
d) Nose cone port system
Answer:a
Clarification: Hypersonic vehicles used the concept of locating multiple pressure ports around an aircraft forebody to extract total pressure and flow direction. This type of configuration is referred to as the flush air data system and has been used at NASA Dryden research flights.

13. Air data measurements in hypersonic flight provide primary flight control parameters.
a) True
b) False
Answer: b
Clarification: Air data measurements in hypersonic flight do not provide any primary flight control parameters. They usually support aerodynamic research to confirm structural loading and aero thermal models.

Avionics for Campus Interviews,

Avionics Multiple Choice Questions on “Modes S System”.

1. How does mode S system differ from mode A and C?
a) Extension of secondary radar
b) Uses pulse technology
c) More data can be transferred
d) One way travel of radio signal
Answer: c
Clarification: The mode S system works similar to that of mode A and mode C system and causes a transponder to reply when its identity is not known. In a mode S system, additional data other than just altitude and identity can be provided in the MTI.

2. What is the frequency of interrogation pulse in secondary radar?
a) 1020 MHz
b) 1300 MHz
c) 1030 MHz
d) 1500 MHz
Answer: c
Clarification: The interrogation pulse is transmitted at 1030 MHz and the reply pulse by the aircraft transponder is given at 1090 MHz. Since the reply is coded information, it does not interfere with other transponders that are using the same frequency.

3. Which mode is used for selective calling?
a) Mode A
b) Mode B
c) Mode C
d) Mode S
Answer: d
Clarification: Different modes have its own unique features; mode A gives only identity, mode c gives identity and altitude, mode B is used in European operations and mode S is used for selective calling of transponders.

4. What are the numbers of non-selective interrogations in mode S system?
a) 5
b) 6
c) 7
d) 8
Answer: b
Clarification: Mode S interrogations are divided into two types: selective and non-selective. The non-selective are the simplest and include familiar techniques. There is a total of 6 non-selective interrogations.

5. How does mode S identify each aircraft?
a) Unique code for each transponder
b) Lapsed time
c) RCS
d) Directly talking to the pilot
Answer: a
Clarification: Unlike the ATCRBS where a squawk code is dialed into the transponder, a mode S transponder has a unique identity permanently assigned to it. The mode S radar system can have more than 16 million different identities, enough for each airplane in the world to have a unique identity.

6. The error caused by two aircraft flying close to each other replying to the same interrogation pulse is called ______
a) Garble
b) Fruit
c) Noise
d) Interference
Answer: a
Clarification: If several aircraft are within the range of the secondary radar and reply to the same interrogation pulse there is a chance that two reply pulses can overlap each other and cause an error in the values of interfere with different aircraft.

7. Transmission of modes reply at regular interval to aid TACS is called ________
a) Garble
b) Squitter
c) Fruit
d) Pulsed Radar
Answer: b
Clarification: An important transponder characteristic that aids TCAS is the transmission of a mode S reply at quasi-random intervals of about 1 sec, which is called squitter.

8. A mode S transponder is silent when it is not interrogated.
a) True
b) False
Answer: b
Clarification: Usually, a transponder remains silent when not interrogated. However, being an integral part of TCAS requires the occasional spontaneous mode S transmissions.

9. Both ATCRBS and mode S have the same range.
a) True
b) False
Answer: a
Clarification: Mode S system and ATCRBS have the same interrogation frequency and the same reply frequency. The transmitter power output is similar to the ATCRBS transponder as is the receiver sensitivity which ensures that mode S transponders will have similar service ranges as ATCRBS transponders.