Category: Avionics Objective Questions

Avionics Multiple Choice Questions on “Hyperbolic Radio Systems – 1”.

1. Hyperbolic navigational systems are called so because of their hyperbolic lines of position.
a) True
b) False
Answer: a
Clarification: Hyperbolic navigational systems are called so because of the hyperbolic lines of the position they produce rather than the circles and radial lines associated with the system that measures distance and bearing.

2. Which one of the following is not a hyperbolic radio system?
a) Loran-C
b) Omega
c) Chayka
d) VOR
Answer: d
Clarification: Loran-C, Omega, Decca, and Chayka are the hyperbolic navigational systems whereas the VOR, DME falls under the point source navigational systems.

3. What does Omega and Decca have in common?
a) Both measure the phase difference
b) Both measure the time difference
c) Both measure the power difference
d) Both operate in the same frequency range
Answer: a
Clarification: Omega and Decca measure the phase differences between the signals transmitted from pairs of stations while the Loran-C and Chayka measure the time difference between the signal from two or more transmitting stations.

4. Loran-C is a line of sight navigational system.
a) True
b) False
Answer: b
Clarification: Loran is a hyperbolic navigational system that came into usage right before the outbreak of world war II. The Loran-C uses ground waves at low frequencies, thereby securing an operating range of over 1000miles, independent of line of sight.

5. Which of the following is not true with respect to Loran-C?
a) Loran stands for Long range navigation
b) Uses ground waves
c) Uses pulse techniques
d) Subjected to site errors
Answer: d
Clarification: The Loran stands for Long range navigation and uses ground waves with pulsed technique to avoid sky wave contamination. Being a hyperbolic system, it is not subjected to the site errors of point source systems.

6. What is the minimum number of transmitter stations required for a Loran system?
a) 3
b) 2
c) 1
d) 5
Answer: a
Clarification: Loran consists of transmitter stations in groups forming chains. At least three transmitter stations make up a chain. One station is designated as the master while the other is called secondaries.

7. Which technique, in Loran, allows the use of single transmitter station in two chains?
a) Blinking
b) Blanking
c) Pulsing
d) Continuous wave transmission
Answer: b
Clarification: The Loran signal format can be modified to accommodate a single transmitter station in two chains. This is accomplished by permitting transmission for one of the chains to take precedence over the other when the signal format calls for simultaneous transmission in both chains. This function is called blanking.

8. What is the pulse frequency used in Loran?
a) 120MHz
b) 150MHz
c) 100kHz
d) 1000MHz
Answer: c
Clarification: Each station transmits signals that have standard pulse leading edge characteristics. Each pulse consists of a 100kHz that rapidly increases in amplitude in a prescribed manner and then decays at a rate that depends on the particular transmitter.

9. _____ is defined as the portion of pules after 65μsec?
a) Pulse trailing edge
b) Final pulse
c) Decay zone
d) Null
Answer: a
Clarification: The pulse trailing edge is defined as that portion of the Loran-C pulse following the peak of the pulse or 65μsec after the pulse is initiated. The pulse trailing edge is controlled in order to maintain spectrum requirements.

10. The short term propagation effects caused by local weather changes are called as ______________
a) Seasonal
b) Temporal
c) Diurnal
d) Noise
Answer: c
Clarification: The diurnal variations are short term propagation effects caused primarily by local weather changes and day/night transitions along the signal path. Variations in a refractive index of the atmosphere versus height from the ground contribute to the short term propagation errors.

11. What is the process of ensuring that the receiver works on ground waves?
a) PGTR identification
b) ECD
c) HCG
d) TOPCO
Answer:a
Clarification: PGTR or Pulse group time reference identification is the process of ensuring that the receiver is operating on the ground wave of the signal. The basic principle behind it is that ground waves travel faster than sky ways.

12. Which of the following is not a function of the monitoring facility in Omega systems?
a) Monitor performance
b) Acts as relays
c) Provide data to phase synchronize the stations
d) Detect solar terrestrial events
Answer: b
Clarification: Each station has a signal monitoring facility that is situated 20 to 50km away from the antenna. These stations perform functions such as monitor signal performance, provide the required data to phase synchronize the stations, and detecting solar terrestrial events that cause shifts in the propagated signal phase.

13. Which of the following is false with respect to Omega systems?
a) Four common transmitted signal frequencies
b) One unique frequency for each station
c) 0.2 sec separation between each of eight transmissions
d) Constant length transmission periods
Answer: d
Clarification: The Omega system has variable length transmission periods. This makes it possible for users to synchronize an Omega receiver to the signal format with no additional extra information.

