Category: Avionics Objective Questions

Avionics Multiple Choice Questions on “ILS/MLS Coupled Landing System and Automatic Landing – 1”.

1. What is ILS?
a) Instrument landing system
b) Indian Levitation System
c) Indian Landing system
d) International Levitation system
Answer: a
Clarification: ILS or instrument landing system is a radio based approach guidance system installed at major airports and airfields where the runway length exceeds 1800 m which provides guidance in poor visibility conditions during the approach to the runway.

2. Which one of the following can provide a reliable and accurate approach path guidance in category II visibility conditions?
a) ILS
b) MLS
c) SBAS
d) VHF
Answer: c
Clarification: SBAS or Satellite Based Augmentation Systems exploiting differential GPS techniques will be able to provide reliable and accurate approach path guidance in category II visibility conditions. This will be increasingly used in the future.

3. What are the two main components of an ILS on the ground?
a) Horizontal alignment transmitter and vertical alignment transmitter
b) Localizer transmitter and glide slope transmitter
c) Center line transmitter and glide slope transmitter
d) Center line transmitter and approach slope transmitter
Answer: b
Clarification: ILS system basically comprises a localiser transmitter and a glide slope transmitter located by the airport runway together with two or three radio marker beacons located at set distances along the approach to the runway. The airborne equipment in the aircraft comprises receivers and antennas for the localiser, glide slope and marker transmissions.

4. What is the frequency used by the localizer transmitter?
a) VHF
b) UHF
c) MF
d) LF
Answer: a
Clarification: The localizer transmitter used VHF range from 108-122 MHz frequency and provides information to the aircraft as to whether it is flying to the left or right of the center line of the runway it is approaching. The localizer receiver output is proportional to the angular deviation γL, of the aircraft from the localizer beam center line which in turn corresponds to the center line of the runway.

5. What is the frequency used by the glide slope transmitter?
a) VHF
b) UHF
c) MF
d) LF
Answer: b
Clarification: The glide slope or glide path transmission is at UHF frequencies from 329.3 to 335 MHz frequency and provides information to the aircraft as to whether it is flying above or below the defined descent path of nominally 2.5◦, for the airport concerned. The glide slope receiver output is proportional to the angular deviation γV, of the aircraft from the center of the glide slope beam which in turn corresponds with the preferred descent path.

6. The use of autopilot for landing using ILS does not depend on _______
a) Visibility category
b) ILS ground installation standard
c) Runway lighting installation
d) Weight of the aircraft
Answer: d
Clarification: The height limits and visibility conditions in which the autopilot can be used to carry out a glide slope coupled approach to the runway depends on the visibility category to which the autopilot system is certified for operation, the ILS ground installation standard, the runway lighting installation and the airport’s runway traffic control capability.

7. Under what category does zero visibility conditions come?
a) Category I
b) Category II
c) Category III
d) Category IV
Answer: c
Clarification: Visibility conditions are divided into three categories, namely Category I, Category II and Category III, depending on the vertical visibility ceiling and the runway visual range (RVR). Category III includes zero visibility conditions.

8. What is DH?
a) Distance Height
b) Direct Height
c) Decision Height
d) Direction height
Answer: c
Clarification: This minimum permitted ceiling for vertical visibility for the landing to proceed is known as the decision height or DH. A very high integrity autopilot system is required for fully automatic landing below a DH of 100 ft – Cat. III conditions.

9. What is the frequency of marker beacon transmission?
a) 100 MHz
b) 75 MHz
c) 1 GHz
d) 1000 MHz
Answer: b
Clarification: The marker beacon transmissions are at 75 MHz. The middle marker beacon is located at a distance of between 1,000 and 2,000 m from the runway threshold and the outer marker beacon is situated at a distance of between 4,500 and 7,500 m from the middle marker.

10. MLS has a wide selection of channels to avoid interference with nearby airports
a) True
b) False
Answer: a
Clarification: Since microwaves are of higher frequency they can have a wide channel from 300 MHz to 300 GHz. This is particularly useful when two airports nearby are aligned equally. The MLS system an all weather guidance system with wide vertical and horizontal beam widths.

11. At what height is the auto flare initiated?
a) 100 ft
b) 50 ft
c) 25 ft
d) 10 ft
Answer: b
Clarification: The auto flare is initiated around 50 ft where the aircraft is over or very near the runway threshold so that the radio altimeter is measuring the height of the aircraft above the runway. Low range radio altimeters are used to ensure accuracy.

12. What type of controller is used in an auto flare control?
a) Proportional plus derivative
b) Proportional plus integral
c) Proportional plus integral plus derivative
d) Proportional only
Answer: b
Clarification: The auto-flare loop is a high-order system; apart from the lags present in the filtered rad.alt. signals there are also the lags present in the response of the pitch attitude command loop. This loop controls and its response is significantly slower at the low speeds during the approach. A proportional plus integral control term is used in the auto-flare controller to ensure accuracy and some phase advance is generally provided to compensate for the lags in the loop and hence improve the loop stability and damping.

13. What is the velocity at the start of flare maneuver if the approach speed is 130 knots?
a) 2.91 m/s
b) 3 m/s
c) 3.56 m/s
d) 10 m/s
Answer: a
Clarification: Vertical velocity at the start of flare = approach speed x glide slope angle. Thus Vv= 66.87* x sin(2.5°) = 2.91 m/s . *[converting knots to m/s].

14. What is the position accuracy that can be achieved by satellite guidance systems?
a) 10 m
b) 5 m
c) 0.5 m
d) 1 m
Answer: d
Clarification: The navigation position accuracy of 1 m which can be achieved with the differential GPS technique is being exploited in the US for landing guidance with a system called the Ground Based Augmentation System, GBAS. The Ground Based Augmentation System, when installed at an airport, will be able to provide the high integrity and accurate guidance necessary for landing in Cat. III visibility conditions.

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,