Category: Avionics Objective Questions

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.

Avionics test on “Transmission Line Basics – 2”.

1. A current carrying conductor is only called a transmission line if the length of the line is _____ long at the signal frequency?
a) 0.1λ
b) 0.3λ
c) 1.2λ
d) 1.5λ
Answer: a
Clarification: A pair of current carrying conductors is not considered to be a transmission line unless it is at least 0.1λlong at the signal frequency.

2. Cables that carry RF signal are not ______
a) Capacitors equivalents
b) Inductors equivalents
c) Simple resistive conductors
d) Covered
Answer: c
Clarification: Cables used to carry RF energy are not simply resistive conductors but are complex equivalents of inductors, capacitors, and resistors. If simple resistive conductors are used then undesirable effects such as noise, power loss occurs.

3. For an operating frequency of 450 MHz, what length of a pair of conductors is considered to be a transmission line?
a) 0.754ft
b) 0.219ft
c) 0.35ft
d) 0.542ft
Answer: b
Clarification: A pair of conductors does not act as a transmission line unless it is at least 0.1λ long.
λ = 984/450 = 2.19 ft
0.1λ = 2.19 x (0.1) = 0.219 ft.

4. Calculate the physical length of the transmission line for an operating frequency of 450MHz and 3⁄8λ long.
a) 0.45
b) 0.84
c) 0.82
d) 0.41
Answer: c
Clarification: λ = 984/450 = 2.19 ft
(3/8)λ = 2.19(3)/8 = 0.82 ft.

5. Which of the following is not a type of coaxial cable connector?
a) PL-259
b) Banana pin connector
c) SMA
d) BNC
Answer: b
Clarification: The most common types of coaxial cable connectors are the PL-259 or UHF, BNC, F, SMA, and N-type connectors. Banana pin connectors are used in power cables.

6. Which of the following is called as UHF connector?
a) PL-259
b) N-type connector
c) SMA
d) BNC
Answer: a
Clarification: The PL-259 is also referred to as a UHF connector, can be used up to low UHF values (less than 500 MHz), although it is more widely used at HF and VHF. It can accommodate both large (up to 0.5-in) and small (0.25-in) coaxial cable.

7. Which of the following is widely used for connecting coaxial cables test instruments?
a) PL-259
b) N-type connector
c) SMA
d) BNC
Answer: d
Clarification: BNC connectors are widely used on 0.25-in coaxial cables for attaching test instruments, such as oscilloscopes, frequency counters, and spectrum analyzers, to the equipment being tested. BNC connectors are also widely used on 0.25-in coaxial cables in LANs and some UHF radios.

8. Which type of coaxial cable connector is the least expensive?
a) PL-259
b) F-type connector
c) SMA
d) BNC
Answer: b
Clarification: The least expensive coaxial cable connector is the F-type connector, which is widely used for TV sets, VCRs, DVD players, and cable TV. The cable plug and its matching chassis jack are shown in Fig. 13-6. The shield of the coaxial cable is crimped to the connector, and the solid wire center conductor of the cable, rather than a separate pin, is used as the connection. A hex-shaped outer ring is threaded to attach the plug to the mating jack.

9. Which is the best type of coaxial cable connector in terms of performance?
a) PL-259
b) N-type connector
c) SMA
d) BNC
Answer: b
Clarification: The best-performing coaxial connector is the N-type connector, which is used mainly on large coaxial cable at the higher frequencies, both UHF and microwave. N-type connectors are complex and expensive, but do a better job than other connectors in maintaining the electrical characteristics of the cable through the interconnections.

10. Two parallel lines appear as a complex impedance when the transmission line is equal to the wavelength of the signal.
a) True
b) False
Answer: b
Clarification: When the length of a transmission line is longer than several wavelengths at the signal frequency, the two parallel conductors of the transmission line appear as a complex impedance. The wires exhibit considerable series inductance whose reactance is significant at high frequencies.

11. Parallel conductors form a distributed inductance with the insulation.
a) True
b) False
Answer: b
Clarification: The parallel conductors, in a transmission line, form a distributed capacitance with the insulation, which acts as the dielectric. This causes characteristic impedance.

To practice all areas of Avionics for tests,

Avionics Multiple Choice Questions on “Point Source Radio Systems”.

1. Which type of direction finders takes bearings on airborne transmitters and advice the aircraft of its bearing from the ground station?
a) Ground based
b) Airborne direction finders
c) Homing adapters
d) Homing beacon
Answer: a
Clarification: Ground based direction finders take bearings on airborne transmitters and advice the aircraft of its bearing from the ground station. Such stations can afford the necessary complex equipment, but the operation is cumbersome and time consuming, and requires airborne transmitter and communication link.

2. Which of the following is not true with respect to airborne direction finder?
a) They are also called homing adapters
b) Uses the simplest of systems
c) Very low error
d) Take bearings on the ground transmitter
Answer: c
Clarification: Airborne direction finder or homing adapters take bearing from ground transmitters. These direction finders typically can afford only the simplest of systems and must, therefore, tolerate large errors.