14. Which of the following techniques reduces the diurnal variation but does little effect on the phase behaviour of signal of equal path length over different environments?
a) Composite Omega
b) Propagation corrections
c) PPC
d) Subtracting the signal phases at two of the frequencies
Answer: a
Clarification: The composite Omega technique reduces the diurnal variation but does little to reduce the wide variation in phase behaviour exhibited by paths of equal length over substantially electromagnetic/geophysical environments.

15. What is the region in which Omega signals are confined according to VLF wave guide model known as?
a) Earth-Atmosphere wave guide
b) Sea-Ionosphere wave guide
c) Earth-Sea wave guide
d) Earth-Ionosphere wave guide
Answer: d
Clarification: In the wave guide model of VLF wave propagation the region in which the Omega signals are confined is known as the Earth-Ionosphere wave guide.

Avionics Multiple Choice Questions on “Doppler Radar”.

1. What is the primary function of the Doppler radar?
a) Determining true airspeed
b) Determining ground speed
c) Determining altitude
d) Detecting stealth aircraft
Answer: b
Clarification: The primary function of a Doppler radar is to determine the velocity vector of an aircraft with respect to the ground. The velocity component can be integrated into distance travelled and hence Doppler radar can be used as a primary sensor for dead reckoning.

2. The Doppler radar works by measuring the ________ of the reflected radio wave.
a) Intensity
b) Frequency
c) Doppler amplitude
d) Doppler shift
Answer: d
Clarification: the velocity is determined by measuring the Doppler shift of microwave transmitted from the aircraft in several narrow beams pointed toward the surface at relatively steep angles, backscattered by the surface and received by the Doppler radar receiver.

3. Which of the following is not true with respect to Doppler radar system?
a) Measures the ground velocity
b) Transmitter power is high and requires ground assistance
c) Velocity is extremely accurate
d) All weather system
Answer: b
Clarification: The Doppler system is self contained, it requires no ground based stations or satellite transmitters. The airborne transmitter power requirements are extremely small, which leads to low weight, size, and cost of equipment.

4. Doppler radar cannot be used in helicopters.
a) True
b) False
Answer: b
Clarification: Helicopters fly at a very slow speed where the conventional pitot tube system may fail. In order to measure accurate airspeed below 40 knots, other systems such as the Doppler radar is used. It is suitable for three dimensional velocity and low velocity measurement, as required for helicopter navigation.

5. What is the change in the observed frequency called?
a) Doppler shift
b) Differential frequency
c) Delta frequency
d) Delta shift
Answer: a
Clarification: In Doppler radar, there is a change in reflected frequency when there is relative motion between the transmitter and the receiver. This change in frequency is called the Doppler shift and is proportional to the relative speed between the transmitter and receiver.

6. The configuration in which the Doppler radar beams are both forward and backward looking is called as?
a) Janus
b) Dual
c) Redundant
d) Wide beam
Answer: a
Clarification: Since the three orthogonal components of velocity are of interest, a minimum of three noncoplanar beams are required to measure the three components. Since such a beam configuration has both forward and rearward looking beams it is called Janus configuration, named after the roman god who has the ability to look backward as well as forward.

7. What is the number of beams that a Doppler radar uses?
a) 4
b) 2
c) 1
d) 3
Answer: a
Clarification: Although only three beams are required to provide the three components of velocity, most modern Doppler radars employ four beams, because of the planar array antennas generally four such beams. The fourth beam shift can be combined with that of the third to obtain a more accurate component of velocity.

8. A Janus configuration has less error than a non-Janus configuration.
a) True
b) False
Answer: a
Clarification: The important advantage that the Janus configuration has over the non-Janus type is a much lower sensitivity of velocity error since the vertical altitude of the aircraft is known.

Avionics Questions and Answers for Freshers on “ILS/MLS Coupled Landing System and Automatic Landing – 2”.

1. What is the beam error in localizer for aircraft CG displacement of 5m at a range of 1,500m?
a) 0.0033 RAD
b) 3.3 RAD
c) 0 RAD
d) 5 RAD
Answer: a
Clarification: The beam error γ = d/R, where d is the displacement of the aircraft’s CG from the beam center line and R is the slant range of the aircraft. γ = 5/1,500 = 0.0033 RAD.

2. The guidance sensitivity decreases as the range decreases
a) True
b) False
Answer: b
Clarification: The beam error of a localizer or glide path transmitter is given by γ =d/R, Where R is the slant range of the aircraft. It is evident that the beam error is inversely proportional to the slant range. As range increases, beam error decreases and hence the guidance sensitivity is more.

