Category: Avionics Objective Questions

Avionics Multiple Choice Questions on “Time Division Multiplexing”.

1. Frequency division multiplexing:Frequency slots::time division multiplexing:?
a) Time slots
b) Coded information
c) Pulsed information
d) Band slots
Answer: a
Clarification: In FDM, multiple signals are transmitted over a single channel, each signal being allocated a portion of the spectrum within that bandwidth. In time-division multiplexing (TDM), each signal occupies the entire bandwidth of the channel. However, each signal is transmitted for only a brief time. In other words, multiple signals take turns transmitting over the single channel.

2. Serial transmission is not possible without time division multiplexing.
a) True
b) False
Answer: a
Clarification: In serial transmission, the data is sent via a single cable. When a clock pulse is applied to the shift register it transmits the information bit by bit in allocated time slots.

3. What device is used to demodulate a time division multiplexed analog wave?
a) High pass filter
b) Low pass filter
c) Band stop filter
d) Attenuator
Answer: b
Clarification: the analog signal is converted to a series of constant-width pulses whose amplitude follows the shape of the analog signal. The original analog signal is recovered by passing it through a low-pass filter. In TDM using PAM, a circuit called a multiplexer (MUX or MPX) samples multiple analog signal sources; the resulting pulses are interleaved and then transmitted over a single channel.

4. Which of the following device was used in early TDM/PAM telemetry systems?
a) Commuter
b) Linear switch
c) Logic gates
d) DSP
Answer: a
Clarification: Multiplexers in early TDM/PAM telemetry systems used a form of rotary switch known as a commutator. Multiple switch segments were attached to the various incoming signals while a high-speed brush rotated by a dc motor rapidly sampled the signals as it passed over the contacts.

5. What is the time allocated for each channel if the number of samples per frame is 4 and the frame rate is 100frames/sec?
a) 1.2ms
b) 3ms
c) 2.5ms
d) 0.54ms
Answer: c
Clarification: Time period for one frame = 1/100 = 0.01s = 10ms. During that 10-ms frame period, each of the four channels is sampled once. Each channel is thus allotted 10/4 = 2.5 ms.

6. What is the purpose of one shot multivibrator?
a) Trigger all AND gates at clock frequency
b) Trigger all OR gates at clock frequency
c) Trigger all AND gates at signal frequency
d) Trigger all OR gates at signal frequency
Answer: a
Clarification: The one-shot multivibrator is used to trigger all the decoder AND gates at the clock frequency. It produces an output pulse whose duration has been set to the desired sampling interval.

7. The circuit used to regenerate clock pulses from the transmitted PAM signals is called ____
a) Clock demodulator circuits
b) Timer circuits
c) Clock receiving circuits
d) Clock recovery circuits
Answer: d
Clarification: Instead of using a free-running clock oscillator set to the identical frequency of the transmitter system clock, the clock for the demultiplexer is derived from the received PAM signal itself. A circuit called the clock recovery circuits are typical of those used to generate the demultiplexer clock pulses.

8. What are used to reduce or stop synchronization problems while receiving?
a) Clock recovery circuits
b) Demodulators
c) Synchronizer
d) Band pass filter
Answer: a
Clarification: Clock recovery circuits are used to remedy the synchronization problem encountered in demultiplexing. The clock pulse is derived from the transmitted signal so that synchronization errors are reduced.

9. In a four channel system, all four signals transmitted contain information.
a) True
b) False
Answer: b
Clarification: After clock pulses of the proper frequency have been obtained, it is necessary to synchronize the multiplexed channels. This is usually done with a special synchronizing (sync) pulse applied to one of the input channels at the transmitter. In the four-channel system discussed previously, only three actual signals are transmitted. The fourth channel is used to transmit a special pulse whose characteristics are unique in some way so that it can be easily recognized.

10. Which of the following is not an advantage of time division multiplexing?
a) Signal interference is less
b) More flexible
c) Full channel can be used for every signal
d) Fast data transfer
Answer: d
Clarification: Since the time available for transmission is shared by all the signals that are modulated, Time division multiplexing is not fast when compared with other multiplexing techniques. However, for applications such as telemetry, with a high sampling rate, the speed of time division multiplexing is sufficient to meet the requirements.

