Category: Avionics Objective Questions

Avionics Multiple Choice Questions on “Pulse Code Modulation”.

1. Which of the following type of multiplexing uses pulse code modulation?
a) Frequency division multiplexing
b) Time division multiplexing
c) Code division multiplexing
d) Amplitude limited multiplexing
Answer: b
Clarification: The most popular form of TDM uses pulse-code modulation (PCM), in which multiple channels of digital data are transmitted in serial form. Each channel is assigned a time slot in which to transmit one binary word of data. The data streams from the various channels are interleaved and transmitted sequentially.

2. Which of the following statements is true with respect to PCM?
a) The parallel binary data is converted into serial before transmission
b) Analog data is transmitted directly
c) Analog signal is amplified before transmission
d) The analog signal is converted into parallel binary data before transmission
Answer: a
Clarification: The analog signal is converted into a digital signal. Since the converted digital signal is parallel bits of data, it has to be converted to serial before it can be multiplexed and transmitted.

3. Which of the following is false with respect to PCM?
a) Reliable
b) Inexpensive
c) Resistant to noise
d) Not easily recoverable
Answer: d
Clarification: When signals have been degraded because of noise, attenuation, or distortion, all the receiver has to do is to determine whether a pulse was transmitted. Amplitude, width, frequency, phase shape, and so on do not affect reception. Thus PCM signals are easily recovered and rejuvenated, no matter what the circumstances.

4. A special PCM system uses 16 channels of data, one whose purpose is identification (ID) and synchronization. Find (a) the number of available data channels.
a) 15
b) 16
c) 14
d) 18
Answer: a
Clarification: 16 (total no. of channels) – 1 (channel used for ID) = 15 (for data).

5. A special PCM system uses 16 channels of data, one whose purpose is an identification (ID) and synchronization. The word length is 6 bits. Find the number of bits per frame.
a) 94
b) 95
c) 96
d) 125
Answer: c
Clarification: Here, Data channels = 16 and word length is 6 bits.
Bits per frame = 6 x 16 = 96.

6. A special PCM system uses 16 channels of data, one whose purpose is an identification (ID) and synchronization. The sampling rate is 3.5 kHz. The word length is 6 bits. Find the serial data rate.
a) 451kHz
b) 326kHz
c) 152kHz
d) 336kHz
Answer: d
Clarification: Serial data rate = sampling rate x no. bits/frame = 3.5 kHz x 96 = 336 kHz.

7. All local and long-distance connections are digital.
a) True
b) False
Answer: a
Clarification: All modern telephone systems use digital transmission via PCM and TDM. The only place where analog signals are still used is in the local loop—the connection between a telephone company’s central office (CO) and the subscriber’s telephone, known as the customer premises equipment (CPE). All local and long-distance connections are digital.

8. In a TDD one channel is sufficient for transmission.
a) True
b) False
Answer: a
Clarification: Time-division duplexing (TDD) means that signals are transmitted simultaneously on a single channel by interleaving them in different time slots. Each time slot may contain one data word, such as 1 byte from an A/D converter or a D/A converter. As long as the serial data rate is high enough, a user will never know the difference. The primary benefit of TDD is that only one channel is needed. It saves spectrum space and cost.

Avionics Multiple Choice Questions on “Navigation Equations”.

1. The navigation equations are used by the on-board computer to calculate position velocity and altitude.
a) True
b) False
Answer: a
Clarification: The navigation equations how the sensor outputs are processed in the on-board computer in order ti calculate the position, velocity and attitude of the aircraft. The navigation equations contain instructions and data and are part of the airborne software that also includes moding, display drivers, failure detection and an operating system.

2. Which one of the following is not an input utilized by the aircraft navigation system?
a) Sonar
b) Radio aids
c) Dead reckoning data
d) Celestial measurements
Answer: a
Clarification: The navigation system utilizes three types of sensor information: position data from radio aids, radar and satellites; Dead reckoning data from inertial sensors; Line of sight direction to stars or celestial measurements.

3. What is the angle between the normal to the reference ellipsoid and the equatorial plane at a point called?
a) Geodetic angle
b) Correction factor
c) Geodetic latitude
d) Earth angle
Answer: c
Clarification: Geodetic latitude of a point is the angle between the normal to the reference ellipsoid and the equatorial plane. Geodetic latitude is our usual understanding of map latitude.

4. Which of the following is false with respect to Earth centered inertial coordinate system?
a) Accuracy is 10^-5/hr
b) Newton’s laws are not valid
c) Angular coordinates of stars are conventionally tabulated in ECI
d) Used in mechanizing inertial navigators
Answer: b
Clarification: The Earth centered coordinate system has three navigational functions. First, Newton’s laws are valid in any ECI coordinate frame, Second, the angular coordinates of stars are conventionally tabulated in ECI. Third, they are used in mechanizing inertial navigators.

5. Which coordinate system is used in satellite based radio navigation?
a) Earth centered, Earth fixed
b) Earth centered inertial
c) Geodetic spherical coordinates
d) Geodetic wander azimuth
Answer: a
Clarification: Satellite based radio navigation system uses ECEF coordinated to calculate satellite and aircraft position. The origin is at the mass center of the earth and its axis lies along the earth’s spin axis.