3. When does an ILS localizer coupling loop become unstable?
a) Aircraft range is infinity
b) Aircraft range is close to zero
c) When too many aircraft are present
d) When no aircraft is present
Answer: b
Clarification: In the localizer coupling loop gain increases as range between aircraft and runway decreases. The gain reaches a point where it becomes too large and the loop becomes unstable. Thus gain scheduling with range is required.

4. What is missing in the localizer coupling loop?
a) Flight computer
b) ILS Localizer receiver
c) ILS glide path receiver
d) ILS Localizer transmitter
Answer: b
Clarification: The Localizer coupling loop is integrated with the heading command loop to manage the heading of the aircraft. It is a closed loop system which continuously corrects with respect to the ILS Localizer. The error in flight path is corrected with respect to the localizer beam by means of a localizer receiver in the aircraft.

5. Which one of the following is not a basic parameter used to define visibility category?
a) Decision height
b) Minimum vertical visibility
c) Runway length
d) Runway visual range
Answer: c
Clarification: The two basic parameters used to define the visibility category are the decision height, that is the minimum vertical visibility for the landing to proceed and the runway visual range. The length of a runway is not used to define visibility category.

6. Which of the following is not one of the operational limits and autopilot requirement for a category IIIc visibility condition?
a) Runway guidance required to taxi point
b) Fully automatic landing system with flare
c) Probability of catastrophic failure of less than 10−7 per hour required
d) Decision height of 5m
Answer: d
Clarification: The autopilot requirements and operational limits for a category III visibility conditions are a probability of catastrophic failure of less than 10−7 per hour, fully automatic landing system with flare, runway guidance required to taxi point. The decision height for a category IIIc condition is 0m. No system is yet certified for category IIIc operation.

7. How is altitude measured when the aircraft is about to land?
a) Pitot static port
b) Pitot stagnation port
c) Radio altimeters
d) Inertial navigation systems
Answer: c
Clarification: When an aircraft is about to land, accurate altitude measurements are needed. Pitot static measurements suffer from errors due to weather and generally have lag. The aircraft height above the ground is measured by very accurate radio altimeters during the landing phase.

8. Why is flare maneuver executed during landing phase?
a) To align with the runway
b) To reduce airspeed and rate of descent
c) To stabilize the aircraft during crosswinds
d) To conserve fuel
Answer: b
Clarification: The flare maneuver is the increase in pitch of the aircraft seconds before touching the ground. This is done to reduce the airspeed and the rate of descent. If the flare is not executed correctly it may result in a runway overrun, landing gear collapse or a tail strike.

9. Why is the kick off drift maneuver executed?
a) To align with the runway
b) To reduce airspeed and rate of descent
c) To stabilize the aircraft during crosswinds
d) To conserve fuel
Answer: a
Clarification: Just prior to touchdown a ‘kick off drift’ maneuver is initiated through the rudder control so that the aircraft is rotated about the yaw axis to align it with the runway. This ensures the undercarriage wheels are parallel to the runway center line so that no sideways velocity is experienced by the wheels when they make contact with the runway.

10. What is HREF in the control law used for auto flare?
a) Height when flare is initiated
b) Height where flare should be initiated
c) Maximum height where flare can be initiated
d) Small negative height
Answer: d
Clarification: In the control law for auto flare, HREF is a small negative height, or bias, which ensures there is still a small downwards velocity at touchdown. This avoids the long exponential ‘tail’ to reach zero velocity and enables a reasonably precise touchdown to be achieved.

11. The final approach path in a satellite landing guidance can be curved
a) True
b) False
Answer: a
Clarification: Since GPS does not use radio beams to align the aircraft to the runway center line the final approach path need not be limited to straight line approaches, but can be curved or stepped, horizontally or vertically. The life cycle costs of a GBAS is also only a fraction of ILS or MLS.

12. What controls the efficient functioning of an engine?
a) Autopilot
b) FMS
c) FBW
d) FADEC
Answer: d
Clarification: The FADEC or Full Authority Digital Engine Control is a system that controls all aspects of the engine performance. Since there are a lot of variables involved in the control of different engines, the workload of the pilot is increased. The FADEC system is used to reduce the workload at the same time giving full control of the engine to the pilot. It also ensures maximum engine performance at different flight conditions.