Avionics Multiple Choice Questions on “Phases of Flight”.

1. What is the speed at which the takeoff is aborted when there is an engine failure?
a) Decision speed
b) Stall speed
c) Rotational speed
d) Takeoff speed
Answer: b
Clarification: At any speed greater than the decision speed, takeoff is not aborted even when there is an engine failure. Stall speed is lower than the decision speed and hence the takeoff is aborted.

2. The distance the aircraft has to cover in the ground to achieve takeoff speed is called?
a) Ground roll
b) Take off distance
c) Runway length
d) Airborne distance
Answer: a
Clarification: The distance the airplane has to cover in the ground before it can lift off is called the ground roll and it varies from aircraft to aircraft. Generally, bigger aircraft have a larger ground roll distance. This is the reason why huge aircraft need longer runways.

3. The distance the aircraft climbs to clear an obstacle of particular height during takeoff is called as?
a) Ground roll
b) Take off distance
c) Runway length
d) Airborne distance
Answer: d
Clarification: The extra distance the aircraft covers airborne but before it covers an obstacle of particular height is called airborne distance. Airborne distance is also added with the ground roll to give the total takeoff distance.

4. What is the takeoff clearance height for a military aircraft?
a) 50ft
b) 35ft
c) 20ft
d) 100ft
Answer: a
Clarification: The height that the aircraft must cover during the airborne distance is different for different aircraft depending upon their usage. The height of the obstacle is generally specified to be 50ft for military aircraft and 35ft for commercial aircraft.

5. The velocity at which yawing motion can be produced by rudder deflection while the aircraft is on the ground is called _______
a) Ground roll speed
b) Ground control speed
c) Minimum control speed on the ground
d) Control speed
Answer: c
Clarification: Minimum control speed on the ground is the minimum speed at which enough aerodynamic force can be generated in the vertical fin by rudder deflection while the aircraft is still rolling on the ground to produce a yawing moment on the aircraft.

6. Departure and approach are subphases of what?
a) Landing phase
b) Takeoff phase
c) Terminal phase
d) Surface phase
Answer: c
Clarification: The terminal phase consists of the departure and the approach phase. Departure starts when the aircraft is in the ground and ends when it is out of the terminal area. The approach starts when the aircraft enters the terminal control area and ends when the aircraft intercepts the landing aids at approach fix.

7. The height at which approach is aborted when the runway is not in sight is called as?
a) Decision height
b) Approach altitude
c) Clearance altitude
d) Landing altitude
Answer: a
Clarification: Decision altitude is the altitude above the runway at which the approach must be aborted if the runway is not sight. Decision height published for each runway at each airport.

8. A non-precision approach has electronic guidance in only in the horizontal direction.
a) True
b) False
Answer: a
Clarification: A non-precision approach has electronic guidance in only in the horizontal direction. An aircraft executing a non-precision approach must abort if the runway is not visible at minimum descent altitude, which is typically 700ft above the runway.

9. What instrument is used to measure the aircraft’s altitude during the approach phase?
a) Pressure altimeter
b) Radio altimeter
c) Sound altimeter
d) Infrared altimeter
Answer: b
Clarification: A radio altimeter can accurately measure the altitude of an aircraft and works on the radar principle. Typically, radio altimeters provide accurate altitude measurements till 5000ft and are used only during takeoff and landing.

10. Which of the following conditions does not lead to missed approaches?
a) Poor visibility
b) Excess fuel
c) Alignment with runway
d) Traffic
Answer: b
Clarification: A missed approach is initiated at the pilot’s option or at the traffic controller’s request, typically because of poor visibility, poor alignment with a runway, equipment failure, or conflicting traffic. A fuel dump is initiated when there is excess fuel in the aircraft.

Avionics Multiple Choice Questions on “Air Data Equations”.