6. Geocentric spherical coordinates are used for short range dead reckoning navigation.
a) True
b) False
Answer: a
Clarification: Geocentric coordinates are sometimes mechanized in short range dead reckoning systems using spherical earth approximation. These are the spherical coordinates of the radius vector R.

7. What is the technique of calculating position from measurements of velocity called?
a) Dead reckoning
b) Doppler radar
c) Secondary surveillance
d) Celestial navigation
Answer: a
Clarification: Dead reckoning is the technique of calculating position from the measurements of velocity. It is the means of navigation in the absence of position fixes and consists in calculating the position by extrapolating measured ground speed.

8. The angle of arrival, relative to the airframe, of a radio signal from an external transmitter is called _________
a) Bearing
b) Heading
c) Phase
d) Approach angle
Answer: a
Clarification: The angle of arrival, relative to the airframe, of a radio signal from an external transmitter is called bearing. Bearing is measured by the difference in phase or time of arrival at multiple antennas on the airframe.

9. ________ related ranging errors to the dispersion in measured position?
a) GEOP
b) GDCP
c) GDOP
d) APEG
Answer: c
Clarification: GDOP or Geometric Dilution of Precision relates ranging errors to the dispersion in measured position. If three range measurements are made in the orthogonal direction, the standard deviation in the aircraft’s position error is the same as those of the three range sensors.

10. What is TDOP?
a) Time Dilution of pseudorange
b) Time Dilution of position
c) Time Dilution of pressure
d) Time Distortion of position
Answer: b
Clarification: TDOP is the Time Dilution of position. It is the contribution of clock error to the error in pseudorange.

Avionics Multiple Choice Questions on “Air Data Computers”.

1. ______ is used for setting stabilizer position and high speed performance boundaries?
a) Mach number
b) True airspeed
c) Calibrated airspeed
d) Altitude
Answer: a
Clarification: Mach number is used for programming stabilizer position for speed stability, defining high speed performance boundaries such as flutter onset, and for defining optimal cruise paths.

2. Which of the following does not come under air data computer?
a) Pressure ports
b) Pressure transducers
c) Computer
d) Output drivers for interfacing
Answer: a
Clarification: A typical central air data computer is a box containing: the pressure transducers, associated excitation circuitry, and signal conditioning circuitry, the computer, and the output drivers that are compatible with interfacing subsystems.

3. An air data computer can be used in multiple aircraft.
a) True
b) False
Answer: a
Clarification: The CADC includes program pin status which is designated connector pins that allow a standard CADC to serve in more than one aircraft. Thus, pin i could recognize that the installation is in the aircraft I, thereby activating its own pitot correction algorithm.

4. Which one of the following is not a part of the processor subassembly?
a) CPU
b) Volatile RAM
c) Non volatile RAM
d) Non erasable ROM
Answer: d
Clarification: The air data equations are solved in the processor assembly, which contains the CPU, memory for the operational flight program which can be electrically erased or EEPROM, data storage memory( usually a volatile RAM), and non volatile RAM.

5. Which of the following includes A/D conversions and packing of discrete signals into logical words?
a) Input processing
b) Output processing
c) Signal processing
d) Mainframe processor
Answer: a
Clarification: Input processing usually involves A/D conversion and packing of discrete signals into logic words, plus implementation of special serial data interfaces.

6. Which system has replaced the CADC by using smaller and cheaper microprocessors?
a) Navigational system
b) Distributed air data system
c) FADEC
d) Inertial system
Answer: b
Clarification: With the advent of smaller and cheaper microprocessors, it became possible to pack them with probes and transducers. The result is a distributed air data system that replaces the CADC. A key feature is the packing of signal processing units with or adjacent to the probes.

7. A distributed air data system uses a centralized flight control computer for calculations.
a) True
b) False
Answer: a
Clarification: A distributed air data system provides corrected pressure, temperature, and angle of attack data to a centralized flight control computer that computes altitude, mach, calibrated and true airspeed.

8. Which of the following is not an advantage of distributed air data system?
a) Elimination of pneumatic plumbing
b) Reliability
c) Easy maintenance
d) Better accuracy
Answer: d
Clarification: Accuracy is independent of the architecture used. The accuracy of a CADC is same as that of a distributed air data system but might suffer from lags occurring from long lengths of tubing.

9. Distributed air data system reduces the overall weight of the aircraft.
a) True
b) False
Answer: a
Clarification: CADC uses long tubing from the probes to the CADC for the transfer of pressure data whereas distributed air data system transmits the data via electric wires. Electric wiring weighs less than tubing and the electrical transmission of pressure information eliminate lags.

10. Which one of the following is not an advantage of centralized architecture?
a) Simple design
b) Software is easily coded
c) Computer is readily accessible
d) Software is flexible
Answer: d
Clarification: In centralized architecture, the software is not flexible. A minor change or update in one of the parts of the software requires the whole software to be written and checked again. Updating the software to the rapidly changing hardware is not possible.

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.