13. The response of a jet engine thrust to the throttle angle is instantaneous.
a) True
b) False
Answer: b
Clarification: The response of the jet engine thrust to throttle angle movement is not instantaneous and approximates to that of a simple first-order filter with a time constant which is typically in the range 0.3 to 1.5 seconds, depending on the thrust setting and flight condition. Clearly, the lag in the throttle servo actuator response should be small compared with the jet engine response.

Avionics for Freshers,

Avionics Assessment Questions and Answers on “Noise Suppression Effects of FM”.

1. Which of the following is not a source of noise in FM?
a) Weather
b) Electronic circuits
c) Lightning
d) Velocity of motion

Answer: d
Clarification: Lightning strikes, ignition systems, motors, electronic circuits and weather create interference signals called noise. They are generally of high frequency and spikes of voltages.

2. Which circuit in FM receivers cancels or filters out noise?
a) Filter circuits
b) Anti-noise circuits
c) Limiter circuits
d) delimiter circuits

Answer: c
Clarification: The receivers in FM contain limiter circuits which restrict the amplitude of the received signal. The noise which only affects the amplitude of the signal causes variations in amplitude of the signal. The limiter circuit clips off any variation in amplitude to filter out most of the noise.

3. Noise introduces a frequency variation into the signal.
a) True
b) False

Answer: a
Clarification: The change in amplitude by the noise introduces a phase shift which is reflected as a small frequency variation in the signal. These frequency variations changes or distorts the signal.

4. What is the frequency deviation produced by noise if the signal to noise ratio is 3:1 and modulating frequency is 800Hz?
a) 152.32Hz
b) 482.5Hz
c) 132.8Hz
d) 271.Hz

Answer: d
Clarification: ɸ=sin^-1(N/S)=sin^-1(1/3)=sin-1(0.3333)=19.47°=0.34rad
δ=ɸ(fm)=0.34(800)=271.8Hz.

5. What is the noise to signal ratio if the phase shift introduced by noise is 0.75°?
a) 0.5
b) 0.966
c) 0.25
d) 1.75

Answer: b
Clarification: ɸ=sin^-1(N/S)
(N/S)=sin(ɸ) =sin(75°)=0.966.

6. What is the frequency deviation produced by noise if the modulating frequency is 400Hz and the phase difference introduced by noise is 0.43rad?
a) 124Hz
b) 163Hz
c) 172Hz
d) 200Hz

Answer: c
Clarification: δ=ɸ(fm)=0.43(400)=172Hz.

7. Calculate the phase difference by noise if the Signal to noise ratio is 7:2.
a) 15.366°
b) 17.5°
c) 13.65°
d) 8.21°

Answer: a
Clarification: ɸ=sin^-1(N/S)=sin^-1(2/7)=sin^-1(0.265)=15.366°.

8. What is the ratio of the shift produced by the noise to the maximum allowed deviation if the modulating frequency is 300Hz and the phase shift by noise is 25° and the maximum allowable deviation is 5kHz?
a) 1.3
b) 0.530
c) 0.153
d) 0.0261

Answer: d
Clarification: δ=ɸ(f_m)= 0.436(300)=130.89Hz
Frequency deviation by noise / maximum deviation = 130.89/5000=0.0261.

9. What is the signal to noise ratio if the maximum allowable frequency deviation is 4kHz and the frequency deviation by noise is 156.42Hz?
a) 0.0364
b) 0.0391
c) 25.75
d) 20.45

Answer: c
Clarification: N/S = Frequency deviation by noise / maximum deviation = 156.42/4000 =0.0391
S/N = (N/S)^-1=25.57.

10. What is the technique in which the high frequency components are amplified more than the low frequency components in FM?
a) Garble
b) Preemphasis
c) Detoriation
d) Selective amplification

Answer: b
Clarification: Preemphasis is a technique in which the high frequency signals are amplified more than the lower frequency signals to have better resistance to noise. It is usually used to transmit sounds from musical instruments which have high frequency harmonics.

11. Which of the following pairs of resistors and capacitors can be used in high pass filters for preemphasis circuits?
a) 100Ω and 0.075μF
b) 100Ω and 0.75μF
c) 150Ω and 0.75μF
d) 16Ω and 0.75μF

Answer: b
Clarification: The time constant for a high pass filter to be used in the preemphasis circuit is 75μs. R x C = 75μs. Thus when R is 100Ω and C is 0.75μF, R x C = 75μs.

12. Find the frequency at which the signal enhancement flattens out in preemphasis circuit if R1=50Ω ,R2=70Ω and C=0.45μF.
a) 12126.09Hz
b) 550Hz
c) 10036..52Hz
d) 9004.56Hz