1. Which of the following is used to obtain the altitude from the static pressure?
a) International atmospheric data
b) Standard atmospheric model
c) International atmospheric tables
d) Standard atmospheric tables
Answer: b
Clarification: To determine the altitude from the measured static pressure, the standard atmospheric model has been established. The standard model gives the relation between a height and the properties of the atmosphere as the solution to a differential equation relating the difference in pressure.

2. What is the height called when the standard temperatures are used?
a) Standard altitude
b) Constant altitude
c) Pressure altitude
d) Total altitude
Answer: c
Clarification: The altitude at which the standard temperatures are used is called the pressure altitude. Pressure altitude is the altitude of the aircraft above sea level. It always takes it’s reference as sea level.

3. Which of the following altitude is measured by the pitot-static tube?
a) Standard altitude
b) Constant altitude
c) Pressure altitude
d) Total altitude
Answer: c
Clarification: Pressure altitude, as the name suggests, is the altitude derived from pressure measurements. It is measured from the static port of the pitot-static probe. The static pressure of air varies with altitude with approximately 101Kpa at sea level.

4. Why is it dangerous to fly over hilly areas with pressure altitude?
a) Affected by temperature
b) Not reliable
c) Costly
d) Not the true altitude
Answer: d
Clarification: Pressure altitude is always referenced from the sea level. When flying over hilly areas, the actual altitude of the aircraft from the ground is definitely lesser than the pressure altitude, thus showing erroneous altitude readings.

5. What does the altitude rate essentially tell us?
a) Vertical speed
b) Rate of climb
c) Rate of glide
d) Rate of decent
Answer: a
Clarification: Air data computers usually provide an output identified as altitude rate which is an outgrowth of the early pneumatic vertical speed indicators which suffer from undesirable lags. Altitude rate combines accelerometer readings and pneumatic readings.

6. Which are the factors that mach number does not depend on?
a) Static pressure
b) Stagnation pressure
c) Static temperature
d) Thrust produced by the aircraft
Answer: d
Clarification: The mach number is computed from the corrected values of static and stagnation pressures. Mach number is comparatively easier to calculate than airspeed in supersonic speeds due to the compression shocks that form in the pitot tube.

7. Why are retractable pitot tubes used in stealth planes?
a) Aerodynamically efficient
b) Stealth configuration
c) Error values in supersonic speeds
d) Use GPS for speed and altitude
Answer: b
Clarification: Stealth planes, such as the B2 spirit stealth bomber, use retractable pitot tubes to maintain their stealth. When not in stealth mode they extend the probes for air data measurements.

8. Why is heating necessary in pitot-static tube?
a) Ice formation
b) Force the flow away from tube
c) Force the flow towards the flow
d) Increase the sensitivity
Answer: a
Clarification: Pitot-static tubes are generally placed with heating elements to stop the formation of ice in the orifice of the probe. Formation of ice on the probes causes erroneous readings and has lead to fatal air crashes.

9. Calibrated speed is the actual speed of the aircraft relative to the wind.
a) True
b) False
Answer: b
Clarification: Calibrated speed is the speed under sea level conditions which would give the same impact pressure. True airspeed is the velocity of the aircraft relative to the local wind.

10. Where will the measurements of static pressure be most accurate?
a) Nose of the aircraft
b) Tail of the aircraft
c) Wing tips of the aircraft
d) Trailing cone
Answer: d
Clarification: The most accurate value of static pressure is when the flow is least affected by the presence of the aircraft itself. In a trailing cone configuration, the probe is far behind the aircraft and is least affected by the flow.

11. What is the speed of the aircraft relative to the ground called as?
a) Relative speed
b) Inertial speed
c) Ground speed
d) True airspeed
Answer: c
Clarification: When you stand on the ground with a radar speed gun pointing at the aircraft, the speed measured is called the ground speed and is measured relative to the ground. True airspeed is measured relative to the wind.

12. Density altitude is the standard parameter for vertical navigation.
a) True
b) False
Answer: b
Clarification: Pressure altitude is the standard parameter used for vertical navigation in controlled navigation. However, density altitude is needed to assess an aircraft’s performance margin.

