Category: Avionics Objective Questions

Avionics Multiple Choice Questions on “Primary and Secondary Radar”.

1. Which of the following cannot be measured in primary radar?
a) Height of the aircraft
b) Speed of the aircraft
c) Direction of the aircraft
d) Position of the aircraft
Answer: a
Clarification: One of the major disadvantages of the primary radar is that it cannot provide a three dimensional fix, that is position and altitude. They are radars that can find the height of the aircraft but can only be used for short distances.

2. Which of the following is false with respect to primary radar?
a) Friend or foe identification is possible
b) Height of the aircraft cannot be determined
c) Has a wide range
d) Requires high power
Answer: a
Clarification: Friend or foe identification is not possible in primary radar and hence it is hard to determine if the aircraft is an enemy or friend. It has a wide range and requires high transmitting power due to the two way travel of the radar signal.

3. What type of radar is the primary radar?
a) Pulse
b) Continuous wave
c) Low frequency
d) Analog
Answer: a
Clarification: The primary radar uses pulse techniques. It sends out short bursts of pulse and the aircraft surface reflects the radio energy back to the radar antenna. It is possible to measure the elapsed time and determine the range or distance of the aircraft.

4. What is the full form of RADAR?
a) Radio and ranging
b) Radio range
c) Range detection radio
d) Radio Detection and Ranging
Answer: d
Clarification: The term Radar was coined by the U.S. navy for as the acronym for Radio Detection and Ranging. The term has entered several languages and is used so extensively that it lost capitalization and is used as a common noun.

5. How well a target backscatters radar depends on ________
a) The objects RCS
b) Power of radar
c) Speed of the object
d) Heat signature of the object
Answer: a
Clarification: When radar pulse strikes the aircraft some radio pass through the object whereas some get reflected back. The amount of radio energy that gets reflected back depends upon the Radar cross section of the aircraft. RCS depends on size, material on the surface of the aircraft and many more parameters.

6. The transmitted and received power is the same in a primary radar.
a) True
b) False
Answer: b
Clarification: The radio energy has to travel two way: to the aircraft and back to the receiver, this caused attenuation by the atmosphere. In addition to this, only a part of the actual transmission radio energy is reflected back to the receiver.

7. What is one of the main objectives of a military aircraft design?
a) High heat signature
b) Low RCS
c) High RCS
d) Highly stable configuration
Answer: b
Clarification: Military aircraft usually wish to have a very low RCS or radar cross section so that they are stealthy and undetectable on the enemy’s primary radar. RCS can be reduced by various methods like using radar energy absorbing materials on the surface of the aircraft and not having sharp corners on the aircraft which are more likely to reflect radio energy back to the radar antenna.

8. Which of the following is false with respect to secondary radar?
a) Requires more power than primary radar
b) Needs a transponder device on the aircraft
c) IFF
d) Better range
Answer: a
Clarification: Secondary radar requires less power than primary radar for the same amount of coverage area. This is because in secondary radar the pulse is one way; it triggers a reply pulse in the transponder of the aircraft which replies with some of the important flight parameters of the aircraft.

9. The ability of the antenna to turn radiated energy into conducted energy is called ____________
a) Return ratio
b) Power ratio
c) Capture area
d) RCS
Answer: c
Clarification: The ability of the antenna to turn radiated energy into conducted energy is called a capture area. As an example, if a power density of mw/m2 illuminates an antenna with 1 m2 capture area, then the antenna provides 1 mw power at the output connector.

10. What is the reflected pulse in primary radar called?
a) Reply
b) Return
c) Echo
d) Radio reply
Answer: c
Clarification: The reflected or backscattered radio energy that reaches the radar antenna is called an echo. In secondary radar it is called a reply since the transponder replies for the interrogation signal.

11. Secondary radar can identify an aircraft flying in its range.
a) True
b) False
Answer: a
Clarification: In secondary radar the transponder present in the aircraft reply for the interrogation pulse with information of the aircraft such as flight name and some of the important flight parameters like airspeed and altitude.

12. The strength of a secondary radar varies inversely with distance to the ______ power.
a) Second
b) Fourth
c) Eighth
d) Zeroth
Answer: a
Clarification: In the secondary radar, the strength of the communication system varies inversely with distance to the second power, compared with primary radar which reduced with the fourth power.

13. Which mode of the secondary radar gives only altitude?
a) Mode A
b) Mode B
c) Mode C
d) Mode S
Answer: a
Clarification: There are two types of interrogations in the ATCRBS: mode A and mode C. Mode A provides identity only and mode C provides identity and altitude of the aircraft.

14. The undesirable responses from buildings and clouds for the radar pulse are called as __________
a) Noise
b) Disturbance
c) Clutter
d) Radio error
Answer: c
Clarification: The primary radar pulse is reflected by buildings and clouds and cause undesirable effects in the actual identification. These are called as clutter and can be eliminated by combing primary radar with secondary radar.