3. Which of the following type of direction finder has a system that rotates the loop, using a servo system until a null is found?
a) Fixed loop
b) Rotatable loop
c) Rotating loop
d) Fixed, crossed loop
Answer: c
Clarification: The rotating loop is driven by a motor and forming part of a servo system that rotates the loop until a null is found and then stops. They are sometimes referred to as the radio compass and are housed inside plastic structures that do not reflect radio waves.

4. Which of the following is not true with respect to loop antennas?
a) Horizontal polarization is assumed
b) Resonance is achieved by a variable capacitor
c) The horizontal antenna pattern is a figure of eight
d) The currents are equal in amplitude and phase when the antenna is 90° with the transmitted signal
Answer: a
Clarification: The loop antenna is a rectangular loop of wire whose inductance is resonated by a variable capacitor. The signal is assumed to be vertically polarized and the horizontal antenna pattern is a figure of eight. When the loop is 90° with respect to the transmitted signal the currents induced are of same amplitude and phase.

5. What can be added to reduce signal ambiguity in a loop antenna?
a) Low noise amplifier
b) Amplifiers
c) Sense antenna
d) Increase the antenna size
Answer: c
Clarification: The horizontal pattern of the loop antenna is a figure of eight which has two null positions 180° apart. This ambiguity will cause the system to give the same indication whether it is pointing toward a station or away from it. A sense antenna can be added when the signal ambiguity must be resolved.

6. What is used to combine the output of the two fixed loop antenna that are placed at right angles to each other?
a) Oscilloscope
b) Spectrum analyzer
c) Mixer
d) Goniometer
Answer: d
Clarification: The goniometer has two sets of fixed windings at right angles to each other, each set connected to one loop. It combines the output from the loop antenna into miniature a magnetic field in which a rotor can operate.

7. The area of reduced signal strength in a non directional beacon is called as ______
a) Cone of silence
b) Zero reception zone
c) Zone of silence
d) Non radio zone
Answer: a
Clarification: The NDB works on an omnidirectional antenna which has the radio pattern of a donut. The zone on the middle which is directly above the antenna is called as the cone of silence. In this region, no signal is transmitted.

8. Which of the following is not an advantage of NDB?
a) Inexpensive
b) Omnidirectional
c) Accuracy depends on airborne equipment
d) No mountain and night effect
Answer: d
Clarification: NDB uses ground waves which follow the curvature of earth. They are inexpensive and independent of accuracy issues but suffer from night effect and mountain effect. Mountain effect is the distortion of ground wave in terrain of nonuniform character.

9. What frequency does the marker beacon use for fixes along the airways?
a) 50MHz
b) 25MHz
c) 500MHz
d) 75MHz
Answer: d
Clarification: Marker beacons were developed for better fixes along the airways in addition to directional information. They operate at 75MHz and radiate a narrow pattern upward from the ground.

10. Which of the following can be used to reinforce the upward beam in a marker beacon?
a) Reflector
b) Amplifier
c) Counterpoise
d) Vertical polarization
Answer: c
Clarification: The marker beacons use an array of half wave radiators that are aligned in line with the airway. A wire mesh counterpoise below this array can be used to reinforce the upward beam by increasing the signal strength in a particular direction.

11. What is the frequency of rotation of the directional beam in a VOR?
a) 20rps
b) 15rps
c) 30rps
d) 50rps
Answer:c
Clarification: The VOR sends out two signals. One is a highly directional signal that rotates at a frequency of 30 rps and an omnidirectional signal. The omnidirectional signal is pulsed every time the directional signal coincides with north.

12. Which of the following generates the same field pattern as a vertical dipole antenna?
a) Yagi antenna
b) Helix antenna
c) Alford loop
d) Microstrip antenna
Answer: c
Clarification: Each radiator in the ground station transmitter is an Alford loop. The Alford loop generates a horizontal polarized signal having the same field pattern as a vertical dipole.

13. What is the time gap between each pulse in an interrogation pulse pair for a DME?
a) 12μsec
b) 5μsec
c) 15μsec
d) 10μsec
Answer: a
Clarification: The interrogation pulses are in pairs. Each pulse is placed 12μsec apart with each pulse lasting 3.5μsec. The pulse pair repetition rate ranges between 5 pulse pairs per sec to a maximum of 150 pulse pairs per sec.

14. Why are pulse pairs used in DME instead of single pulses?
a) Reduce interference
b) Better range
c) Reduced noise
d) Redundancy
Answer: a
Clarification: Pulse systems can easily interfere with each other even if they are of different frequencies. Paired pulses are used in DME to reduce interference with other pulsed systems.

15. What is the frequency in which the DME transmits?
a) 50Mhz above or below the received frequency
b) 20Mhz above the received frequency
c) 40Mhz below the received frequency
d) 63Mhz above or below the received frequency
Answer: d
Clarification: The Ground beacon in the DME receives the airborne pulses, and after a 50μs delay, retransmits them back to the aircraft on a frequency 63Mhz above or below the airborne transmitting frequency. Here the frequency received by the DME is the Airborne transmitting frequency.