Avionics Multiple Choice Questions on “Electronic Scanned Array Radar”.

1. What is the fundamental limitation of the radar that is overcome by Electronic scanned array radars?
a) More clarity
b) Situation awareness
c) Better range
d) Less noise
Answer: b
Clarification: The fundamental limitation in military radars is situation awareness. This limitation is overcome by the electronically scanned array which has a very high rate of scanning.

2. How is the radar beam moved in the ESA?
a) Phase shift
b) Mechanical gimbal
c) Mechanical servos
d) Hydraulics
Answer: a
Clarification: In ESA, the antenna’s beam is moved electronically by setting the phase angles of phase shifters located at each radiating element to provide a linear phase taper across the array surface.

3. Which of the following determines the direction of the antenna beam in ESA?
a) Physical direction of antenna
b) Direction of aircraft nose
c) Slope of the phase taper
d) Mechanical gimbal
Answer: c
Clarification: The ESA controls the beam by phase shifters. The slope of the phase taper determines the direction in which the antenna beam will be pointed.

4. The ESA radar beam can be repositioned in a few _______
a) Microseconds
b) Nanoseconds
c) Milliseconds
d) Seconds
Answer: a
Clarification: Since the antenna’s phase shifter settings can be changed in a few microseconds, an ESA’s beam can be repositioned almost instantly.

5. The ESA is more _________ since it has no moving parts.
a) Prone to failure
b) Powerful
c) Reliable
d) Accurate
Answer: c
Clarification: The antenna reliability is dramatically improved in an ESA since there is no moving parts present. The antennas face one direction but the radar beam can be steered electronically to a very wide angle.

6. Which of the following are present in an ESA?
a) Gimbals
b) Electronic motors
c) Phase shifter
d) Potentiometer
Answer: c
Clarification: The ESA has no mechanical moving parts, such as antenna gimbals, potentiometers, rotary joints, or hydraulic/electronic motors. It uses phase shifters to electronically steer the beam.

7. Both AESA and PESA are ________ radars?
a) Pulse
b) Continuous wave
c) Low frequency
d) Analog
Answer: a
Clarification: Both AESA and PESA that are currently in use are pulse radars. Its basic principle is that shorts bursts of radio pulse are emitted which reflect off a target and the reflected pulse gives the information about the object.

8. Which of the following is false with respect to PESA?
a) PESA can only have one main beam and one transmit power source
b) All elements combine to a single receiver. Each element steers the beam through phase shifters
c) Because it uses a, multiple frequencies, it is less susceptible to enemy jammers
d) PESA is more technically simple
Answer: c
Clarification: In a PESA only one transmitter drives the whole antenna and hence only one frequency can be achieved at a time. This allows it to be more susceptible to jammers.

9. What is the number of transmitting modules in a common AESA?
a) 20 to 50
b) 1000 to 2000
c) 150 to 200
d) 1
Answer: b
Clarification: A typical airborne active array contains 1000 to 2000 transmitting modules, each capable of transmitting 5 to 20 watts of power.

10. Which one of the following is common between AESA and PESA?
a) Number of transmitting modules
b) Number of frequencies that can be used
c) Power required by each transmitter
d) Electronic scanning capability
Answer: d
Clarification: One of the basic common features between the AESA and PESA is that both provide electronic scanning. This allows the use of narrow beams since the rate of scanning is very high.

11. AESA is more reliable than PESA.
a) True
b) False
Answer: a
Clarification: An AESA solves many of the reliability and maintainability problems due to TWT and power supply problems. 4 to 6% of the T/R module could fail without noticeable degradation in antenna performance.

12. AESA can alter its transmitting and receiving patterns.
a) True
b) False
Answer: a
Clarification: In AESA a new control variable is introduced. The gain of each Tx and Rx module can be controlled and hence the array radars can adaptively alter their transmit and receive antenna pattern to maximize system performance.

13. Which of the following can track multiple targets with ease?
a) AESA
b) PESA
c) ESA
d) Conical radar antenna
Answer: a
Clarification: AESA radars can produce multiple radar frequencies with multiple beams. This allows the radar to have faster scan rates and can track multiple targets with ease.