15. What is the time separation for P1 and P3 for mode A secondary radar?
a) 8 μs
b) 21 μs
c) 6 μs
d) 4 μs
Answer: a
Clarification: The time delay between P1 and P3 determines the type of interrogation. When the Time lag is 8μs, it corresponds to mode A whereas a separation of 21 μs corresponds to mode C.

Avionics Multiple Choice Questions on “Fly-by-Light Flight Control”.

1. Which of the following affects the Fly-By-Wire system?
a) EMI
b) Lightning strikes
c) Flying in powerful Radio/Radar region
d) Bad weather
Answer: d
Clarification: One of the common modes of failure in the Fly-By-Wire system is EMI. Since the whole aircraft runs on electricity, Electromagnetic Interference heavily affects the FBW system. It is caused by lightning strikes and in regions of high power radio or radar.

2. What is EMP?
a) Electromagnetic pulse
b) Electromagnetic Power
c) Electromagnetic protection
d) Electrically Matched power line
Answer: a
Clarification: An EMP, abbreviated as Electromagnetic pulse is a sudden change in the magnitude of an electrical field(a pulse) with strengths of hundreds of volts per meter. It occurs in a very wide range of electromagnetic radiation spectrum and can damage any electrical equipment in its vicinity.

3. What is EMC?
a) Electromagnetic capture
b) Electromagnetic compatibility
c) Electromagnetic combat
d) Electromagnetic course
Answer: b
Clarification: EMC or Electromagnetic compatibility is the ability of the aircraft equipment to withstand high levels of EMI. This is a necessary condition as many cases have occurred of aircraft experiencing severe transients in the automatic flight control system placing the aircraft temporarily out of control whilst overflying high power radar/radio transmitters. This has been due to the susceptibility of the flight control system analogue electronics to EMI because of inadequate electro-magnetic screening.

4. What is nuclear hardness?
a) Ability to survive a nuclear explosion
b) Ability to survive in the vicinity of nuclear explosion
c) Ability to defend nuclear weapons
d) Ability to drop nuclear weapons with precision
Answer: b
Clarification: Vicinity to a nuclear explosion and consequent emission of gamma radiation and an EMP of very high electric field strength. Military avionic equipment is specially designed to survive up to a certain (classified) level of these effects. This attribute is referred to as ‘nuclear hardness’ and is achieved by the design of the circuits, use of nuclear hard components, circumvention techniques and very efficient electromagnetic screening.

5. Where is an EMC vault used?
a) Housing of FBW computer
b) Housing of all electronics
c) Screened enclosure around the connector
d) Shield for wires
Answer: c
Clarification: The wires from the cable connectors and the EM filters are housed within a screened enclosure around the connector known as an ‘EMC vault’ so that there is no aperture for EMI radiation to enter the box. The wires connecting different electrical equipment are the most affected by EMI.

6. Fiber optic cables are used in aircraft only for faster data transmission.
a) True
b) False
Answer: b
Clarification: In addition to high data rates in the optical transmission medium, another important advantage is its ability to survive EMI. Fiber optics have very less interference by EMI and are used in aircraft mainly for this reason.

7. Which one of the following is false with respect to fiber optic cables?
a) High speed
b) Heavy compared to normal wires
c) Less EM interference
d) Costly
Answer: b
Clarification: Fiber optics weigh comparatively very low than normal wires. Wires require shielding from EMI which also adds up to the overall weight of the connectivity system. They also offer high speed data transfer and are relatively costly.

8. Optical fiber is an insulator.
a) True
b) False
Answer: a
Clarification: Fiber optic cables carries a digitally modulated light intensity signal. IT has no necessity to be made of an conductor as the only condition for light to pass through is an opaque medium. Optical fibers are always made with an insulating material as they offer less EMI.

9. What type of multiplexing is used in optical fiber lines?
a) Time division multiplexing
b) Code division multiplexing
c) Frequency division multiplexing
d) wavelength division multiplexing
Answer: d
Clarification: Fibre optic data transmission use ‘wavelength division’ multiplexing whereby a single fibre can be used to transmit several channels of information as coded light pulses of different wavelengths (or colours) simultaneously. The individual data channels are then recovered from the optically mixed data by passing the light signal through wavelength selective passive optical filters which are tuned to the respective wavelengths. WDM has a very high integrity as the multiplexed channels are effectively optically isolated.

10. Where was the first Fly-By-Light system used?
a) Fighter aircraft
b) Bomber Aircraft
c) Airships
d) Spacecrafts
Answer: c
Clarification: Non-rigid airships may be acquiring a new lease of life as long duration platforms for airborne radar surveillance systems. The airship is able to carry a large high power radar antenna within its envelope and is able to remain airborne for several days. Since the antenna could severely interfere with FBW systems FBL system was used.

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,