Avionics Multiple Choice Questions on “Vertical & Heading References”.

1. How to determine the true vertical reference?
a) Differentiating
b) Time average of apparent vertical
c) Integration
d) Sum of all the apparent vertical
Answer: b
Clarification: The simplest approach to the determination of the true vertical is to time-average the direction of the apparent vertical(vector sum of gravity and vehicle acceleration).

2. When is the gyro interrupted in erection cutoff method?
a) Deviation is large
b) Deviation is low
c) Never interrupted
d) Always interrupted
Answer: a
Clarification: The error caused in the vertical referencing can be rectified by introducing nonlinearities. One such method is the erection cutoff where the precession of the gyroscope is interrupted when a large deflection is sensed.

3. Which one of the following is the basic vertical reference in all forms of heading and attitude devices?
a) Line perpendicular to artificial horizon
b) Earth’s gravitational field
c) Line perpendicular to actual horizon
d) Earth’s magnetic field
Answer: b
Clarification: The basic vertical reference in all forms of heading and attitude reference devices is the Earth’s gravitational field. The direction of Earth’s gravitational field can be sensed with great accuracy in stationary systems. However, when the platform moves, all the instruments indicate the vector sum vehicle acceleration and local gravity.

4. The error that occurs due to the rotation of Earth as well as the aircraft’s own motion is called as ______
a) Asymmetrical error
b) Precession rate error
c) Rate compensation error
d) Standoff constant error
Answer: d
Clarification: A stand off constant error exists since the direction of the local vertical changes as a result of both the rotation of the Earth and the aircraft’s own motion around the Earth.

5. A special mode for initial leveling is needed when using erection cutoff mode.
a) True
b) False
Answer: a
Clarification: When erection cutoff is used, there is a need for a special mode of operation that provides initial leveling, because no precession rate would exist in the presence of large initial tilt.

6. What can be done to reduce to avoid poor acceleration error averaging?
a) Use higher order leveling loop
b) Use lowest order leveling loop
c) Use switch gravity sensors
d) Use differential leveling loop
Answer: a
Clarification: The stand off error depends upon the gravity sensors switch’s dead and gives poor acceleration errors. Significant improvement can be obtained in the averaging vertical reference by using a high order leveling loop.

7. The relative acceleration of a vehicle moving with respect to a rotating Earth can be divided into three parts.
a) True
b) False
Answer: a
Clarification: When a vehicle moves at a velocity relative to the rotating Earth, the inertial acceleration can be divided into three parts: Acceleration relative to the surface of Earth, Coriolis acceleration caused by rotation of Earth, and a correction that depends on the coordinate frame in which the gyro is constrained.

8. Inertial heading references are better than a magnetic compass.
a) True
b) False
Answer: a
Clarification: The best airborne heading references are the inertial navigators. Less expensive, smaller and less accurate references are those that depend on the Earth’s magnetic field, those that depend on the use of gyroscopes to retain a preset azimuth, and those that use subinertial gyros to maintain a three axis reference.

9. Which of the following is not a unit of the magnetic field?
a) Gamma
b) Gauss
c) Tesla
d) Maxwell
Answer: d
Clarification: The magnetic fields B are measured in Gauss and Tesla(10⁴ Gauss). In geomagnetism, Units of “Gamma”(10^-5 )are used.

10. What is the orientation of the Earth’s magnetic axis with respect to its spin axis?
a) 2°
b) 0°
c) 11°
d) 24°
Answer: c
Clarification: The Earth’s magnetic field is produced by a bar magnet oriented 11°away from the spin axis and passing within a hundred kilometers of its center.

11. Which of the following is not the angle between the horizontal component of B and true north?
a) Magnetic declination
b) Magnetic variation
c) Dip angle
d) Magvar
Answer: c
Clarification: The magnetic declination, also called the magnetic variation or magvar, is the angle between the horizontal component of B and true north. Navigators use ‘magvar’ in order not to confuse astronomical declination with magnetic declination.

12. Why does magvar have diurnal change?
a) Magnetic pole migration
b) Static buildup error
c) Sensitivity of the instruments
d) Variation in aircraft magnetic effects
Answer: a
Clarification: Magvar has a diurnal change, a long term change due to the migration of magnetic poles, and random changes because of magnetic storms. The magnitude of the magnetic field changes about 0.5% per year.

13. Which of the following is used to sense aircraft magnetic fields?
a) Magnetic compass
b) Airborne magnetometer
c) Accelerometer
d) Gyroscope
Answer: b
Clarification: Aircraft have magnetic fields that add to the Earth’s field and are sensed by the airborne magnetometers. The induced error, called deviation, can be 10° or more.

14. Magnetic compass is also called as whiskey compass.
a) True
b) False
Answer: a
Clarification: The simplest compasses measure the orientation of the nearly free permanent magnet attached to a buoyant body floating in a liquid. Since the liquid is usually alcohol, it is also called as whisky compass.