14. The possibility of one phased array radar to act as multiple radars is called as _______
a) ESA
b) Multimode
c) Digital
d) interleaving radar modes
Answer: d
Clarification: Beam agility makes it possible for one phased array radar to act as multiple radars each with its own beam shape and scan pattern! This is referred to as interleaving radar modes. The same radar can be tracking for airborne threats using one beam shape and scan pattern while searching for ground targets using another beam shape and scan pattern.

15. In an aircraft ESA the beam shape and direction is controlled by _______
a) Navigator
b) Computer
c) Ground controller
d) Cannot be controlled
Answer: b
Clarification: The beam shape and direction is controlled by an on-board computer which gives the necessary phase shift to the phase shifters to change the beam direction and shape.

Avionics Multiple Choice Questions on “Digital Implementation”.

1. How does the digital implementation in the FBW system reduce weight?
a) Lightweight materials
b) Fewer components
c) System integration
d) Automated control
Answer: c
Clarification: In a typical Digital FBW system, One computer can control all three axes of control, whereas an analog system requires dedicated hardware for each axis of control. The reduction in hardware weight and volume, due to system integration, is of the order of 5:1 for a system of even modest complexity. The more complex systems could not be implemented economically using analogue technology.

2. Which of the following is not a result of digital implementation?
a) Hardware economy
b) Flexibility in updating
c) More power
d) Built in test capabilities
Answer: c
Clarification: The digital implementation in FBW only affects the control and stability of the aircraft. Power is not, in any way, related to the FBW system.

3. What is it called when the sampled signal is of lower frequency than the original signal?
a) Aliasing
b) Overshoot
c) Steady state error
d) Damping
Answer: a
Clarification: When the sampled signal is of lower frequency than the original signal it is known as aliasing. Aliasing induces data loss and errors. It can be reduced by a process called anti-aliasing.

4. How is aliasing prevented?
a) Sampling at the same frequency of the original signal
b) Sampling at double the frequency of the original signal
c) Sampling at less than double the frequency of the original signal
d) Sampling at more than double the frequency of the original signal
Answer: d
Clarification: Aliasing occurs when the sampling rate is lower than the frequency of the original signal. The Nyquist theorem states that the sampling rate should be more than twice the frequency of the original signal to avoid aliasing.

5. How is data staleness overcome?
a) Time synchronisation
b) Down conversion
c) Up conversion
d) Using multiple channels
Answer: a
Clarification: Data staleness problem it is necessary to have some form of time synchronisation of the computer iteration periods. This iteration period synchronisation is generally implemented through software.

6. Which of the following is not an advantage of using a digital data bus?
a) Self test
b) Multiplexing
c) less weight
d) Not affected by electromagnetic interference
Answer: d
Clarification: Very large reductions in the weight of cabling are achieved by the use of multiplexed data transmission and data bus networks. High integrity data transmission can be achieved with very comprehensive self checking and data validation capabilities. However, they are heavily affected by electromagnetic interference.

7. What happens when the output of one channel is delayed by one iteration period before comparing with other channels in a quadruplex system?
a) Latency
b) Staleness
c) Aliasing
d) High noise
Answer: b
Clarification: Data staleness occurs when a channel is delayed by, say, one iteration, before comparison with others. To overcome this problem it is necessary to have some form of time synchronisation of the computer iteration periods.

8. Which of the following is true with respect to the passive optical sensor?
a) Not affected by EMI
b) Does not require an electrical supply
c) Output from the sensor is an optical signal
d) Processing this signal does not require any electrical device
Answer: d
Clarification: Passive optical sensors offers particular advantages as these sensors are not affected by EMI. Passive optical sensors are defined as sensors which do not require electrical supplies or any electronic processing at the sensor, the output of the sensor being an optical signal which is modulated by the quantity being measured. The processing of this optical signal is carried out by a separate electronic unit which is fully screened from EMI, the only connection between the sensor and the electronic unit being an optical fibre cable.